Tyrosinase and ocular diseases: Some novel thoughts on the molecular basis of oculocutaneous albinism type 1

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Progress in Retinal and Eye Research 26 (2007) 323–358

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Tyrosinase and ocular diseases: Some novel thoughts on themolecular basis of oculocutaneous albinism type 1

Kunal Ray�, Moumita Chaki, Mainak Sengupta

Molecular and Human Genetics Division, Indian Institute of Chemical Biology, 4 Raja S. C. Mullick Road, Jadavpur, Kolkata 700 032, India

Abstract

Tyrosinase (TYR) is a multifunctional copper-containing glycoenzyme (�80 kDa), which plays a key role in the rate-limiting steps of

the melanin biosynthetic pathway. This membrane-bound protein, possibly evolved by the fusion of two different copper-binding

proteins, is mainly expressed in epidermal, ocular and follicular melanocytes. In the melanocytes, TYR functions as an integrated unit

with other TYR-related proteins (TYRP1, TYRP2), lysosome-associated membrane protein 1 (LAMP1) and melanocyte-stimulating

hormone receptors; thus forming a melanogenic complex. Mutations in the TYR gene (TYR, 11q14-21, MIM 606933) cause

oculocutaneous albinism type 1 (OCA1, MIM 203100), a developmental disorder having an autosomal recessive mode of inheritance. In

addition, TYR can act as a modifier locus for primary congenital glaucoma (PCG) and it also contributes significantly in the eye

developmental process. Expression of TYR during neuroblast division helps in later pathfinding by retinal ganglion cells from retina to

the dorsal lateral geniculate nucleus. However, mutation screening of TYR is complicated by the presence of a pseudogene–TYR like

segment (TYRL, 11p11.2, MIM 191270), sharing �98% sequence identity with the 30 region of TYR. Thus, in absence of a full-proof

strategy, any nucleotide variants identified in the 30 region of TYR could actually be present in TYRL. Interestingly, despite extensive

search, the second TYR mutation in 15% of the OCA1 cases remains unidentified. Several possible locations of these ‘‘uncharacterized

mutations’’ (UCMs) have been speculated so far. Based on the structure of TYR gene, its sequence context and some experimental

evidences, we propose two additional possibilities, which on further investigations might shed light on the molecular basis of UCMs in

TYR of OCA1 patients; (i) partial deletion of the exons 4 and 5 region of TYR that is homologous with TYRL and (ii) variations in the

polymorphic GA complex repeat located between distal and proximal elements of the human TYR promoter that can modulate the

expression of the gene leading to disease pathogenesis.

r 2007 Elsevier Ltd. All rights reserved.

Keywords: Albinism; Oculocutaneous albinism type 1; OCA1; Tyrosinase; TYR

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 324

2. TYR: the gene, the protein and evolution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 325

2.1. Structure of the gene and its regulation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 325

2.2. Protein structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 326

2.3. Protein expression. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 328

2.4. Evolutionary significance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 328

2.4.1. Evolution of TYR . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 328

2.4.2. TYR vs. its pseudogene. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 330

2.4.3. TYR and TYRPs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 331

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ARTICLE IN PRESSK. Ray et al. / Progress in Retinal and Eye Research 26 (2007) 323–358324

3. Processing and maturation of TYR. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 332

3.1. ER maturation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 332

3.1.1. Translocation in the ER lumen and signal sequence cleavage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 332

3.1.2. BiP binding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 332

3.1.3. Glycosylation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 334

3.1.4. Lectin chaperone binding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 335

3.1.5. Oligomerization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 336

3.1.6. Disulfide bond formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 336

3.1.7. ER quality-control and ER associated degradation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 337

3.2. Transport from ER to Golgi . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 337

3.3. Golgi maturation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 337

3.3.1. Glycosylation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 337

3.3.2. Copper loading. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 337

3.4. Protein targeting to melanosome . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 338

3.5. Melanosomal factors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 339

4. Functional aspects of TYR. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 340

4.1. Pigmentary system and melanin biosynthesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 340

4.2. Visual system development . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 341

4.2.1. Axonal projections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 342

4.2.2. Development of the central retina and the rod photoreceptors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 343

4.3. Retinal network adaptation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 344

4.4. Modifier locus in developmental glaucoma . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 344

5. TYR and oculocutaneous albinism (OCA) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 345

5.1. Pigmentation genes and OCA types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 345

5.2. TYR in OCA type 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 346

5.2.1. Mutation profile . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 349

5.2.2. OCA1 cases lacking the second mutation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 349

6. Future directions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 351

Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 352

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 352

1. Introduction

The most important element in the human pigmentarysystem is melanin—a light-absorbing high molecularweight biopolymer (Nordlund et al., 1998). Thoughinitially it was thought to be exclusively composed ofindole 5,6-quinone, it is now known that physiologicalmelanins are much more heterogeneous in nature. Mela-nins are involved in a variety of important functions,such as, coloration, giving protection against ultraviolet(UV) radiation and participating in the developmentalprocesses (Spritz and Hearing, 1994). Melanogenesisis a multistep process in which tyrosine is first hydroxy-lated to form 3,4-dihydroxyphenylalanine (DOPA); DOPAis then oxidized to dopaquinone. Thereafter a cascadeof reactions occurs in multiple directions, which ultimatelygives rise to the formation of eumelanin (black andbrown polymers) and pheomelanin (red and yellowpolymers).

The most important enzyme in melanogenesis is tyrosi-nase (TYR) that is expressed in epidermal, follicular andocular melanocytes and catalyses the first two rate-limitingsteps of melanin biosynthesis (Lerner et al., 1950). The firsthint of TYR activity was noted in 1895, when the Frenchnaturalists, Bourquelot and Bertrand observed that toad-stool Russula nigricans bore a colorless substance, whichblackened upon exposure to air. A year following this

discovery, Bertrand recognized that colorless substance astyrosine. In 1904, it was discovered that extracts of amammalian melanoma could convert tyrosine to melanin.These observations paved the way for the research ofmelanogenesis and in subsequent years it was revealedthat TYR was the enzyme, which acted upon tyrosine,ultimately leading to the formation of melanin. In 1908,Sir Archibald Garrod, a British physician and scientistsuggested that albinism, the disease characterized by thepartial or whole loss of pigmentation, was probably dueto the failure of an intracellular enzyme. In late 1950s,it was conclusively proved that albinism (at least theform that was being studied) was indeed due to theinactivity of TYR. This happened to be a much-neededlink between albinism and the loss of TYR activity(Gerritsen, 2004).Human TYR, encoded by the gene TYR (11q14-21,

MIM 606933), is a membrane-bound copper enzyme—a multifunctional monophenol monoxygenase. Mutationsin TYR lead to the absence or decreased synthesis ofmelanin and thus cause oculocutaneous albinism type 1(OCA1, MIM 203100). OCA, in general, is a heteroge-neous group of autosomal recessive disorders representingcongenital hypopigmentation, associated with other devel-opmental abnormalities of eye characterized by fovealhypoplasia, abnormally low visual acuity, nystagmus,strabismus and optic nerve misrouting at the chiasm.

ARTICLE IN PRESSK. Ray et al. / Progress in Retinal and Eye Research 26 (2007) 323–358 325

Although previously it was believed that OCA1 is solelycaused by the deficiency of TYR activity, it is currentlyknown that some forms of OCA1 are also endoplasmicreticulum (ER) retention diseases. In the later cases, themisfolded mutant TYRs are retained by the ER quality-control machinery during the protein maturation processand are degraded by subsequent proteolysis. The minimalstructural requirements that allow TYR and other mela-nosomal proteins to exit ER are still not fully elucidated(Olivares et al., 2003). Hence, it is essential to characterizeas well as to understand the sorting signals recognized byER quality-control components and at the same time it isalso important to have the detailed information aboutTYR sorting. It must be mentioned here that TYR is notonly the key enzyme in melanin formation, but alsocontributes significantly in the eye development processby a yet unknown mechanism. Its expression duringneuroblast division helps in later pathfinding of the retinalganglion cell (RGC) axons from retina to the dorsal lateralgeniculate nucleus (dLGN) of the brain. Moreover, it canalso act as a modifier locus in case of primary congenitalglaucoma (PCG). This review focuses on TYR—itsstructure and evolution, protein maturation as well asfunction and most importantly its role in the developmentof OCA1. Interestingly, in 15% of the OCA1 cases thesecond TYR mutation remains unidentified despite ex-tensive mutation screening. Those ‘‘uncharacterizedmutations’’ (UCMs) have been suggested to be locatedupstream of the gene within the locus control like regionor alternatively the defect may be present at a secondlocus (digenic scenario). We propose two additionalpossible locations, which on further investigations mightshed light on these hidden mutations in TYR of OCA1patients.

Fig. 1. Schematic representation of the promoter region of human tyrosinase

including the putative E-Box element (*), which is to be validated by functional

E-box motifs within Tyrosinase Distal Element (TDE) and Tyrosinase Proxima

regulatory sequence. The sequence of the GA complex repeat (236 bp) reveals th

an arrowhead). The locations of the promoter elements are indicated by their

2. TYR: the gene, the protein and evolution

2.1. Structure of the gene and its regulation

TYR is the first albinism gene that was cloned. Kwonet al. (1987) screened a lambda-gt11 human melanocytecDNA library with antibodies against hamster TYR toisolate a clone for human TYR (Kwon et al., 1987).A subsequent cross-species comparison showed that TYR

cDNA mapped to the mouse albino c-locus (chromo-some 7). The involvement of TYR in albinism wasdemonstrated through its ability to rescue the albinophenotype in transgenic animals (Beermann et al., 1990;Tanaka et al., 1990; Sturm et al., 1998). Human TYR gene(TYR, 11q14-q21, MIM 606933) consists of 5 exons andspans �65 kb of the genomic region. The 2082 bp transcript(Accession No: NM_000372.4) of TYR encodes a �80 kDaglycoprotein (Accession No: NP_000363.1) composed of529 amino acids.The mechanisms controlling pigment cell specific gene

expression appear to be conserved from fish to mammals,since promoters of heterologous origins are active in eithermouse or fish which in turn, indicate the conservation ofthe transcription factors as well as their binding sites inthose cells (Murisier and Beermann, 2006). Using acombination of DNaseI footprinting and band shift assayscoupled with the mutagenesis data of specific DNAelements, the requirements for the melanocyte-specificexpression of the human TYR (hTYR) promoter havebeen examined (Bentley et al., 1994). The promoter region

of the hTYR has an interesting architecture, which governsthe tissue-specific expression of the gene by the complexinteraction of specific transcription factors with thepromoter cis-elements (Fig. 1). Functional analysis of the

gene. The sequence (Contig No: NT_008984.16) shows the cis-elements

assays. The transcription start site is indicated by +1. The sequence of the

l Element (TPE) are underlined. h5’URS represents the human 50 upstream

at it contains a direct repeat of a stretch of 43 nucleotides (underlined with

relative position but not drawn to scale.


hTYR promoter demonstrated the presence of an initiatorE-box (�12 to �7), which is highly conserved throughoutevolution, an M-box (�107 to �97) conserved within themelanocyte-specific promoters, one distal E-box element(�1861 to �1842) termed as TYR distal element (TDE)and one Sp1 site (�45 region). It has been speculated thathere the transcription factor SP1 might act as a sensor ofdifferentiation instead of a cell-specific transcriptionactivator (Murisier and Beermann, 2006). The E-boxcontains a 6 bp core sequence ‘CANNTG’, which isrecognized by a family of basic-helix-loop-helix leucinezipper (bHLH-LZ) transcription factors. The initiatorE-box has been shown to be essential for the transcrip-tional activation of hTYR by recruiting the microphthal-mia associated transcription factor (MITF), a member ofbHLH-LZ family. The M-box (AGTCATGTGCT), whichis a part of the TPE i.e. TYR proximal element (�112 to–94), is thought to be required for the efficient expressionof the hTYR in pigment cells. This M-box also contains acore E-box motif. However, the third E-box element i.e.TDE has been shown to direct maximal pigment cellspecific expression of TYR in vitro (Yasumoto et al., 1994,1997). For the transactivation of the pigment cell specificgenes, the indispensable transcription factor is MITF(human homolog of the mouse microphthalmia (Mi) geneproduct). MITF, belonging to the MYC supergene familyof proteins, is also needed for the melanocyte and retinalpigment epithelium (RPE) development (Takemoto et al.,2002). In mouse RPE, Mitf acts in synergy with Otx2, apaired-type homeobox transcription factor that bindsspecifically to a bicoid motif in the murine TYR promoter(Martinez-Morales et al., 2003). Aksan and Goding (1998)proposed that a 50 flanking T residue, present adjacent tothe E-box motif, may be required for the binding of MITFto the E-box (Aksan and Goding, 1998). It is worthwhile tomention here that MITF is necessary but might not besufficient for TYR activation. Several major isoforms ofMITF have been identified in different cell types, whichdiffer only at their amino termini, otherwise sharing all theimportant functional domains of the protein. In mousemodel, it has been shown that mast cell-specific Mitf-mccould not transactivate TYR in spite of binding to itspromoter element. Such observation suggests the possibi-lity of differential recruitment of protein complexes tothe amino termini of the Mitf isoforms (Takemoto et al.,2002). This data opens up a new facet in the area oftranscriptional regulation of TYR by other yet to beidentified transcription factor(s) that might interact withthe unique amino terminal end of melanocyte-specific Mitf-m ensuring tissue specific expression. In vitro experimentshave demonstrated that TFE3 (transcription factor E3),having high homology with MITF, also stimulates TYR

expression through binding to its M-Box and initiator E-Box in the promoter. However, no such observation hasbeen made with the endogenous TFE3. Therefore, it hasbeen proposed that in spite of being expressed in themelanocytes, TFE3 binding to the M-box is prevented by

other HLH proteins devoid of DNA-binding domains(Verastegui et al., 2000). Later in 2001, it was demonstratedthat the ubiquitously expressed USF1 (upstream transcrip-tion factor 1) could mediate UV-induced TYR expression(Galibert et al., 2001), which raises the speculation thatwhile MITF constitutively transactivates TYR expression;USF1 might be involved in the UV mediated response. Inaddition to the cis-elements mentioned above, using in

silico approach, we could identify a stretch of sequence(�1972 to �1967) upstream to the TDE (Fig. 1), whichcould be a putative E-box. However, its functionality is yetto be validated.There is a GA complex repeat (�868 and �633) between

TPE and TDE of hTYR (Fig. 1), which contains a 43nucleotides direct repeat from �824 to �782 and from�775 to �733. The GA repeat has been reported to bepolymorphic in Caucasian and Japanese population(King et al., 2001; Tanita et al., 2002). However, theregulatory role of this repeat, if any, has not beeninvestigated. Additionally, similar to the b-globin regula-tory region (DNase hypersensitive region), a distal en-hancer element called locus control region (LCR) has beencharacterized for murine TYR, which is situated about15 kb upstream of the gene (Giraldo et al., 2003). TheLCRs are great genetic insulators (scaffold attachmentregions) and play a key role in chromatin organizationsurrounding the gene. Indeed, transgenic mouse lacking theLCR has been shown to exhibit position effect variegation(PEV) in RPE and skin. Comparative genomics analysisbetween mouse and human has served as a powerfulapproach in identifying a LCR like region in hTYR, knownas human 50 upstream regulatory sequence (h50URS,

between �8 and �10Kb), which has been functionallyvalidated (Regales et al., 2003) (Fig. 1). Moreover, anenhancer region exhibiting cell lineage specific DNase

hypersensitivity site has been described 9 kb upstream ofthe gene (Fryer et al., 2003). All these observations indicatea potential role of this h50URS in transcriptional regulationof TYR.

