The roles of cytoplasmic poly(A)-binding proteins in regulating gene

Barbara Gorgoni

works at the Human Genetics

Unit of the Medical Research

Council in Edinburgh, Scotland

and is an AICR post-doctoral

fellow.

Nicola K. Gray

works at the Human Genetics

Unit of the Medical Research

Council in Edinburgh, Scotland

and is funded by an MRC

Career Development Award.

Keywords: PABP, mRNAtranslation, mRNA stability,poly(A), development,RNA�protein interactions

Nicola K. Gray,

MRC Human Genetics Unit,

Western General Hospital,

Crewe Road,

Edinburgh,

EH4 2XU, UK

Tel: +44 (0) 131 332 2471

Fax: +44 (0) 131 467 8456

E-mail: [email protected]

The roles of cytoplasmicpoly(A)-binding proteins inregulating gene expression:A developmental perspectiveBarbara Gorgoni and Nicola K. GrayReceived: 10th June 2004

Abstract

Poly(A)-binding proteins (PABPs) are central to the regulation of messenger RNA (mRNA)

translation and stability; however, the roles and contributions of different PABP family

members in controlling gene expression are not yet fully understood. In this paper, the current

state of knowledge of the different cytoplasmic PABP proteins and their function in animal cells

will be summarised, with particular reference to their roles in development. Possible

regulatory mechanisms and potential new roles for these proteins in the control of specific

mRNAs are also highlighted.

INTRODUCTIONPoly(A)-binding proteins (PABPs) are a

family of proteins characterised by their

ability to bind to poly(A) RNA with a KD

of approximately 2�7 nM.1–3 PABPs

require 12 adenosines to bind, but protect

between 25 and 27 adenosine residues and

can multimerise along a poly(A) tract.1,4,5

These proteins are present in yeast, plants

and animals but are not conserved in

prokaryotes. PABPs have been divided

into two broad categories, nuclear and

cytoplasmic, based on intracellular

location and phylogeny. Nuclear PABPs

(PABPN1) bear little resemblance to their

cytoplasmic counterparts (see Figure 1),

function in the adenylation and

maturation of pre-messenger RNAs (pre-

mRNAs) and are beyond the scope of this

paper (recently reviewed in refs. 6 and 7).

Metazoa typically contain several genes

encoding cytoplasmic PABPs and the

function of one of these proteins, PABP1,

has been intensively studied in many

organisms. In humans, three additional

family members — testis PABP (tPABP),

inducible PABP (iPABP) and PABP5 —

have been identified (Figure 1). Recently,

two novel PABP proteins — embryonic

PABP (ePABP) and ePABP2 — were

identified in Xenopus laevis,8,9 which may

play a specific role during development.

Although the presence of multiple PABP

pseudogenes complicates their

identification in the human genome, at

least one of these (ePABP2) appears to

have a human homologue (Figure 1).

PHENOTYPESASSOCIATED WITH PABPsThe function of PABPs was first addressed

in yeast, where they were found to be

essential for viability.1 To date,

surprisingly few studies in higher

eukaryotes have used genetic

manipulations to analyse the role of

PABPs in development. Drosophila

encodes only one cytoplasmic PABP

(pAbp) and its deletion leads to

embryonic lethality. P-element insertions

into the untranslated regions (UTRs) of

pAbp result in a meiotic defect during

spermatogenesis10 and in a neuromuscular

junction phenotype.11 In Caenorhabditis

elegans, two cytoplasmic PABPs are

present and RNA interference (RNAi)

phenotypes have been reported for one of

these (Pab-1), including embryonic

lethality at 50�80 per cent, sterile

progeny and slow growth.12,13

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PABP1PABP1 (also referred to as PABP, PAB1,

PAB and PABPC1) appears to be

ubiquitously expressed and is the only

PABP whose function in mRNA

translation and stability has been

extensively addressed. PABP1 is

predominantly cytoplasmic,2 but has been

reported to shuttle to the nucleus14 —

although the biological relevance of this

awaits clarification. PABP1 is composed

of four non-identical RNA recognition

motifs (RRMs) and a C-terminal domain

that does not bind RNA (Figure 1).

RRMs 1�2 bind poly(A) with high

affinity and, when bound, form a globular

structure composed of four-stranded anti-

parallel �-sheets backed by two Æ-helices.This structure allows one face of the

domain to bind poly(A), leaving the other

face free for protein�protein

interactions.15 RRMs 3�4 also have the

capacity to bind RNA and, while this

binding is often described as non-

specific,3,16–18 recent reports suggest that

at least one preferential binding sequence

is AU rich.19,20 The C-terminus of

PABP1 is composed of a less-conserved

proline-rich linker region and a carboxyl

terminal domain. The proline-rich region

is predicted to be relatively unstructured

and is implicated in PABP1

homodimerisation,3,21 which may

facilitate ordered binding to poly(A)

stretches.3,5 The carboxyl terminal

domain, broadly referred to as PABC, is

composed of five Æ-helices.22,23

Intriguingly, it bears homology to a subset

of HECT E3 (homologous to E6AP

carboxyl terminus E3) ubiquitin-protein

ligases,23 although no function in protein

degradation has been ascribed. This

domain is, however, important in

promoting protein�protein

interactions.24–29

OTHER CYTOPLASMICPABPstPABP (PABPC2 in mouse and PABPC3

in human) is encoded by an intron-free

gene that appears to have arisen from

PABP1 by retrotransposition.30,31

Consequently, tPABP is highly related to

PABP1, maintaining the same overall

structure (Figure 1). Amino acid

substitutions within the RRM regions

may underlie the modestly reduced

affinity of tPABP for poly(A).31,32

Compared with PABP1, the proline-rich

region of tPABP contains several small

deletions31 but their significance remains

unclear. In mouse and human, tPABP

mRNA is only abundant in the testis31,32

and can be detected in particular types of

male germ cells, suggesting that it may

play an important role in

spermatogenesis.31,32 The close homology

of tPABP to PABP1 makes it likely that it

functions in mRNA translation and/or

stability.

iPABP (also called PABPC4 or APP-1)

was identified separately as a protein

whose mRNA expression is upregulated

in activated T cells33 and as a protein

expressed on the surface of activated

platelets.34 Its mRNA is also expressed in

a wide variety of other tissues.33 iPABP is

PABP1 is composed offour non-identical RRMsand a C-terminaldomain that does notbind RNA

The close homology oftPABP and iPABP toPABP1 makes it likelythat these proteinsfunction in mRNAtranslation and/orstability

Figure 1: General structure of human poly(A)-binding protein (PABP)family members. PABP1 (PABP, PABPC1, PAB1, PAB) has four non-identical RNA recognition motifs (RRMs) linked by an unstructuredproline-rich region (P-rich) to a globular C-terminal domain (PABC).tPABP (PABPC2 in mouse and PABPC3 in humans) and iPABP (PABPC4or APP-1) maintain a similar structure. By contrast, PABP5 (PABPC5)lacks the proline-rich linker and PABC. PABPN1 (oculopharyngealmuscular dystrophy (OPMD), PAB2, PABP2, PABPII) and ePABP2 containonly one RRM and have a long acidic N-terminus and a short arginine-richC-terminus. Chromosomal locations of the genes are indicated on theright of the figure

1 2 6 & HENRY STEWART PUBLICATIONS 1477-4062. BRIEF INGS IN FUNCTIONAL GENOMICS AND PROTEOMICS . VOL 3. NO 2. 125–141. AUGUST 2004

Gorgoni and Gray


closely related to PABP1 (Figure 1) but

has a number of amino acid substitutions

in the RRM motifs and shows

considerable divergence in the proline-

rich linker region.33 iPABP has a

predominantly cytoplasmic location and

binds to poly(A)33 and eukaryotic release

factor (eRF)3,27 but no function in

translation or stability has been

established.

PABP5 (also known as PABPC5) is

encoded by a gene on the X chromosome

in humans and mice.35 Its mRNA is

present at low levels in a variety of human

tissues and at a slightly higher level in the

ovary.35 In contrast to PABP1, tPABP

and iPABP, PABP5 lacks the proline-rich

linker region and the PABC domain

(Figure 1).35 As yet, PABP5 has not been

shown to bind poly(A), to be localised in

the cytoplasm or to function in translation

or stability; however, since the C-

terminus of PABP1 is not absolutely

required for viability in yeast or

translation in Xenopus, PABP5 may be a

functional PABP protein.1,24

ePABP was identified in Xenopus as a

protein that binds AU-rich sequences8

and as an eRF3-interacting protein.36

Like Xenopus PABP1, ePABP binds

poly(A)36 and protects mRNAs from

deadenylation.8 ePABP maintains the

same general structure as PABP1 but

shows considerable divergence, especially

in RRM3 and the proline-rich linker

region.8 ePABP is present at higher levels

than PABP1 during most of oogenesis and

early embryogenesis,8,36 its levels

decreasing as PABP1 levels increase at the

onset of zygotic transcription.36 Thus, it

may play a specific developmental role in

protecting mRNA from deadenylation

and/or in poly(A)-mediated translation,

although a function in translation has yet

to be described. While no ePABP gene

has been identified in mammals, searches

of the database reveal potential open

reading frames (ORFs) with homology to

Xenopus ePABP.

