The Curious Status of the Golgi Apparatus
1998; Cell Press; Volume: 95; Issue: 7 Linguagem: Inglês
10.1016/s0092-8674(00)81713-4
ISSN1097-4172
AutoresBenjamin S. Glick, Vivek Malhotra,
Tópico(s)Lipid Membrane Structure and Behavior
ResumoAn organelle is a membrane-bounded intracellular structure with a unique molecular composition and function. Where do organelles come from? Most biologists would state that each organelle forms by the growth and division of a preexisting organelle of the same type. This view has dominated cell biology for decades (46Palade G.E Membrane biogenesis an overview.Methods Enzymol. 1983; 96: 29-55Google Scholar, 69Warren G Wickner W Organelle inheritance.Cell. 1996; 84: 395-400Abstract Full Text Full Text PDF PubMed Scopus (220) Google Scholar). However, recent evidence suggests that only a subset of the organelles fit this picture, and that a different perspective is needed. The nucleus matches this standard description of an organelle. Certain protein and nucleic acid species are found exclusively in the nucleus, and the entire contents of the nucleus double and divide with each cell cycle. A similar pattern is seen with mitochondria and chloroplasts (6Birky Jr., C.W The partitioning of cytoplasmic organelles at cell division.Int. Rev. Cytol. [Suppl.]. 1983; 15: 49-89PubMed Google Scholar). This simple paradigm is often extended to include all of the organelles in the cell. Many organelles consist of multiple compartments, and each compartment is also viewed as a permanent structure with a unique composition. However, the standard view of organelle biogenesis is being challenged by studies of the endomembrane system, which comprises the organelles of the exocytic and endocytic pathways. These organelles exchange material by means of transport vesicles (56Rothman J.E Wieland F.T Protein sorting by transport vesicles.Science. 1996; 272: 227-234Crossref PubMed Scopus (1010) Google Scholar). Such vesicles selectively incorporate a subset of the proteins and lipids present in a donor compartment, and then deliver their contents by fusing with an acceptor compartment. As a result, proteins of the endomembrane system are rarely localized to a single compartment (39Mellman I Simons K The Golgi complex in vitro veritas?.Cell. 1992; 68: 829-840Abstract Full Text PDF PubMed Scopus (377) Google Scholar). While this point is widely appreciated, vesicular transport also has deeper implications (24Helenius A Mellman I Wall D Hubbard A Endosomes.Trends Biochem. Sci. 1983; 8: 245-250Abstract Full Text PDF Scopus (376) Google Scholar). For example, what would happen if vesicles were to fuse not with a preexisting acceptor compartment, but with one another? As shown in Figure 1A, the result would be a membrane-bounded structure with a unique composition (i.e., a new compartment). The properties of this compartment could then be modified by subsequent vesicular transport events. By producing and/or receiving vesicles, the compartment could undergo compositional changes in a process termed "maturation" (Figure 1B). By fragmenting into transport vesicles, the compartment could simply disappear (Figure 1C). This picture profoundly affects the way we think about the endomembrane system. Some compartments may be definable only as intermediates in a continuous process of transformation (24Helenius A Mellman I Wall D Hubbard A Endosomes.Trends Biochem. Sci. 1983; 8: 245-250Abstract Full Text PDF Scopus (376) Google Scholar). Instead of regarding compartments as separate entities, we are forced to view them as part of an integrated whole. The question becomes: assuming that compartments can form de novo, then mature, then disappear, how does the cell maintain a stable endomembrane system? And what mechanisms allow this system to undergo reversible changes during growth and differentiation? To explore these issues, we will focus our discussion on the Golgi apparatus. The Golgi is typically represented as a series of stable compartments, with transport vesicles serving to carry the secretory cargo forward from one compartment to the next. By contrast, early morphologists proposed that Golgi compartments are transitory structures (4Beams H.W Kessel R.G The Golgi apparatus structure and function.Int. Rev. Cytol. 