2.2. Protein structure

TYR (monophenol monoxygenase EC 1.14.18.1) is amelanosomal membrane bound glycoenzyme with a type-3copper active site. It is synthesized as a nascent protein of�60 kDa that on glycosylation attains a final size of�80 kDa. TYR is an extremely stable protein with a longbiological half-life (�10 h in vivo) and a relatively lowisoelectric point (pH 4.3). It is highly resistant to attack byheat or proteases (King et al., 2001). The mature TYRpolypeptide (529 amino acids long) includes an 18-aminoacid long N-terminal signal peptide that targets thenascent polypeptide to the ER for folding, modificationand sorting; six N-glycosylation sites (discussed indetails in the Section 3); two copper binding sites(CuA and CuB) and one transmembrane (TM) domainfollowed by a relatively short carboxyl tail that contains the


essential signals for sorting and targeting to themelanosomes.

Mammalian TYR in its pure and complete form has notbeen crystallized so far. The presence of N-glycans and aTM region makes the crystallization of this protein verydifficult. In 2002, Garcia-Borron et al. proposed a reason-able model of this enzyme with the aid of the following fourdifferent types of complementary evidences: (1) crystal-lographic data are available on hemocyanin (Gaykemaet al., 1984) and plant catechol oxidase (Klabunde et al.,1998) that share similar metal-binding property with TYR.Moreover, the phytoenzyme (catechol oxidase) evenpossesses partial TYR activity and produce brown poly-phenolic catechol melanin; (2) sequence comparison dataof TYRs from different sources has identified the invariantresidues likely to play crucial roles as structural or func-tional determinants; (3) sequence comparison of TYR andTYR-related proteins (TYRPs) can further discriminatethe conserved residues contributing to the commonproperties of this protein family as well as the variantpositions that may be responsible for the characteristicproperties of each member; (4) relevant data have beencollected on TYR protein structure from the extrapolationof the functional effects of naturally occurring orartificially created mutations. However, further sequencecomparison, site-directed mutagenesis data and informa-tion obtained from TYRs of lower organisms couldimprove the three-dimensional model of the protein aswell as the knowledge in terms of enzyme–substrateinteraction.

The active site of TYR sports two antiferromagneticallycoupled (i.e. magnetic field creating parallel but opposingspins varying with temperature) copper atoms, each ofwhich is attached to 3–4 histidine residues at CuA and CuBsites, forming a binuclear center. The CuA and CuB,separated by a distance of 3.6 A at the type-3 active site(Gerritsen, 2004), bind to the histidine-based regionshaving sequence motifs H–x(n)–H–x(8)–H and H–x(3)–H–x(n)–H, respectively (where ‘n’ is a variable number ofresidues) (Garcia-Borron and Solano, 2002). Interestingly,despite the catalytic activities (i.e. hydroxylase and oxidaseactivities) of TYR being well characterized (Raper, 1928;Mason, 1948; Lerner et al., 1949), the mechanisms are notclearly understood. It is known, however, that hydroxyla-tion initiates with the binding of a dioxygen to the copperatoms at the active site. It has long been debated whetherthe tyrosine-hydroxylase and DOPA-oxidase activities ofTYR share a common catalytic site. Indeed, L-DOPAproduced by the consumption of L-tyrosine (in the 1st stepof melanin biosynthetic reaction) acts as the substrate forthe 2nd step of the reaction. Since for both the activitiesthere are different substrates (L-tyrosine and L-DOPA), ithas been suggested that the catalytic mechanisms for eitherof the substrates must differ. The speculation is furthersupported by the mutational mapping data of humanTYR, which suggest that although some common structur-al domains account for both activities of the enzyme,

distinct ones should also be involved. It is still unclearwhether L-DOPA is an active site-residing intermediate ofthe 1st step reaction or is released from the active site(Garcia-Borron and Solano, 2002). Olivares et al. (2002)introduced some modifications to the classical catalyticmechanism of TYR: (i) for TYR hydroxylation, L-tyrosinebinds to oxy-TYR by interaction of the phenolic groupwith CuA; (ii) for DOPA oxidase, L-DOPA is first dockedto CuB (Olivares et al., 2002).Besides the active site, TYR also possesses several other

important regions. The TM region anchors the bulk of theprotein inside the melanosomal lumen and also supportsthe C-terminal peptide to be oriented to the cytosol.However, unlike the mammalian forms, TYRs fromStreptomyces origin do not contain the TM domain. Thisobservation suggests that the TM region appeared late inTYR evolution, possibly when the enzyme had to beinserted in the membrane (thylakoidal membrane in plantchloroplasts and melanosomal membrane in animalmelanocytes). Again, for TYR and all other endolysoso-mal/melanosomal proteins, there exist two well-knownC-terminal signals (Letourneur and Klausner, 1992) thatare involved in the final targeting of the protein. These are:(1) a dileucine motif (LL) and its variants (L/I)(L/I/V/A)(Sandoval et al., 1994) and (2) a tyrosine-based motif(YXXB), where B is any bulky hydrophobic residue(Honing et al., 1996). Besides the TM region and theC-terminal signal, another notable feature of TYR is thecysteine (Cys) residues, the number and importance ofwhich are variable within TYRs across different species.Streptomyces TYR is devoid of Cys, whereas mammalianTYRs have 17 Cys residues, 15 of which are perfectlyconserved in all the TYRPs. These Cys residues ofmammalian TYR are clustered in three regions of theprotein–the first two are N-terminal to CuA and the thirdone lies between CuA and CuB. The first cluster thatcontains 4–5 Cys is characterized by a consecutive CC pair;the second one, termed as epidermal growth factor (EGF)-like region consists of 5 Cys residues and the third cluster isalso comprised of 4–5 Cys residues. In contrast to themammalian TYRs, plant catechol oxidases contain the firstcluster only. As per the available crystallographic datafrom plant catechol oxidase, it is clear that the first clusterplays an essential role in correct folding and maintenanceof the N-terminal domain (Garcia-Borron and Solano,2002). Davis (1990) presumed that the second cluster(EGF-like region) is involved in protein–protein interac-tion (Davis, 1990); which was further supported by thefindings of Kobayashi et al. (1998), who demonstrated theinteraction between TYR with TYRP1 and TYRP2(Kobayashi et al., 1998). However, this interaction couldbe mediated by the C-terminal regions also (Manga et al.,2000) as both TYR and TYRP1 with one Cys residue intheir C-termini have tighter molecular interaction witheach other than with TYRP2 that lacks the Cys in theequivalent position. The third Cys cluster is possiblyneeded for chaperone interactions (Garcia-Borron and

ARTICLE IN PRESS

Fig. 2. Schematic representation of the transcript variants of tyrosinase gene. Panel A shows the gene with the cryptic splice sites (in exon 1) and the six

splice variants observed in lymphoblastoid cell lines (Fryer et al., 2001); Panel B illustrates a magnified version of exon 1 where the cryptic splice sites are

marked by the arrowheads and their adjacent sequences at codons 53, 118 and 266 are also shown. The figure is not drawn to scale.

K. Ray et al. / Progress in Retinal and Eye Research 26 (2007) 323–358328

Solano, 2002). The high number of mutations in theseCys-rich regions in TYR negative albinism underlines theirimportance in TYR function.

2.3. Protein expression

TYR is mainly expressed in two cell types: (a)Melanocytes, derived from migrating neural crest cells thatcolonize within iris, cochlea, skin and choroid and (b) RPE

cells, derived from the optic cup. During mouse embryo-genesis, the expression of TYR can first be detected (by insitu hybridization) from +10.5 days postcoitum (d.p.c.)onwards in the RPE cells and from +16.5 d.p.c. onwardsin the skin melanocytes (Beermann et al., 1992; Steel et al.,1992). The absence of TYR expression from the eye resultsin erroneous pathway selection of optic fibers at the chiasmand hence TYR has been implicated as a developmentalregulator of CNS pathway. A series of studies usingtransgenic TYR reporter mice have been publisheddocumenting lacZ expression in developing and adultmouse brain under the control of murine TYR promoterand 50 upstream regulatory sequences (Tief et al., 1996a, b,1998; Camacho-Hubner and Beermann, 2001). Usingindirect methods, it was proposed that TYR might alsobe expressed in the developing and adult brain, whichcould have been very significant in terms of its potential

wider role in axonal pathway finding. However, in contrastto the previous observations, Gimenez et al. (2003) couldnot find any detectable TYR in developing, perinatal andadult mouse brains using in situ hybridization and real-time PCR techniques (Gimenez et al., 2003). But consider-ing the presence of neuromelanin (Xu et al., 1997; Ikemotoet al., 1998; Tief et al., 1998; Matsunaga et al., 1999) andcatecholamine synthesis within the brain of tyrosinehydroxylase (TH)-deficient mice (Rios et al., 1999), theputative existence of TYR-like activity in some areas of thebrain could not be ruled out.In the lymphoblastoid cell lines, in addition to the

normal-sized transcript, five smaller transcripts of TYR

were identified (Fig. 2). Normal melanocytes in primaryculture and a human melanoma cell line (MNT1) were alsofound to produce the same pattern of TYR transcriptswhich indicates that hTYR pre-mRNA has the potential toproduce a number of splice variants (Fryer et al., 2001).

2.4. Evolutionary significance

2.4.1. Evolution of TYR

Early in the evolution of life, when the environment wasanaerobic (Canfield and Teske, 1996); oxygen productionduring photosynthesis was toxic to many creatures. So, toneutralize the oxygen by oxidation reactions, a variety of

ARTICLE IN PRESS

Fig. 4. A possible evolutionary map of the type-3 copper proteins. Tyrosin

Haemocyanins (HCNs), than Arthropod (ARTH) HCNs. Polyphenoloxidas

van Holde et al., 2001. J Biol Chem 276, 15566.

Fig. 3. A cartoon of bi-copper center present in type-3 copper proteins.

Each copper (Cu) is attached with three histidine moieties (His). Straight

lines correspond to the ‘His’ that are in the same plane as that of the ‘Cu’

atoms, whereas the solid and the hashed lines demarcate residues those are

above and below the plane, respectively. The figure has been drawn in

accordance with the Fischer’s projection rule.

K. Ray et al. / Progress in Retinal and Eye Research 26 (2007) 323–358 329

enzymes were evolved which used two metal ions—eitheriron or copper for this purpose. In case of the copperproteins, a type-3 copper center was evolved, in which twocoppers reversibly bonded oxygen in a side-on configura-tion as in case of peroxide (Magnus et al., 1994; Cuff et al.,1998) (Fig. 3). TYRs represent an extremely ancient classof binuclear copper proteins, which possibly evolved fromthe ancestral copper protein whose function was to protectprimitive organisms from oxygen toxicity. Gradually,with the emergence of major metazoan phyla, as aerobicmetabolism became well established a circulating oxygentransport protein became indispensable, which led to theemergence of the arthropod and molluscan hemocyaninprecursors from the already existing type-3 copper proteinsi.e. the primitive polyphenoloxidases (PPOs) and theprimitive TYRs, respectively. The high molecular weightmultiple-subunit oxygen transporters segregated into pro-to-arthropodan and proto-molluscan lines, which thenindependently evolved to yield arthropod hemocyanin andmolluscan hemocyanin, with quite distinct sequences asfound today. Parallely with the haemocyanins, mammalianTYRs evolved in a separate line from the primitive TYRs

ases (TYRs) appear to be more closely related to Molluscan (MOLL)

es are shown in the figure as PPOs. The figure has been adapted from


(van Holde et al., 2001) (Fig. 4). Recent structural analysisand sequence comparison show that arthropod hemocya-nin exhibits some sequence similarity to arthropodphenoloxidases (Aspan et al., 1995; Burmester andScheller, 1996), whereas the molluscan hemocyanin moreclosely resembles mammalian TYRs (van Gelder et al.,1997) and plant catechol oxidase (Klabunde et al., 1998).The greater sequence dissimilarity between the molluscanhemocyanin and TYRs as compared with that betweenarthropod hemocyanin and insect phenoloxidases suggeststhat the molluscan hemocyanin and TYR split is moreancient (Fig. 4).

It has been also speculated that while a simpleduplication of a primordial copper B site gave rise to thearthropod copper-binding region; the molluscan copper-binding domain was evolved by the fusion between twogenes encoding copper A and copper B types of structures,respectively (van Holde et al., 1992; van Holde and Miller,1995; Decker and Terwilliger, 2000). Duplication theorygains support by the observed similarity in the placementof histidine-moieties at locations 1B and 1A, as well as 3Band 3A and additionally, the location of phenylalanine isalso conserved. Histidine 2 is the only important residue tobe shifted significantly (Fig. 5).

Among the above-mentioned type-3 proteins, whileTYR catalyzes the o-hydroxylation of the phenoliccompounds followed by the oxidation of the diphenolicproducts, catechol oxidase catalyzes the oxidation steponly and in contrast to these enzymes, hemocyanins use thecopper-binding sites for binding and transporting oxygen(Jaenicke and Decker, 2004). Interestingly, it has beendemonstrated that the oxygen-carrying hemocyanins couldbe converted to oxidases (viz. TYR and catecholoxidase)by some structural alterations (Decker and Rimke, 1998;Nagai and Kawabata, 2000; Decker et al., 2001; Nagai

Fig. 5. Sequence comparison of the CuA and CuB sites of type-3 copper prot

ncTYR—Neurospora crassa tyrosinase), catechol oxidases (IbCO—Ipomoea ba

hemocyanin, functional unit g; Odg—Octopus dofleini hemocyanin, function

hemocyanins (Pia—Panulirus interruptus hemocyanin, subunit a; LpII—Limu

phenol oxidase; DrpPO—Drosophila melanogaster phenol oxidase). Histidine 2

the Cu-A site. This figure has been updated from van Holde et al., 2001. J B

residues between CuA and CuB sites.

et al., 2001). It is believed that during evolution ofhemocyanins from primitive type-3 copper proteins, thesubstrate-binding property of the enzymes must have beeninhibited by additional residues that block the active sitefor large substrates, which would not affect its oxygen-binding capacity (van Holde et al., 2001).

2.4.2. TYR vs. its pseudogene

In situ chromosomal hybridization revealed that a TYR

cDNA probe hybridized to two locations on humanchromosome 11. The major site of hybridization wasfound at band q14–q21—the location of authentic TYR

gene, whereas the second hybridization site was detected onthe short arm of chromosome 11 at p11.2-cen. This secondlocus was eventually designated as TYR-related segment(TYRL, 11p11.2, MIM 191270) (Barton et al., 1988)—a pseudogene of TYR, which contains sequences similar toexons IV and V of the authentic gene. It is thought thatduplication of TYR exons 4 and 5 regions followed by11q:11p translocation has given rise to the TYRL segment.However, during the hominoid evolution, as the averagenucleotide substitution rate is estimated to be 1.1� 10�9

substitutions/site/year in unconstrained sequences (Koopet al., 1989), the very low (2.6%) nucleotide sequencedivergence between the noncoding regions of 11q and 11psegments places the TYRL origination as a relatively recentevent (�24 million years ago). This is consistent with theobservation that both the 11q TYR and the 11p TYRL

exist in all of the human ethnic groups that have beenstudied so far and it suggests that the presence versusabsence of the 11p segment may be a useful discriminatorymarker for studies of prehuman primate evolution (Giebelet al., 1991). In fact, when TYR and TYRL were analyzedin primates to understand the evolution of human TYR, itwas found that exons IV and V of chimpanzee and gorilla

eins. Gr-I (Group-I) represents TYRs (hsTYR—Homo sapiens tyrosinase,

tatas catechol oxidase) and molluscan hemocyanins (Hpg—Helix pomatia

al unit g) and Gr-II (Group-II) includes arthropod phenol oxidases and

lus polyphemus hemocyanin, subunit II; PapPO—Pacifasticus leniusculus

shows the significant shift between the Gr-I and Gr-II copper proteins at

iol Chem 276, 15565, who originally showed the conservation of specific


TYR are very similar to human, with the gorilla sequencebeing more similar than chimpanzee and it is gorilla butnot chimpanzee that contains a TYRL locus similar tohuman TYRL (Oetting et al., 1993).