A second embryonic PABP (ePABP2),

with an expression pattern reminiscent of

ePABP, has recently been identified in

Xenopus.9 It appears to have mammalian

homologues and its mRNA expression

pattern in mouse is similar to that in

Xenopus.9 ePABP2 binds to poly(A) and is

predominantly cytoplasmic despite its

resemblance to nuclear rather than

cytoplasmic PABPs (Figure 1).9 The

function of this protein remains

enigmatic; future work will determine if

it plays a role similar to nuclear PABPs in

determining the length of the poly(A) tail

added to mRNAs, during cytoplasmic

rather than nuclear polyadenylation.6,7

Alternatively, ePABP2 may function in

mRNA translation or in protecting

mRNAs from deadenylation despite its

divergence from PABP1.

In conclusion, many cell types appear

to express multiple PABP proteins;

however, the reasons for this remain

unclear. Several possibilities can be

envisaged: (1) their basic functions may be

divergent; (2) the activities of these

proteins may be differentially regulated;

(3) some cells may require elevated levels

of PABPs to achieve high levels of gene

expression; and (4) different PABPs may

preferentially regulate specific subsets of

mRNAs.

PABP1 IN TRANSLATIONINITIATIONTranslation initiation is a complex event

that ends in the formation of a ribosome

competent to begin elongation at the

initiation codon. It has been described in

detail elsewhere (see ref. 37). A simplified

version of the mRNA-dependent steps

highlighting the factors discussed below

can been seen in Figure 2. While the

poly(A) tail is not absolutely required for

translation,38 polyadenylated mRNAs are

translated with a much greater efficiency

and it is now generally accepted that the

poly(A) tail functions synergistically with

the 59 cap to promote the initiation of

translation.39,40

The function of PABP1 in translation

has been investigated in many species,

both in vivo and in vitro, resulting in a

model in which interactions between

PABP1 on the poly(A) tail and factors

Many cell types appearto express differentPABPs but it is unclearwhether these proteinsare functionallyredundant

Interactions betweenPABP1 on the poly(A)tail and factors locatedat the 59 end bring theends of the mRNA intoclose proximity formingan ‘end-to-end complex’or ‘closed loop’


The roles of cytoplasmic poly(A)-binding proteins in regulating gene expression


located at the 59 end bring the ends of the

mRNA into close proximity forming an

‘end-to-end complex’ or ‘closed

loop’.41,42 One interaction, which is

conserved in many species, is between

RRMs 1�2 of PABP1 and eukaryotic

initiation factor (eIF)4G. eIF4G is part of

the eIF4F complex that binds to the

m7GpppG cap via eIF4E (Figure 2).

Thus, the simultaneous interaction of

eIF4G with eIF4E and PABP1 physically

links the ends of the mRNA, forming

circular structures that can be visualised by

microscopy.43 The importance of the

PABP1�eIF4G interaction is widely

accepted and the supporting evidence has

been reviewed elsewhere (see refs. 6, 7,

38 and 42). This interaction enhances

both the affinity of eIF4E for the cap

structure44–47 and PABP1 for poly(A),48

effectively locking proteins onto both

ends of the mRNA. As a result, this

association may in part underlie the ability

of PABP1 to promote small ribosomal

(40S) subunit recruitment,49 which is

aided by the interaction between eIF4G

and eIF3 (Figure 2).

Several lines of evidence show that the

effects of PABP1 on translation are not

limited to interactions with eIF4G.6,42 For

instance, mutations in eIF4G that

eliminate binding to Pab1p do not disrupt

the viability or growth rate of yeast

cells.50 In light of this, it is interesting to

note that several other translation factors

interact with PABP1.

eIF4B, an initiation factor that aids the

processivity of the eIF4A RNA helicase,37

has been shown to interact with PABPs in

plants and mammalian cells.48,51 In plants,

this interaction enhances both poly(A)/

PABP binding and eIF4A/eIF4B helicase

activity,48,52 presumably promoting 40S

ribosomal recruitment (Figure 2). In

mammalian cells, the C-terminal region of

PABP1 contacts eIF4B and this interaction

is disrupted during apoptosis and by some

viral proteases, suggesting it may play an

important physiological role.51

PABP-interacting protein 1 (Paip1)

shares considerable homology with the

central region of eIF4G and interacts with

Figure 2: mRNA-dependent steps of cap-dependent initiation. (A) Acomplex of proteins, eukaryotic initiation factor (eIF)4F, binds to them7GpppG cap (indicated as a black circle) at the 59 end. This complex iscomposed of eIF4E (4E) that binds the cap directly; eIF4G (4G), a largescaffolding protein; and the RNA-dependent helicase eIF4A (4A). eIF4B(4B) is also present and stimulates eIF4A helicase activity, thought tounwind secondary structure within the 59 untranslation region. The openreading frame is depicted by an open box. Poly(A)-bound poly(A) bindingprotein (PABP1) (PABP) associates directly with eIF4G, forming a closedloop. PABP1 can also interact with eIF4B and PABP-interacting protein 1(Paip1) (indicated by arrows), which in turn associate with eIF4A,providing additional links between the 59 and 39 ends of the mRNA. (B)The small ribosomal subunit (40S) binds at or near the cap. Recruitmentis assisted by the interaction of eIF3, located on the 40S, and eIF4G. (C)The 40S subunit, aided by associated factors, migrates to the initiationcodon, normally the first AUG, in a process often referred to as scanning.(D) Initiation factors are released (not depicted) and the large ribosomalsubunit (60S) joins to form the 80S ribosome, which is competent toinitiate translation. PABP1 may also be involved in the 60S joining step, byan as yet undefined mechanism. The figure is schematic and does notdepict all the factors involved, nor is it meant to indicate the spatialrelationship between the proteins or the full extent of RNA�protein andprotein�protein interactions


Gorgoni and Gray


PABP1 via RRMs 1�2 and PABC,24,25

at a 1:1 stochiometry.25 Paip1, like

eIF4G, interacts with eIF4A (Figure 2)

but, by contrast, does not contact eIF4E,

and an interaction with eIF3 remains to

be proven.53 Thus, a role for Paip1 in

PABP1-mediated translation is less clear

than for eIF4G.24,53

Several lines of evidence suggest that

other factors may also promote PABP1-

mediated translation. First, RRMs 3�4 of

PABP1 are sufficient to stimulate

translation in tethered function assays

even though they are not implicated in

binding any of the factors discussed.24

Moreover, effects of PABP/poly(A) on

60S ribosomal subunit joining have also

been reported.54,55 Future work will be

required to understand further the factors

and mechanisms involved and whether

the different factors are utilised in

different cell types.

LINKS BETWEENTRANSLATIONTERMINATION AND PABP1The PABC domain can interact with

eRF3 in both yeast and mammals.26–28

eRF3 is a GTPase that enhances the

activity of eRF1, which catalyses

translation termination.56 The eRF3/

PABP1 interaction may promote

recycling of terminating ribosomes from

the 39 to 59 end, facilitating multiple

rounds of initiation on an mRNA.28

Alternatively, it may link translation to

mRNA decay, as eRF3 appears to

interfere with the ability of PABP1 to

multimerise on poly(A),26 potentially

leading to PABP1 dissociation,

deadenylation and, ultimately, turnover.

PABP1 IN mRNA STABILITYPABPs also play an important role in

mRNA stability. The majority of mRNA

decay studies have been carried out in

yeast, although the pathways appear to be

conserved in higher eukaryotes.57,58

Consequently, the roles of PABP1 in

mRNA stability are less defined than in

translation. Nonetheless, several lines of

evidence suggest that PABP1 can

influence both the deadenylation-

dependent decapping pathway and

exosome-mediated 39!59 degradation.58

Deadenylation appears to be the initial

step in both these decay pathways and, in

many cases, seems to be rate limiting.58,59

PABP1 can protect mRNAs from

deadenylation by inhibiting the action of

deadenylases such as poly(A) ribonuclease

(PARN),60–64 and incremental

deadenylation products equivalent to the

size of PABP footprints can be

detected.62,65

One simple model for explaining the

role of PABP1 in protecting mRNAs

from decay is the formation of the

eIF4E�eIF4G�PABP1 complex (Figure

2). By linking proteins tightly to both

ends of the mRNA, this complex can

simultaneously prevent the access of

deadenylases and can prevent decapping.

As discussed earlier, eRF3 may play a role

in disrupting these complexes, linking

translation to decay.26 Once translation

has ceased, PABP1 has been proposed to

protect mRNAs independently of eIF4E

by binding directly to the 59 cap;66,67

however, in vivo clarification of this latter

observation awaits. PABP1 may,

therefore, have additional functions in

stability, influencing steps subsequent to

deadenylation, such as remodelling of

mRNPs,6,7 and may even promote the

activity of certain deadenylases.68 A more

detailed knowledge of the factors that

mediate decay in vertebrate cells may first

be required in order to gain a fuller

insight into the role of PABP1 in mRNA

stability.

PABPs may also be involved in

regulating mRNAs containing specific

sequences that control their stability, such

as AU-rich elements (AREs) and mCRD

elements (see later, c-fos). AREs can be

bound by stabilising or destabilising

factors.69 Some destabilising factors

recruit components of the decay

machinery such as the exosome and

PARN;70-72 however, others might

function by disrupting interactions

between PABP1 and the poly(A) tail or

factors at the 59 end. Interestingly, some

PABPs may alsoregulate the stability ofspecific mRNAs

PABP1 may influenceboth the deadenylation-dependent decappingpathway and exosome-mediated 39 to 59degradation




PABPs appear to bind AU-rich

sequences, suggesting a more direct role

in the stability of these mRNAs,8,20

although it is not clear whether they can

discriminate between AREs and other

AU-containing RNAs.19 In a few cases,

PABP1 has also been implicated in the

regulation of endonucleolytic cleavage —

a pathway that is responsible for the

degradation of a relatively small number

of mRNAs (see later, Æ–globin) —although, in most cases, cleavage is

independent of poly(A)/PABP function

(eg see ref. 73).

