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PHOSPHATIDYLCHOLINE (PtdCho; seeFig. 1) is the major membrane phospho-lipid in eukaryotic cells1–3. In addition tobeing the major structural componentof cellular membranes, PtdCho servesas a reservoir for several lipid mess-engers: it is the source of the bioactivelipids lysoPtdCho, phosphatidate, diacyl-glycerol, lysophosphatidate, platelet-activating factor and arachidonic acid.The generation of these lipid mess-engers from PtdCho depends on metab-olism of the latter. Recent studies ofSaccharomyces cerevisiae mutants and mammalian systems have uncov-ered novel interrelationships betweenthe two pathways for PtdCho metab-olism, as well as interactions betweenthese pathways and those for CTP syn-thesis and secretion through the Golgiapparatus. Here, we focus on theseinterrelationships in yeast and in mammalian systems, and raise ques-tions about the way in which the cell regulates the involvement of PtdCho metabolism with these cellularprocesses.

Complex interactions in phosphatidylcholinesynthesis

PtdCho is synthesized by two alterna-tive pathways1–4: the CDP-choline path-way and the phosphatidylethanolamine(PtdEtn)-methylation pathway (Fig. 2).In mammalian cells, all three steps inthe PtdEtn-methylation pathway arecatalysed by one enzyme, whereas, inyeast, the PEM1/CHO2-encoded methyl-transferase catalyses the first methyl-ation step and the PEM2/OPI3-encodedmethyltransferase catalyses the last twomethylation steps. The PtdEtn used inthe methylation pathway is derived fromphosphatidylserine (PtdSer; see Fig. 3).In yeast, PtdSer is synthesized fromCDP-diacylglycerol and serine (Fig. 3),

whereas, in mammalian cells, PtdSer issynthesized by an exchange reaction be-tween PtdEtn or PtdCho and serine4.Nearly all of the genes that encode theenzymes involved in PtdCho metab-olism in these two types of organismhave been isolated and characterized,and many of the enzymes have been purified and studied1,3–8.

In S. cerevisiae, PtdCho is synthesizedprimarily by the PtdEtn-methylationpathway (shown in red in Fig. 3).However, the CDP-choline pathway(shown in blue in Fig. 3) becomes essen-tial for PtdCho synthesis if the enzymesin the PtdEtn-methylation pathway aredefective. For example, mutants inwhich synthesis of PtdSer or PtdEtn, ormethylation of PtdEtn, is defective re-quire exogenous choline for growth, be-cause they must synthesize PtdChothrough the CDP-choline pathway3.Mutants in which PtdSer or PtdEtn synthesis is defective can also synthe-size PtdCho if they are supplementedwith ethanolamine, which is used for PtdEtn synthesis through the CDP-ethanolamine pathway (Fig. 3)3. ThePtdEtn subsequently is methylated toform PtdCho in the PtdEtn-methylationpathway.

The prevailing view has been that theCDP-choline pathway is a salvage path-way used by cells when PtdEtn methyl-ation is compromised9. However, recentwork on mutants that have defectivePtdCho synthesis and lack phospho-lipase D (PLD) has shown that the CDP-choline pathway contributes to PtdChosynthesis even when wild-type cells aregrown in the absence of exogenouscholine10. The PtdCho synthesizedthrough the PtdEtn-methylation path-way is constantly turned over to yieldfree choline and phosphatidate (Fig.3)10. This reaction is catalysed by PLD(Fig. 3)11. The free choline is then incor-porated back into PtdCho through the

Interactions among pathways forphosphatidylcholine

metabolism, CTP synthesis andsecretion through the Golgi

apparatus

Claudia Kent and George M. CarmanPhosphatidylcholine is the major phospholipid in eukaryotic cells. It servesas a structural component of cell membranes and a reservoir of severallipid messengers. Recent studies in yeast and mammalian systems haverevealed interrelationships between the two pathways of phosphatidyl-choline metabolism, and between these pathways and those for CTP syn-thesis and secretion via the Golgi. These processes involve the regulationof the CDP-choline and phosphatidylethanolamine-methylation pathways ofphosphatidylcholine synthesis, CTP synthetase, phospholipase D and thephospholipid-transfer protein Sec14p.

C. Kent is at the Dept of BiologicalChemistry, University of Michigan MedicalSchool, Ann Arbor, MI 48109, USA; andG. M. Carman is at the Dept of Food Science, Rutgers University, New Brunswick,NJ 08901, USA.

