The Debate about Transport in the Golgi—Two Sides of the Same Coin?
2000; Cell Press; Volume: 102; Issue: 6 Linguagem: Inglês
10.1016/s0092-8674(00)00060-x
ISSN1097-4172
AutoresHugh R.B. Pelham, James E. Rothman,
Tópico(s)Pancreatic function and diabetes
ResumoHistorical precedent in biology suggests that when divergent views are held on the basis of divergent observations that do not actually contradict each other, then the essence of both views is probably correct and the truth lies somewhere in between. The Golgi appears to be no exception. A vigorous controversy has evolved over the past five years concerning the nature of transport of secretory and other cargo across the Golgi stack: is it carried out by transport vesicles budding and fusing among static cisternae, or do the cisternae themselves move with no vesicles being involved? For various reasons, each of these mechanisms had proven difficult to establish or to rule out in simple and compelling ways. As a result there has been a considerable polarization of the field. Now, it now seems that the essence of both views is likely to be correct. Here, we review recent data that bring the two previous extreme views into accord while resolving the difficulties associated with each of them. While cisternal maturation can nicely explain rapid protein secretion in budding yeast, it operates too slowly in the tightly stacked Golgi of animal cells to explain the transport of most proteins, yet it is important for certain large macromolecular aggregates that may not be accommodated in vesicles. Transport vesicles provide the "fast track" taken across the Golgi by most proteins in animal cells. But, rather than moving uniquely forward (or backward), as originally envisioned by proponents of vesicle (or cisternal maturation) models, it now appears that transport vesicles may "percolate" up-and-down the stack in a kind of bidirectional random walk. The controversy about Golgi transport occurs against a background of well-established core molecular principles that explain how transport vesicles bud and fuse to carry out the fundamental cellular activity of intercompartmental transport (59Rothman J.E Wieland F.T Protein sorting by transport vesicles.Science. 1996; 272: 227-234Crossref PubMed Scopus (1010) Google Scholar, 40Mellman I Warren G The road taken past and future foundations of membrane traffic.Cell. 2000; 100: 99-112Abstract Full Text Full Text PDF PubMed Scopus (346) Google Scholar). Neither these molecular mechanisms nor, in particular, the physiological importance of vesicles for Golgi function are at issue. Indeed these vesicles are central to any discussion of the Golgi. Typically 70–90 nm in diameter, they bud from each of the 4–6 cisternae that comprise the Golgi stack in a typical mammalian cell, ranging from the cis face (where proteins enter the stack from the ER) to the opposite trans face of the stack (where proteins can exit for later compartments) (45Orci L Glick B.S Rothman J.E A new type of coated vesicular carrier that appears not to contain clathrin its possible role in protein transport within the Golgi stack.Cell. 1986; 46: 171-184Abstract Full Text PDF PubMed Scopus (328) Google Scholar, 47Orci L Stamnes M Ravazzola M Amherdt M Perrelet A Söllner T.H Rothman J.E Bidirectional transport by distinct populations of COPI-coated vesicles.Cell. 1997; 90: 335-349Abstract Full Text Full Text PDF PubMed Scopus (337) Google Scholar). They are named COPI vesicles because their budding is driven by the assembly of the COPI coat from cytoplasmically derived subunits (coatomers). The coatomers attach to Golgi membranes via the GTPase ARF1 (when it is in its GTP-bound form) and then polymerize into a COPI coat, forming an approximately spherical template that shapes the adherent membrane into a vesicle. When ARF hydrolyzes its bound GTP, the coatomers dissociate and the now uncoated vesicle can fuse when its now exposed v-SNAREs (packaged during budding) partner cognate t-SNAREs on the target membrane (Golgi or ER) (59Rothman J.E Wieland F.T Protein sorting by transport vesicles.Science. 1996; 272: 227-234Crossref PubMed Scopus (1010) Google Scholar, 5Barlowe C Traffic COPs of the early secretory pathway.Traffic. 