REGULATED EXPRESSIONOF PABP PROTEINSPABP1 mRNA is expressed at different

levels in many tissues (eg see ref. 32);

furthermore, the expression of PABP1

protein is tightly regulated by two

independent mechanisms. A 59-terminal

oligopyrimidine tract (59-TOP) controls

expression in response to cell growth.74

TOPs are also present in other

components of the translational

machinery and allow for coordinated

growth regulation (reviewed in ref. 75).

Additionally, PABP1 autoregulates its

own mRNA by binding an A-rich

sequence within its 59UTR,76,77 leading

to repression when PABP1 levels are high.

The translation of other PABP

mRNAs may also be subject to

autoregulation. Some iPABP

complementary DNA (cDNA) sequences

appear to have an A-rich stretch in their

59 UTR.78 Interestingly, iPABP seems to

have more than one promoter,79

suggesting that transcription from

alternate promoters may allow differential

regulation by generating a subset of

mRNAs without the poly(A) stretch.

tPABP mRNAs do not appear to undergo

autoregulation,31 but polysome analysis in

mouse suggests that they may be subject

to translational control by an as yet

unidentified mechanism.32

Database analysis of cDNAs reveals

potential alternative splice variants that

alter the protein-coding region of iPABP

and PABP1, often in the proline-rich

region.78 This raises the possibility that

additional PABP isoforms may be

generated by alternative splicing, further

increasing the complexity of PABPs

produced. Future work will be required

to elucidate whether these isoforms are

expressed, and, if they are, what their

importance is during development.

REGULATION OF PABPPROTEINSProtein modificationPABP proteins can be subject to both

phosphorylation and methylation.

Phosphorylation of translation factors,

including eIF4E and eIF2, is a well-

established means of controlling global

translation (reviewed in refs. 80�82).

Multiple phosphorylation forms of PABPs

have been described in plants, yeast and

sea urchins,83,84 but the functional effects

of these phosphorylation events are best

understood in plants. Phosphorylation of

wheat germ PABP enhances its

cooperative binding to poly(A),85

suggesting that its ability to homodimerise

is increased. Moreover, the ability of

eIF4G and eIF4B to promote PABP/

poly(A) interactions appears to be

differentially influenced by the

phosphorylation state of PABP.85

Strong evidence for phosphorylation of

PABPs in vertebrates is still lacking;

however, serum stimulation, which

activates a variety of kinase pathways,

enhances the PABP1�eIF4G interaction

in cultured Xenopus kidney cells86 and

human PABP1 can be phosphorylated by

the p38 mitogen-activated protein kinase

pathway in vitro.20 While phosphorylation

of PABPs has not been extensively

analysed during development, a

preliminary analysis in Xenopus eggs

suggests that neither PABP1 nor ePABP is

phosphorylated.36

PABP1 is also a substrate for

coactivator-associated arginine

methyltransferase (CARM1), which

methylates arginines in the proline-rich

linker region in vitro and in HeLa cells.87

The function of this modification is

unclear but it may affect

Additional PABPisoforms may begenerated byalternative splicing,further increasing thecomplexity of PABPproteins

PABP proteins can besubject to bothphosphorylation andmethylation

Expression of PABP1protein is tightlyregulated


Gorgoni and Gray


homodimerisation, RNA or protein

interactions or even its intracellular

localisation.

Protein regulatorsOther proteins can also affect the activity

and functions of PABP proteins. While

this form of regulation was first identified

in viruses, cellular proteins that utilise this

strategy have now been identified.

Rotavirus non structural protein 3

(NSP3) was the first protein shown to

modulate PABP1 activity by competing

for binding to eIF4G. This disrupts the

PABP1�eIF4G interaction and excludes

adenylated cellular mRNAs from

translation.88 Interestingly, NSP3 is

required for the translation of viral

mRNAs as it binds their non-adenylated

39 end, leading to circularisation by

mimicking PABP1 function.89

The cellular protein Paip2 acts as a

negative regulator of PABP1 by binding

to sites within RRMs 2�3 and PABC,29

inhibiting its interaction with poly(A) and

Paip1.90 A recent study points to a role

for Paip2 in development, showing that

Drosophila Paip2 inhibits cell growth in

various tissues in larvae and adults by

inhibiting translation.91 Interestingly,

recent reports also suggest that Paip2 may

have an additional role in mRNA

stability.92,93

PABPs may also be subject to

proteolytic cleavage during viral

infections and apoptosis. This has been

characterised in enteroviruses where

protease 3Cpro cleaves PABP1 at three

sites within the proline-rich linker region.

Cleavage releases the PABC domain from

the RRMs,51,94 leading to the inhibition

of poly(A)-dependent translation.95 Since

cleaved PABP1 retains its interaction with

eIF4G and with the poly(A) tail, these

observations confirm the importance of

interaction with other factors or with

itself in PABP-mediated translational

stimulation.

During apoptosis, caspase-mediated

cleavage of several translation factors

correlates with a partial inhibition of

translation (reviewed in ref. 96). Cleavage

of eIF4G and eIF4B disrupts their

interaction with PABP1 and formation of

the end-to-end complex. PABP1

degradation during apoptosis has also

recently been suggested, although it

appears not to be caspase mediated97 and

its relative contribution to translational

inhibition during apoptosis remains to be

determined.

CYTOPLASMIC POLY(A)TAIL LENGTH CHANGES INDEVELOPMENTIn animal cells, mRNAs initially receive a

poly(A) tail of between 200 and 250

nucleotides in the nucleus. Upon entry to

the cytoplasm, the poly(A) tail is normally

slowly removed and this eventually signals

the mRNA for degradation. During early

development, transcription is often

quiescent; thus, changes in the pattern of

protein synthesis rely on the activation,

repression or destruction of pre-existing

maternal mRNAs.98 These changes are

often accompanied by dramatic changes

in poly(A) tail length which occur in the

cytoplasm.38,99 In general, mRNAs are

stored with short poly(A) tails and are

polyadenylated concomitantly with their

translational activation, while

deadenylation is associated with

translational silencing.

Cytoplasmic polyadenylation requires

specific 39 UTR sequences that have been

defined during oocyte maturation

(cytoplasmic polyadenylation elements

[CPEs]100,101 or adenylation control

elements [ACEs]102) and embryogenesis

(embryonic CPEs).103,104 The importance

of cytoplasmic polyadenylation has been

demonstrated in a number of species.

Gene inactivation of mouse CPE-binding

protein-1 (CPEB-1) or testis poly(A)

polymerase (tPAP), which promote

cytoplasmic polyadenylation, blocks

oogenesis and/or spermatogenesis.105,106

Elegant experiments also underline the

important role of polyadenylating

individual mRNAs during development.

For instance, polyadenylation of c-mos in

Xenopus and mouse is essential for oocyte

maturation.107,108 Similarly, in embryos,

Paip2 acts as a negativeregulator of PABP1 byinhibiting its interactionwith poly(A) and Paip1

During earlydevelopment, mRNAsare stored with shortpoly(A) tails and arepolyadenylatedconcomitantly withtheir translationalactivation

PABPs may be subjectto proteolytic cleavageduring viral infectionsand apoptosis




polyadenylation of bicoid is required for

the establishment of anterior�posterior

axis formation in Drosophila,109 and

polyadenylation of activin mRNA is

required for mesoderm formation in

Xenopus.110

It is widely accepted that the function

of the poly(A) tail is to bind PABP

proteins to mRNAs; however, the

mechanism by which cytoplasmic

polyadenylation promotes translation is

less clear.38 In some cases, it may enhance

the translation of mRNAs that are already

being translated at a low level,or it may

activate the translation of silent

mRNAs.38,98 This may be, in part,

determined by the PABP status of the

poly(A) tails. For instance, PABPs may be

bound to the short poly(A) tails and the

role of polyadenylation may be to recruit

additional PABP molecules. Alternatively,

the short tails may be devoid of PABPs

either because PABP binding is blocked

or because only long poly(A) tails are

sufficient to compete for limited amounts

of PABPs.

An additional layer of complexity arises

because the length of polyadenylation

varies between mRNAs. For instance,

polyadenylation of cyclin B1 and c-mos

results in an increase from 30 to 250 and

from 50 to 120 residues, respectively.111

The extent to which polyadenylation

affects the translation of individual

mRNAs is also determined by other

factors.38 First, the basal translatability of

mRNAs may be important, such that

poorly translated mRNAs may be

stimulated to the highest degree by

PABPs. In support of this, mRNAs

containing structured 59 UTRs were

found to be more sensitive to levels of

PABP1 than mRNAs containing

unstructured 59 ends.112 Secondly, the

presence of repressor proteins can

influence the apparent effects of

polyadenylation.38 Several models to

explain the relationship between

polyadenylation and 39 UTR repressors

exist.38,113 For instance, loss of a repressor

protein and polyadenylation may occur

independently and, while the two events

are not linked, their additive effects are

responsible for the overall change in

translation. Alternatively, the repressor

protein may exert a dominant effect over

the poly(A) tail by preventing the binding

of PABPs or by interfering with their

function. Loss of the repressor would

allow the poly(A) tail to participate in

end-to-end complexes. Finally,

polyadenylation may precede and be

required for the loss of the repressor

protein (see later, maskin).