R2 C

O CH2

O C H

O C R1

CH2 CH2O P O CH2 N1 CH3

CH3

CH3

O

O

O2

Figure 1Structure of phosphatidylcholine. R1 and R2 are fatty acids; usually, R1 is saturated and R2is unsaturated.

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CDP-choline pathway, and the phos-phatidate is recycled back into PtdChoby the PtdEtn-methylation pathway andinto other phospholipids – for example,phosphatidylinositol (PtdIns) – via CDP-diacylglycerol (Fig. 3)10,12.

Whereas, in yeast, PtdCho is derivedprimarily by methylation of PtdEtn, themajor pathway in all mammalian tissuesis the CDP-choline pathway. PtdEtnmethylation is, however, active in mam-malian liver13. From 5% to 40% of hepaticPtdCho is estimated to be formed bymethylation of PtdEtn14,15. Why is thispathway so active in liver but not inother cells? To investigate the role ofPtdEtn methylation in liver, Vance andco-workers16 produced a mouse inwhich PtdEtn methylation is defective(i.e. a mouse that lacked the gene thatencodes PtdEtn N-methyltransferase).Under normal laboratory conditions,the homozygous knockouts have no ob-vious phenotype. However, when theyare fed a choline-poor diet, the mice suf-fer dramatic liver damage within threedays17. The PtdCho content of the liverdecreases by ~50%, and there is an extensive loss of serum lipoproteins –an observation consistent with a defectin the ability of the PtdCho-deficientliver to secrete lipoproteins normally. A possible explanation for this pathol-ogy is that the PtdEtn-methylation path-way is a necessary supplement to the

CDP-choline pathway in the liver intimes of stress, such as starvation17. Thedemand for PtdCho biosynthesis in liveris much greater than in other tissues because liver synthesizes PtdCho for

export in serum lipoproteins and bile.The inability of the methyltransferase-deficient mice to synthesize PtdCho forsecretion might result in accumulationof deleterious metabolic intermediatesor toxic products normally secreted inbile.

In cells other than hepatocytes, littleif any PtdCho is made by methylation ofPtdEtn. In fact, expression of PtdEtn N-methyltransferase cannot be induced innon-hepatic cells, even when the cells aredeprived of choline18,19. Expression ofexogenous PtdEtn methyltransferase can,however, affect the CDP-choline path-way in non-hepatic cells. When cellsthat normally do not express PtdEtn N-methyltransferase are transfected witha cDNA that encodes it, they express the enzyme and synthesize appreciablequantities of PtdCho through methyl-ation of PtdEtn20,21. In response to the elevated levels of PtdCho, choline-phosphate cytidylyltransferase (CCT)levels decrease in proportion to the activity of PtdEtn N-methyltransferase;this regulation appears to occur at thelevel of gene expression21. We are onlybeginning to elucidate the factors thatcontrol the expression of the gene thatencodes CCT; this system will thereforebe very useful if we are to decipher howthe PtdCho content of a cell can affectCCT synthesis.

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CDP-choline pathway(a)

Choline

ATP

ADPCK

Choline-PCTP

PPi

CCT

CDP-choline

DAG

CMPCPT

PtdCho

PtdEtn-methylation pathway(b)

PtdEtn

AdoMet

AdoHcyPEMT

PtdMME

AdoMet

AdoHcyPEMT

PtdDME

AdoMet

AdoHcyPEMT

PtdCho

Figure 2Phosphatidylcholine (PtdCho) is synthesized by the CDP-choline (a) and phos-phatidylethanolamine (PtdEtn)-methylation pathways (b). Choline kinase (CK), choline-phos-phate (choline-P) cytidylyltransferase (CCT), and choline phosphotransferase (CPT) catalysereactions in the CDP-choline pathway (shown in blue). PtdEtn N-methyltransferase (PEMT)catalyses the three-step methylation of PtdEtn in the PtdEtn-methylation pathway (shown inred). In yeast, one methyltransferase enzyme catalyses the first methylation reaction and asecond methyltransferase enzyme catalyses the last two methylation reactions. AdoHcy,adenoasylhomocysteine; AdoMet, S-adenosylmethionine; PtdMME, phosphatidylmethyl-ethanolamine; PtdDME, phosphatidyldimethylethanolamine.