2.4.3. TYR and TYRPs

In melanin biosynthetic pathway, apart from TYR, theother two important melanosomal enzymes (Fang et al.,2002) are—(a) TYR-related protein 1 (TYRP1) [the genemaps to the mouse brown (b) locus (Jackson, 1988;Shibahara et al., 1991)] and (b) TYR-related protein 2(TYRP2) [the gene located at the slaty (slt) locus in mice(Jackson et al., 1992; Tsukamoto et al., 1992)]. Sequencecomparison of these three proteins reveals that all of themcontain an EGF like region, two metal-binding domainsthat form the catalytic site, one C-terminal TM domainwith a short cytoplasmic tail and certain conserved Cysresidues that may be involved in protein–protein interac-tions (Fig. 6A) (Sturm et al., 2001). These three enzymesrepresent a family of closely related gene products (overall�40% amino acid identity) that share a common tertiarystructure (Jimenez-Cervantes et al., 1998; Kobayashi et al.,1998) and hence have been grouped together to form theTYRP family of genes (Hearing and Jimenez, 1989;Jackson et al., 1992; Kwon, 1993). The physical character-ization of human TYRP1 and TYRP2 transcription unitsas well as that of the TYR locus (Giebel et al., 1991;Ponnazhagan et al., 1994) also suggests a commonstructure of this gene family. Human TYR (Giebel et al.,1991; Ponnazhagan et al., 1994), TYRP1 (Box et al., 1998)and TYRP2 (Sturm et al., 1995) possess five, seven andeight exons, respectively and analysis of these exonicregions and the splice junctions, as shown in Fig. 6B,reveals only one site that is identical in all three genes—theone between the penultimate and the final exon thatcontains the sequence for the C-terminal TM region (Sturm

Fig. 6. Comparison of TYR with TYRP1 and TYRP2. Panel A demonstrates

positions of signal peptide, EGF-like region, two metal-binding domains (A

comparative representation of TYR, TYRP1 and TYRP2 in terms of their ex

boxes; intron numbers are indicated by roman numerals above the junctions wh

as 0 (between two codons), 1 (between the first and second nucleotides of a codo

has been reproduced from Sturm et al., 2001. Gene 277, 55.

et al., 1995). Again, each of the TYR–TYRP1 andTYRP1–TYRP2 gene pairs share another common splicejunction; e.g. TYR intron II is positioned homologous tothat of TYRP1 intron IV and TYRP1 intron II is homo-logous to TYRP2 intron III. It has been suggested that theintrons in equivalent positions within the human TYR,TYRP1 and TYRP2 loci have been acquired late in theirevolution (Mattick, 1994). A sequence-based cladisticanalysis of these proteins among the vertebrates suggeststhat the TYRP1 and 2 sequences are more closely related toeach other than either is to TYR (Morrison et al., 1994).Comparative genomics including the exon–intron bound-ary study between these genes along with their sequencesimilarity data clearly indicates that the TYRP gene familyhas evolved from one common ancestral gene by duplica-tion and subsequent divergence (Budd and Jackson, 1995;Sturm et al., 2001). It is thought that TYR gave rise toTYRP1, which then duplicated to form TYRP2. However,it has been suggested that the triplication of the TYRfamily has occurred during the early radiation of chordatesi.e. before the evolution of mammals and this is evidentfrom the fact that while the birds, fish and axolotl haveboth TYRP-like genes; ascidians posses a single copy only(Sato et al., 1999).Functionally, TYRP1 and TYRP2 act downstream of

TYR in the melanin biosynthetic pathway and all thesethree can associate to form a higher order melanosomalprotein complex (Jimenez-Cervantes et al., 1998; Kobayashiet al., 1998). In addition to their role in pigmentation, TYRfamily proteins also influence the biology of melanocytesand melanoma. Based on the observation that human TYR

and TYRP1 share a common enhancer element and atranscription factor, Shibata et al. (1992) hypothesized thatthese genes are regulated in a coordinated manner duringmelanocyte development and differentiation (Shibata et al.,1992). However, it is now known that in melanocyte and

the common structure of the human TYRP family member proteins. The

and B) and the C-terminal TM domain are indicated. Panel B is the

on/intron boundaries and phase interruptions. The exons are denoted by

ereas the intron phases are shown below. The phase interruptions are listed

n) and 2 (between the second and third nucleotides of a codon). This figure


melanoma TYRP1 can be regulated independently ofTYR (Sturm et al., 1994; Kobayashi et al., 1995b;Vijayasaradhi et al., 1995a; Fang and Setaluri, 1999; Fanget al., 2001).

3. Processing and maturation of TYR

Efficient and rapid folding of polypeptide is an essentialcellular process, as it is required for the biological functionof the native protein. In course, proteins may undergomodifications viz. cleavage and covalent attachment ofcarbohydrates and lipids that are critical for their functionand correct localization within the cell. The eukaryoticprotein maturation process is mostly monitored in the ERby a stringent quality-control machinery that helps in theco- and post-translational protein folding, modificationand assembly of nascent proteins to their native structures,so that the properly folded proteins can sort to the Golgiaccording to their need and subsequently be packaged intothe transport vesicles (vesicular transport) that carry themto their ultimate targets i.e. the sites of action (cytosol,intracellular organelles, cell membrane etc). ER quality-control system is a complex process involving correctrecognition of any misfolded or incompletely assembledproteins, aggregated side products and folding intermedi-ates followed by their ER retention, cytosolic retro-translocation, deglycosylation and finally ubiquitinmediated proteolysis (ER-associated protein degradation;ERAD). The maturation of proteins depends on thecoupled action of molecular chaperones, a variety offolding enzymes, folding sensors and other modifiers.These factors may be ubiquitous (primary quality-controlsystem), though cell-specific chaperones interacting withselected proteins (secondary quality-control system) arealso found.

TYR, the membrane bound melanosomal glycoprotein,comprises upto �0.4% of the total cellular protein in themelanocytes, where the secondary quality-control system islikely to play a critical role. TYR is known to require along processing time for maturation (Halaban et al., 1997),most of which is required for protein folding (Halabanet al., 1983). In contrast to the normal matured glycoforms(70–84 kDa), melanoma TYR is Endoglycosidase H sensi-tive and appears as a distinct 70 kDa doublet, which is

Fig. 7. Schematic representation of TYR sorting through ER and Golgi. P

translocated within the ER lumen through the sec61 translocon and BiP bindin

the glycan moiety, Glc3Man9GlcNAc2 (represented by the dotted circle) that is

binding occurs when two trimmed glycans (Glc1Man9GlcNAc2) are added to th

addition of the 3rd glycan. ERp57 being associated with CNX or CRT

homodimerized, although hetero-oligomeric interactions (shown with ‘?’ mark

However, the un/misfolded protein re-enters the CNX/CRT cycle via UGGT (U

correctly, is sorted out to the cytosol by EDEM (ER degradation enhancing

demonstrates the role of glycan-trimming during the ER maturation of

Glc3Man9GlcNAc2; Glc2 form, Glc2Man9GlcNAc2 and Glc1 form, Glc1Man

TYR—(1) transport of ER-processed TYR (correctly folded) via CopII vesicles

(3) Cu-loading, which may occur directly through Menkes copper transporter

TYR from trans Golgi network (TGN) to stage II melanosome via direct or i

thought to represent a premature form of TYR. It wasindeed found that in normal melanocytes, the newlysynthesized TYR appeared as a 70 kDa doublet that wasslowly processed to the larger species (Halaban et al.,1997). However, the entire process of sorting, folding,targeting and intracellular trafficking of TYR works out ina highly concerted manner, which has been demonstratedthrough cartoons in Fig. 7 and described in the text underspecific heads.

3.1. ER maturation

ER provides a protective folding environment and likemany other proteins, ER maturation of TYR begins co-translationally (Fig. 7A).

3.1.1. Translocation in the ER lumen and signal sequence

cleavage

The 60 kDa TYR nascent peptide enters the ER lumenthrough the sec61 translocon. Actually, it is the N-terminalsignal sequence of the polypeptide chain, which bindsthe signal receptor protein (SRP) and thus targets theribosome-nascent chain complex to the ER membrane(Wang and Hebert, 2006). The sec61 translocon alongwith its associated proteins provide a protective environ-ment to the maturing polypeptide so that any oppor-tunity for aberrant processing of the protein is stericallyminimized (Chen and Helenius, 2000). Subsequently,the N-terminal signal sequence is removed in the ERlumen (Kwon et al., 1987; Ruppert et al., 1988; Bouchardet al., 1989; Yamamoto et al., 1989). This is an essentialstep, specially in case of the type I membrane proteinslike TYR; otherwise the polypeptide would remainassociated with the ER membrane by the hydrophobicregion, which in turn would severely affect the mobilityof the maturing polypeptide (Marquardt and Helenius,1992).

3.1.2. BiP binding

Prior to the initiation of the protein folding, as thetranslocation process is still in progress, one of themost abundant cellular chaperones, Bip—an Hsp familyof proteins, binds preferentially to the hydrophobic patchesof the nascent polypeptide. Bip acts to maintain the

anel Ai shows the ER processing of TYR—(1) the nascent peptide is

g occurs transiently, the protein becomes glycosylated with the addition of

trimmed later to facilitate the lectin chaperone binding; (2) calnexin (CNX)

e polypeptide chain and (3) calreticulin (CRT) binding is initiated with the

supports the disulfide bond formation; (4A) properly folded TYR is

) cannot be ruled out and (5) then the protein exits ER to reach Golgi.

DP-glucose: glycoprotein glucosyl transferase) (4B) and if still fails to fold

—mannosidase-like protein) for proteasomal degradation (4C). Panel Aii

TYR. Different forms of glycans shown are as follows: Glc3 form,

9GlcNAc2. Panel B schematically demonstrates the Golgi processing of

to the cis-Golgi; (2) further modification of TYR within Golgi, followed by

(MNK) or may be aided by a carrier protein; (4) sorting out of matured

ndirect pathway.



permeability barrier of ribosome-free translocons duringthe protein translocation process (Crowley et al., 1994;Hamman et al., 1998) and also protects the protein fromaggregation at that stage (Bukau and Horwich, 1998). Bipin course associates with ATP for its proper function(Helenius et al., 1992), which is regulated by the J-domaincontaining proteins that initiate ATP hydrolysis and arealso thought to deliver the substrate to Bip (Bukau andHorwich, 1998). In the year 2005, Wang et al. found thatwhen the nascent TYR chain reaches a length of �70amino acids, the Bip-mediated seal is released (Wang et al.,2005). However, in those cases where a potential glycosyla-tion site is located within the first �50 amino acids of thematuring glycoprotein, Bip binding can be bypassed. Butthis is certainly not true for TYR because here the first

Fig. 8. Glycosylation sites in tyrosinase. Panel A, Schematic representation

domains and the glycosylation sites (the drawing is not in scale); Panel B, det

glycan is added at Asn68 position (Wang et al., 2005). TYRhas several potential Bip-binding sites, but for TYR, Bipbinding is a transient phenomenon immediately followedby the glycosylation of the protein.

3.1.3. Glycosylation

Human TYR contains seven N-linked glycosylation sitesamong which six are located N-terminal to the perfectlyconserved CuB region (Fig. 8A). During translocation ofthe nascent polypeptide, each of these potential glycosyla-tion sites (Asn–X–Ser/Thr) is conjugated with a large(14-member), 2.5 kDa, flexible, hydrophilic N-linked gly-can (Glc3Man9GlcNAC2) (Fig. 8B), thus giving the proteina 70 kDa structure (Kornfeld and Kornfeld, 1985; Hebertet al., 2005). For this glycan-transfer reaction, each of the

of tyrosinase polypeptide showing the potentially important functional

ailed architecture of a glycan moiety.


glycosylation consensus sequences needs to form a turn sothat the OH-containing side chains of Ser/Thr are able toincrease the nucleophilic property of the Asn amide sidechain that is otherwise relatively inert (Helenius and Aebi,2004). However, this process is aided by the action ofoligosaccharyl transferases (OST), located at the ERmembrane proximally with respect to the sec61 translocon.All the glycosylation sites of TYR are utilized efficiently,except the Asn290 site (Asn–Gly–Thr–Pro), where anadjacent proline residue provides the steric hindrance andthis results in formation of a heterogeneous population ofTYR, possessing either six or seven glycans. Though thisphenomenon does not influence the ability of TYR to exitER and reach Golgi, it still remains to be determined thatwhether the additional glycan can confer any maturationalor functional advantage to the protein. In this context, it isworth mentioning that processing of murine TYR includesN-glycosylation of at least four out of the six availableglycosylation sites and no inefficient glycosylation site ispresent there (Branza-Nichita et al., 1999). Once added, theglycan residues not only ensure the correct local position-ing of the peptide segments to which they are bound butmay also discourage the erroneous binding of anymolecular chaperones and also act sterically to protectthe protein from proteolysis or antigenic recognition(Hebert et al., 2005). If these glycans are ablated, transientaggregates formation has been observed that contained thenon-native disulfide bonds requiring ATP hydrolysis fordisassembly (Svedine et al., 2004). Experimental evidencessuggest that the rate of protein translation has a role in theglycosylation of the potential sites of human TYR. In fact,in the melanoma cells, the faster translation rate of TYRthan that of the normal melanocytes has been shown toimpair the glycosylation at the inefficient site (Asn290)(Ujvari et al., 2001). Thus, it has been inferred that humanTYR contains two types of N-glycan acceptor sites that aredistinguishable on the basis of their kinetics of glycosyla-tion: (1) sites of rapid, co-translational glycosylation and(2) sites of slow, conformation-dependent glycosylation(Olivares et al., 2003).