1968; 23: 209-276Crossref PubMed Scopus (228) Google Scholar). New cisternae appeared to be forming at the cis-face of the Golgi stack by the coalescence of ER-derived membranes, while cisternae of the trans-Golgi network (TGN) appeared to be fragmenting into secretory vesicles. The implication was that cisternae move progressively across the stack from the cis- to the trans-face. Some of the strongest evidence for this "cisternal progression" model came from analyzing cell-surface scales in algae (5Becker B Melkonian M The secretory pathway of protists spatial and functional organization and evolution.Microbiol. Rev. 1996; 60: 697-721PubMed Google Scholar). These scales are large electron-dense glycoconjugates that are synthesized within Golgi stacks. Scales are not seen in the vesicles that bud from the rims of Golgi cisternae, and in some species the scales can be more than 20 times the size of a typical transport vesicle. These observations suggested that scales are carried through the Golgi stack by the cisternae themselves. Why were the images of algal scales not taken as conclusive evidence for cisternal progression? One alternative view is that algae have evolved a unique mechanism for intra-Golgi transport (15Farquhar M.G Palade G.E The Golgi apparatus (complex) - (1954–1981) - from artifact to center stage.J. Cell Biol. 1981; 91: 77s-103sCrossref PubMed Google Scholar). This explanation seems unlikely, because various animal cells and higher plant cells also transport large electron-dense structures through the Golgi stack, and these structures also appear to be excluded from Golgi-associated vesicles (8Clermont Y Xia L Rambourg A Turner J.D Hermo L Transport of casein submicelles and formation of secretion granules in the Golgi apparatus of epithelial cells of the lactating mammary gland of the rat.Anat. Rec. 1993; 235: 363-373Crossref PubMed Scopus (61) Google Scholar, 58Schnepf E Golgi apparatus and slime secretion in plants the early implications and recent models of membrane traffic.Protoplasma. 1993; 172: 3-11Crossref Scopus (34) Google Scholar). In some cells the secretory cargoes form aggregates that induce large distensions of the Golgi cisternae (41Mollenhauer H.H Morré D.J Perspectives on Golgi apparatus form and function.J. Electron Microsc. Tech. 1991; 17: 2-14Crossref PubMed Scopus (57) Google Scholar). Unfortunately, most of these model systems share the problem that the secretory cargoes are poorly characterized (57Schekman R Mellman I Does COPI go both ways?.Cell. 1997; 90: 197-200Abstract Full Text Full Text PDF PubMed Scopus (72) Google Scholar). For instance, it is conceivable that an immature algal scale can disassemble into subunits, which are carried in transport vesicles to the next cisterna and then reassembled. Another problem is that Golgi organization is ambiguous in many cell types. Thus, one could argue that in an algal Golgi apparatus, all of the scale-containing cisternae actually represent an extended TGN compartment, and that no progressive transport occurs through this portion of the stack (39Mellman I Simons K The Golgi complex in vitro veritas?.Cell. 1992; 68: 829-840Abstract Full Text PDF PubMed Scopus (377) Google Scholar). A new paper by 7Bonfanti L Mironov Jr., A.A Martìnez-Menàrguez J 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 (this issue): 993-1003Abstract Full Text Full Text PDF PubMed Scopus (307) Google Scholar( [this issue of Cell]) provides the best evidence to date for cisternal progression. The authors chose to work with collagen-secreting vertebrate fibroblasts because the biochemistry of collagen synthesis is well understood. Procollagen fibers are stable triple helices about 300 nm long; once formed, these fibers almost certainly do not disassemble. Aggregates of procollagen fibers appear as electron-dense swellings of the Golgi cisternae (31Leblond C.P Synthesis and secretion of collagen by cells of connective tissue, bone and dentin.Anat. Rec. 1989; 224: 123-138Crossref PubMed Scopus (91) Google Scholar). By using a drug that specifically and reversibly inhibits the exit of procollagen from the ER, 7Bonfanti L Mironov Jr., A.A Martìnez-Menàrguez J 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 (this issue): 