Choline

Choline-P

CDP-ChoCDP-EtnCDP-DAG

PtdChoPtdEtnPtdSer

PtdIns

CK

CCT

CPT

CDS

PEMTPSD

PSS PLD

CTPS

PA

DAG

UTP

CTPPA

Figure 3CTP synthetase and phospholipase D play important roles in the synthesis and turnover ofphosphatidylcholine (PtdCho). The CDP-choline pathway enzymes choline kinase (CK),choline-phosphate (choline-P) cytidylyltransferase (CCT) and choline phosphotransferase(CPT) are shown in blue. The enzymes CDP-diacylglycerol (CDP-DAG) synthase (CDS), PtdSersynthase (PSS), PtdSer decarboxylase (PSD), and the phosphatidylethanolamine (PtdEtn)methyltransferases (PEMT) catalyse reactions that lead to the formation of PtdCho by thePtdEtn-methylation pathway (shown in red). The reactions catalysed by CTP synthetase(CTPS) and phospholipase D (PLD) are shown in black. CDP-Etn, CDP-ethanolamine; PA,phosphatidate; PtdIns, phosphatidylinositol; PtdSer, phosphatidylserine.

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CTP synthesis and phosphatidylcholinemetabolism

CTP is essential for phospholipid syn-thesis in yeast and in mammalian cells. Itis the immediate precursor of the acti-vated, energy-rich phospholipid-pathwayintermediates CDP-diacylglycerol, CDP-ethanolamine and CDP-choline (Fig. 3).Proper production of CTP, therefore,should be critical for normal PtdCho biosynthesis. Novel insights into themodulation of PtdCho synthesis by CTPin yeast have come from studies of aGlu161→Lys mutant of the URA7-encoded CTP synthetase22. In yeast, CTPsynthetase is an essential enzyme that isresponsible for the synthesis of CTP fromUTP (Fig. 3). The Glu161→Lys mutant isnot as sensitive to product inhibition byCTP as the wild-type enzyme22. Cells thatcarry this mutation exhibit elevated lev-els of CTP and alterations in overall lipidmetabolism22. The major affect of the mutation on phospholipid synthesis is an increase in the use of the CDP-cholinepathway for PtdCho synthesis. Thecholine required for this synthesis comesfrom turnover of PtdCho that is synthe-sized by the PtdEtn-methylation path-way. The Glu161→Lys mutation in CTPsynthetase causes significant increasesin the rate of synthesis of PtdCho andphosphatidate, and a decrease in the rateof PtdSer synthesis. McDonough et al.23

have attributed the increased use of theCDP-choline pathway to an increase inthe availability of CTP for the CCT reac-tion. The decrease in the rate of PtdSersynthesis is consistent with a decrease inthe rate of PtdCho synthesis through thePtdEtn methylation pathway. The mecha-nism for this regulation might be directinhibition of PtdSer synthase activity byCTP (Ref. 23).

The Glu161→Lys mutation also causesan increase in total neutral-lipid contentat the expense of total phospholipid22.The increase in total neutral lipids is dueto increases in triacylglycerol, free fattyacids, and ergosterol esters22. Thesechanges are reminiscent of the changesin lipid metabolism that occur when wild-type cells enter the stress-like conditionsof stationary phase9. Thus, the activationof the CDP-choline pathway as a conse-quence of the Glu161→Lys mutation inCTP synthetase is detrimental to the regu-lation of lipid metabolism, and properregulation of PtdCho synthesis throughthe CDP-choline pathway is important foryeast-cell physiology.

We have known for a long time thatCTP levels are an important factor in de-termining the rate of PtdCho synthesis

in mammalian cells. Choy and Vance24

provided the first demonstration thatCCT is rate-limiting in the CDP-cholinepathway: they showed that the elevatedrate of PtdCho synthesis in poliovirus-infected HeLa cells is due to the el-evated level of CTP (Ref. 24). Subsequentstudies have suggested that exogenouscytidine stimulates PtdCho synthesis byincreasing the levels of CTP25,26, a CCTsubstrate. Of course, the activity of anyenzyme will be modified by changes in the concentration of its substrate – if that concentration is not saturating.Enzymological studies of purified CCT,however, have suggested a mechanismfor the relationship between cellularCTP levels and CCT activity that is more elaborate than this. CCT is a lipid-activated phosphoenzyme that be-comes dephosphorylated upon acti-vation in the cell1. The kinetic effect ofthe binding of lipids to CCT is a signifi-cant decrease in the Km for CTP (Ref.27). Moreover, the phosphorylationstate of CCT affects lipid binding: the de-phosphoenzyme has a higher affinity forlipids than does the phosphorylated en-zyme28. The phosphorylation state ofthe enzyme, therefore, can affect the Kmfor CTP via modulation of lipid binding.Thus, this enzyme can be sensitive toand can balance the importance of CTPlevels, the membrane composition andthe state of its own phosphorylation inthe cell.