2000; 1: 371-377Crossref PubMed Scopus (82) Google Scholar). Transport into the Golgi from the ER is carried out by a compositionally distinct coated vesicle, termed COPII, which also employs a GTP-switch mechanism for coating and uncoating (6Barlowe C Orci L Yeung T Hosobuchi M Hamamoto S Salama N Rexach M.F Ravazzola M Amherdt M Schekman R COPII a membrane coat formed by Sec proteins that drive vesicle budding from the endoplasmic reticulum.Cell. 1994; 77: 895-908Abstract Full Text PDF PubMed Scopus (996) Google Scholar, 64Springer S Spang A Schekman R A primer on vesicle budding.Cell. 1999; 97: 145-148Abstract Full Text Full Text PDF PubMed Scopus (220) Google Scholar, 5Barlowe C Traffic COPs of the early secretory pathway.Traffic. 2000; 1: 371-377Crossref PubMed Scopus (82) Google Scholar). COPII coats bud (or possibly tubulate) membranes containing secretory cargo from transitional zones of ER. The ER-derived vesicles uncoat and join to form or fuse with vesicular-tubular clusters (VTCs), also termed intermediate compartments (ICs). These large carriers then deliver their cargo to the cis face of the stack, either by maturing into or fusing with preexisting cis-Golgi membranes (4Bannykh S.I Balch W.E Membrane dynamics at the endoplasmic reticulum–Golgi interface.J. Cell Biol. 1997; 138: 1-4Crossref PubMed Scopus (191) Google Scholar, 54Presley J.F Cole N.B Schroer T.A Hirschberg K Zaal K.J Lippincott-Schwartz J ER-to-Golgi transport visualized in living cells.Nature. 1997; 389: 81-85Crossref PubMed Scopus (3) Google Scholar, 60Scales S.J Pepperkok R Kreis T.E Visualization of ER-to-Golgi transport in living cells reveals a sequential mode of action for COPII and COPI.Cell. 1997; 90: 1137-1148Abstract Full Text Full Text PDF PubMed Scopus (400) Google Scholar, 71Wooding S Pelham H.R The dynamics of golgi protein traffic visualized in living yeast cells.Mol. Biol. 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These same cargo depart the stack from the trans face, now sorted into a series of distinct carriers for delivery to one or another target membrane. Within the stack the cargo exists as an unfractionated mixture, present in every cisterna at essentially the same concentration. Resident proteins of the ER constitute a special class of protein that can also enter the Golgi along with cargo, but when they do they have a different fate. Residents of the lumen of the ER are invariably marked via a common signal peptide (KDEL) (44Munro S Pelham H.R.B A C-terminal signal prevents secretion of luminal ER proteins.Cell. 1987; 48: 899-907Abstract Full Text PDF PubMed Scopus (1514) Google Scholar). This insight led to the discovery of retrograde transport, the process by which these escaped ER resident proteins are returned from the Golgi to the ER (33Lewis M.J Sweet D.J Pelham H.R The ERD2 gene determines the specificity of the luminal ER protein retention system.Cell. 1990; 61: 1359-1363Abstract Full Text PDF PubMed Scopus (157) Google Scholar, 61Semenza J.C Hardwick K.G Dean N Pelham H.R ERD2, a yeast gene required for the receptor-mediated retrieval of luminal ER proteins from the secretory pathway.Cell. 1990; 61: 1349-1357Abstract Full Text PDF PubMed Scopus (374) Google Scholar). A KDEL-specific receptor awaits its ligand in the Golgi where it sequesters it into transport vesicles targeted to the ER. The ER and Golgi are marked with distinct t-SNAREs, containing Ufe1p (Syntaxin 18 in animals; 34Lewis M.J Rayner J.C Pelham H.R A novel SNARE complex implicated in vesicle fusion with the endoplasmic reticulum.EMBO J. 1997; 16: 3017-3024Crossref PubMed Scopus (119) Google Scholar, 26Hatsuzawa K Hirose H Tani K Yamamoto A Scheller R.H Tagaya M Syntaxin 18, a SNAP receptor that functions in the endoplasmic reticulum, intermediate compartment, and cis-Golgi vesicle trafficking.J. Biol. Chem. 2000; 275: 13713-13720Crossref PubMed Scopus (84) Google Scholar) and Sed5p (Syntaxin 5 in animals; 23Hardwick K Pelham H.R.B SED5 encodes a 39-kD integral membrane protein required for vesicular transport between the ER and the Golgi complex.J. Cell Biol. 1992; 119: 513-521Crossref PubMed Scopus (250) Google Scholar, 9Bennett M.K Garcia-Arraras J.E Elferink L.A Peterson K Fleming A.M Hazuka C.D Scheller R.H The syntaxin family of vesicular transport receptors.Cell. 1993; 74: 863-873Abstract Full Text PDF PubMed Scopus (579) Google Scholar), respectively, to allow anterograde and retrograde pathways to operate between the ER and the Golgi without interference. Two related questions lie at the center of the current debate (Rothman and Wieland, 1996; 18Glick B.S Malhotra V The curious status of the Golgi apparatus.Cell. 1998; 95: 883-889Abstract Full Text Full Text PDF PubMed Scopus (185) Google Scholar, 53Pelham H.R Getting through the Golgi complex.Trends Cell Biol. 1998; 8: 45-49Abstract Full Text PDF PubMed Scopus (98) Google Scholar) about how proteins are transported in the Golgi: (1) Do COPI vesicles mediate anterograde (i.e., cis-to-trans) transport? (it is clear that COPI vesicles mediate retrograde transport from Golgi to ER); Or (2), is anterograde transport instead mainly due to en bloc movement of entire cisternae (i.e., cisternal progression) up the stack? When this controversy arose in 1994, it had been widely accepted for about a decade that vesicles (3Balch W.E Glick B.S Rothman J.E Sequential intermediates in the pathway of intercompartmental transport in a cell-free system.Cell. 1984; 39 (b): 525-536Abstract Full Text PDF PubMed Scopus (119) Google Scholar, 17Farquhar M.G Progress in unraveling pathways of Golgi traffic.Annu. Rev. Cell Biol. 1985; 1: 447-488Crossref PubMed Scopus (390) Google Scholar)—and in particular COPI vesicles (45Orci L Glick B.S Rothman J.E A new type of coated vesicular carrier that appears not to contain clathrin its possible role in protein transport within the Golgi stack.Cell. 1986; 46: 171-184Abstract Full Text PDF PubMed Scopus (328) Google Scholar)—mediate anterograde transport. Even in the earliest studies of COPI vesicles (45Orci L Glick B.S Rothman J.E A new type of coated vesicular carrier that appears not to contain clathrin its possible role in protein transport within the Golgi stack.Cell. 1986; 46: 171-184Abstract Full Text PDF PubMed Scopus (328) Google Scholar) it was noted that about half of the COPI vesicles budding from Golgi cisternae contain the VSV G protein, a viral-encoded cargo protein that is targeted to the plasma membrane, suggesting that these vesicles carry the cargo across the Golgi stack. The event that precipitated the current controversy was the unexpected result from yeast genetics that COPI vesicles mediate retrograde transport from the Golgi to the ER (15Cosson P Letourneur F Coatomer interaction with di-lysine endoplasmic reticulum retention motifs.Science. 1994; 263: 1629-1631Crossref PubMed Scopus (472) Google Scholar, 31Letourneur F Gaynor E.C Hennecke S Demolliere C Duden R Emr S.D Riezman H Cosson P Coatomer is essential for retrieval of dilysine-tagged proteins to the endoplasmic reticulum.Cell. 1994; 79: 1199-1207Abstract Full Text PDF PubMed Scopus (646) Google Scholar). As COPI vesicles are the only vesicles observed to bud within the confines of the Golgi stack, how could the same vesicle (assuming all COPI vesicles are functionally equivalent) carry ER residents backward while also carrying different cargo forward (52Pelham H.R About turn for the COPs?.Cell. 1994; 79: 1125-1127Abstract Full Text PDF PubMed Scopus (106) Google Scholar)? A new possibility thereby emerged (really, an old possibility resurrected): there is no vesicle for anterograde (cis-to-trans) transport. The alternative, moving cisternae, is a model originally derived largely from electron microscopic studies of plants and algae (7Beams H.W Kessel R.G The Golgi apparatus structure and function.Int. Rev. Cytol. 1968; 23: 209-276Crossref PubMed Scopus (228) Google Scholar, 8Becker B Melkonian M The secretory pathway of protists spatial and functional organization and evolution.Microbiol. Rev. 1996; 60: 697-721PubMed Google Scholar). Cisternal "progression" had been in favor from the 1960s up to the 1980s when COPI vesicles were identified and shown to contain cargo. Cisternal progression, or flow, requires that cisternae form de novo at the cis face and disassemble at the trans face. More recently, it was recognized that for the Golgi to retain its complement of designated residents (including processing enzymes like glycosyltransferases) in the face of the continuous loss from the trans face that a cisternal progression would create, transport vesicles operating within the Golgi must be postulated to carry such proteins retrograde from later to earlier cisternae. As a result, new cisternae forming at the cis face from fusing COPII vesicles and/or VTCs would "mature" as they progress across the stack: gaining glycosyltransferases, losing ER residents by retrograde transport, and retaining cargo. This new and critical aspect of the cisternal progression model is encapsulated in the currently preferred term, cisternal progression/maturation (19Glick B.S Elston T Oster G A cisternal maturation mechanism can explain the asymmetry of the Golgi stack.FEBS