3.1.4. Lectin chaperone binding

Each glycan has three glucose moieties associated withnine mannose residues, but the glucose trimming mustoccur in the ER before the interaction with the concernedmolecular chaperones, viz. membrane bound calnexin(CNX) and soluble calreticulin (CRT, the paralogue ofCNX). In this process of glucose trimming, two enzymescome into play—the first glucose residue of the TYRnascent polypeptide is trimmed by a-glucosidase I while thesecond one is removed by a-glucosidase II, thus producingmonoglucosylated oligomannosidic glycans—a prerequi-site step for CNX binding to TYR (Hebert et al., 1995).Experimentally, it has been proved that if the action ofa-glucosidase I is inhibited, CNX will not bind to TYRswith glycans having more than one glucose residue andas a result, the folding would be accelerated, eventually

resulting in a totally inactive protein lacking the coppermolecules. CNX binding to TYR initiates after theaddition of two glycans to the nascent polypeptide chainwhereas CRT binding begins only after addition of a thirdglycan moiety (Wang et al., 2005). The CNX/CRTinteraction is thought to help the nascent protein to attainits proper protease-resistant conformation. In fact, nascentTYR polypeptide remains bound with the chaperones,until the proper 3D structure of the protein is attained andthe intramolecular disulfides are in place. However, in spiteof being bound to CNX and CRT, if TYR fails to foldcorrectly, a-glucosidase II cleaves the third and finalglucose residue from the glycan upon which TYR isdissociated from the lectin chaperones. It is interesting tonote that a-glucosidase II does not cleave the third glucosemoiety at its first exposure, otherwise there would be nochaperone binding at all. So, there must be a selection biasand a specific mechanism, which does not allow removal ofthe third glucose before chaperone binding; it may sohappen that the lectin chaperone binding is a strictly timedependent process. However, on release from the chaper-one cycle, the free unfolded TYR quickly enters a cycle ofreglucosylation by the ER sensor UDP-glucose: glycopro-

tein glucosyl transferase (UGGT), followed by the reasso-ciation with CNX and CRT and further deglucosylation bya-glucosidase II, until the protein attains its maturedconformation (Sousa and Parodi, 1995; Rodan et al.,1996; Zapun et al., 1997; Branza-Nichita et al., 1999). Incourse, TYR interacts with these two lectin chaperones andis released from the binding cycle only when a-glucosidase

II cleaves the third and final glucose residue of the N-linkedglycans (Hebert et al., 2005).Addition of post translational glucosidase inhibitors trap

the glycans in their monoglucosylated state and therebyprevents the substrate release from the chaperones, whichinhibits the overall protein folding and oxidation of thetrapped substrate. This indicates that global folding ofproteins may occur only after release from the lectinchaperones, which may actually play an important part inslowing down the folding process for sake of the overallfolding efficiency (Hebert et al., 2005). It must bementioned here that during the early events of folding,some mutant TYRs were shown to be extensivelyassociated with these chaperones (mostly CNX) andthereby retained in the ER (Halaban et al., 2000). Recently,it has been found that instead of being associated withCNX, soluble albino TYRs are retained in the ER by CRTand Bip (Popescu et al., 2005). These observations indicatethat though previously it was assumed that CNX and CRTcould substitute for each other in the ER quality-control(Molinari et al., 2004), in reality, they should have distinctroles to play. It is currently under investigation whetherthere is any particular domain in the protein that plays anactive role in the selection of the appropriate chaperone(Popescu et al., 2005).At the end of the folding process, the distantly located

CuA and CuB sites of TYR are juxtaposed, thus forming


the active center of the enzyme. This is essential for loadingthe two copper atoms in the active site that actually rendersthe catalytic activity of the enzyme. Also, the acquisition ofthe glycosylation-competent correct conformation mostlikely relies on the specific intramolecular interactionsbetween these two parts (Olivares et al., 2002). After thematured 3D protein conformation is achieved, L-DOPAand L-tyrosine—the substrates of TYR, stabilize theproperly folded protein so that it can no longer serve asa substrate for reglucosylation and subsequently bereleased from the ER. Thus, in addition to L-DOPA’s rolein TYR activation during melanin biosyntyhesis, which hasbeen known for about 70 years (Raper, 1928; Lerner et al.,1949), L-DOPA and L-tyrosine may also act as bioregula-tors (Halaban et al., 2001). It has been experimentallydemonstrated that TYR having oxidized copper atoms(without bound oxygen) can oxidize L-DOPA to itscorresponding quinone (dopaquinone); thus the copperatoms in the enzyme active site get reduced, enabling thegeneration of the active, oxygen-bound form of the enzyme(Cooksey et al., 1997; Riley, 1998, 1999). So, if L-DOPA isactually needed for proper TYR sorting in the ER andthere is some sort of interaction between L-DOPA andTYR, then TYR activation by L-DOPA may very welloccur in the ER. In that case, it can be speculated that onlythe oxygen-bound active enzyme is competent to exit ERand any condition that blocks its formation, such asmutations producing inactive protein and reported to causeOCA1, can result in ER retention as well as subsequentdegradation of the mutant TYR. But, in case this wholemechanism is operative, copper loading of TYR mustoccur in ER. Indeed, three loss-of-function albino mutantsof TYR have been demonstrated to accumulate in the ERand a copper transporter has also been found to belocalized within the ER membrane. But, there is no recordof its role in supplying copper to the TYR and moreimportantly, the Menkes copper transporter (MNK)located in the trans Golgi network (TGN) is more likelyto play this specific role (discussed in details in the Section3.3.2). Halaban and group (2001) opined that theautocatalytic activation of TYR by L-tyrosine and thesubsequent build up of L-DOPA induced proper folding ofER glycoforms of the enzyme and being a slow process, itcould explain the long delay of TYR release from the ER.Thus, TYR is thought to exert a self-regulatory mechanismensuring continuous trafficking of properly folded proteins,only under conditions favorable for catalytic activity andmelanin production (Halaban et al., 2001).

3.1.5. Oligomerization

Mutational analysis experiments have established thatprior to attaining proper folded conformation, TYRrequires both homo- and hetero-oligomeric interactions(Wang and Hebert, 2006). Interestingly, formation of TYRhomodimers has been demonstrated in ER by gel filtrationchromatography, electrophoresis and immunoprecipitationstudies (Jimenez-Cervantes et al., 1998) and although

chaperone binding is not actually indispensable for thisprocess, that may influence the overall efficiency. Earlier itwas thought that TYR dimerization resulted from anintermolecular disulfide bond formation and Cys500,located in the cytoplasmic tail of TYR, might play acrucial role in the process (Francis et al., 2003). However,recently Wang and Hebert (2006) have noted that TYRdimers are not naturally covalently linked. Interestingly,TYR dimerization requires melanocyte-specific factors likeTYRP1 (Wang and Hebert, 2006). It has been proposedthat all the TYR-related proteins might interact in a multi-enzyme complex and their EGF motifs (Cys rich) arethought to mediate the protein–protein interaction(Jackson et al., 1992). It has been speculated that TYRP1and possibly TYRP2 are essential for stabilizing TYRduring its folding and modification in the ER milieu andhence mutations in any one can influence the maturationand degradation of the other (Kobayashi et al., 1998;Manga et al., 2000). Both in vivo and in vitro studies havesubstantiated this view. Kobayashi et al. (1998) investi-gated the in vivo effect of a particular TYRP1 mutation(within the EGF domain) on the stability of TYR and bypulse-chase experiments it has been demonstrated thatTYR degraded more rapidly in TYRP1 mutant cells.Moreover, the cell phenotypes as well as the lesser stabilityof TYR could be partly rescued, when the TYRP1 mutantcells were transfected with the wild type TYRP1 cDNA.Studies have also demonstrated that oligomerization andstabilization of TYR need its own enzymatic activity aswell as TYRP1 (Francis et al., 2003). As predicted frommutational analysis, TYRP2 also presumably binds (even ifweakly and transiently) to the TYR—TYRP1 complex,thereby forming an oligomeric protein assembly. However,the molecular mechanism, precise site and level of theinteraction of these melanocyte-specific proteins are not yetwell understood. As suggested by the experimentalevidences, the formation of oligomeric protein complexescan reduce the cytotoxicity of melanogenic intermediatesagainst melanocytes and hence it is likely that to minimizeany unfavorable reaction, all these melanogenic proteinsshould fold simultaneously as well as transport together ina complex (Toyofuku et al., 2001b).

3.1.6. Disulfide bond formation

In a maturing protein, the disulfide bonding betweenthe Cys residues helps it to attain the native confor-mation. TYR possesses multiple intramolecular disulfidesthat stabilize its structure and modulate the normal exitof TYR from ER to reach Golgi, but the bondingpattern as well as the precise roles of the lumenal Cysresidues still remain unsolved. Though mutational ana-lysis suggested that these residues may have a part toplay in TYR maturation (Halaban et al., 2001), alkylationstudies have shown that majority of the Cys residuesin TYR are unavailable for modification (Aroca et al.,1990; Wang et al., 2005). Interestingly, Wang et al. (2005)recently opined that the disulfide linkages are proximal


and do not involve any large covalent loop (Wang et al.,2005).

It has been demonstrated that interaction of ERoxidoreductase ERp57 with TYR coincides with theformation of disulfide bond within the protein (Wanget al., 2005). ERp57 is a member of the protein disulfideisomerase (PDI) family and is required to maintain theproper oxidizing conditions in the ER (Hebert et al., 2005),which is of particular interest in relation to the disulfidebond formation (Helenius et al., 1992). Thus, it has beenhypothesized that ERp57 catalyzes the disulfide bondformation, which is an oxidation reaction. However, sincethe ERp57 is always associated with the lectin chaperones,it is thought that when the nascent polypeptide chain entersinto ER lumen, the lectin chaperone binding recruitsERp57 at the distinct locations of the polypeptide and thendisulfide bonding commences co-translationally (Wang andHebert, 2006). The ternary complex formed by the nascentchain, ERp57 and either CNX or CRT is thought toprovide the electrons to ERp57 to support the disulfidebond formation.

3.1.7. ER quality-control and ER associated degradation

The misfolded TYR molecules are subjected to the ERquality-control machinery that leads to their eventualdegradation in the cytoplasm and thereby generatesantigenic peptides which are presented to the immunesystem by HLA class I molecules (Halaban et al., 1997).TYR degradation is ubiquitin-proteasome mediated andregulated by phospholipase D2. Proteins that ultimately failto reach the native state even after repeated cycles of CNX/CRT binding, are believed to exit the chaperone-bindingcycle through the action of ER mannosidase I, whichgenerates the Man8 glycoform. Actually, this mannose-trimming step is the pre-requisite for sorting of glycopro-teins to the proteasome (Svedine et al., 2004). However,many proteins that are folded normally are also mannose-trimmed before leaving ER, which suggests that themannose removal is not sufficient for the ER associateddegradation (ERAD) (Helenius and Aebi, 2001). Recently,a mammalian ER type II membrane glycoprotein—ERdegradation enhancing mannosidase-like protein (EDEM)has been implicated as a mannose specific quality-controlreceptor, which is a lectin chaperone that extracts proteins(Man8 forms) from CNX-binding cycle and sorts them fordestruction (Molinari et al., 2003; Oda et al., 2003).

Mosse et al. (2001) observed that prior to the proteaso-mal degradation, most of the reverse translocated TYRmolecules were still glycosylated and the proteolysisoccurred with a cleavage at the C-terminus. But it is veryunlikely that the proteasome can degrade the heavilyglycosylated proteins, as its pore is too small to allow suchproteins to pass through it (Lowe et al., 1995; Groll et al.,1997). Therefore, it is believed that the deglucosylationand proteasomal degradation processes should be coupledin some manner. However, it is apparent from their studythat the ERAD of TYR occurs via two distinct cytosolic

intermediates—one is glycosylated and partially proteo-lysed and the other is full length deglucosylated (Mosseet al., 2001).

3.2. Transport from ER to Golgi

In the absence of any ER-retention signal, properlyfolded melanosomal proteins are incorporated into theCOPII transport vesicles that bud from the smooth ER toreach Golgi (Wang and Hebert, 2006). This release mayalso be due to certain export signals. However, the exactstructural requirements needed for the ER-exit is stillunknown. It has been shown that the mannose-bindingtype I membrane lectin ‘ERGIC-53’ assists in the transportof glycoproteins from the ER to ERGIC (ER–Golgiintermediate compartment) by virtue of the traditionalanterograde pathway (Hebert et al., 2005). From theERGIC, the protein is then sorted to the cis-Golgicompartments. It can be mentioned here that the transportof TYR from ER and its subsequent processing depends onthe neutralization of pH in the Golgi (Watabe et al., 2004).

3.3. Golgi maturation

Once in the Golgi, the protein is slowly processed (t1/2of 3–4 h) to its final form i.e. the enzymatically activeconformation (Halaban et al., 1983; Jimenez et al., 1988;Vijayasaradhi et al., 1991) (Fig. 7B).

3.3.1. Glycosylation

Within the Golgi, Golgi mannosidases I and II removethe redundant mannose residues from the glycans andcomplex sugar modifications (by glycosyl transferases)further increase the molecular mass of TYR to �80 kDa(Wang and Hebert, 2006). At the end of this process,hybrid and complex types of N-linked sugars as well asO-linked sugars are found on TYR (Halaban et al., 1983;Roux and Lloyd, 1986; Vijayasaradhi et al., 1991). Thepresence of O-linked sugars might have a so far unknownimplication on Golgi sorting of TYR.

3.3.2. Copper loading

TYR is a copper-containing glycoenzyme, whose cata-lytic activity depends upon the two copper atoms residingin the CuA and CuB sites of the protein. Hence, it isevident that at some point of time during proteinmodification, these two Cu atoms need to be loaded atthe required sites of TYR and for this purpose Cu must betransported from the cytoplasm into the secretory path-way. ATP7A (MNK) encodes a P-type ATPase—Menkescopper transporter (MNK) localized at the TGN thattransfers cations through the lipid bilayer of TGN (Petriset al., 2000). When MNK is defective it leads to the MenkesDisease—an X-linked recessive disorder of copper meta-bolism, where patients suffer from a systemic copperdeficiency, hypopigmentation being one of its clinicalfeatures. These findings led to the hypothesis that MNK


may have a part to play in copper transport to copper-dependent enzymes including TYR that are synthesizedwithin the secretory pathway. Subsequently it was foundthat in the immortalized fibroblasts with defective ATP7A

(Menkes disease fibroblasts), TYR expressed from a cDNAconstruct had a very low level of activity, which was greatlyenhanced when the MNK protein was co-expressed. Again,when a mutation predicted to inhibit the copper transportwas introduced into the highly conserved phosphorylationsite of MNK, TYR activity was impaired due to chelationof copper in the cell culture medium. These observationsstrongly suggested that MNK has a role in deliveringcytoplasmic copper into the secretory pathway for TYR,but till date the precise mechanism of copper loading is notknown. Petris and group (2000) proposed two models forthe incorporation of copper into the active site of TYR. Inthe first model, copper emerging from the MNK proteininto the lumen of the secretory pathway may be directlyadded into apo-TYR that is specifically bound to thelumenal regions of the MNK protein. Alternatively, copperentering the secretory pathway may be transferred fromMNK protein to TYR apo-enzyme via intermediate copperchaperones. An experiment to test whether the lumenalloops of MNK are involved in binding TYR would beexpected to shed light on the exact molecular mechanism ofcopper loading in TYR.