Phosphatidylcholine metabolism and thesecretory pathway

Yeast. Trafficking of proteins from theGolgi complex involves considerablemembrane flux and, given that PtdCho is the major membrane phospholipid,there is probably a regulatory link be-tween secretion and PtdCho metabolism.Recent studies, in fact, have suggestedthat there is an interaction betweenPtdCho metabolism and the secretorypathway that involves enzymes in theCDP-choline pathway, a phospholipid-transfer protein (Sec14p) and PLD(Pld1p). In vitro, Sec14p catalyses thetransfer of PtdIns and PtdCho betweenmembrane surfaces29. In vivo, Sec14pfunction is essential for cell viability andbudding of vesicles from the Golgi com-plex (Fig. 4)29. The sec14 mutation issuppressed (i.e. bypassed) by mu-tations in the genes that encode theCDP-choline-pathway enzymes cholinekinase (cki1), CCT (cct1) and cholinephosphotransferase (cpt1) – the cki1sec14 double mutant, for example, is viable29. In other words, the synthesis of

PtdCho by the CDP-choline pathway islethal in the absence of a functionalSec14p. Synthesis of too much PtdChoby the CDP-choline pathway appears tobe detrimental to the secretory process.Sec14p could regulate the PtdIns:PtdChoratio in Golgi membranes and downregu-late PtdCho synthesis by directly in-hibiting CCT activity (Fig. 4)30 and/or bypreventing consumption of the diacyl-glycerol used for PtdCho synthesisthrough the CDP-choline pathway31. Theenzymes in the PtdEtn-methylationpathway do not appear to be involvedwith Sec14p function: mutations in thispathway do not bypass its essentialfunction29.

Recent work by Henry and co-work-ers10,11 has indicated that PLD activityplays a role in connecting the CDP-choline pathway and secretion. Cellsthat carry mutations in the CDP-cholinepathway enzymes exhibit a choline-excretion phenotype10. This phenotypedepends on PLD1/SPO14-encoded-PLD-mediated turnover of PtdCho synthe-sized by PtdEtn methylation, and the inability of cells to reincorporate freecholine into PtdCho through the CDP-choline pathway10,11. The choline-excre-tion phenotype is exacerbated if mu-tations in genes for CDP-choline-pathway enzymes are combined with atemperature-sensitive sec14 (sec14ts)mutation (when grown at the restrictivetemperature)10. The increase in cholineexcretion in these mutants requires acti-vation of PLD (Ref. 11). Furthermore,such mutations fail to bypass the sec14ts

phenotype if the gene that encodes PLDis also deleted, and secretion is not re-stored in these triple mutants11. Theseresults indicate that the correct levels of the CDP-choline-pathway enzymesand PLD are important for secretion andviability in yeast.

Henry and co-workers10,11 haveargued that phosphatidate derived fromthe PLD-mediated turnover of PtdCho in the cki1 sec14ts double mutant is thebasis for the suppression of the sec14mutation. This hypothesis is supportedby the work of Bankaitis and co-work-ers32, who have shown that phosphati-date levels increase in response to PLDactivity. These workers have previouslyproposed that diacylglycerol plays anessential role in the Golgi secretoryfunction31. They now propose that PLDactivity yields sufficient phosphatidatefor phosphatidate-phosphatase en-zymes to provide a diacylglycerol poolthat mediates protein secretion32.However, a specific requirement for

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phosphatidate rather than diacyl-glycerol is further supported by studiesin mammalian systems33,34 (see below).