Thus, the effects of poly(A)/PABPs on

the translation of mRNAs during early

development are complex and need to be

defined for individual mRNAs.

Cytoplasmic polyadenylation also occurs

in other cell types, such as neurones,114,115

and studies of early development provide

a paradigm for understanding its function.

During development, subsets of

mRNAs undergo deadenylation at

specific times.99 This process is linked to

translational silencing, since PABP1

overexpression prevents both silencing

and deadenylation.64 While deadenylation

has been studied in a number of species,

including Drosophila116 and mice,102 it has

perhaps been most extensively

characterised in Xenopus.117 During

oocyte maturation, mRNAs that lack a

CPE, such as ribosomal proteins and

actin,118,119 undergo deadenylation. This

is termed ‘default deadenylation’ since

specific sequences are not required and it

is, in part, promoted by the release of

PARN from the nucleus.120 In contrast to

deadenylation in somatic cells, mRNAs

that undergo default deadenylation can

remain stable and be degraded later in

development.117 It is unclear whether

deadenylation causes translational

silencing or whether silencing by an

unknown mechanism initiates

deadenylation. In the latter scenario,

deadenylation would promote loss of

PABP proteins, resulting in further

silencing and deadenylation.

A second type of deadenylation

requires specific cis-acting sequences and

has been best characterised in the early

embryo.99 Several elements have been

The mechanism bywhich cytoplasmicpolyadenylationpromotes translation isnot clear

During development,subsets of mRNAsundergo deadenylationand translationalsilencing at specifictimes

Polyadenylation doesnot affect thetranslation of allmRNAs equally


Gorgoni and Gray


identified and appear to recruit different

39 UTR-binding proteins.117 Two main

models have been proposed to explain

how these elements function.117 In the

first, 39 UTR-binding proteins recruit or

promote the activity of a deadenylase

other than PARN.8,117,121 In the second

model, binding of the regulatory protein

leads to translational repression and this,

in turn, leads to deadenylation. While the

role of PABP proteins in mRNA specific

deadenylation may be primarily to protect

the poly(A) tail, they may also have a

more active role, since ePABP was first

identified as a protein that can bind to

ARE motifs.8

REGULATION OF SPECIFICmRNAs BY PABPsIn addition to their roles in translation

described above, PABP proteins are also

involved in the regulation of specific

mRNAs. Few cases have been

documented; however, the mechanisms

described suggest a more widespread role

for PABP as a specific regulator.

One well-studied example is cyclin B1,

which is part of the maturation-

promoting factor (MPF) required for

oocyte maturation in Xenopus. Prior to

maturation, cyclin B1 mRNA has a short

poly(A) tail and is maintained in a

translationally repressed state by a

complex of proteins. Surprisingly,

repression requires CPE elements and

CPEB,122,123 which later direct

polyadenylation. CPEB at the 39 end and

eIF4E at the 59 end are bound in an

inactive complex by the bridging protein

maskin (Figure 3).124 This prevents the

association of eIF4E and eIF4G, blocking

translation initiation.124 Upon

progesterone stimulation, cyclin B1

mRNA undergoes cytoplasmic

polyadenylation,111,125 leading to the

increased binding of PABPs and

eIF4G.126 The PABP�eIF4G complex

displaces maskin from eIF4E, allowing the

initiation of translation. More recent

work suggests that Xenopus Pumilio may

also be involved in repression of cyclin B1

in oocytes by interacting with CPEB to

block polyadenylation.127

Repression of cyclin B1mRNA is overcomewhen the PABP–e1F4Gcomplex displaces therepressor maskin frome1F4E

Figure 3: Model of maskin-mediated repression/derepression. (A) In oocytes, cyclin B1mRNA has a relatively short poly(A) tail and is translationally silent. The cytoplasmicpolyadenylation location in the 39 UTR recruits CPE binding protein (CPEB), which associateswith maskin. Maskin, in turn, interacts with eukaryotic initiation factor (eIF)4E, precluding itsaccess to eIF4G and inhibiting the formation of the initiation complex. The cleavage andpolyadenylation specificity factor (CPSF) may be loosely associated with its target elementAAUAAA and with CPEB. The poly(A) tail may bind a limited number of poly(A)-bindingproteins (PABPs). For simplicity, the role of Pumilio is not depicted. (B) During oocytematuration, the kinase aurora (also known as Eg2) is activated and phosphorylates (-P) CPEB.This leads to stabilisation of the CPEB/CPSF complex and recruitment of poly(A) polymerase(PAP) to the end of the mRNA where it directs polyadenylation. Subsequently, the elongatedpoly(A) tail recruits one or more PABP molecules, which in turn associates with eIF4G. PABP-bound eIF4G displaces maskin from eIF4E and translation is enhanced. The figure is schematicand is not meant to indicate the spatial relationship between the proteins or the full extent ofRNA�protein and protein�protein interactions




Translational repression by maskin also

occurs during early embryogenesis, where

cyclin B1 synthesis is tightly regulated by

polyadenylation to ensure cell-cycle

progression.128 Similarly, in neuronal

dendrites, CPEB and maskin are

suggested to repress mRNAs such as ÆCa2þ-calmodulin-dependent protein

kinase II (ÆCAMKII) prior to synaptic

activation.129,130 Consequently, PABPs

may mediate relief of repression in these

as well as in other cells.

PABP proteins do not always seem to

perform their function through the

poly(A) tail.24,131 For instance, in vitro

experiments suggest that translation of

YB-1 mRNA, encoding a cold-shock

domain-containing protein, is enhanced

by the binding of PABP1 to an A-rich

sequence within its 39 UTR,132

presumably by promoting end-to-end

complexes.

By contrast, when PABP1 is bound to

an A-rich sequence within its own 59

UTR, it acts as a repressor by stalling the

migration of the 40S subunit.133

Repression requires the proline-rich

region21 implicated in homodimerisation

and cooperative binding to poly(A),

raising the possibility that PABP1 must

multimerise on the 59 UTR to impede

ribosome scanning.

PABP1 has also been implicated in the

repression of other mRNAs, such as the

neuropeptide vasopressin, prior to

dendrite activation. PABP1 associates

with a dendritic localiser sequence (DLS)

that spans part of the coding region and

the 39 UTR of this mRNA.134 DLS-

bound PABP1 is proposed to interfere

with PABP1 molecules on the poly(A)

tail, either directly or indirectly, inhibiting

the formation of the end-to-end complex

(reviewed in ref. 135) by an undefined

mechanism.

The role of PABPs in regulating

specific mRNAs is not only limited to

translation but it is also important in

stability. This appears to be the case

with Æ-globin mRNA, which is

stabilised by the binding of Æ complex

protein (ÆCP)1 (heterogenous nuclear,

ribonucleoprotein (hnRNP) E1) and ÆCP2 (hnRNP E2) proteins to its 39

UTR.136 Interaction of ÆCP1 and

ÆCP2 with PABP1 leads to an increase

in their affinity for the mRNA,

precluding access to an endonuclease

(ErEn).137 ÆCP1 and ÆCP2 can, in turn,

stabilise the association of PABP1 with

the poly(A) tail, protecting the transcript

from deadenylation.137 Thus, PABP1

simultaneously protects Æ-globin mRNA

from both endonucleolytic cleavage and

39!59 degradation.

Stability of c-fos mRNA, which

encodes a transcription factor, is also

modulated by PABP1. This transcript

contains a major protein-coding region

determinant of instability (mCRD) in the

ORF that directs accelerated

deadenylation prior to degradation.138

The mCRD interacts with a protein

complex that contains PABP1, Paip1,

hnRNPD, NS1-associated protein 1

(NSAP1) and upstream of N-ras (Unr),

which is proposed to bring the mCRD

and poly(A) tail in to close proximity,

preventing deadenylation by stabilising

the PABP/poly(A)tail interaction.139

During translation, ribosomal movement

across the mCRD displaces or reorganises

the complex, and is proposed to

destabilise PABP1 binding to the poly(A)

tail, exposing the 39 end to nuclease

attack.

PERSPECTIVESGiven the wide variety of roles played by

PABPs in regulating specific mRNAs, it is

expected that additional examples and

mechanisms will continue to be

uncovered. This is supported by several

observations. First, several PABP proteins

can bind AU-rich sequences8,19,20 and the

affinity for AU of one of these, iPABP, is

only 2-fold lower than its affinity for

poly(A), suggesting that some PABP

proteins may regulate mRNAs containing

AU-rich sequences. Secondly, tethering

PABP1 to the 39 UTR promotes

translation,24 raising the possibility that

indirect recruitment of PABPs to the

mRNA by PABP-binding proteins could

PABP1 can stabilise Æ-globin mRNA bysimultaneouslyprotecting it fromendonucleolyticcleavage and 39 to 59degradation

PAB1 binding to an A-rich sequence within itsown 59 UTR causesrepression of PABP1translation


Gorgoni and Gray


be a novel way to control translation.

Lastly, an extensive list of proteins

containing the PABP-interaction motif 2

(PAM2), known to bind the PABC

domain, has recently been published.140

Many of these proteins do not have a

described role in translation or stability, or

even a proven interaction with PABP.