3.4. Protein targeting to melanosome

Based on the electron microscopic and immunohisto-chemical studies, it has been proposed that after theloading of two copper atoms into the active site of theenzyme, the fully matured and enzymatically active TYR issorted from the TGN to the early (Stage-II) melanosomesvia coated secretory vesicles and thereafter becomelocalized in the limiting membrane of these organelles(Costin et al., 2003). Although not much is known aboutthe exact molecular mechanism of TYR transport, differentgroups of scientists have tried to find out the particularmolecular signals needed for melanosome targeting;whether such signals belong to a separate class of moleculeor reside in the TYR itself is yet unresolved. However, the

Fig. 9. Common conserved motifs at the C-terminal region of the tyrosinase a

mouse.

slower rate of transit from the Golgi may well regulate theoutput of mature TYR to melanosomes (Vijayasaradhiet al., 1991).In this context, the role of glycosphingolipid (a carbo-

hydrate moiety that is attached to a ceramide—a lipidanchor with two hydrophobic tails) has been investigatedby Sprong and group (2001a). They used glycosphingolipiddeficient GM95 cells to explore its role in membranetransport and observed that these cells were unable tomake melanin because of the sorting failure of TYR, whichinstead of being targeted to the melanosome was ratheraccumulated in the Golgi itself. This accumulated proteinwas, however, fully active in vitro. Glycosphingolipids,such as, glucosylceramide (GlcCer) and galactosylceramide(GalCer) have been found to be required for sorting ofmelanosomal proteins in the Golgi complex (Sprong et al.,2001a). The same group also proposed that the lateralmicrodomains of the glycosphingolipids and their choles-terol-enriched domains in the lumenal leaflet of the TGNare involved in the sorting of membrane proteins (Simonsand van Meer, 1988; Sprong et al., 2001b). But, how theglycosphingolipids enable TYR transport is still unknown.It is possible that these molecules may form domains on thecytosolic surface of the TGN, which are involved inmelanosomal vesicle budding. Again, these can also recruitcoat proteins (e.g. glycolipid transfer protein) from thecytosol (Mattjus et al., 2000).Melanosomes are also considered as modified peroxi-

somes (Halaban and Moellmann, 1990) and like severalperoxisomal enzymes TYR possesses the peroxisomaltargeting sequence (Ser–His–Leu) at the C-terminus(Bouchard et al., 1989) (Fig. 9). The possible functionalimplication of the peroxisomal targeting sequence wasobtained from a mutant TYR molecule that due to asingle base insertion in the gene lost the ‘Ser–His–Leu’sequence and was not detectable in the melanosome. Laterstudies revealed that for proper targeting of the melano-somal membrane proteins (including TYR), a dileucinemotif (E510KQPLL515) in the cytosolic tail is required(Vijayasaradhi et al., 1995b; Calvo et al., 1999; Simmenet al., 1999) (Fig. 9) and by a surface plasmon resonancestudy, it has been further demonstrated that the dileucine

nd tyrosinase related proteins (TYR, TYRP1 and TYRP2) in human and


motif of TYR interacts with the medium chain subunit m3of heterotetrameric AP-3 adaptor complex (Honing et al.,1998). AP-3, shown to be associated with clathrin(Dell’Angelica et al., 1998), binds to the cytosolic tails ofthe membrane proteins (Odorizzi et al., 1998) and pearlmouse with a mutation in AP-3’s b3A subunit has beenshown to express hypopigmentation (Feng et al., 1999).For the sorting and trafficking of TYR, the ‘tyrosine-basedsignal’ (Y521HSL524) with YXXy structure (y being a bulkyhydrophobic amino acid) may also play some importantrole (Fig. 9). This signal may be essential for the finaltargeting step of TYR to the melanosome, provided theprotein trafficking proceeds through an indirect pathwayvia the cell surface (Simmen et al., 1999). In fact, Simmenand group (1999) proposed that TYR might take anindirect route via the cell surface, but at the same time theyalso did not exclude the possibility of a fraction beingdirectly delivered from the TGN to the melanosome. Thedileucine motif and the tyrosine-based motif, conservedwithin the cytosolic tails of different vertebrate TYRs andTYRPs, could be the parts of a combined signal and theextreme C-terminal amino acids (which are however, notconserved among the melanosomal proteins) might play arole in the interaction with the two sorting signals (Simmenet al., 1999). Both of these signals are always preceded byacidic amino acids, possibly contributing in signal recogni-tion. Interestingly enough, these two signals are the sortingdeterminants for the late endosomes, lysosomes as well asmelanosomes (the specialized form of late endosome thatcontain a specific cohort of resident proteins); but theremust be a still unknown signal, which targets themelanosomal proteins specifically to the melanosomes inpigmented cells (Calvo et al., 1999).

3.5. Melanosomal factors

Several melanosomal factors that have the potential toinfluence the TYR sorting phenomenon are discussedbelow:

The P protein, present in the large granular fraction ofthe melanocytes (Rinchik et al., 1993), plays a critical rolein sorting and intracellular trafficking of TYR, asevidenced by the mislocalization of TYR due to a defectin the P protein (Potterf et al., 1998). Manga et al. (2001)experimentally proved that in the melanocytes lacking P,TYR appeared to undergo proteolysis, then accumulatedin vesicles and was secreted subsequently. This suggeststhat P functions at an organelle prior to melanosome—possibly as early as when the protein is in the Golgi or ER,because TYR degradation takes place early in the transportpathway. However, the precise point at which P actsremains to be elucidated (Manga et al., 2001). The role of Pin TYR trafficking was further confirmed by Chen et al.(2002) when they demonstrated that such misrouting ofTYR could be corrected by transfection of an expressionvector encoding an epitope-tagged wild-type P transcript.They also demonstrated that the root of this mislocaliza-

tion resides in the ER, as a high percentage of TYR with ashorter half-life possibly remained trapped in ER (TYR inP-null melan-p1 cells were actually found to be localized toperinuclear compartment rather than to melanosomes)(Chen et al., 2002). It has been hypothesized that to getproperly folded, P actually provides TYR the optimalfolding conditions in the ER, but exactly how P generatesthis condition is yet to be determined. Interestingly, P isthought to have a role in the generation and transport ofglycosphingolipids, which are important for melanosomalprotein trafficking (Sprong et al., 2001a). P may also play arole in the alkalization of the melanosomal pH, thusfavoring optimal TYR activity. In contrast to the previousprediction of P being a melanosomal protein (Rosemblatet al., 1994), using percoll and sucrose gradient fractiona-tion in combination with the immunofluorescence micro-scopy, Chen et al. (2002) demonstrated that ER contains asignificant amount of P-protein (Chen et al., 2002) thoughit is still possible that a fraction of the total P is also locatedin other organelles.In the underwhite (uw, mouse homolog of MATP i.e.

membrane associated transporter protein) mutant mice,though TYR processing through the ER and Golgi isnormal, the protein trafficking to the melanosomes isaberrant. As a result, a significant amount of Tyr and othermelanogenic enzymes are secreted out from the cells. Thus,it may be predicted that MATP functions in the sorting ofTyr from the trans-Golgi network to Stage-II melano-somes; but like P, the exact cellular level, where MATPfunctions is not yet been deciphered. Mutations in theMATP gene cause OCA4 with greatly reduced Tyr activity.Costin et al. (2003) opined that although MATP mutantcells produce normal levels of Tyr, but a large fraction ofTyr along with Tyrp1 and Tyrp2 is secreted out of themelanocytes, thus resulting in low level of melaninproduction. Similar phenomenon has been observed incase of P gene mutations, where a greater fraction of Tyr islost, ultimately giving the OCA2 phenotype. The molecularbasis of OCA4 is similar to OCA2 but MATP is presumedto work at a later stage in the melanosomal maturationpathway than that of P. In contrast to the OCA2 andOCA4 cases, in OCA1 and OCA3, the misfolded Tyr in theER sorting pathway, is degraded immediately by theproteasome (Fig. 10) (Costin et al., 2003). However, in caseof OCA1, even if the correctly folded protein reaches themelanosome, it remains catalytically inactive. It will beinteresting to find out if a similar scenario is maintained incase of human beings.Interestingly, by the pulse chase analysis, Hall and group

(2005) demonstrated that phenylthiourea (PTU) inducesdegradation of TYR, following complete maturation of theenzyme (Hall and Orlow, 2005). Their data suggest thatthere must be a protease-mediated late or post-Golgicheckpoint at which TYR can be degraded. In fact, tocheck the incomplete proteins that evade ER quality-control system, there is evidence for additional pathways inthe literature. In yeasts, these proteins are degraded by

ARTICLE IN PRESS

Fig. 10. Correlation between melanosomal protein sorting and OCA types. The disruption of TYR trafficking occurs at the ER level in case of OCA1 and

OCA3, when the mis/unfolded protein is ultimately degraded by the proteasome. In OCA2 and OCA4, the enzyme is abnormally secreted from the cells—

for OCA2 this is an immediate post-Golgi event, whereas in case of OCA4, this occurs later before being delivered to the early melanosomes. This figure

has been reproduced from Costin et al., 2003. J Cell Sci 116, 3210.


vacuoles, thereby suggesting a quality-control systembeyond ER (Hong et al., 1996; Holkeri and Makarow,1998; Jorgensen et al., 1999; Coughlan et al., 2004). Thesame process is also operative in the mammalian system(Arvan et al., 2002). It is possible that the PTU-induceddegradation of TYR appears to follow such a system andin that case, it can be inferred that a point of quality-control exists near the Golgi complex that is specific toTYR. But the conditions for this checkpoint activation andthe proteins that are degraded through this system are notknown (Hall and Orlow, 2005).

Over expression or abnormal synthesis of melanosomalproteins may also very well trigger the signals, which arenormally unavailable to the cytoplasmic machinery thatsorts proteins to other intracellular locations. For example,the recognition of the C-terminal SHL motif (a perox-isomal sorting signal) by the cytosolic peroxisomal target-ing machinery leads to the retro-translocation of TYR tocytosol (Subramani, 1996).

There are some evidences that Rabs, v-SNARES andt-SNARES might play some role in orchestrating thejourney of melanosomal proteins from the moment of theirsynthesis in melanocytes until their export to the neighbor-ing granules (Huang et al., 1999). In fact, recentlyWasmeier et al. (2006) showed that Rab38 and Rab32regulate a critical step in the trafficking of mousemelanogenic enzymes, particularly, TYR, from the TGNto melanosomes. In Rab 38/Rab 32-deficient cells, Tyrappeared to be mistargeted and degraded after exit fromthe TGN (Wasmeier et al., 2006). But in human, no suchrecord of their direct association with TYR has beenestablished till date.

4. Functional aspects of TYR

4.1. Pigmentary system and melanin biosynthesis

The pigmentary system of mammals is dependent onthe synthesis of a light-absorbing biopolymer melanin(Nordlund et al., 1998), which serves a variety of importantfunctions—as a cosmetic entity participating in pro-tective coloration, in sexual attraction within species, abarrier protecting against UV radiation, a scavengerof cytotoxic radicals as well as a metabolic inter-mediate and a participant in developmental processes(Spritz and Hearing, 1994). Melanin is synthesized withinmelanosome—a membrane-bound intracellular organelle,produced by the melanocytes (i.e. pigment cells).Two types of melanin are produced in mammalian

skin—eumelanin (black or brown), derived from themetabolites of dopachrome and pheomelanin (red oryellow), derivative of the metabolites of 5-S-cysteinylDOPA. The rate-limiting enzyme for melanin formationis TYR —the copper containing monophenol monoox-ygenase (Zhao and Boissy, 1994). It catalyses threereactions: (a) hydroxylation of tyrosine to 3,4-dihydrox-yphenylalanine (DOPA)—the first and the most critical(rate-limiting) reaction; (b) oxidation of DOPA to dopa-quinone (in the absence of TYR, DOPA can autooxidizespontaneously to dopaquinone, but at a slower rate) (Kinget al., 2001) and (c) conversion of 5,6-dihydroxyindoleto indole-5,6-quinone (Fig. 11). For the initial critical step(i.e. hydroxylation of tyrosine), TYR needs DOPA asa cofactor (King et al., 1978). The rates of tyrosinehydroxylation in absence of the DOPA are negligible,

ARTICLE IN PRESS

Fig. 11. Schematic representation of the melanin biosynthetic pathway. TYR catalyses three reactions: (a) hydroxylation of tyrosine to 3,4-

dihydroxyphenylalanine (DOPA)—the rate limiting step in melanin biosynthesis (b) oxidation of DOPA to dopaquinone; and (c) conversion of 5,6-

dihydroxyindole to indole-5,6-quinone. It is thought that TYR may also have the potential to oxidize DHICA to indole-5,6-quinone carboxylic acid,

which is primarily the function of TYRP1.


which raises the question on the origin of the initial DOPAcofactor, since DOPA is normally not available within thecell. Several hypotheses have been drawn to elucidate thepossible origin of the DOPA cofactor or any alternativemethod of the enzyme activation, but this question stillremains unresolved (King et al., 2001). However, in theeumelanin pathway, the dopaquinone is then transformedinto dopachrome and further transformation of thisproduct needs the distinct catalytic properties of TYRP1and TYRP2. TYRP2 (also called DCT i.e. dopachrometautomerase) catalyzes the conversion of dopachrome todihydroxyindole carboxylic acid (DHICA) (Tsukamotoet al., 1992; Kroumpouzos et al., 1994; Yokoyama et al.,1994), whereas TYRP1 not only acts as a DHICA oxidase(Jimenez-Cervantes et al., 1994; Kobayashi et al., 1994),but also stabilizes TYR. Current thought is that TYR mayalso have the potential to oxidize DHICA to indole-5,6-quinone carboxylic acid, which is the primary function ofTYRP1 (Murisier and Beermann, 2006; Wang and Hebert,2006). Indeed, TYR forms a melanocyte-specific melano-genic complex in association with TYRPs (TYRP1 andTYRP2), lysosome associated membrane protein 1(LAMP1) and melanocyte-stimulating hormone receptors,which act as an integrated unit (Orlow et al., 1994).

In this context, it is worthwhile to mention that in situTYR activity requires an appropriate ionic environmentand the melanosome is an acidic organelle (pH: 3.0–5.0)

(Bhatnagar et al., 1993). But it has been demonstrated thatmammalian TYRs show optimal activity at neutral pH.The P protein that shares structural similarity with theE. coli Na+/H+ anti-porter was previously suggested tomaintain the melanosomal pH. P was thought to functionas a channel to reduce the proton concentration inside themelanosomes and in this way to regulate the TYR activity(Ancans et al., 2001). But according to Puri et al. (2000),P-null melanocytes are not acidic and an acidic melanoso-mal environment is important for melanin biosynthesis(Puri et al., 2000; Brilliant, 2001). However, recently, it hasbeen reported that human melanocytes do express all theseven isoforms of sodium–hydrogen exchangers (NHEs)—a family of vital integral membrane antiporters that areknown to regulate the intracellular and intraorganellar pHby catalyzing the electroneutral exchange of extracellularNa+ for intracellular H+ (Smith et al., 2004).

4.2. Visual system development

Disturbances of melanin formation and/or melanosomefunction have been implicated in numerous pigmentarydisorders (such as different forms of albinism). Interest-ingly, most of these disorders are characterized by reducedvisual acuity, loss of binocular vision (strabismus) andpresence of nystagmus (Oetting and King, 1999). In thealbinos, the connections between the eye and the brain are


systematically disrupted due to some developmentalanomaly of the axonal behavior, thereby leading to severeimpairments in vision. Moreover, in some regions of theeye, the patterns of cell production are also disrupted anddelayed (Lavado et al., 2006). Based on these information,it has been speculated that TYR might be involved in thenormal visual development.