Mammals. One might expect that mecha-nisms that interconnect secretion andPtdCho biosynthesis would be found inmammals as well as in yeast. As in yeast,mammalian phospholipid-transfer pro-teins have been implicated in secre-tion35,36, but direct interactions betweenthese proteins and PtdCho biosynthesishave not been demonstrated. An indirectrelationship might exist, however. Vesiclebudding requires hydrolysis of PtdCho byPLD33,34. Degradation of PtdCho by PLDwould then necessitate PtdCho biosyn-thesis to achieve homeostasis. CCT is activated in response to PtdCho degra-dation37,38, presumably because its lipid-binding domain is sensitive to the compo-sition of PtdCho-deficient membranes. It isnot unreasonable, therefore, to expectactivation of CCT in response to endo-cytosis-associated PtdCho degradation.

Work by Weinhold and co-workerssuggested that there is another connec-tion between PtdCho metabolism andsecretion. They discovered that trans-cytosis-associated protein (TAP; alsoknown as p115) interacts with CCT39–43.TAP/ p115 is necessary for vesicle trans-port within the Golgi apparatus43, fortranscytosis42 and possibly for post-mitotic vesicle fusion44. The effect of thebinding of TAP/p115 to CCT on the activ-ities of the two proteins is not yet clear.

It might be difficult to envision a trulyphysiological role for interactions be-tween CCT and either secretory vesiclesor TAP/p115, because CCT is nuclear inmany situations1 whereas secretoryvesicles and TAP/p115 are cytoplasmic.Recently, however, Lykidis and co-work-ers45 discovered another isoform of CCT,CCTb. This new isoform is similar to thenuclear CCT in its catalytic and regula-tory domains, but lacks the nuclear-targeting sequence and appears to be

cytoplasmic45. Although interactions between CCTb and TAP/p115 or phos-pholipid-transfer proteins have not yetbeen investigated, we suspect thatCCTb plays a role in connecting vesicletraffic with PtdCho metabolism.

Conclusions and outlookStudies in yeast and mammalian sys-

tems have revealed complex inter-relationships among PtdCho-synthesispathways, CTP-synthesis enzymes, PLDand Sec14p. A unifying theme is evident:although the CDP-choline pathway playsan important role in PtdCho synthesis,its misregulation is detrimental to cellphysiology. Figure 4 summarizes theconclusions that can be drawn from thestudies discussed here. The processesshown are an oversimplification of thecomplex regulation of PtdCho metab-olism, CTP synthesis and the secretorypathway. Clearly, pathways for metab-olism of other phospholipids, particu-larly PtdIns and its derivatives, playroles in metabolic regulation and cellu-lar trafficking; these are discussed elsewhere29.

Several questions require further ex-amination. The CDP-choline and PtdEtn-methylation pathways appear to beregulated coordinately through the ac-tions of CTP synthetase, PLD andSec14p. How are these proteins regu-lated, and how does this regulation im-pact on the secretory process and cellphysiology? Multiple forms of CTP syn-thetase, PLD and Sec14p exist; multipleforms of several other lipid-biosynthesisenzymes are common in eukaryotic sys-tems4. Are the multiple forms of CTPsynthetase, PLD and Sec14p localized todifferent cellular compartments, andhow would such compartmentation im-pact on regulation? What mechanismscontrol the communication betweenpathways if they are localized in dif-ferent places? We still have much tolearn about the relationships betweenphospholipid metabolism and othermetabolic processes. This is an excitingtime in science because the moleculartools for addressing such relationshipsare becoming more available.

References1 Kent, C. (1997) Biochim. Biophys. Acta 1348,

79–902 Vance, D. E. (1996) in Biochemistry of Lipids,

Lipoproteins and Membranes (Vance, D. E. andVance J. E., eds), pp. 153–181, Elsevier

3 Henry, S. A. and Patton-Vogt, J. L. (1998) Prog.Nucleic Acid Res. 61, 133–139

4 Vance, J. E. (1998) Trends Biochem. Sci. 23,423–428

5 Ishidate, K. (1997) Biochim. Biophys. Acta 1348,

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Choline Choline-P CDP-Choline