993-1003Abstract Full Text Full Text PDF PubMed Scopus (307) Google Scholar could document that procollagen fibers move through the entire Golgi stack from the cis- to the trans-face. A careful morphological and immunocytochemical analysis documented that procollagen remains within Golgi cisternae throughout this transport process. The combined data argue convincingly that procollagen transits through the Golgi by cisternal progression. This conclusion is extremely important, because for the past 15–20 years, most Golgi researchers have considered the cisternal progression idea to be defunct. The cisternal progression model had been discarded for three main reasons (14Farquhar M.G Progress in unraveling pathways of Golgi traffic.Annu. Rev. Cell Biol. 1985; 1: 447-488Crossref PubMed Scopus (390) Google Scholar). First, it failed to explain why vesicles are associated with the rims of Golgi cisternae. Second, whereas secretory cargo molecules pass through the Golgi stack in minutes, resident Golgi proteins often have a half-life of many hours. If the cisternae were simply moving through the stack in a "conveyor belt" fashion, Golgi proteins should be swept out of the organelle at the same rate as the secretory cargo. Third, Golgi stacks are polarized, with certain resident proteins being concentrated in cis-cisternae and others in medial- or trans-cisternae. It was unclear how a cisternal progression mechanism could preserve this polarity. These considerations led to the stable compartments model for the Golgi (12Dunphy W.G Rothman J.E Compartmental organization of the Golgi stack.Cell. 1985; 42: 13-21Abstract Full Text PDF PubMed Scopus (275) Google Scholar, 14Farquhar M.G Progress in unraveling pathways of Golgi traffic.Annu. Rev. Cell Biol. 1985; 1: 447-488Crossref PubMed Scopus (390) Google Scholar). In this view, different Golgi proteins are anchored in distinct compartments, and secretory cargo molecules are carried through the stack in anterograde vesicles. Support for the stable compartments notion came from the groundbreaking work of Rothman and colleagues, who reconstituted intra-Golgi transport in a cell-free system and then used this system to characterize a novel class of transport vesicles (55Rothman J.E Mechanisms of intracellular protein transport.Nature. 1994; 372: 55-63Crossref PubMed Scopus (1965) Google Scholar). These intra-Golgi transport vesicles contain a coat protein complex known as COPI; a second coat protein complex, termed COPII, was subsequently identified on ER-derived transport vesicles (56Rothman J.E Wieland F.T Protein sorting by transport vesicles.Science. 1996; 272: 227-234Crossref PubMed Scopus (1010) Google Scholar, 30Kuehn M.J Schekman R COPII and secretory cargo capture into transport vesicles.Curr. Opin. Cell Biol. 1997; 9: 477-483Crossref PubMed Scopus (105) Google Scholar). But the stable compartments model is now showing signs of strain. For one thing, resident Golgi proteins are not exclusively localized to specific compartments, as initially believed (39Mellman I Simons K The Golgi complex in vitro veritas?.Cell. 1992; 68: 829-840Abstract Full Text PDF PubMed Scopus (377) Google Scholar, 67Velasco A Hendricks L Moremen K.W Tulsiani D.R.P Touster O Farquhar M.G Cell type-dependent variations in the subcellular distribution of a-mannosidase I and II.J. Cell Biol. 1993; 122: 39-51Crossref PubMed Scopus (272) Google Scholar, 50Rabouille C Hui N Hunte F Kieckbusch R Berger E.G Warren G Nilsson T Mapping the distribution of Golgi enzymes involved in the construction of complex oligosaccharides.J. Cell Sci. 1995; 108: 1617-1627Crossref PubMed Google Scholar, 54Roth J Topology of glycosylation in the Golgi apparatus.in: Berger E.G Roth J The Golgi Apparatus. Birkhäuser Verlag, Basel1997Google Scholar). Moreover, individual Golgi proteins are not immobilized in the cisternae, but they can diffuse rapidly within the plane of the membrane (10Cole N.B Smith C.L Sciaky N Terasaki M Edidin M Lippincott-Schwartz J Diffusional mobility of Golgi proteins in membranes of living cells.Science. 1996; 273 (b): 797-801Crossref PubMed Scopus (393) Google Scholar) and can also travel between different portions of the stack (22Harris S.L Waters M.G Localization of a yeast early Golgi mannosyltransferase, Och1p, involves retrograde transport.J. Cell Biol. 1996; 132: 985-998Crossref PubMed Scopus (120) Google Scholar, 33Linstedt A.D Mehta A Suhan J Reggio H Hauri H.