4.2.1. Axonal projections

The first step towards guiding the retinal axons to theirspecific regions of the optic tectum takes place within theretina itself. After the differentiation of the retinal ganglioncells (RGCs), millions of these RGC axons grow along theretinal inner surface towards the optic disk (the head of theoptic nerve). Formation of the mature optic nerve isachieved by fasciculation of these axons. Emerging fromthe optic disc, the ultimate destination of the mammalianaxons is the dorsal lateral geniculate nuclei (dLGN) of themidbrain. However, upon entering the brain, as the RGCaxons reach the optic chiasm, they have to ‘‘decide’’ if theyare to continue straight towards the dLGN of thecontralateral side or to turn 901 towards the ipsilateralside (Fig. 12). Normally, ganglion cell axons from nasalretina reach the contralateral hemisphere while manytemporal retinal fibers go the other way round (von demHagen et al., 2005). It appears that those axons that are notdestined to reach the contralateral side of the brain arerepulsed from doing so when they enter the optic chiasm(Godement et al., 1990), but the molecular basis of thisrepulsion is not known. This pattern of partial decussations(crossing) at the optic chiasm forms the anatomical basisfor the stereoscopic vision in mammals.

In albinism, a portion of the RGC axons, originallydestined to the ipsilateral route, misproject to thecontralateral side of the brain, thereby resulting in the

Fig. 12. Pathfinding of RGC axons. A, B, C and D represents the

sequential steps for axonal growth; A, Establishment of retinal layers; B,

Directed axonal growth; C, Orderly progression into optic nerve; and D,

Decision to cross or turn. The figure has been adapted from Gilbert, Scott

F., 2000. Developmental Biology, sixth ed.

disruption of binocular vision (Lund, 1965; Guillery, 1971;Cooper and Pettigrew, 1979; Drager and Olsen, 1980;Lavado and Montoliu, 2006) (Fig. 13). This anomaly hasbeen documented in a range of albino mammals includingrodents (Lund, 1965), rabbits (Giolli and Guthrie, 1969),cats (Guillery, 1969), ferrets (Guillery, 1971) as well ashumans (Neveu et al., 2003). In the albinos, however, thegrowth patterns and the cellular organization of the RGCaxons in the optic chiasm appear normal (Marcus et al.,1996), which suggests that the defects associated with theaxonal guidance occur at a point upstream of the opticchiasm—either within the optic nerve or within the retinaitself. If the differential distribution of melanin is taken asthe cause of this abnormality, it is worth mentioning thatthe cells that produce melanin are located in severalpositions of the eye—choroid, optic stalk and RPE, wherethey can potentially affect the development of the visualpathway. However, the melanin-producing cells in thechoroids are not likely to be involved in this pathway, asthey develop after the abnormality at the chiasm becomesapparent (Chan et al., 1993; Chan and Guillery, 1993).Again, normally pigmented rats lacking pigmented cellsspecifically in the optic stalk have been found to exhibitnormal decussation patterns of the optic chiasm(Horsburgh and Sefton, 1986). Therefore, it is the thirdand the last candidate i.e. the RPE, which could directlyinfluence the decussating pattern of the optic chiasm. Infact, a direct link between the melanin levels in RPE andthe proportion of the uncrossed retinal axon projection haslong been recognized in mammals (Sanderson et al., 1974;LaVail et al., 1978; Balkema and Drager, 1990). Pigmenta-tion of RPE that regulates the proliferation and differ-entiation of the neural retina during development is crucialfor normal vision. Even a low level of melanin in RPE hasbeen shown to be sufficient to allow the establishment ofnormal (uncrossed) chiasmatic pathway (Rachel et al.,2002b). However, how these RPE cells impart a signalupon the RGC axons is still unclear. It is now known thatretinal neuronal mitosis takes place when the cells are atthe ventricular margin next to the RPE (Young, 1983) andat this point, cells of the neural retina form transitory gapjunctional connections with the neuroepithelial and thepigmented epithelial cells. These gap junctions might be aplausible answer to the cell-to-cell communication betweenRGC and RPE (Fujisawa et al., 1976; Hayes, 1976;Townes-Anderson and Raviola, 1981), but it has not beenproved yet (Rice et al., 1999). Thus, even if it seems thatmelanin production is the cue to visual development, theprecise relationship between the amount of melaninproduction by RPE and the visual system defects inalbinism remains unclear (Rachel et al., 2002b). Recently,the combination of transgenic and endogenous TYR wasused in mice to show that only 35% of TYR activity wasrequired to accumulate normal levels of RPE melanin,beyond which melanin synthesis appears to be saturatedand thus increasing TYR activity has no observable effecton RPE pigmentation. Furthermore, the same study

ARTICLE IN PRESS

Fig. 13. Comparison of the normal and albino individuals in terms of binocular vision. The upper boxes denote the visual object (the input) and the lower

boxes represent the visual signals received by the eyes, which clearly indicate how the signal from the same object is differentially processed by the normal

and albino individuals. Blue and yellow colors represent the visual inputs (through the retinal ganglion cell axons) to the right and left hemispheres of the

brain, respectively. Different shades of blue and yellow are used to demarcate visual inputs from the left and the right eye. Altered distribution of retinal

ganglion cell axons causes loss of binocular vision (strabismus) in the albino individual.


suggested that over-expression of TYR did not produce asignificant increase in the size of the uncrossed retinalprojection. It is also speculated that a differential regula-tory pattern of TYR exists between the RPE and neuralcrest derived melanocytes (Rachel et al., 2002b).

All these background information suggests TYR, the keymelanogenic enzyme, as the principal candidate for being amolecular signal, which is needed for the proper projectionof RGCs towards the dLGN on the appropriate side of thebrain. In fact, there is a significant body of evidence thatcan qualify TYR to be responsible for the axonalmisprojections as found in albinism: (1) in vivo experimentscreating point mutations in TYR or deletions of the entirelocus result in visual defects along with hypopigmentation(Ruppert et al., 1988; del Marmol and Beermann, 1996); (2)transgenic replacement of Tyr in albino mice not onlyrestores the normal pigmentation (Beermann et al., 1990;Tanaka et al., 1990) but also the normal visual projectionsfrom the retina to the dLGN (Jeffery et al., 1994a, b, 1997);(3) experimental evidences suggest that TYR is firstexpressed on embryonic day 10 (E10) (Beermann et al.,1992) in the RPE and pigment formation starts at E11. Theonset of melanin formation is graded across the retina, withperipheral regions becoming pigmented first. By E12.5 theentire RPE becomes pigmented (Drager, 1985) and thisoccurs at the same time as the initial pattern of neuroblastdivisions (E11.5–E16.5), eventually giving rise to ipsilat-erally projecting ganglion cells. Therefore, it is assumedthat the TYR expression pattern along with the gradedonset of pigment formation might be a developmentalsignal that sets up positional information in the retina,committing the ganglion cells produced from the neuro-blasts to either ipsilateral or contralateral projections.

4.2.2. Development of the central retina and the

rod photoreceptors

In absence of melanin, two additional deficits are foundin retinal development. Firstly, the central retina specia-lized by the presence of fovea (the area of maximum visualacuity), fails to develop fully such that the cellular layer isabnormally thin (foveal hypoplasia) (Elschnig, 1913;Jeffery and Kinsella, 1992; Jeffery et al., 1994a, b). It hasnow been observed that in the albinos, the medial-lateralwidth of the optic nerves, tracts and the chiasm aresignificantly decreased, presumably because of the reducednumber of ganglion cells from the underdeveloped centralretina. This may be the reason of the decreased corticalvolume at the occipital poles in the albino patients (vondem Hagen et al., 2005). Secondly, there is a cellular deficitin nearly all layers of the retina, most notably a 30%reduction in the rod photoreceptor numbers (Jeffery et al.,1994a, b; Ilia and Jeffery, 2000). In albino animals, duringdevelopment of the neural retina, there is an alteration ofthe cell cycle events, reflected by a transient increase ofproliferation and a subsequent increase of apoptosis thatresults in the cell deficit (Ilia and Jeffery, 1996, 1999;Gimenez et al., 2001; Rachel et al., 2002a). Jeffery et al.(1997) introduced functional TYR into albino embryos ofmice and showed that the transgenic animals not onlyexhibited normal pigmentation but also all the visualabnormalities were reverted (Jeffery et al., 1997). Thisconfirms the role of TYR in the development of centralRGC layer as well as the rod photoreceptors. It is possiblethat the action of TYR is related to the temporal pace ofcell production, perhaps by influencing the cell cycle.A recent comparison of the patterns of cell production inthe ganglion cell layer of pigmented and albino rats, has


shown that there is a temporal delay in these patterns inalbinos (Ilia and Jeffery, 1996).

All the above-mentioned observations suggest the role ofTYR in visual system development; but there may beinvolvement of other factor(s) whose presence correlateswith the TYR activity. Currently, a model points towardsL-DOPA as one such factor, whose accumulation isreduced in albinos (Ilia and Jeffery, 1999) and which canalso control the cell cycle (Akeo et al., 1994). It wasobserved that L-DOPA addition to the developing albinomouse eyes could correct proliferation and cell-deathdefects (Ilia and Jeffery, 1999). Moreover, there is anincreasing number of indirect evidence, related to othervisual abnormalities supporting this idea (Kobayashi et al.,1995a; Rios et al., 1999; Eisenhofer et al., 2003; Libbyet al., 2003). Recently, Lavado et al. (2006) have shownthat in transgenic albino mice lacking functional TYR,ectopic expression of TH displayed corrected photorecep-tor numbers as well as normal chiasmatic pathway similarto the pigmented animals. However, the albino phenotypecould not be corrected, which proves that L-DOPAsynthesized by tyrosine hydroxylase in absence of TYR,may play an important role in retinal developmentalpathway. Interestingly, since unlike TYR, tyrosine hydro-xylase cannot oxidize L-DOPA to dopaquinone, melanincould not be produced and the transgenic mice retainedtheir albino phenotype. This observation indeed suggeststhat substitution of independent but biochemically relatedenzymes might help to overcome developmental abnorm-alities (Lavado et al., 2006).

4.3. Retinal network adaptation

In an animal’s environment, although the light intensitymay vary by 10 orders of magnitude, its visually drivenbehaviors remain stable as the visual system adjusts itssensitivity to a wide range of light intensities. Retinalnetwork adaptations are thought to be controlled by light-dependent changes in neuromodulator levels. In fact,dopamine (a neurotransmitter) acts as a paracrine factorin this regard and its secretion is thought to be critical forlight adaptation (Witkovsky, 2004). In fish, exogenousdopamine mimics the effect of ambient light on the dark-adapted outer retina. Dopamine is synthesized in twosequential steps—(1) hydroxylation of the amino acidL-tyrosine to L-DOPA by tyrosine hydroxylase and(2) decarboxylation of L-DOPA to dopamine by aromaticamino acid decarboxylase (AADC).

Zebra fish having mutations in one of the albino genessdy, lack melanin in their eyes and they do exhibit severelyaffected visual performance (Neuhauss et al., 1999); but thesdy visual defects could not be attributed for the ocularanomalies commonly found in the albino mammals, viz.lack of a fovea, rod deficit or misrouting of the opticpathway (Jeffery, 1997). However, retinal light adaptationis disrupted in the sdy mutants and as a result they areunable to properly set the light sensitivity with the change

of background luminance. Interestingly, the sdy gene codesfor TYR, which like tyrosine hydroxylase can convertL-tyrosine to L-DOPA—the critical intermediate of thedopamine biosynthesis pathway. Using behavioral andelectrophysiological measurements such as the optokineticresponse assay (OKR) (Roeser and Baier, 2003)—a valuable tool to quantify behavioral visual performancesuch as visual acuity, contrast sensitivity and lightadaptation; it has been shown that following the onset oflight, the sdy mutants initially do not response and recoverwith a sluggish time course. The same defect was observedin fully pigmented zebrafish, treated with the TYRinhibitor phenylthiourea (PTU) and interestingly, albinomutants who lack melanin but possess functional TYR donot exhibit such defect (Haffter et al., 1996). Theseexperiments suggest that a TYR product other thanmelanin is responsible for the light adaptation. In fact,the molecular nature of sdy suggests that the light-adaptivesignal missing in the mutant could be dopamine. Hence, itis believed that TYR may play an important role fordopamine production in the retina through a novelpathway (Page-McCaw et al., 2004). However, in zebrafish, AADC is expressed in photoreceptors, dopaminergicinterplexiform cells (IPCs) and possibly in the horizontalcells, whereas TYR expression is found in the RPE thatsurrounds and supports the photoreceptor cells. Therefore,in a simple model, it can be stated that—the RPE mayproduce L-DOPA that may be secreted into the extra-cellular space surrounding the photoreceptors, which thentake it up and convert it to dopamine. Dopamine may thenbe released into the extracellular space, where it acts ondopamine receptors at several sites in the retinal circuitry.This signal might reset the retinal gain, enabling the visualcircuitry to cope with the brighter light. Therefore, anyloss-of-function (particularly tyrosine-hydroxylase func-tion) mutations of TYR are expected to remove theL-DOPA supply from RPE, ultimately resulting in thechronic deficiency of dopamine (Page-McCaw et al., 2004).There are also reports that in the striatum of mice, Tyrcontributes to dopamine synthesis complementing thecanonical pathway via tyrosine hydroxylase (Rios et al.,1999). A similar additive effect of the TYR (in RPE) andtyrosine hydroxylase (in IPC) pathways may quite well beat work in the retina.

4.4. Modifier locus in developmental glaucoma

Glaucoma represents a heterogeneous group of opticneuropathies with a complex genetic basis. These neuro-pathies gradually reduce the vision without any warningand often there is no symptom. According to the latestestimates, glaucoma affects about 67 million peopleworldwide (Quigley, 1996). Mutations in the CYP1B1

(Cytochrome P450 family 1, subfamily B, polypeptide 1)gene cause human primary congenital glaucoma (PCG)—a subset of glaucoma characterized by the developmentaldefects in the trabecular meshwork that drains fluid from


the eye resulting in an elevated intraocular pressure (IOP),corneal edema, photophobia and ocular enlargement(Ritch et al., 1996; Sarfarazi and Stoilov, 2000; Gouldand John, 2002). In PCG patients, existence of a modifierlocus affecting the development of glaucoma has beenspeculated. In this context, Libby et al. (2003) testeddifferent mouse strains to find out the strain-specificmodifier genes that might suppress or enhance glaucoma-related ocular abnormalities (e.g. angle abnormalities). Incourse, Cyp1b1 mutant mice were crossed with differentmouse strains and it was found that albino mice lackingTyr were more severely affected with PCG-related ocularproblems (such as focal abnormalities and angle abnorm-alities) than the pigmented ones. It was also observed thateven in absence of a Cyp1b1 mutation, TYR deficiencyitself could cause mild histopathological defects in theiridocorneal angle of the albino mice (van Dorp et al.,1984; Gould et al., 2004). Interestingly, the abnormalitiesof the albino mice were more severe when the Cyp1b1

mutation was added on, suggesting a synergistic effect ofCYP1B1 and TYR in PCG. All these results indicatedthat Tyr might play a protective role against the develop-mental abnormalities in Cyp1b1 mutated mice. Tyr

was also found to be a modifier locus for the anteriorsegment dysgenesis (ASD) phenotype, caused by the nullmutation in Foxc1 (Forkhead Box C 1) gene, which wasalso implicated in human PCG and similar to Cyp1b1

studies, TYR deficient FoxcI+/� mice had more severeabnormalities than their pigmented counterparts (Libbyet al., 2003).