Lett. 1997; 414: 177-181Abstract Full Text PDF PubMed Scopus (123) Google Scholar, 18Glick B.S Malhotra V The curious status of the Golgi apparatus.Cell. 1998; 95: 883-889Abstract Full Text Full Text PDF PubMed Scopus (185) Google Scholar, 53Pelham H.R Getting through the Golgi complex.Trends Cell Biol. 1998; 8: 45-49Abstract Full Text PDF PubMed Scopus (98) Google Scholar). The requirement for retrograde transport (both within the stack and from the stack to the ER) provides a compelling and complete role for COPI vesicles in this model. But, what about the observation that many COPI vesicles budding from isolated Golgi contain cargo and can carry them between stacks (46Orci L Malhotra V Amherdt M Serafini T Rothman J.E Dissection of a single round of vesicular transport sequential intermediates for intercisternal movement in the Golgi stack.Cell. 1989; 56: 357-368Abstract Full Text PDF PubMed Scopus (178) Google Scholar, 51Ostermann J Orci L Tani K Amherdt M Ravazzola M Elazar Z Rothman J.E Stepwise assembly of functionally active transport vesicles.Cell. 1993; 75: 1015-1025Abstract Full Text PDF PubMed Scopus (228) Google Scholar)? Ardent "progressionists" took the view that this was either an in vitro artifact of some kind, or the result of the spilling of abundant anterograde-moving cargo into backward-moving vesicles, although the purpose of this was not apparent. Equally ardent vesicle "secessionists" regarded this as a COP-out; they insisted that there was no direct evidence that cisternae move at all. This debate was focused recently by two principal observations. First, immunoelectron microscopy of intact cells (47Orci L Stamnes M Ravazzola M Amherdt M Perrelet A Söllner T.H Rothman J.E Bidirectional transport by distinct populations of COPI-coated vesicles.Cell. 1997; 90: 335-349Abstract Full Text Full Text PDF PubMed Scopus (337) Google Scholar) revealed that there are actually two distinct populations of COPI vesicles: one population contains the KDEL receptor but little if any cargo; the other contains cargo (VSV G protein or proinsulin) but little if any KDEL receptor. Both types bud throughout the Golgi stack. While it could still not be formally ruled out that both types of COPI vesicles move exclusively backward, it became very hard to see how cargo could accidentally spill-over into one but not the other vesicle population. And if the cargo-containing COPI vesicles move anterograde—even to a limited extent—then a cisternal progression would no longer be required to explain cargo transport (but could occur in parallel). Second, in a quantitative pulse–chase experiment employing electron microscopy, it was definitively shown that biosynthetic aggregates (collagen precursors) much larger than COPI vesicles nonetheless traverse the Golgi stack in animal cells (11Bonfanti L Mironov Jr., A.A Martinez-Menarguez J.A Martella O Fusella A Baldassarre M Buccione R Geuze H.J Mironov A.A Luini A Procollagen traverses the Golgi stack without leaving the lumen of cisternae evidence for cisternal maturation.Cell. 1998; 95: 993-1003Abstract Full Text Full Text PDF PubMed Scopus (307) Google Scholar). Since the aggregates could not fit in COPI vesicles, they must be carried in the anterograde direction by a different mechanism. Since serial sections indicated that the aggregates were retained within cisternae—as distinct from being enclosed in some sort of "megavesicle"—this result constituted compelling evidence for cisternal flow. However, the rate of aggregate movement was far slower than that of most proteins—and even bulk lipids—studied in other cells. These cross the stack in 5–15 min, while fully 20%–25% of the aggregates originally in the cis half of the stack remain there even after 1 hr, hinting that a COPI vesicle pathway may coexist and provide an even faster track for anterograde transport than cisternal flow. In sum, two alternative models for anterograde transport were widely considered, each with its own apparent problems: Cisternal progression/maturation naturally explained the transport of aggregates and the fixed concentration of cargo throughout the stack. While it provided a compelling role for COPI vesicles in retrograde transport, it did not provide a compelling explanation for the existence of a population of COPI vesicles carrying the cargo that exits the trans face of the stack. Moreover, the rate of transport of aggregates across the stack was substantially slower than that reported for most cargo