-P Sequence and overexpression of GPP130/GIMPc evidence for saturable pH-sensitive targeting of a type II early Golgi membrane protein.Mol. Biol. Cell. 1997; 8: 1073-1087Crossref PubMed Scopus (98) Google Scholar). Finally, some morphologists have continued to insist that cisternal progression must be occurring, at least in some cell types (41Mollenhauer H.H Morré D.J Perspectives on Golgi apparatus form and function.J. Electron Microsc. Tech. 1991; 17: 2-14Crossref PubMed Scopus (57) Google Scholar, 8Clermont Y Xia L Rambourg A Turner J.D Hermo L Transport of casein submicelles and formation of secretion granules in the Golgi apparatus of epithelial cells of the lactating mammary gland of the rat.Anat. Rec. 1993; 235: 363-373Crossref PubMed Scopus (61) Google Scholar, 5Becker B Melkonian M The secretory pathway of protists spatial and functional organization and evolution.Microbiol. Rev. 1996; 60: 697-721PubMed Google Scholar). With the present study by 7Bonfanti L Mironov Jr., A.A Martìnez-Menàrguez J 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 (this issue): 993-1003Abstract Full Text Full Text PDF PubMed Scopus (307) Google Scholar, these morphological arguments have become impossible to ignore. The apparently conflicting results can be accomodated by a unified model that incorporates both cisternal progression and vesicular transport. According to this "cisternal maturation" model, the secretory cargo is carried forward by cisternal progression, while COPI vesicles travel in the retrograde direction to recycle resident Golgi proteins (Figure 2) (58Schnepf E Golgi apparatus and slime secretion in plants the early implications and recent models of membrane traffic.Protoplasma. 1993; 172: 3-11Crossref Scopus (34) Google Scholar, 2Bannykh 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, 40Mironov A.A Weidman P Luini A Variations on the intracellular transport theme maturing cisternae and trafficking tubules.J. Cell Biol. 1997; 138: 481-484Crossref PubMed Scopus (129) Google Scholar, 48Pelham H.R.B Getting through the Golgi complex.Trends Cell Biol. 1998; 8: 45-49Abstract Full Text PDF PubMed Scopus (98) Google Scholar). As a cisterna progresses through the stack, it matures by exporting "early" Golgi proteins to younger cisternae while receiving "late" Golgi proteins from older cisternae. How does this model account for the polarity of the Golgi stack? If Golgi proteins compete with one another for incorporation into COPI vesicles, the strongest competitors will be recycled efficiently from cis-cisternae, whereas weaker competitors will only be recycled efficiently from medial- or trans-cisternae; as a result, each Golgi protein will be concentrated in the part of the stack at which retrograde vesicular transport of this protein balances cisternal progression (18Glick 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). The basis of this postulated competition is unknown, but it might involve differential partitioning of Golgi proteins into membrane subdomains or differences in the oligomerization state of Golgi proteins (16Füllekrug J Nilsson T Protein sorting in the Golgi complex.Biochim. Biophys. Acta. 1998; 1404: 77-84Crossref PubMed Scopus (34) Google Scholar, 42Munro S Localization of proteins to the Golgi apparatus.Trends Cell Biol. 1998; 8: 11-15Abstract Full Text PDF PubMed Scopus (215) Google Scholar). Although the work of 7Bonfanti L Mironov Jr., A.A Martìnez-Menàrguez J 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 (this issue): 993-1003Abstract Full Text Full Text PDF PubMed Scopus (307) Google Scholar provides compelling evidence for cisternal progression, the maturation of individual cisternae has not been directly demonstrated. It may soon be feasible to observe cisternal maturation in vivo, by labeling an early and a late Golgi marker with distinct spectral variants of the green fluorescent protein (72Wooding S Pelham H.R.B The dynamics of Golgi protein traffic visualized in living yeast cells.Mol. Biol. Cell. 1998; 9: 2667-2680Crossref PubMed Scopus (156) Google Scholar). A given cisterna