All these findings in mice suggest a possible associationbetween glaucoma and albinism. However, the question ishow can TYR affect the severity of the glaucomaphenotype. Libby et al. (2003) speculates that it might bedue to the defect in the tyrosine-hydroxylase activity ofTYR, as PITX2 and PITX3, whose mutations causedevelopmental glaucoma, indeed transactivate the THgene in their normal form. They hypothesized that TYRmight affect angle development through modulation ofL-DOPA levels. L-DOPA is the precursor of catechola-mines, which are very important developmental regulators(Thomas et al., 1995; Zhou et al., 1995; Ilia and Jeffery,1999). In fact, L-DOPA treatment was found to prevent thesevere angle dysgenesis present in untreated mice lackingboth Cyp1b1 and Tyr. This experiment suggests a potentialpathway involving L-DOPA (or an L-DOPA metabolite) inthe angle formation and the fact that any disturbance inthis pathway can be treated by L-DOPA administrationopens the door to a potential new therapy for the treatmentof some forms of glaucoma. The TYR–CYP1B1 andTYR–FOXC1 synergism may very well occur in humansalso, but then it must represent a very rare case, far lesscommon than even the rare forms of glaucoma like PCG(Alward, 2003). However, the outcome of a recent studyconducted by Bidinost et al. (2006), did not support TYR

as a modifier of the CYP1B1-associated PCG phenotype inthe Saudi Arabian population (Bidinost et al., 2006).

5. TYR and oculocutaneous albinism (OCA)

5.1. Pigmentation genes and OCA types

Early in the 20th century, Gertrude and CharlesDavenport first attempted to study the basis of humanpigmentation by examining the inheritance pattern of eye,hair and skin color (Davenport and Davenport, 1907,1909, 1910). Thereafter, Sewall Wright (1918) recognizedthat all these traits were physiologically connected andhence, while discussing inheritance, those should beconsidered together (Wright, 1918). Relatively recentstudies provide definitive understanding of the ways inwhich the underlying genes determine human pigmenta-tion. Studies on hypopigmented human phenotypes arebeing used to track down and characterize the dysfunc-tional genes as well as the genetic variations across diversehuman races.The pigmentary system of human is based on the

formation of melanin by the melanocytes. Melaninaccumulates within the specialized organelle—melanosomes and the genes that direct the formation,transport, distribution and function of melanosomes arecalled ‘pigmentation genes’. Defects in the melaninproduction due to dysfunction of melanocytes, primarilyin the skin and eyes result in the development of congenitalhypopigmentary diseases, including oculocutaneous albin-ism (OCA). Tomita and Suzuki (2004) classified the geneticand molecular bases of various types of congenitalpigmentary disorders (Tables 1–4) as: (1) disorders ofmelanoblast migration in the embryo from the neural crestto the skin–e.g. piebaldism and Waardenburg Syndrometypes 1–4 (WS1–4); (2) disorders of melanosome formationin the melanocyte–e.g. Hermansky-Pudlak syndrome types1–7 (HPS1–7) and Chediak-Higashi syndrome 1 (CHS1);(3) disorders of melanin synthesis in the melanosome—e.g.oculocutaneous albinism types 1–4 (OCA1–4); and (4)disorders of mature melanosome transfer to the tips of thedendrites–e.g. Griscelli syndrome types 1–3 (GS1-3)(Tomita and Suzuki, 2004).OCA is a heterogeneous group of autosomal recessive

disorders of melanin synthesis, which is characterized by acongenital reduction or absence of melanin and associatedwith common developmental abnormalities of the eye.Apart from hypopigmentation, other changes to the opticsystem include decreased visual acuity secondary to fovealhypoplasia, photophobia, iris transillumination, nystag-mus, pigment deficiency in peripheral retina and misrout-ing of optic nerves at the chiasm. Mutations in those genesthat regulate the multistep process of melanin synthesis anddistribution of melanin are the basis of this disease and tillnow at least 16 different genes have been identified that onmutation can cause different forms of albinism (Tomitaand Suzuki, 2004). Albinos share the common visualdefects irrespective of the fact that whether the mutation ispresent in TYR (i.e. TYR negative albinism, OCA1) or not(i.e. TYR positive albinisms) (Sanderson et al., 1974;

ARTICLE IN PRESSTable

1

Disordersofmelanoblast

migrationfrom

theneuralcrestinto

theskin

Disease

Inheritance

pattern

Locusname

MIM

no.

Chromosomal

location

Protein

Function

Disease

characteristics

Piebaldism

aAD

KIT

172800

4q11–q12

Plasm

amem

branereceptor

forstem

cellgrowth

factor

Melanoblast

proliferation

andmigration

Mentalretardation,short

stature,and

whiteforelock

andabsence

of

pigmentationofthemedialportionofthe

forehead,eyebrows,andchin

andofthe

ventralchest,abdomen,andextrem

ities

(Thebordersofunpigmentedareasare

hyperpigmented)

WS1

AD

PA

X3

193500

2q35

Paired

boxgene3

Transcriptionfactorof

MIT

F

Whiteforelock

andskin

patches

more

frequent,Dystopia

canthorum

present

WS2

AD

MIT

F193510,600193,

606662

3p14.1–p12.3

Microphthalm

iaassociated

transcriptionfactor


differentmelanosomal

proteinslikeTYR

Nodystopia

canthorum,Sensorineural

hearingloss

andheterochromia

irides

more

frequent

WS3

AD

PA

X3

148820

2q35

Paired

boxgene3


MIT

F

Dystopia

canthorum

andlimbanomalies

WS4b

AD

SO

X1

0277580

22q13

SRY-relatedHMG-box

gene10


MIT

F

Aganglionic

megacolon

aSomecasesofpiebaldism

are

causedbymutationin

thegeneencodingthezincfinger

transcriptionfactorSNAI2,locatedonchromosome8q11.

bSomeWS4patients

havemutationsin

Endothelin-3

(EDN3)orEndothelin-B

receptor(EDNR3)genes.


Balkema and Drager, 1990). The underlying geneticheterogeneity resulting in the observed range of over-lapping phenotypic variation renders difficulty in theclinical diagnosis of albinism subtypes (Garrison et al.,2004). For example, (a) it has been reported that HPS andCHS sometime manifest overlapping phenotypes; (b)albinos with P-gene mutation could show HPS-relatedsymptoms instead of more commonly observed OCA2phenotype; (c) it is difficult to distinguish OCA1B fromOCA2 phenotype and (d) while OCA4 patients in theTurkish population exhibit OCA2 phenotype, amongJapanese, the hypopigmentation pattern and clinicalphenotype of OCA4 are variable depending on the mutantgenotype (Inagaki et al., 2004). Thus, the possibility ofmisleading interpretation based on the clinical symptomsunderscores the importance of molecular diagnosis of thedisease pathogenesis.

5.2. TYR in OCA type 1

One of the most common types of albinism is TYR-related OCA (i.e. OCA1), which results from the loss offunction of the gene TYR. Defects in TYR have beenreported in almost all populations of the world and OCA1occurs in approximately 1 per 40,000 individuals in mostpopulations (King et al., 2001). Tomita et al. (1989) firstreported a pathological mutation in TYR of an OCApatient (Tomita et al., 1989). Since then, a large numberof mutations causing OCA1 have been reported (Gotoet al., 2004).The classical OCA1 cases can be characterized by the

total absence of melanin in the skin, hair and eyes at birth.The irides can be light blue and translucent so that thewhole iris appears pink or red in ambient light, which later(with age) may acquire some pigmentation. The skinremains white or become slightly pigmented with time; butif remains unprotected, may develop erythema on sunexposure. However, in general, different phenotypes ofOCA1 are observed, which depend on the amount ofresidual activity of the enzyme, produced by the mutantTYR alleles. A fast, convenient but empirical determina-tion of OCA1 is done by the ‘hair-bulb assay’ which isdependent on the conversion of freshly picked hair-bulb todark-brown color due to presence of TYR activity innormal but not OCA1 patients’ sample (King and Witkop,1976). Based on the level of TYR activity in the hair-bulb,OCA1 has been classified into two categories—(a) OCA1A:when the enzyme activity is completely lacking and (b)OCA1B: when some residual activity is retained. Forexample, TYR null and in/del mutations would be expectedto cause OCA1A while most but not all of the missensemutations would result in OCA1B. OCA1A is the mostsevere form of OCA. The visual acuity of the OCA1Apatients is greatly reduced such that most of the patientsare recorded as legally blind; nystagmus, strabismus,photophobia are usually severe and the translucent iridesthat appear pink early in life, often become gray-blue with

ARTICLE IN PRESS

Table

2

Disordersofmelanosomeform

ationin

themelanocyte

Disease

Inheritance

pattern

Locusname

MIM

no.

Chromosomal

location

Protein

Function

Disease

characteristics

HPS1

AR

HP

S1

203300,604982

10q23.1

HPS1(transm

embrane

protein)

WithHPS4form

alysosomal

complex—

BLOC3(biogenesisof

lysosome-relatedorganellescomplex-

3)thatisinvolved

inthebiogenesisof

lysosomal-relatedorganellesthrough

amechanism

distinct

from

that

operatedbytheAP3complex

OCA

features,mildto

severebleeding

diathesisandceroid

storagedisease

HPS2

AR

AD

TB

3A

608233

5q14.1

Adaptor-relatedprotein

complex3

Encodes

thebeta3A

subunitof

heterotetrameric

complexAP-3

(adaptorprotein-3)thathasrole

in

form

ationofcoatedvesicles

HPS3

AR

HP

S3

606118

3q24

HPS3protein

Notknown

HPS4

AR

HP

S4

606682

22q11.2–q12.2

HPS4protein

Form

ationofBLOC3withHPS1

HPS5

AR

HP

S5

607521

11p15–p13

a-integrin-binding

protein

63

Directlyinteractsin

acomplex

referred

as‘biogenesisoflysosome-

relatedorganellescomplex-2’or

BLOC2

HPS6

AR

HP

S6

607522

10q24.32

HPS6

Directlyinteractsin

BLOC2complex

HPS7

AR

DT

NB

P1

607145

6p22.3

Dystrobrevin-binding

protein

1

Playspart

inthebiogenesisof

lysosome-relatedorganelles

CHS1

AR

LY

ST

214500

1q42.1-q42.2

CHS1

Cytosolicprotein

witharolein

vesicle

transport

Severeim

munedeficiency,OCA

features,bleedingtendencies,

recurrentpyogem

icinfection,

progressiveneurologic

defects

anda

lymphoproliferativesyndrome


ARTICLE IN PRESS

Table 3

Disorders of melanin synthesis

Disease Inheritance

pattern

Locus name MIM no. Chromosomal

location

Protein Function Disease characteristics

OCA1 AR TYR 203100,

606952

11q14–q21 Tyrosinase Oxidation of

tyrosine and DOPA

Reduction or complete

absence of melanin in the

skin, hair and eyes;

photophobia, strabismus,

moderate to severely reduced

visual acuity, nystagmus,

optic nerve misrouting at the

chiasm

OCA2 AR P 203200 15q11.2–q12 P Not yet been

clarified, sorting of

TYR (?)

Variable phenotype, a patient

with complete loss of melanin

is indistinguishable from an

OCA1A patient and one with

bon hair resembles OCA1B

patient

OCA3 AR TYRP1 203290 9p23 Gp75/Tyrosinase

related protein 1

DHICA oxidase,

TYR stabilization

Decreased melanogenesis,

mild phenotype

OCA4 AR MATP 606574 5p13.3 Membrane

associated

transporter protein

Not yet been

clarified, sorting of

TYR (?)

Variable phenotype, often

similar to OCA2

Table 4

Disorder of mature melanosome transfer in the melanocyte

Disease Inheritance

pattern

Locus name MIM# Chromosomal

location

Protein Function Disease characteristics

GS1 AR MYO5A 214450 15q21 Myosin 5A Organelle motor

protein

OCA1 characteristics, severe

primary neurological

impairment with

developmental delay and

mental retardation

GS2 AR RAB27A 607624 15q21 Rab27a RAS family protein OCA1 characteristics,

immune defects and

hemophagocytic syndrome

GS3 AR MLPH 609227 2q37 Melanophilin Interacts with

MyoVa, Rab27a

Only pigmentary dilution of

skin and hair


age. OCA1A phenotype is same in all ethnic groupsirrespective of age with absence of melanin throughout thelife of the patient. In contrast, in OCA1B, although there islittle or no apparent melanin at birth (hence the individualsappear to have OCA1A), progressive melanization mayoccur with time. The range of pigmentation in OCA1Bvaries from very little cutaneous pigment to nearly normalskin pigmentation and the phenotype is influenced by theethnicity and family pigment patterns. Occasionally, amoderate amount of residual TYR activity leads to normalskin pigmentation and is wrongly diagnosed as ocularalbinism (in ocular albinism, the eye features remain sameas in other types of albinism while the skin pigment patternappears almost normal). OCA1B is called ‘yellow OCA’due to the color of the hair, produced by the pheomelaninsynthesis. Dopaquinone, a product of TYR activity, hashigh affinity for sulfhydryl compounds. It is believed thatlow activity of TYR in OCA1B patient results in small

amount of dopaquinone, which on binding to thesulfhydryl compounds produce pheomelanin and is man-ifested as the yellow color of hair. Yellow OCA was firstidentified in the Amish of Indiana (USA) and thensubsequently in other population groups. In addition toOCA1A and OCA1B, there is another subtype of OCA1,called temperature sensitive albinism (OCA1TS). In thistype of OCA1, temperature sensitive TYR is produced thatpossesses �25% activity of the normal enzyme at 37 1C;but the activity improves at lower temperatures (Toyofukuet al., 2001a). It is worthwhile to mention here that Kinget al. (2001) proposed a fourth subtype of OCA1, termed‘minimal pigment OCA’ (OCA1MP), also called ‘dark-eyed albinism’ (King et al., 2001). All the OCA1MP caseshave been recognized within the Caucasians only and itsoccurrence and phenotypic variation in other ethnic groupshas not yet been reported. This type of OCA1 is associatedwith the formation of small amount of melanin primarily in


the iris and the other features remain similar to OCA1A(King et al., 2001). But it is not clear that whether it is trulya distinct type of OCA1. It is possible that the OCA1MPcases actually represent a special form of OCA1B, wherethe mutant allele produces an enzyme with less residualactivity than a typical OCA1B allele or alternatively,OCA1MP is caused due to the expression of a partiallyactive enzyme from a mutant TYR allele that is limited tothe eye tissue.

5.2.1. Mutation profile

As per Human Genome Mutation Database (http://www.hgmd.cf.ac.uk/ac/search/120476.html), at present atotal of 189 OCA1 mutations have been reported in TYR,which include 148 missense or nonsense mutations, 23small deletions, 8 small insertions, 2 in/del type, 1 complexrearrangement and 7 splice site alterations. Similar to mostof the autosomal recessive disorders, a large number ofOCA1 patients have been characterized as compoundheterozygotes for the mutant alleles of TYR. However, incontrast to the world literature, we have recently reportedthat OCA1 among Indians are caused primarily by a fewfounder mutations (Chaki et al., 2005a, 2006). Analysis ofall the missense OCA1 mutations identified so far revealsthat most of those are clustered corresponding to five areasof TYR, among which two involve the CuA and CuB sites(Fig. 14). Mutations in these Cu-binding regions arepredicted to disrupt the metal ion–protein interaction,necessary for the function of the enzyme. The other threeclusters are located N-terminal to the CuA domain,between the CuA and CuB domains and between theCuB and TM domains. Thus it appears that these sites mayrepresent important domains of the enzyme in terms of itsfunction as well as processing i.e. may be responsible forsubstrate (tyrosine or DOPA) binding (Oetting and King,1992b) or interact with and/or be the part of the active siteof the enzyme (Gaykema et al., 1984). In fact, computermodeling of the active site based on the known 3Dstructure of the copper containing oxygen transporterhemocyanin from the spiny lobster (Panulirus interruptus)has revealed that each of the Cu-binding sites consists oftwo a-helical regions containing three histidine residues inhemocyanin. Based on homology modeling, it has been

Fig. 14. Clustering of missense mutations in TYR. Schematic representation

missense mutations, marked by the dotted triangles (the drawing is not in sca

proposed that similar molecular configuration exists inTYR. These studies have also shown that mutations in theCu-binding region alter the conformation of the a-helicalregions (Oetting and King, 1992a) as well as the position ofthe histidine residues, which either may prevent correctbinding of Cu to the histidine ligands or TYR-Cuinteraction or also may affect the Cu–Cu distance, resultingin an inactive enzyme. However, the juxtaposition of thetwo Cu atoms is also critical, as this region is needed forperoxide formation with dioxygen, necessary for thecatalytic activity of TYR. Again, the binding of tyrosineor DOPA must occur next to the Cu atoms and even aminimal change in the chemical nature of an amino acid inthis region is also likely to alter the enzymatic activityby reducing the substrate binding. This may account forthe N-terminal mutation cluster (codon 42–89) of TYR(King et al., 2001). However, small deletions or insertionsresulting in frameshift and nonsense mutations, whichultimately result in the loss of important functional domainor premature translational termination of the protein, aredistributed throughout the coding region and do notappear in cluster.