proteins. Vesicle transport by a specialized population of COPI vesicles provided a compelling explanation for the existence of two types of COPI vesicles, but did not explain how aggregates much larger than these vesicles could be transported. It was also mechanistically unclear how two sets of cargo could be packaged in separate COPI vesicles that bud using the same coat protein. Moreover, selective vesicles moving exclusively anterograde would be expected to generate a cis < trans concentration gradient of their cargo, and this is not observed. None of the data favoring cisternal progression/maturation explicitly exclude the possibility of simultaneous vesicle transport, and vice versa. In fact, during the last year or so, new information has emerged that constrains both models away from their previous extremes and suggests that both may operate. In contrast to the natural procollagen aggregates, artificial protein aggregates (formed by tandem repeats of a spontaneously dimerizing mutant of the FK506 binding protein), which accumulate in the cis-most cisternae during a 15 degree temperature block, transit the stack on a fast track when the block is released (69Volchuk A Amherdt M Ravazzola M Brugger B Rivera V.M Clackson T Perrelet A Söllner T.H Rothman J.E Orci L Megavesicles implicated in the rapid transport of intracisternal aggregates across the Golgi stack.Cell. 2000; 102: 335-348Abstract Full Text Full Text PDF PubMed Scopus (89) Google Scholar). And unlike the procollagen aggregates, the artificial aggregates actually become enveloped into megavesicles, possibly drastic size variants of COPI vesicles, which then mediate their anterograde transport (69Volchuk A Amherdt M Ravazzola M Brugger B Rivera V.M Clackson T Perrelet A Söllner T.H Rothman J.E Orci L Megavesicles implicated in the rapid transport of intracisternal aggregates across the Golgi stack.Cell. 2000; 102: 335-348Abstract Full Text Full Text PDF PubMed Scopus (89) Google Scholar). The studies of 11Bonfanti L Mironov Jr., A.A Martinez-Menarguez J.A Martella O Fusella A Baldassarre M Buccione R Geuze H.J Mironov A.A Luini A Procollagen traverses the Golgi stack without leaving the lumen of cisternae evidence for cisternal maturation.Cell. 1998; 95: 993-1003Abstract Full Text Full Text PDF PubMed Scopus (307) Google Scholar and 69Volchuk A Amherdt M Ravazzola M Brugger B Rivera V.M Clackson T Perrelet A Söllner T.H Rothman J.E Orci L Megavesicles implicated in the rapid transport of intracisternal aggregates across the Golgi stack.Cell. 2000; 102: 335-348Abstract Full Text Full Text PDF PubMed Scopus (89) Google Scholar are complementary in that they suggest that when aggregates can enter megavesicles they will be on the same fast track as COPI vesicles, but when they cannot enter any vesicle they will transit at the slower pace of cisternal progression. An initial comparison (presented in a preliminary form; 10Bonfanti L Martella O Miranov A Luini A Comparative analysis of Procollagen I and VSV G protein in the same cell.Mol. Biol. Cell (Suppl.). 1999; 10: 114aGoogle Scholar) of the rate of transport of the VSV G protein with that of procollagen aggregates in the same Golgi confirms the expectation from the published work (11Bonfanti L Mironov Jr., A.A Martinez-Menarguez J.A Martella O Fusella A Baldassarre M Buccione R Geuze H.J Mironov A.A Luini A Procollagen traverses the Golgi stack without leaving the lumen of cisternae evidence for cisternal maturation.Cell. 1998; 95: 993-1003Abstract Full Text Full Text PDF PubMed Scopus (307) Google Scholar) that procollagen aggregates traverse on a "slow track" as compared to other cargo, establishing the pace of cisternal progression. The more rapid rate of transport of most cargo must then be due to movement between cisternae, which we presume involves the COPI population containing these same cargo, which would then have to fuse in the anterograde direction at least some of the time. Immunoelectron microscopy of intact cells (49Orci L Amherdt M Ravazzola M Perrelet A Rothman J.E Exclusion of Golgi residents from transport vesicles budding from Golgi cisternae in intact cells.J. Cell Biol. 2000; in press (a)Google Scholar) and cell-free budding reactions (62Sönnichsen B Watson R Clausen H Misteli T Warren G Sorting by COPI-coated vesicles under interphase and mitotic conditions.J. Cell Biol. 1996; 134: 1411-1415Crossref PubMed Scopus (87) Google Scholar) reveals that while steady-state Golgi residents like