should then change color as it matures. Such an experiment will require a method for following individual cisternae by time-lapse fluorescence microscopy. Assuming that the Golgi does operate by vesicle-mediated cisternal maturation, a host of novel questions arise. To highlight these questions, it is useful to consider the various compartments of the ER-Golgi system in turn and to examine the current state of knowledge about each of these compartments. According to the maturation model, the first step in Golgi biogenesis is the formation of ER-derived COPII transport vesicles (30Kuehn M.J Schekman R COPII and secretory cargo capture into transport vesicles.Curr. Opin. Cell Biol. 1997; 9: 477-483Crossref PubMed Scopus (105) Google Scholar, 3Barlowe C COPII and selective export from the endoplasmic reticulum.Biochim. Biophys. Acta. 1998; 1404: 67-76Crossref PubMed Scopus (99) Google Scholar). These vesicles bud from specific ER regions known as transitional ER (tER) sites or ER exit sites (45Palade G Intracellular aspects of the process of protein synthesis.Science. 1975; 189: 347-358Crossref PubMed Scopus (2267) Google Scholar, 2Bannykh 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). Although COPII vesicle formation is being intensively studied, nothing is known about how vesicle budding is restricted to tER sites. Interestingly, Golgi stacks in many cell types are directly apposed to tER sites (71Whaley W.G The Golgi Apparatus. Springer-Verlag, Vienna1975Crossref Google Scholar, 15Farquhar M.G Palade G.E The Golgi apparatus (complex) - (1954–1981) - from artifact to center stage.J. Cell Biol. 1981; 91: 77s-103sCrossref PubMed Google Scholar), suggesting that the mechanisms that define the tER will be key to understanding the assembly and positioning of Golgi structures. What is the destination of COPII vesicles? The term "ER-to-Golgi transport" suggests the existence of a stable, well-defined acceptor compartment. A plausible alternative is that COPII vesicles fuse with one another (Figure 2), and perhaps also with larger tubules that pinch off from the tER (40Mironov A.A Weidman P Luini A Variations on the intracellular transport theme maturing cisternae and trafficking tubules.J. Cell Biol. 1997; 138: 481-484Crossref PubMed Scopus (129) Google Scholar). The result of this homotypic fusion would be the vesiculo-tubular clusters (VTCs) that make up the ER-Golgi intermediate compartment, or ERGIC (23Hauri H.-P Schweizer A The endoplasmic reticulum-Golgi intermediate compartment.Curr. Opin. Cell Biol. 1992; 4: 600-608Crossref PubMed Scopus (168) Google Scholar, 2Bannykh 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). COPI vesicles would then bud from the ERGIC to recycle selected components back to the ER (2Bannykh 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, 37Lowe M Kreis T.E Regulation of membrane traffic in animal cells by COPI.Biochim. Biophys. Acta. 1998; 1404: 53-66Crossref PubMed Scopus (79) Google Scholar). In vertebrate cells, entire ERGIC clusters travel along microtubules to the juxtanuclear Golgi region (34Lippincott-Schwartz J Cytoskeletal proteins and Golgi dynamics.Curr. Opin. Cell Biol. 1998; 10: 52-59Crossref PubMed Scopus (153) Google Scholar). The model shown in Figure 2 proposes that a new cis-Golgi cisterna forms when ERGIC clusters fuse with one another while receiving Golgi proteins from older cisternae. Hence, "ER-to-Golgi transport" assays, which rely upon the encounter of the secretory cargo with Golgi proteins (59Schwaninger R Plutner H Davidson H.W Pind S Balch W.E Transport of protein between endoplasmic reticulum and Golgi compartments in semiintact cells.Methods Enzymol. 1992; 219: 110-124Crossref PubMed Scopus (13) Google Scholar, 73Wuestehube L.J Schekman R.W Reconstitution of transport from endoplasmic reticulum to Golgi complex using endoplasmic reticulum-enriched membrane fraction from yeast.Methods Enzymol. 