Nonetheless, it is appealing to envisage

that additional factors may modulate

PABP’s function in specific

developmental and environmental

situations, by previously described or

novel mechanisms.

Moreover, fundamental issues in

understanding the function of PABP

proteins still remain. While the formation

of end-to-end complexes clearly seems

important, as yet researchers do not

appear to have identified all the

components, or to understand their

relative contributions or how complex

formation is regulated. Likewise, the

function of PABP proteins, other than

PABP1, remains to be directly addressed,

as does their contribution to

development. The importance of

elucidating the respective roles and

functions of each family member is

underlined by the recent implication of

two family members, PABPN17 and

PABP5,35 in human disease. Research in

the next few years is likely to focus on

these unresolved issues and may reveal

novel roles in regulating gene expression

or even in other biological processes.

Acknowledgments

We apologise to our colleagues whose work we

could not include due to space constraints. We

thank Brian Collier, Kris Dickson, Tom Van

Agtmael and Gavin Wilkie for critical reading of

the manuscript and Sandy Bruce for the

preparation of figures.

References

1. Sachs, A. B., Davis, R. W. and Kornberg,R. B. (1987), ‘A single domain of yeastpoly(A)-binding protein is necessary andsufficient for RNA binding and cell viability’,Mol. Cell. Biol., Vol. 7, pp. 3268–3276.

2. Gorlach, M., Burd, C. G. and Dreyfuss, G.(1994), ‘The mRNA poly(A)-bindingprotein: localization, abundance, and RNA-

binding specificity’, Exp. Cell Res., Vol. 211,pp. 400–407.

3. Kuhn, U. and Pieler, T. (1996), ‘Xenopuspoly(A) binding protein: functional domainsin RNA binding and protein-proteininteraction’, J. Mol. Biol., Vol. 256,pp. 20–30.

4. Baer, B. W. and Kornberg, R. D. (1980),‘Repeating structure of cytoplasmic poly(A)-ribonucleoprotein’, Proc. Natl. Acad. Sci.USA, Vol. 77, pp. 1890–1892.

5. Baer, B. W. and Kornberg, R. D. (1983),‘The protein responsible for the repeatingstructure of cytoplasmic poly (A)-ribonucleoprotein’, J. Cell Biol., Vol. 96, pp.717–721.

6. Mangus, D. A., Evans, M. C. and Jacobson,A. (2003), ‘Poly(A)-binding proteins:multifunctional scaffolds for the post-transcriptional control of gene expression’,Genome Biol., Vol. 4, p. 223.

7. Kuhn, U. and Wahle, E. (2004), ‘Structureand function of poly(A) binding proteins’,Biochim. Biophys. Acta, Vol. 1678, pp. 67–84.

8. Voeltz, G. K., Ongkasuwan, J., Standart, N.et al. (2001), ‘A novel embryonic poly(A)binding protein, ePAB, regulates mRNAdeadenylation in Xenopus egg extracts’, GenesDev., Vol. 15, pp. 774–788.

9. Good, P. J., Abler, L., Herring, D. andSheets, M. D. (2004), ‘Xenopus embryonicpoly(A) binding protein 2 (ePABP2) defines anew family of cytoplasmic poly(A) bindingproteins expressed during the early stages ofvertebrate development’, Genesis, Vol. 38,pp. 166–175.

10. Fasulo, B., Becattini, R., Cenci, G. et al.(1999), ‘Doppio fuso (duo), a gene requiredfor spindle pole assembly during Drosophilamale meiosis’, Flybase, URL:http://fly.ebi.ac.uk:7081/.

11. Sigrist, S. J., Thiel, P. R., Reiff, D. F. et al.(2000), ‘Postsynaptic translation affects theefficacy and morphology of neuromuscularjunctions’, Nature, Vol. 405, pp. 1062–1065.

12. Simmer, F., Moorman, C., van der Linden,A. M. et al. (2003), Wormbase,http://www.wormbase.org/.

13. Kamath, R. S., Fraser, A. G. and Ahringer,J. A. (2003), Wormbase,http://www.wormbase.org/.

14. Afonina, E., Stauber, R. and Pavlakis, G. N.(1998), ‘The human poly(A)-binding protein1 shuttles between the nucleus and thecytoplasm’, J. Biol. Chem., Vol. 273, pp.13015–13021.

15. Deo, R. C., Bonanno, J. B., Sonenberg, N.et al. (1999), ‘Recognition of polyadenylateRNA by the poly(A)-binding protein’, Cell,Vol. 98, pp. 835–845.




16. Burd, C. G., Matunis, E. L. and Dreyfuss, G.(1991), ‘The multiple RNA-binding domainsof the mRNA poly(A)-binding protein havedifferent RNA-binding activities’, Mol. CellBiol., Vol. 11, pp. 3419–3424.

17. Nietfeld, W., Mentzel, H. and Pieler, T.(1990), ‘The Xenopus laevis poly(A) bindingprotein is composed of multiple functionallyindependent RNA binding domains’,EMBO J., Vol. 9, pp. 3699–3705.

18. Deardorff, J. A. and Sachs, A. B. (1997),‘Differential effects of aromatic and chargedresidue substitutions in the RNA bindingdomains of the yeast poly(A)-bindingprotein’, J. Mol. Biol., Vol. 269, pp. 67–81.

19. Sladic, R. T., Lagnado, C. A., Bagley, C. J.and Goodall, G. J. (2004), ‘Human PABPbinds AU-rich RNA via RNA-bindingdomains 3 and 4’, Eur. J. Biochem., Vol. 271,pp. 450–457.

20. Bollig, F., Winzen, R., Gaestel, M. et al.(2003), ‘Affinity purification of ARE-bindingproteins identifies polyA-binding protein 1 asa potential substrate in MK2-induced mRNAstabilization’, Biochem. Biophys. Res.Commun., Vol. 301, pp. 665–670.

21. Melo, E. O., Dhalia, R., Martins de Sa, C.et al. (2003), ‘Identification of a C-terminalpoly(A)-binding protein (PABP)-PABPinteraction domain: role in cooperativebinding to poly (A) and efficient cap distaltranslational repression’, J. Biol. Chem., Vol.278, pp. 46357–46368.

22. Kozlov, G., Trempe, J. F., Khaleghpour, K.et al. (2001), ‘Structure and function of theC-terminal PABC domain of humanpoly(A)-binding protein’, Proc. Natl. Acad.Sci. USA, Vol. 98, pp. 4409–4413.

23. Deo, R. C., Sonenberg, N. and Burley, S. K.(2001), ‘X-ray structure of the humanhyperplastic discs protein: an ortholog of theC-terminal domain of poly(A)-bindingprotein’, Proc. Natl. Acad. Sci. USA, Vol. 98,pp. 4414�94419.

24. Gray, N. K., Coller, J. M., Dickson, K. S.et al. (2000), ‘Multiple portions of poly(A)-binding protein stimulate translation in vivo’,EMBO J., Vol. 19, pp. 4723–4733.

25. Roy, G., De Crescenzo, G., Khaleghpour, K.et al. (2002), ‘Paip1 interacts with poly(A)binding protein through two independentbinding motifs’, Mol. Cell Biol., Vol. 22, pp.3769–3782.

26. Hoshino, S., Imai, M., Kobayashi, T. et al.(1999), ‘The eukaryotic polypeptide chainreleasing factor (eRF3/GSPT) carrying thetranslation termination signal to the 39-poly(A) tail of mRNA. Direct association oferf3/GSPT with polyadenylate-bindingprotein’, J. Biol. Chem., Vol. 274, pp.16677–16680.

27. Cosson, B., Berkova, N., Couturier, A. et al.(2002), ‘Poly(A)-binding protein and eRF3are associated in vivo in human and Xenopuscells’, Biol. Cell, Vol. 94, pp. 205–216.

28. Uchida, N., Hoshino, S. I., Imataka, H. et al.(2002), ‘A novel role of the mammalianGSPT/eRF3 associating with poly(A)-binding protein in cap/poly(A)-dependenttranslation’, J. Biol. Chem., Vol. 277, pp.50286–50292.

29. Khaleghpour, K., Kahvejian, A., DeCrescenzo, G. et al. (2001), ‘Dual interactionsof the translational repressor Paip2 withpoly(A) binding protein’, Mol. Cell Biol., Vol.21, pp. 5200–5213.

30. Kleene, K. C., Mulligan, E., Steiger, D. et al.(1998), ‘The mouse gene encoding the testis-specific isoform of poly(A) binding protein(Pabp2) is an expressed retroposon:intimations that gene expression inspermatogenic cells facilitates the creation ofnew genes’, J. Mol. Evol., Vol. 47, pp.275–281.

31. Feral, C., Guellaen, G. and Pawlak, A.(2001), ‘Human testis expresses a specificpoly(A)-binding protein’, Nucleic Acids Res.,Vol. 29, pp. 1872–1883.

32. Kleene, K. C., Wang, M. Y., Cutler, M. et al.(1994), ‘Developmental expression of poly(A)binding protein mRNAs duringspermatogenesis in the mouse’, Mol. Reprod.Dev., Vol. 39, pp. 355–364.

33. Yang, H., Duckett, C. S. and Lindsten, T.(1995), ‘iPABP, an inducible poly(A)-bindingprotein detected in activated human T cells’,Mol. Cell Biol., Vol. 15, pp. 6770–6776.