glycosyltransferases are present in COPI vesicles, they are at a lower concentration than in the cisternae from which the vesicles bud. If cisternal flow were the only means of anterograde transport, then the transferases and other residents would need to be substantially concentrated in the retrograde-moving COPI vesicles to allow cargo to remain with a nonrecycled portion of cisternal membrane which can depart the trans face for post-Golgi locations (19Glick B.S Elston T Oster G A cisternal maturation mechanism can explain the asymmetry of the Golgi stack.FEBS Lett. 1997; 414: 177-181Abstract Full Text PDF PubMed Scopus (123) Google Scholar). This constraint relaxes when cisternal flow is slower than most anterograde vesicular transport. Therefore, the lack of enrichment of resident proteins independently suggests the existence of anterograde vesicular traffic. One study (30Lanoix J Ouwendijk J Lin C.C Stark A Love H.D Ostermann J Nilsson T GTP hydrolysis by arf-1 mediates sorting and concentration of Golgi resident enzymes into functional COP I vesicles.EMBO J. 1999; 18: 4935-4948Crossref PubMed Scopus (175) Google Scholar) has claimed up to 10-fold enrichment of glycosyltransferases in putative COPI vesicles produced in vitro, but this figure can be challenged because it depends critically on the nature and purity of the membranes from which the vesicles bud. Since at least 90% of the proteins in Golgi cisternae are residents (55Quinn P Griffiths G Warren G Density of newly synthesized plasma membrane proteins in intracellular membranes II. Biochemical studies.J. Cell Biol. 1984; 98: 2142-2147Crossref PubMed Scopus (115) Google Scholar), it is hard to imagine a mechanism by which much further concentration would be possible. There is mounting evidence that distinct biochemical mechanisms can be used to create COPI vesicles of different composition, even from the same membrane. On one hand, the KDEL receptor oligomerizes and binds the GTPase-activating protein ARFGAP (1Aoe T Cukierman E Lee A Cassel D Peters P.J Hsu V.W The KDEL receptor, ERD2, regulates intracellular traffic by recruiting a GTPase-activating protein for ARF1.EMBO J. 1997; 16: 7305-7316Crossref PubMed Scopus (143) Google Scholar), which in turn could provide a foothold for both ARF and coatomer subunits. This active role of the KDEL receptor may contribute directly to Golgi function, explaining why it is needed for this (at least in yeast; 61Semenza J.C Hardwick K.G Dean N Pelham H.R ERD2, a yeast gene required for the receptor-mediated retrieval of luminal ER proteins from the secretory pathway.Cell. 1990; 61: 1349-1357Abstract Full Text PDF PubMed Scopus (374) Google Scholar). 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While other genetic (65Springer S Chen E Duden R Marzioch M Rowley A Hamamoto S Merchant S Schekman R The p24 proteins are not essential for vesicular transport in Saccharomyces cerevisiae.Proc. Natl. Acad. Sci USA. 2000; 97: 4034-4039Crossref PubMed Scopus (93) Google Scholar) and biochemical evidence (68Szafer E Pick E Rotman M Zuck S Huber I Cassel D Role of coatomer and phospholipids in GTPase-activating protein-dependent hydrolysis of GTP by ADP-ribosylation factor-1.J. Cell Biol. 2000; 275: 23615-23619Scopus (57) Google Scholar) suggests that the picture will be complex and that much remains to be learned, the striking biochemical results noted above give clues as to how the sorting of proteins into two parallel sets of COPI vesicles—those destined ultimately for the ER and those restricted to intra-Golgi movement—might be achieved according to kinetic mechanisms. COPI vesicles containing cargo have been found to possess their own unique SNARE, GOS28, which is undetectable in the other, KDEL receptor–containing population of vesicles (50Orci L Ravazzola M Volchuk A Engel T Gmachl M Amherdt M Perrelet A Söllner T.H Rothman J.E Anterograde flow of cargo across the Golgi stack potentially mediated via bidirectional "percolating" COPI vesicles.Proc. Natl. Acad. Sci. USA. 2000; in press (b)Google Scholar). GOS28 and the syntaxin with which it participates in a SNARE complex, Syntaxin5 (Sed5p in yeast), are both approximately evenly distributed throughout the entire stack (27Hay J.C Klumperman J Oorschot V Steegmaier M Kuo C.S Scheller R.H Localization, dynamics, and protein interactions reveal distinct roles for ER and Golgi SNAREs.J. Cell Biol. 1998; 141 (Erratum: J. Cell Biol. 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