1992; 219: 124-136Crossref PubMed Scopus (80) Google Scholar), may actually measure reactions involved in cisternal formation. As the secretory cargo is carried forward by cisternal progression, resident Golgi proteins must stay behind (16Füllekrug J Nilsson T Protein sorting in the Golgi complex.Biochim. Biophys. Acta. 1998; 1404: 77-84Crossref PubMed Scopus (34) Google Scholar, 42Munro S Localization of proteins to the Golgi apparatus.Trends Cell Biol. 1998; 8: 11-15Abstract Full Text PDF PubMed Scopus (215) Google Scholar). It has generally been assumed that Golgi proteins are retained in static cisternae. But according to the maturation model, Golgi proteins are mobile and are localized to the stack exclusively by COPI vesicle-mediated retrieval. As predicted by this model, Golgi proteins can be packaged into COPI vesicles (61Sönnichsen B Watson R Clausen H Misteli T Warren G Sorting by COPI-coated vesicles under interphase and mitotic conditions.J. Cell Biol. 1996; 134: 1411-1425Crossref PubMed Scopus (87) Google Scholar, 44Orci 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, 36Love H.D Lin C.-C Short C.S Ostermann J Isolation of functional Golgi-derived vesicles with a possible role in retrograde transport.J. Cell Biol. 1998; 140: 541-551Crossref PubMed Scopus (74) Google Scholar). A second, untested prediction is that blocking COPI vesicle formation should cause resident Golgi proteins to redistribute to later secretory compartments. The maturation model assumes that COPI vesicles mediate retrograde transport. Genetic studies indicated that COPI vesicles carry material from the Golgi back to the ER (32Letourneur F Gaynor E.C Hennecke S Démolière 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 (645) Google Scholar, 17Gaynor E.C Graham T.R Emr S.D COPI in ER/Golgi and intra-Golgi transport do yeast COPI mutants point the way?.Biochim. Biophys. Acta. 1998; 1404: 33-51Crossref PubMed Scopus (74) Google Scholar), but crucial questions remain about vesicular traffic within the Golgi. The simplest possibility is that COPI vesicles move backward one cisterna at a time. It has been suggested that long coiled-coil proteins tether COPI vesicles to the Golgi membranes, thereby restricting vesicle movement to adjacent cisternae (43Orci L Perrelet A Rothman J.E Vesicles on strings morphological evidence for processive transport within the Golgi stack.Proc. Natl. 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Obvious candidates for intra-Golgi vesicle targeting molecules are the SNARE proteins (56Rothman J.E Wieland F.T Protein sorting by transport vesicles.Science. 1996; 272: 227-234Crossref PubMed Scopus (1010) Google Scholar, 19Götte M Fischer von Mollard G A new beat for the SNARE drum.Trends Cell Biol. 1998; 8: 215-218Abstract Full Text Full Text PDF PubMed Scopus (79) Google Scholar). Although analysis of protein secretion in yeast indicated that intra-Golgi transport involves a surprisingly limited number of SNARE interactions (48Pelham H.R.B Getting through the Golgi complex.Trends Cell Biol. 1998; 8: 45-49Abstract Full Text PDF PubMed Scopus (98) Google Scholar), combinatorial associations of the SNAREs may boost this number (70Weimbs T Mostov K Low S.H Hofmann K A model for structural similarity between different SNARE complexes based on sequence relationships.Trends Cell Biol. 1998; 8: 260-262Abstract Full Text Full Text PDF PubMed Scopus (115) Google Scholar). This uncertainty leaves open the possibility that COPI vesicles also travel through the Golgi in the anterograde direction, as originally proposed. Certain secretory proteins appear to be packaged into COPI vesicles (44Orci 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); such proteins might be on a "fast track" that would carry them forward through the Golgi at a rate faster than cisternal progression. This idea could be tested by rigorously comparing the rates at which various secretory cargo molecules transit through the stack. The final stage of the ER-Golgi system is the TGN. This compartment serves as a sorting station where cargo molecules are packaged into either secretory vesicles, regulated secretory granules, or vesicles destined for the endosomal/lysosomal/vacuolar system (39Mellman I Simons K The Golgi complex in vitro veritas?.Cell. 1992; 68: 829-840Abstract Full Text PDF PubMed Scopus (377) Google Scholar). Transport to endosomes and lysosomes/vacuoles involves the budding of clathrin- and adaptor-containing vesicles from the TGN. Regulated secretory granules apparently form by a different mechanism: large precursor granules pinch off