34. Houng, A. K., Maggini, L., Clement, C. Y.and Reed, G. L. (1997), ‘Identification andstructure of activated-platelet protein-1, aprotein with RNA-binding domain motifsthat is expressed by activated platelets’, Eur. J.Biochem., Vol. 243, pp. 209–218.

35. Blanco, P., Sargent, C. A., Boucher, C. A.et al. (2001), ‘A novel poly(A)-bindingprotein gene (PABPC5) maps to an X-specific subinterval in the Xq21.3/Yp11.2homology block of the human sexchromosomes’, Genomics, Vol. 74, pp. 1–11.

36. Cosson, B., Couturier, A., Le Guellec, R.et al. (2002), ‘Characterization of the poly(A)binding proteins expressed during oogenesisand early development of Xenopus laevis’, Biol.Cell, Vol. 94, pp. 217–231.

37. Hershey, J. W. B. and Merrick, W. C.(2000), ‘Pathway and mechanism of initiationof protein synthesis’, in Sonenberg, N.,Hershey, J. W. B. and Mathews, M. B. (Eds),‘Translational Control of Gene Expression’,Cold Spring Harbor Laboratory Press, NewYork, NY, pp. 33–88.

38. Gray, N. K. and Wickens, M. P. (1998),


Gorgoni and Gray


‘Control of translation initiation in animals’,Ann. Rev. Cell Dev. Biol., Vol. 14, pp.399–458.

39. Gallie, D. R. (1998), ‘A tale of two termini: afunctional interaction between the termini ofan mRNA is a prerequisite for efficienttranslation initiation’, Gene, Vol. 216,pp. 1–11.

40. Sachs, A. (2000), ‘Physical and functionalinteractions between the mRNA capstructure and the poly(A) tail’, in Sonenberg,N., Hershey, J. W. B. and Mathews, M. B.(Eds), ‘Translational Control of GeneExpression’, Cold Spring Harbor LaboratoryPress, New York, NY, pp. 447–466.

41. Jacobson, A. (1996), ‘Poly(A) metabolism andtranslation: the closed-loop model’ inHershey, J. W. B., Mathews, M. B. andSonenberg, N. (Eds), ‘Translational Control’,Cold Spring Harbor Laboratory Press, NewYork, NY, pp. 451–480.

42. Wilkie, G. S., Dickson, K. S. and Gray,N. K. (2003), ‘Regulation of mRNAtranslation by 59- and 39-UTR-bindingfactors’, Trends Biochem. Sci., Vol. 28, pp.182–188.

43. Wells, S. E., Hillner, P. E., Vale, R. D. et al.(1998), ‘Circularization of mRNA byeukaryotic translation initiation factors’,Mol.Cell, Vol. 2, pp. 135–140.

44. Haghihat, A. and Sonenberg, N. (1997),‘eIF4G dramatically enchances the binding ofeIF4E to the mRNA 5’-cap structure’, Biol.Chem., Vol. 272, pp. 21677–21680.

45. Ptushkina, M., von der Haar, T., Vasilescu, S.et al. (1998), ‘Cooperative modulation byeIF4G of eIF4E-binding to the mRNA 59cap in yeast involves a site partially shared byp20’, EMBO J., Vol. 17, pp. 4798–4808.

46. Borman, A. M., Michel, Y. M. and Kean,K. M. (2000), ‘Biochemical characterisationof cap-poly(A) synergy in rabbit reticulocytelysates: the eIF4G�PABP interactionincreases the functional affinity of eIF4E forthe capped mRNA 59-end’, Nucleic AcidsRes., Vol. 28, pp. 4068–4075.

47. Luo, Y. and Goss, D. J. (2001), ‘Homeostasisin mRNA initiation: wheat germ poly(A)-binding protein lowers the activation energybarrier to initiation complex formation’,J. Biol. Chem., Vol. 276, pp. 43083–43086.

48. Le, H., Tanguay, R. L., Balasta, M. L. et al.(1997), ‘Translation initiation factors eIF-iso4G and eIF-4B interact with the poly(A)-binding protein and increase its RNAbinding activity’, J. Biol. Chem., Vol. 272, pp.16247–16255.

49. Tarun, S. Z. and Sachs, A. B. (1995), ‘Acommon function for mRNA 59 and 39 endsin translation initiation in yeast’, Genes Dev.,Vol. 9, pp. 2997–3007.

50. Tarun, S. Z., Wells, S. E., Deardorff, J. A.et al. (1997), ‘Translation initiation factoreIF4G mediates in vitro poly(A) tail-dependent translation’, Proc. Natl. Acad. Sci.USA, Vol. 94, pp. 9046–9051.

51. Bushell, M., Wood, W., Carpenter, G. et al.(2001), ‘Disruption of the interaction ofmammalian protein synthesis eukaryoticinitiation factor 4B with the poly(A)-bindingprotein by caspase- and viral protease-mediated cleavages’, J. Biol. Chem., Vol. 276,pp. 23922–23928.

52. Bi, X. and Goss, D. J. (2000), ‘Wheat germpoly(A)-binding protein increases the ATPaseand the RNA helicase activity of translationinitiation factors eIF4A, eIF4B, and eIF-iso4F.’ J. Biol. Chem., Vol. 275, pp.17740–17746.

53. Craig, A. W., Haghighat, A., Yu, A. T. et al.(1998), ‘Interaction of polyadenylate-bindingprotein with the eIF4G homologue PAIPenhances translation’, Nature, Vol. 392, pp.520�9523.

54. Munroe, D. and Jacobson, A. (1990),‘mRNA poly (A) tail, a 39 enhancer oftranslational initiation’, Mol. Cell Biol., Vol.10, pp. 3441–3455.

55. Searfoss, A., Dever, T. E. and Wickner, R.(2001), ‘Linking the 39 poly(A) tail to thesubunit joining step of translation initiation:relations of Pab1p, eukaryotic translationinitiation factor 5b (Fun12p), and Ski2p-Slh1p’, Mol. Cell Biol., Vol. 21, pp.4900–4908.

56. Welch, E. M., Wang, W. and Peltz, S. W.(2000), ‘Translation termination: it’s not theend of the story’, in Sonenberg, N., Hershey,J. W. B. and Mathews, M. B. (Eds),‘Translational Control Of Gene Expression’,Cold Spring Harbor Laboratory Press, NewYork, NY, pp. 467–486.

57. Schwartz, D. C. and Parker, R. (2000),‘Interaction of mRNA translation andmRNA degradation in S. cerevisiae’, inSonenberg, N., Hershey, J. W. B. andMathews, M. B. (Eds), ‘Translational ControlOf Gene Expression’, Cold Spring HarborLaboratory Press, New York, NY,pp. 807–825.

58. Tourriere, H., Chebli, K. and Tazi, J. (2002),‘mRNA degradation machines in eukaryoticcells’, Biochimie, Vol. 84, pp. 821–837.

59. Ross, J. (1995), ‘mRNA stability inmammalian cells’, Microbiol. Rev., Vol. 59, pp.423–450.

60. Bernstein, P., Peltz, S. W. and Ross, J.(1989), ‘The poly(A)-poly(A)-bindingprotein complex is a major determinant ofmRNA stability in vitro’, Mol. Cell Biol.,Vol. 9, pp. 659–670.

61. Ford, L. P., Bagga, P. S. and Wilusz, J.




(1997), ‘The poly(A) tail inhibits the assemblyof a 39-to-59 exonuclease in an in vitro RNAstability system’, Mol. Cell Biol., Vol. 17, pp.398–406.

62. Korner, C. G. and Wahle, E. (1997),‘Poly(A) tail shortening by a mammalianpoly(A)-specific 39-exoribonuclease’, J. Biol.Chem., Vol. 272, pp. 10448–10456.

63. Korner, C. G., Wormington, M.,Muckenthaler, M. et al. (1998), ‘Thedeadenylating nuclease (DAN) is involved inpoly(A) tail removal during the meioticmaturation of Xenopus oocytes’, EMBO J.,Vol. 17, pp. 5427–5437.

64. Wormington, M., Searfoss, A. M. andHurney, C. A. (1996), ‘Overexpression ofpoly(A) binding protein prevents maturation-specific deadenylation and translationalinactivation in Xenopus oocytes’, EMBO J.,Vol. 15, pp. 900–909.

65. Wang, Z., Day, N., Trifillis, P. and Kiledjian,M. (1999), ‘An mRNA stability complexfunctions with poly(A)-binding protein tostabilize mRNA in vitro’, Mol. Cell Biol., Vol.19, pp. 4552–4560.

66. Gao, M., Wilusz, C. J., Peltz, S. W. andWilusz, J. (2001), ‘A novel mRNA-decapping activity in HeLa cytoplasmicextracts is regulated by AU-rich elements’,EMBO J., Vol. 20, pp. 1134–1143.

67. Khanna, R. and Kiledjian, M. (2004),‘Poly(A)-binding-protein-mediatedregulation of hDcp2 decapping in vitro’,EMBO J., Vol. 23, pp. 1968–1976.

68. Uchida, N., Hoshino, S. and Katada, T.(2004), ‘Identification of a humancytoplasmic poly(A) nuclease complexstimulated by poly(A)-binding protein’,J. Biol. Chem., Vol. 279, pp. 1383–1391.

69. Bevilacqua, A., Ceriani, M. C., Capaccioli, S.et al. (2003), ‘Post-transcriptional regulationof gene expression by degradation ofmessenger RNAs’, J. Cell Physiol., Vol. 195,pp. 356–372.