5.2.2. OCA1 cases lacking the second mutation

Interestingly, despite OCA being an autosomal recessivedisorder, even after sequencing the entire coding region ofTYR including the splice site junctions and the promoterregion, in 15% cases of OCA1 the second mutation couldnot be identified. These putative mutations are referred asuncharacterized mutations (UCMs) (Fryer et al., 2003).There may be several explanations for difficulty in findingthe UCMs. It is possible that at least in some cases thesecond mutation is present in a second locus other thanTYR. In fact, digenic mutations have already beenidentified in cases of ocular albinism and Waardenburgsyndrome (Morell et al., 1997; Ming and Muenke, 2002).King et al. (2003) proposed the following possible locationsfor these cryptic (or hidden) mutations: (1) the regulatoryregion of TYR that is not sequenced using currentmethods, (2) the locus control like region, (3) intronmutation that alters the normal splicing pattern of theRNA, (4) intronic regulatory region of the gene that hasnot been detected yet by the current methods and (5) whole

of tyrosinase polypeptide showing the locations of the five clusters of

le).

http://www.hgmd.cf.ac.uk/ac/search/120476.html

http://www.hgmd.cf.ac.uk/ac/search/120476.html


gene deletion that has been identified in one family so far(King et al., 2003). Based on our observation, we suggesttwo additional possibilities for these UCMs.

5.2.2.1. Heterozygous deletion in 30 region of TYR. The30-region of TYR (�68 kb) shares �98.55% sequenceidentity with the pseudogene TYRL. TYRL arose almost24 million years ago from TYR by gene duplicationfollowed by translocation and it includes the last twoexons (exons 4 and 5) of the authentic gene (Giebel et al.,1991). Thus, in a PCR using genomic DNA as template,one has to be sure that the primers are TYR specific. PCRamplification of these regions using primer-sequencescommon to both loci may result in co-amplification ofTYR and TYRL and would lead to misinterpretation of theresults. We observed that there are a large number ofpublicly available nucleotide variants of TYR in thisregion, which are same as the bases present in the identicallocations in the pseudogene. Thus, the authenticity of thereported 204 SNPs and 27 TYR mutations in OCA1patients in 30-region of the gene is questionable. Weresolved this long-standing problem using locus-specificamplification conditions from genomic DNA (Chaki et al.,2005b), which could be used to identify explicitly all themutations and SNPs in exons 4 and 5 of TYR along withthe proximal flanking sequences. Our simple PCR strategyrenders extra subsequent steps [cloning and selection of

Fig. 15. Detection of a gross deletion encompassing exons 4 and 5 region of T

Chromosome 11, and the region (�13 kb) of homology (�98.55% identity) be

Panel B, Polyacrylamide gel electrophoresis pattern of PCR products from a c

homologous regions of TYRL using locus specific primers as well as commo

amplicons were estimated based on 100-bp ladder used in the gel and was f

Polyacrylamide gel electrophoresis pattern of PCR products from the genomic

of TYR, and homologous regions of TYRL using locus specific primers as well

of the amplicons were estimated based on 100-bp ladder used in the gel, and

amplification product was obtained with TYR specific primers showing deletio

TYR specific clones by RFLP analysis (Giebel et al., 1991)]redundant even if those were done appropriately in some ofthe studies but were not apparent from the description ofmethods in the publications. We argue that none of thereported nucleotide variants in this region of TYR could bejustified as locus specific without proper experimentalevidence.Using this strategy we have detected a homozygous gross

deletion at the 30 region of the TYR (Fig. 15) in one of ourpatients, which would have been wrongly characterized asa homozygote for a reported TYR mutation (P406L) ifgene-specific amplification were not undertaken (Chakiet al., 2006). In fact, applying common primers of TYR andTYRL, Passmore et al (1999) identified P406L mutation inhomozygous condition in an OCA1 patient (Passmoreet al., 1999), which is likely to be a homozygote for a grossdeletion in the 30-region in TYR. However, detection ofdeletion in this region of the gene in heterozygouscondition would require PCR-based gene dosage experi-ment by amplification of only TYR alleles (and not TYRL).A carefully planned strategy combining allele specific PCRand quantitative real-time PCR using SYBR green canaccount for the heterozygous gene deletion in the suspectedsamples. We argue that occurrence of deletion in the30-region of the gene is expected to be in higher frequencyand will be detected if appropriate strategy is taken whilescreening for mutation in the TYR in OCA1 patients.

YR, Panel A, Relative location of TYR (gene) and TYRL (pseudogene) in

tween the two loci encompassing exons 4 and exon 5 of TYR are shown.

ontrol DNA obtained by amplification of the exons 4 and 5 of TYR, and

n pair of primers that would co-amplify from both loci. The sizes of the

ound to be of expected size in each case (Chaki et al., 2005b). Panel C,

DNA of one OCA1 patient obtained by amplification of the exons 4 and 5

as common pair of primers that would co-amplify from both loci. The sizes

was found to be of expected size in each case (Chaki et al., 2005b). No

n in the 30 region of the gene (including exons 4 and 5).


There is enough evidence in the literature to suggest thathomologous recombination mediated gene deletion occursin higher frequency when a duplicated region of the gene ispresent within the same chromosome e.g. steroid sulfatasegene (STS) in patients with X-linked ichthyosis, factor VIIIgene (F8) in patients with hemophilia A. Therefore, wehypothesize that those OCA1 patients in whom the secondmutation has not been identified are worth lookingfor a TYR allele which harbor a deletion in the 30-regionof the gene.

5.2.2.2. Promoter polymorphism (GA-repeat): a potential

site for altered gene expression. Kikuchi et al. (1989) firstreported the presence of a large complex GA repeat in the50 flanking sequence of the human TYR promoter (between�863 and �633 nucleotide) containing a 43 nucleotidesdirect repeat from �824 to �782 and from �775 to �733,respectively (Fig. 1). They identified this repeat element asa ‘‘characteristic sequence of (GA:TC)n, which can assumea hinged-DNA structure’’ (Kikuchi et al., 1989). In fact,both the poly ‘‘A’’ stretch and the GAA repeat (alter-natively combined sequence) have been postulated to formtriplex structure (Bidichandani et al., 1998; Gacy andMcMurray, 1998; Sakamoto et al., 1999), which iskinetically probable as well as thermodynamically stableand these features also increase with length of the repeat. Itis interesting to note that this complex GA repeat of hTYR

promoter is not conserved through evolution. We analyzedthe available sequence data from Ensembl GenomeBrowser (http://www.ensembl.org/Homo_sapiens/index.html) and noticed that while the promoters of chimpanzee,mouse and rat TYRs possess simple and much shorterversions of this GA repeat, no such repeat is present incow, dog and chicken. However, zebra fish contains a 50 bpGA–GT repeat. Subsequently, a few studies revealed thatthis GA repeat is polymorphic in certain populations, e.g.in Caucasians and Japanese (Morris et al., 1991; Tanitaet al., 2002) and proposed it as a useful marker for trackingthe TYR alleles for molecular diagnosis of OCA1. Since therepeat is located within the promoter and between twocharacterized cis-elements (TPE and TDE), it is likely thatthe repeat might regulate the transcriptional efficiencyof the TYR, mediated by variation in the repeat numbers inthe region.

Simple sequence repeats (SSRs) is widely dispersed in thegenome and among these, the dinucleotide-repeats havebeen reported to form alternative DNA structures such asZ-DNA, hinged DNA and cruciform DNA (Rothenberg,2001). Again, GA repeat serves as cis element in thepromoter regions of certain genes where specific trans

acting factors could bind. For example, GA repeat bindingprotein b2 has been proposed to influence the expression ofcytochrome oxidase and nuclear control of mitochondrialfunction. Based on all these information, we propose thatthe GA repeat in the TYR proximal promoter region islikely to control the expression of the gene. In depth studyis needed to elucidate the DNA dynamics and transcription

factor-induced transcriptional regulation of TYR, whichwould reveal the local DNA conformation of TYR in thecontext of the extensive GA repeat i.e. whether or not itattains any specific microloop or other physical orienta-tion. One could hypothesize that if an allelic variant of therepeat blocks the expression of the gene, its presence incombination with another mutant allele of TYR bearingmore obvious gene-defect (e.g., in/del and nonsensemutation) would result in albino phenotype. To validatethe hypothesis, the spectrum of naturally occurring allelicvariants due to GA repeat has to be characterized andinfluence of such variants on the expression of the genesneed to be tested. It is thus possible that some of UCMs inOCA1 would be characterized on deciphering the under-lying role of the repeat element present in the TYR

promoter.

6. Future directions

A systematic review of the studies made on TYRfunctions as well as the dysfunctions that cause aberrantcellular events leading to disease phenotype, would helpone to appreciate the areas that remain still unexplored inthe melanin biosynthetic pathway. We propose the follow-ing areas for future exploration, which are important butby no means all inclusive.A fair deal of knowledge is to be acquired regarding the

transcriptional regulation of the TYR. In spite of theidentification of a DNase hypersensitive region (homo-logous to mouse LCR), till date, there is no direct evidenceof a well-characterized functional LCR in human TYR. Itis interesting to note that despite extensive functionalstudies on TYR promoter region, an in depth analysis canstill point towards the existence of unknown factor(s),presumably important in tissue specific expression of TYR.Since it is known that a mast-cell specific MITF isoformcould bind to the corresponding cis-elements in the TYR

promoter but could not initiate transcription, this observa-tion suggests that there must be some other still undiscov-ered transcription factor(s) which are tissue specific and actby interacting with the amino terminal end of the pigmentcell specific MITF isoform—the only region which varieswithin the different MITF isoforms. The tissue specificexpression of TYR can also be explained by the presenceof certain negative regulator(s) in the nonpigmented cells(i.e. cells other than melanocytes and RPE). In fact, forTYRP1 promoter, there is a report of a negative regulator(melanocyte stimulating factor, MSF), which shuts off thegene expression in the nonpigmented cells. It is possiblethat the same mechanism is operative for TYR promoter aswell. Again, it is known that in mice, Mitf and Otx2 thatare capable of interacting through the bHLH domain ofMitf, act in synergistic fashion and these two factors co-localize within the nuclei of murine RPE cells. It has alsobeen demonstrated that Otx2 can bind and activate murineTyr promoter and in mice deficient for Otx2 or Mitf, RPEdevelopment is impaired with the loss of melanogenic gene

http://www.ensembl.org/Homo_sapiens/index.html

http://www.ensembl.org/Homo_sapiens/index.html


expression. Naturally, it will be interesting to investigatewhether the same process is operative for human TYR alsoand if it is true, then OTX2 will be considered as an RPE-specific transcription factor in human (because its expres-sion is restricted to the RPE during adulthood). Again,during the murine eye development, the Mitf and Pax6

genes are known to express across the entire optic vesicle(including both presumptive retina and RPE regions) (Boraet al., 1998; Nakayama et al., 1998; Nguyen and Arnheiter,2000) and Mitf has also been shown to interact with Pax6in vitro, resulting in the mutual inhibition of theirtranscriptional properties (Planque et al., 2001). It wouldbe interesting to examine whether mutual inhibition ofthese two proteins have any regulatory role in TYRexpression. Last but not the least, the existence of thecomplex repeat in the human TYR promoter resulting invariable alleles is certainly worth examining for itsimplication in gene regulation.

There is a large body of literature that enlightens us onthe mechanism of TYR protein processing and it also pointsto the direction that remains to be explored. Till dateseveral sorting studies on TYR have been performed takingit as a model for all type-3 glycoproteins and some of theTYR missense mutations causing OCA1 have also beendemonstrated to undergo ER associated degradation.Therefore, it is a pertinent question to ask as to what isthe minimal structural requirement that would allow TYRto exit the ER and the same is true for Golgi also. Again,the exact mechanisms of disulfide bonding and copperloading are not known for TYR. It will be also necessary toknow the exact locations and the precise functions ofMATP and P proteins that will not only be helpful forunderstanding of the TYR sorting pathway but alsoimportant from the disease point of view. It is wellestablished that TYR undertakes AP-3 dependent directroute from Golgi to melanosome and this involves thedileucine motif of the newly matured protein. It has beenalso suggested that TYR can be transported through anindirect route via AP-1 that is likely to involve the tyrosine-based motif. However, the exact need for the indirect routeas well as the circumstances under which the cell decides tofollow this route is still not clear.

Lastly, the studies to unravel the molecular event(s) that

would identify the second mutation (UCM) in 15% OCA1cases appear to be extremely important. As mentionedearlier, in addition to characterization of the potentiallocus control like region (LCR), two other directions couldbe explored parallelly for this purpose. Firstly, theprobability of the GA complex repeat to harbor the secondmutation should be investigated. Secondly, the possibilityof the presence of a gross deletion in the 30-region of TYR

in heterozygous condition is also to be considered.However, it must be mentioned here that as suggestedearlier, the UCMs in OCA1 might not be restricted to theTYR itself and may be present in another gene, whoseproduct participates in melanin metabolic pathway orsomehow interacts with TYR or any other important

factors of the pathway. Thus mutations in two differentloci (TYR and the second locus) might cause OCA indigenic condition. In support of this hypothesis, it can bestated that for MATP also, there are reports where thesecond mutations have not been found in some OCA4patients. Further, it is known that splicing aberrations isnot restricted to the canonical splice junctions of the geneand presence of regulatory regions within the introns ofdifferent genes have also been documented so far. There-fore, until we study the TYR in a greater detail, some of themutant alleles will remain refractory to our currentendeavor on mutation screening. Another important aspectof work is to decipher the relation between OCA1 and PCG,more precisely the exact level at which TYR influencesCYP1B1 activity.

Acknowledgments

The authors would like to thank Prof. ParthaP. Majumder, Prof. Nitai P. Bhattacharyya, Dr. SusantaRoychoudhury and Dr. Arijit Mukhopadhyay for theirhelpful comments in reviewing the manuscript. Theauthors’ research described in the review was supportedby the Council of Scientific and Industrial Research, India(Grant No. CMM-0016).

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Tyrosinase and ocular diseases: Some novel thoughts on the molecular basis of oculocutaneous albinism type 1