from the TGN, then condense and mature by the vesicle-mediated export of selected components (66Tooze S.A Biogenesis of secretory granules in the trans-Golgi network of neuroendocrine and endocrine cells.Biochim. Biophys. Acta. 1998; 1404: 231-244Crossref PubMed Scopus (177) Google Scholar). It seems likely that a similar process generates secretory vesicles. Such vesicles are often large and pleiomorphic structures that resemble the swollen portions of Golgi cisternae (41Mollenhauer H.H Morré D.J Perspectives on Golgi apparatus form and function.J. Electron Microsc. Tech. 1991; 17: 2-14Crossref PubMed Scopus (57) Google Scholar, 35Lippincott-Schwartz J Smith C Insights into secretory and endocytic membrane traffic using green fluorescent protein chimeras.Curr. Opin. Neurobiol. 1997; 7: 631-639Crossref PubMed Scopus (51) Google Scholar, 52Rambourg A Clermont Y Three-dimensional structure of the Golgi apparatus in mammalian cells.in: Berger E.G Roth J The Golgi Apparatus. Birkhäuser Verlag, Basel1997Crossref Google Scholar). Formation of secretory vesicles involves reactions of lipid metabolism (13Fang M Rivas M.P Bankaitis V.A The contribution of lipids and lipid metabolism to cellular functions of the Golgi complex.Biochim. Biophys. Acta. 1998; 1404: 85-100Crossref PubMed Scopus (35) Google Scholar) as well as novel membrane–cytoskeleton interactions (29Kreis T.E Goodson H.V Perez F Rönnholm R Golgi apparatus-cytoskeleton interactions.in: Berger E.G Roth J The Golgi Apparatus. Birkhäuser Verlag, Basel1997Google Scholar, 64Stow J.L Heimann K Vesicle budding on Golgi membranes regulation by G proteins and myosin motors.Biochim. Biophys. Acta. 1998; 1404: 161-171Crossref PubMed Scopus (50) Google Scholar). According to the maturation model, secretory vesicles are the remnants of Golgi cisternae that have exported all of the components not destined for secretion. This concept illustrates how the underlying model can influence the questions that are asked. For many years, researchers have sought to identify a coat complex that drives the budding of secretory vesicles (64Stow J.L Heimann K Vesicle budding on Golgi membranes regulation by G proteins and myosin motors.Biochim. Biophys. Acta. 1998; 1404: 161-171Crossref PubMed Scopus (50) Google Scholar, 66Tooze S.A Biogenesis of secretory granules in the trans-Golgi network of neuroendocrine and endocrine cells.Biochim. Biophys. Acta. 1998; 1404: 231-244Crossref PubMed Scopus (177) Google Scholar). The model shown in Figure 2 implies that secretory vesicles form not by budding from a stable TGN compartment, but rather by the terminal maturation of TGN cisternae. Many different drugs or mutations cause a loss of recognizable Golgi structures (e.g., 65Takizawa P.A Yucel J.K Veit B Faulkner D.J Deerinck T Soto G Ellisman M Malhotra V Complete vesiculation of Golgi membranes and inhibition of protein transport by a novel sea sponge metabolite, ilimaquinone.Cell. 1993; 73: 1079-1090Abstract Full Text PDF PubMed Scopus (190) Google Scholar, 21Guo Q Vasile E Krieger M Disruption in Golgi structure and membrane traffic in a conditional lethal mammalian cell mutant are corrected by ε-COP.J. Cell Biol. 1994; 125: 1213-1223Crossref PubMed Scopus (128) Google Scholar, 72Wooding S Pelham H.R.B The dynamics of Golgi protein traffic visualized in living yeast cells.Mol. Biol. Cell. 1998; 9: 2667-2680Crossref PubMed Scopus (156) Google Scholar). The fragility of the Golgi is not surprising in the context of the maturation model, because the Golgi is seen as the product of a dynamic balance between anterograde and retrograde membrane flow. Any treatment that selectively inhibits one or the other of these pathways will disrupt Golgi organization. This perspective has practical implications. For instance, reagents that block COPII vesicle formation (60Shima D.T Cabrera-Poch N Pepperkok R Warren G An ordered inheritance strategy for the Golgi apparatus visualization of mitotic disassembly reveals a role for the mitotic spindle.J. Cell Biol. 1998; 141: 955-966Crossref PubMed Scopus (160) Google Scholar, 63Storrie B White J Röttger S Stelzer E.H.K Suganuma T Nilsson T Recycling of Golgi-resident glycosyltransferases through the ER reveals a novel pathway and provides an explanation