70. Chen, C. Y., Gherzi, R., Ong, S. E. et al.(2001), ‘AU binding proteins recruit theexosome to degrade ARE-containingmRNAs’, Cell, Vol. 107, pp. 451–464.

71. Lai, W. S., Kennington, E. A. andBlackshear, P. J. (2003), ‘Tristetraprolin andits family members can promote the cell-freedeadenylation of AU-rich element-containing mRNAs by poly(A) ribonuclease’,Mol. Cell Biol., Vol. 23, pp. 3798–3812.

72. Gherzi, R., Lee, K. Y., Briata, P. et al.(2004), ‘A KH domain RNA bindingprotein, KSRP, promotes ARE-directedmRNA turnover by recruiting thedegradation machinery’, Mol. Cell, Vol. 14,pp. 571–583.

73. Binder, R., Horowitz, J. A., Basilion, J. P.

et al. (1994), ‘Evidence that the pathway oftransferrin receptor mRNA degradationinvolves an endonucleolytic cleavage withinthe 39 UTR and does not involve poly(A) tailshortening’, EMBO J., Vol. 13, pp. 1969–1980.

74. Hornstein, E., Git, A., Braunstein, I. et al.(1999), ‘The expression of poly(A)-bindingprotein gene is translationally regulated in agrowth-dependent fashion through a 59-terminal oligopyrimidine tract motif’, J. Biol.Chem., Vol. 274, pp. 1708–1714.

75. Meyuhas, O. and Hornstein, E. (2000),‘Translational control of TOP mRNAs’, inSonenberg, N., Hershey, J. W. B. andMathews, M. B. (Eds), ‘Translational Controlof Gene Expression’, Cold Spring HarborLaboratory Press, New York, NY,pp. 671–694.

76. Wu, J. and Bag, J. (1998), ‘Negative controlof the poly(A)-binding protein mRNAtranslation is mediated by the adenine-richregion of its 59-untranslated region’, J. Biol.Chem., Vol. 273, pp. 34535–34542.

77. De Melo Neto, O. P., Standart, N. andMartins De Sa, C. (1995), ‘Autoregulation ofpoly(A)-binding protein synthesis in vitro.’Nucleic Acids Res., Vol. 23, pp. 2198–2205.

78. Strausberg, R. L., Feingold, E. A., Grouse,L. H. et al. (2002), ‘Generation and initialanalysis of more than 15,000 full-lengthhuman and mouse cDNA sequences’, Proc.Natl. Acad. Sci. USA, Vol. 99, pp.16899–16903.

79. Thierry-Mieg, D., Thierry-Mieg, J.,Potdevin, M. et al. (2003), ‘Identification andfunctional annotation of cDNA-supportedgenes in higher organisms using AceView’,URL: http://www.ncbi.nlm.nih.gov/IEB/Research/Acembly.

80. Raught, B., Gingras, A. and Sonenberg, N.(2000), ‘Regulation of ribosomal recruitmentin eukaryotes’, in Sonenberg, N., Hershey, J.W. B. and Mathews, M. B. (Eds),‘Translational Control of Gene Expression’,Cold Spring Harbor Laboratory Press, NewYork, NY, pp. 245–293.

81. Scheper, G. C. and Proud, C. G. (2002),‘Does phosphorylation of the cap-bindingprotein eIF4E play a role in translationinitiation?’, Eur. J. Biochem., Vol. 269, pp.5350–5359.

82. Clemens, M. J. (2001), ‘Initiation factor eIF2alpha phosphorylation in stress responses andapoptosis’, Prog. Mol. Subcell. Biol., Vol. 27,pp. 57–89.

83. Drawbridge, J., Grainger, J. L. and Winkler,M. M. (1990), ‘Identification andcharacterization of the poly(A)-bindingproteins from the sea urchin: a quantitativeanalysis’, Mol. Cell Biol., Vol. 10, pp.3994–4006.


Gorgoni and Gray


84. Gallie, D. R., Le, H., Caldwell, C. et al.(1997), ‘The phosphorylation state oftranslation initiation factors is regulateddevelopmentally and following heat shock inwheat’, J. Biol. Chem., Vol. 272, pp.1046–1053.

85. Le, H., Browning, K. S. and Gallie, D. R.(2000), ‘The phosphorylation state ofpoly(A)-binding protein specifies its bindingto poly(A) RNA and its interaction witheukaryotic initiation factor (eIF) 4F, eIFiso4F,and eIF4B’, J. Biol. Chem., Vol. 275, pp.17452–17462.

86. Fraser, C. S., Pain, V. M. and Morley, S. J.(1999), ‘The association of initiation factor 4Fwith poly(A)-binding protein is enhanced inserum-stimulated Xenopus kidney cells’,J. Biol. Chem., Vol. 274, pp. 196–204.

87. Lee, J. and Bedford, M. T. (2002), ‘PABP1identified as an arginine methyltransferasesubstrate using high-density protein arrays’,EMBO Rep., Vol. 3, pp. 268–273.

88. Piron, M., Vende, P., Cohen, J. et al. (1998),‘Rotavirus RNA-binding protein NSP3interacts with eIF4GI and evicts the poly(A)binding protein from eIF4F’, EMBO J., Vol.17, pp. 5811–5821.

89. Vende, P., Piron, M., Castagne, N. et al.(2000), ‘Efficient translation of rotavirusmRNA requires simultaneous interaction ofNSP3 with the eukaryotic translationinitiation factor eIF4G and the mRNA 39end’, J. Virol., Vol. 74, pp. 7064–7071.

90. Khaleghpour, K., Svitkin, Y. V., Craig, A.W. et al. (2001), ‘Translational repression by anovel partner of human poly(A) bindingprotein, Paip2’, Mol. Cell, Vol. 7, pp.205–216.

91. Roy, G., Miron, M., Khaleghpour, K. et al.(2004), ‘The Drosophila poly(A) bindingprotein-interacting protein, dPaip2, is a noveleffector of cell growth’, Mol. Cell Biol., Vol.24, pp. 1143–1154.

92. Gouyon, F., Onesto, C., Dalet, V. et al.(2003), ‘Fructose modulates GLUT5 mRNAstability in differentiated Caco-2 cells: role ofcAMP-signalling pathway and PABP(polyadenylated-binding protein)-interactingprotein (Paip) 2’, Biochem. J., Vol. 375, pp.167–174.

93. Onesto, C., Berra, E., Grepin, R. et al. (inpress), ‘Poly (A) binding protein-interactingprotein 2, a strong regulator of vascularendothelial growth factor mRNA’, J. Biol.Chem.

94. Kuyumcu-Martinez, N. M., Joachims, M.and Lloyd, R. E. (2002), ‘Efficient cleavageof ribosome-associated poly(A)-bindingprotein by enterovirus 3C protease’, J. Virol.,Vol. 76, pp. 2062–2074.

95. Kuyumcu-Martinez, N. M., Van Eden, M.

E., Younan, P. et al. (2004), ‘Cleavage ofpoly(A)-binding protein by poliovirus 3Cprotease inhibits host cell translation: a novelmechanism for host translation shutoff’, Mol.Cell Biol., Vol. 24, pp. 1779–1790.

96. Clemens, M. J., Bushell, M., Jeffrey, I. W.et al. (2000), ‘Translation initiation factormodifications and the regulation of proteinsynthesis in apoptotic cells’, Cell. DeathDiffer., Vol. 7, pp. 603–615.

97. Marissen, W. E., Triyoso, D., Younan, P.et al. (2004), ‘Degradation of poly(A)-bindingprotein in apoptotic cells and linkage totranslation regulation’, Apoptosis, Vol. 9, pp.67–75.

98. Mendez, R. and Richter, J. D. (2001),‘Translational control by CPEB: a means tothe end’, Nat. Rev. Mol. Cell Biol., Vol. 2, pp.521–529.

99. Richter, J. D. (1996), ‘Dynamics of poly (A)addition and removal during development’,in Hershey, J. W. B., Mathews, M. B. andSonenberg, N. (Eds), ‘Translational Control’,Cold Spring Harbor Laboratory Press, NewYork, NY, pp. 481�9503.

100. Fox, C. A., Sheets, M. D. and Wickens,M. P. (1989), ‘Poly (A) addition duringmaturation of frog oocytes: distinct nuclearand cytoplasmic activities and regulation bythe sequence UUUUUAU’, Genes Dev., Vol.3, pp. 2151–2162.

101. McGrew, L. L., Dworkin-Rastl, E.,Dworkin, M. B. et al. (1989), ‘Poly (A)elongation during Xenopus oocyte maturationis required for translational recruitment and ismediated by a short sequence element’, GenesDev., Vol. 3, pp. 803–815.

102. Huarte, J., Stutz, A., O’Connell, M. L. et al.(1992), ‘Transient translational silencing byreversible mRNA deadenylation’, Cell, Vol.69, pp. 1021–1030.

103. Simon, R., Tassan, J. and Richter, J. D.(1992), ‘Translational control by poly(A)elongation during Xenopus development:differential repression and enhancement by anovel cytoplasmic polyadenylation element’,Genes Dev., Vol. 6, pp. 2580–2591.

104. Simon, R. and Richter, J. D. (1994), ‘Furtheranalysis of cytoplasmic polyadenylation inXenopus embryos and identification ofembryonic cytoplasmic polyadenylationelement-binding proteins’, Mol. Cell. Biol.,Vol. 14, pp. 7867–7875.