CDP-DAG

PtdCho

CCT

PLD

PA

Sec14p

PA

DAG

CDP-choline pathway

PtdEtn-methylation pathway

Golgi-derivedsecretory vesicles

Golgi complex

UTP

CTP

PtdSer PtdIns

PtdEtn

PtdCho

Figure 4Involvement of phosphatidylcholine (PtdCho) metabolism with CTP synthesis and the secre-tory pathway. The choline used for the synthesis of PtdCho through the CDP-choline pathway(shown in blue) is derived from transport of exogenous choline into the cell or the phospholi-pase D (PLD)-mediated turnover of PtdCho. The phosphatidate (PA) derived from PtdChoturnover is recycled back into PtdCho, and other phospholipids such as phosphatidylinositol(PtdIns), through CDP-diacylglycerol (CDP-DAG). The synthesis of CTP is essential to PtdChosynthesis by the CDP-choline and phosphatidylethanolamine (PtdEtn)-methylation pathways.Elevated CTP synthesis favors increased use of the CDP-choline pathway. However, this might be detrimental to cell physiology. In yeast, synthesis of PtdCho by the CDP-cholinepathway is lethal in the absence of a functional Sec14p. Sec14p, a PtdIns/PtdCho transfer protein, is essential for cell viability and the formation of Golgi-derived secretoryvesicles. Sec14p could down-regulate choline-phosphate (choline-P) cytidylyltransferase ac-tivity, the rate-limiting step in the CDP-choline pathway37,38. PLD also plays an essential rolein the secretory pathway. Data indicate that the PA derived from the PLD-mediated turnoverof PtdCho mediates secretory-vesicle budding. CCT, choline-P cytidylyltransferase.

AS PART OF the immune response topathogenic invaders, several types ofvertebrate cell present proteolytic frag-ments of endocytosed foreign proteins inthe context of specialized peptide recep-tors, the major histocompatibility com-plex (MHC) class II molecules at the cellsurface1. The foreign peptide cargo can berecognized by T lymphocytes, which are

activated, produce cytokines and providehelp for activation and differentiation ofantibody-producing B cells.

For the generation of antigenic pep-tides from endocytosed proteins, MHC-class-II-positive antigen-presenting cells(APCs) generally exploit the conven-tional proteolytic system of endosomaland lysosomal organelles2. The lysosomalenvironment is characterized by low pH,reducing conditions and high proteolyticactivity, and thus provides optimal con-ditions for protein unfolding and degra-dation, thereby giving rise to peptides.However, in order to be loaded onto

MHC class II molecules, peptides mustfulfil some structural requirements. (1)They must possess certain amino acidside chains at so-called ‘anchor’ pos-itions; these side chains allow bindingto the groove in the MHC class II mol-ecule. This requirement is differentamong the 50–100 allelic variants of theMHC class II molecule, owing to the highdegree of variation in the antigenic-peptide-binding groove. (2) They mustbe at least 13–15 residues in length.Consequently, proteolysis of antigenicproteins has to be limited in order togenerate and preserve such peptides.

How can this be achieved in a com-partment that has an extremely highproteolytic activity that normally cleavesproteins to di- and tri-peptides and aminoacids? One possibility is that the pro-cessing compartments contain special-ized peptide shuttles that bind inter-mediary oligopeptides, prevent theirterminal degradation and deliver themto MHC class II molecules. In order toserve all types of MHC class II variants,the specificity of any such protectivemolecules would have to be rather broad.Repeatedly, investigators have postu-lated that such peptide-binding moleculesinterfere with antigen processing in theendocytic system3, but no proof of theirinvolvement has been obtained.

An alternative, and more direct, wayto capture antigenic fragments beforequantitative proteolysis is for MHC classII molecules themselves to be present inproteolytic compartments; this is in factthe case in APCs. Consequently, binding

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HLA-DM – an endosomal andlysosomal chaperone for the

immune system

Anne B. Vogt and Harald KropshoferMolecular chaperones are involved in a variety of cellular processes, in-cluding stabilization of newly synthesized polypeptide chains, assembly ofoligomers, transport of proteins and organelle biogenesis. They are knownto exert their activity in the cytosol, endoplasmic reticulum, mitochondriaand chloroplasts. HLA-DM is the first example of a molecular chaperonethat operates in lysosomes: it plays a crucial role in endosomal and lyso-somal compartments during loading of major histocompatibility complex(MHC) class II molecules, specialized peptide receptors that are expressedby antigen-presenting cells of the immune system.

A. B. Vogt and H. Kropshofer are at the Deptof Molecular Immunology, German CancerResearch Center, Im Neuenheimer Feld 280,D-69120 Heidelberg, Germany.Email: a.vogt@dkfz-heidelberg.de