105. Tay, J. and Richter, J. D. (2001), ‘Germ celldifferentiation and synaptonemal complexformation are disrupted in CPEB knockoutmice’, Dev. Cell, Vol. 1, pp. 201–213.

106. Kashiwabara, S., Noguchi, J., Zhuang, T.et al. (2002), ‘Regulation of spermatogenesisby testis-specific, cytoplasmic poly(A)




polymerase TPAP’, Science, Vol. 298, pp.1999–2002.

107. Gebauer, F., Xu, W., Cooper, G. M.et al. (1994), ‘Translational control bycytoplasmic polyadenylation c-mos mRNA isnecessary for oocyte maturation in themouse’, EMBO J., Vol. 13, pp. 5712–5720.

108. Sheets, M. D., Wu, M. and Wickens, M. D.(1995), ‘Polyadenylation of c-mos as a controlpoint in Xenopus meiotic maturation’, Nature,Vol. 374, pp. 511–516.

109. Salles, F. J., Lieberfarb, M. E., Wreden, C.et al. (1994), ‘Regulated polyadenylation ofmaternal mRNAs allows coordinate initiationof Drosophila development’, Science, Vol. 266,pp. 1996�91998.

110. Simon, R., Wu, L. and Richter, J. D. (1996),‘Cytoplasmic polyadenylation of activinreceptor mRNA and the control of patternformation in Xenopus development’, Dev.Biol., Vol. 179, pp. 239–250.

111. Sheets, M. D., Fox, C. A., Hunt, T. et al.(1994), ‘The 39-untranslated regions of c-mosand cyclin mRNAs stimulate translation byregulating cytoplasmic polyadenylation’,Genes Dev., Vol. 8, pp. 926–938.

112. Gallie, D. R., Ling, J., Niepel, M. et al.(2000), ‘The role of 59-leader length,secondary structure and PABP concentrationon cap and poly(A) tail function duringtranslation in Xenopus oocytes’, Nucleic AcidsRes., Vol. 28, pp. 2943–2953.

113. Wickens, M., Goodwin, E. B., Kimble, J.et al. (2000), ‘Translational control ofdevelopmental decisions’, in Hershey, J. W.B., Mathews, M.B. and Sonenberg, N. (Eds),‘Translational Control Of Gene Expression’,Cold Spring Harbor Laboratory Press, NewYork, NY, pp. 295–370.

114. Richter, J. D. (2000), ‘Influence ofpolyadenylation-induced translation onmetazoan development and neural synapticfunction’, in Sonenberg, N., Hershey, J. W.B. and Mathews, M. B. (Eds), ‘TranslationalControl of Gene Expression’, Cold SpringHarbor Laboratory Press, New York, NY,pp. 785–805.

115. Steward, O. and Schuman, E. M. (2003),‘Compartmentalized synthesis anddegradation of proteins in neurons’, Neuron,Vol. 40, pp. 347–359.

116. Wreden, C., Verotti, A. C., Schisa, J. A. et al.(1997), ‘Nanos and pumilio establishembryonic polarity in Drosophila bypromoting posterior deadenylation ofhunchback mRNA’, Development, Vol. 124,pp. 3015–3023.

117. Paillard, L. and Osborne, H. B. (2003), ‘Eastof EDEN was a poly(A) tail’, Biol. Cell, Vol.95, pp. 211–219.

118. Fox, C. A. and Wickens, M. (1990), ‘Poly(A)

removal during oocyte maturation: a defaultreaction selectively prevented by specificsequences in the 39 UTR of certain maternalmRNAs’, Genes Dev., Vol. 4, pp.2287–2298.

119. Varnum, S. M. and Wormington, W. M.(1990), ‘Deadenylation of maternal mRNAsduring Xenopus oocyte maturation does notrequire specific cis-sequences: a defaultmechanism for translational control’, GenesDev., Vol. 4, pp. 2278–2286.

120. Copeland, P. R. and Wormington, M.(2001), ‘The mechanism and regulation ofdeadenylation: identification andcharacterization of Xenopus PARN’, RNA,Vol. 7, pp. 875–886.

121. Paillard, L., Omilli, F., Legagneux, V. et al.(1998), ‘EDEN and EDEN-BP, a cis elementand an associated factor that mediatesequence-specific mRNA deadenylation inXenopus embryos’, EMBO J., Vol. 17, pp.278–287.

122. Barkoff, A. F., Dickson, K. S., Gray, N. K.et al. (2000), ‘Translational control of cyclinB1 mRNA during meiotic maturation:coordinated repression and cytoplasmicpolyadenylation’, Dev. Biol., Vol. 220, pp.97–109.

123. de Moor, C. H. and Richter, J. D. (1999),‘Cytoplasmic polyadenylation elementsmediate masking and unmasking of cyclin B1mRNA’, EMBO J., Vol. 18, pp. 2294–2303.

124. Stebbins-Boaz, B., Cao, Q., deMoor, C. H.et al. (1999), ‘Maskin is a CPEB-associatedfactor that transiently interacts with eIF-4E’,Mol. Cell, Vol. 4, pp. 1017–1027.

125. Mendez, R., Murthy, K. G., Ryan, K. et al.(2000), ‘Phosphorylation of CPEB by Eg2mediates the recruitment of CPSF into anactive cytoplasmic polyadenylation complex’,Mol. Cell, Vol. 6, pp. 1253–1259.

126. Cao, Q. and Richter, J. D. (2002),‘Dissolution of the maskin-eIF4E complex bycytoplasmic polyadenylation and poly(A)-binding protein controls cyclin B1 mRNAtranslation and oocyte maturation’, EMBO J.,Vol. 21, pp. 3852–3862.

127. Nakahata, S., Kotani, T., Mita, K. et al.(2003), ‘Involvement of Xenopus Pumilio inthe translational regulation that is specific tocyclin B1 mRNA during oocyte maturation’,Mech. Dev., Vol. 120, pp. 865–880.

128. Groisman, I., Jung, M. Y., Sarkissian, M.et al. (2002), ‘Translational control of theembryonic cell cycle’, Cell, Vol. 109, pp.473–483.

129. Huang, Y. S., Jung, M. Y., Sarkissian, M.et al. (2002), ‘N-methyl-D-aspartate receptorsignaling results in Aurora kinase-catalyzedCPEB phosphorylation and alpha CaMKII


Gorgoni and Gray


mRNA polyadenylation at synapses’, EMBOJ., Vol. 21, pp. 2139–2148.

130. Huang, Y. S., Carson, J. H., Barbarese, E. etal. (2003), ‘Facilitation of dendritic mRNAtransport by CPEB’, Genes Dev., Vol. 17, pp.638–653.

131. Coller, J. M., Gray, N. K. and Wickens,M. P. (1998), ‘mRNA stabilization bypoly(A)-binding protein is independent ofpoly(A) and requires translation’, Genes Dev.,Vol. 12, pp. 3226–3235.

132. Skabkina, O. V., Skabkin, M. A., Popova,N. V. et al. (2003), ‘Poly(A)-binding proteinpositively affects YB-1 mRNA translationthrough specific interaction with YB-1mRNA’, J. Biol. Chem., Vol. 278, pp.18191–18198.

133. Bag, J. (2001), ‘Feedback inhibition ofpoly(A)-binding protein mRNA translation.A possible mechanism of translation arrest bystalled 40S ribosomal subunits’, J. Biol. Chem.,Vol. 276, pp. 47352–47360.

134. Mohr, E., Prakash, N., Vieluf, K. et al.(2001), ‘Vasopressin mRNA localization innerve cells: characterization of cis-actingelements and trans-acting factors’, Proc. Natl.Acad. Sci. USA, Vol. 98, pp. 7072–7079.

135. Mohr, E., Kachele, I., Mullin, C. et al.(2002),‘Rat vasopressin mRNA: a model system to

characterize cis-acting elements and trans-acting factors involved in dendritic mRNAsorting’, Prog. Brain Res., Vol. 139, pp.211–224.

136. Wang, Z., Day, N., Trifillis, P. andKiledjvan, M. (1999), ‘An mRNA stabilitycomplex functions with poly(A)-bindingprotein to stabilize mRNA in vitro’, Mol. Cell.Biol., Vol. 19, pp. 4552–4560.

137. Wang, Z. and Kiledjian, M. (2000), ‘Thepoly(A)-binding protein and an mRNAstability protein jointly regulate anendoribonuclease activity’, Mol. Cell Biol.,Vol. 20, pp. 6334–6341.

138. Shyu, A. B., Belasco, J. G. and Greenberg,M. E. (1991), ‘Two distinct destabilizingelements in the c-fos message triggerdeadenylation as a first step in rapid mRNAdecay’, Genes Dev., Vol. 5, pp. 221–231.

139. Grosset, C., Chen, C. Y., Xu, N. et al.(2000), ‘A mechanism for translationallycoupled mRNA turnover: interactionbetween the poly(A) tail and a c-fos RNAcoding determinant via a protein complex’,Cell, Vol. 103, pp. 29–40.

140. Albrecht, M. and Lengauer, T. (2004),‘Survey on the PABC recognition motifPAM2’, Biochem. Biophys. Res. Commun.,Vol. 316, pp. 129–138.




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The roles of cytoplasmic poly(A)-binding proteins in regulating gene