for nocodazole-induced Golgi scattering.J. Cell Biol. 1998; 143 (in press)Crossref PubMed Scopus (288) Google Scholar) will cause an imbalance in membrane traffic and may therefore alter the distribution of resident Golgi proteins. In broader terms, viewing the Golgi as a dynamic structure raises the question of how membrane flow through this organelle is regulated. One component of this regulatory system in mammalian cells appears to be heterotrimeric G proteins (25Jamora C Takizawa P.A Zaarour R.F Denesvre C Faulkner D.J Malhotra V Regulation of Golgi structure through heterotrimeric G proteins.Cell. 1997; 91: 617-626Abstract Full Text Full Text PDF PubMed Scopus (102) Google Scholar, 64Stow J.L Heimann K Vesicle budding on Golgi membranes regulation by G proteins and myosin motors.Biochim. Biophys. Acta. 1998; 1404: 161-171Crossref PubMed Scopus (50) Google Scholar), suggesting that signal transduction pathways help to modulate Golgi membrane traffic (53Reynolds T.B Hopkins B.D Lyons M.R Graham T.R The high osmolarity glycerol response (HOG) MAP kinase pathway controls localization of a yeast Golgi glycosyltransferase.J. Cell Biol. 1998; 143: 935-946Crossref PubMed Scopus (33) Google Scholar). If Golgi organization is so easily destroyed, can cells recover by creating entire Golgi stacks de novo from the ER? The experimental data regarding this question are inconclusive (69Warren G Wickner W Organelle inheritance.Cell. 1996; 84: 395-400Abstract Full Text Full Text PDF PubMed Scopus (220) Google Scholar). In cells treated with brefeldin A (BFA), Golgi components redistribute to the ER, and when the drug is removed, Golgi stacks are rapidly regenerated (27Klausner R.D Donaldson J.G Lippincott-Schwartz J Brefeldin A insights into the control of membrane traffic and organelle structure.J. Cell Biol. 1992; 116: 1071-1080Crossref PubMed Scopus (1518) Google Scholar). However, BFA-treated cells might contain Golgi remnants that nucleate the formation of new stacks. Another potential example of de novo Golgi formation comes from studies of nocodazole-treated vertebrate cells. Nocodazole depolymerizes microtubules, thereby causing the juxtanuclear Golgi ribbon to redistribute into individual stacks spread throughout the cytoplasm (34Lippincott-Schwartz J Cytoskeletal proteins and Golgi dynamics.Curr. Opin. Cell Biol. 1998; 10: 52-59Crossref PubMed Scopus (153) Google Scholar). One interpretation is that the Golgi ribbon simply breaks apart in nocodazole-treated cells (60Shima D.T Cabrera-Poch N Pepperkok R Warren G An ordered inheritance strategy for the Golgi apparatus visualization of mitotic disassembly reveals a role for the mitotic spindle.J. Cell Biol. 1998; 141: 955-966Crossref PubMed Scopus (160) Google Scholar). However, recent studies raise the possibility that nocodazole treatment induces the return of Golgi components to the ER, followed by the generation of new Golgi stacks at peripheral tER sites (9Cole N.B Sciaky N Marotta A Song J Lippincott-Schwartz J Golgi dispersal during microtubule disruption regeneration of Golgi stacks at peripheral endoplasmic reticulum exit sites.Mol. Biol. Cell. 1996; 7 (a): 631-650Crossref PubMed Scopus (391) Google Scholar, 63Storrie B White J Röttger S Stelzer E.H.K Suganuma T Nilsson T Recycling of Golgi-resident glycosyltransferases through the ER reveals a novel pathway and provides an explanation for nocodazole-induced Golgi scattering.J. Cell Biol. 1998; 143 (in press)Crossref PubMed Scopus (288) Google Scholar). These results are suggestive of de novo assembly of Golgi from the ER, although again it is conceivable that peripheral Golgi stacks arise from preexisting "seeds" present at tER sites. Golgi stacks also break down and reform during vertebrate mitosis. The mechanism of mitotic Golgi reorganization is controversial (1Acharya U Mallabiabarrena A Acharya J.K Malhotra V Signaling via mitogen-activated protein kinase kinase (MEK1) is required for Golgi fragmentation during mitosis.Cell. 1998; 92: 183-192Abstract Full Text Full Text PDF PubMed Scopus (162) Google Scholar, 26Jesch S.A Linstedt A.D The Golgi and endoplasmic reticulum remain independent during mitosis in HeLa cells.Mol. Biol. Cell. 1998; 9: 623-635Crossref PubMed Scopus (76) Google Scholar, 38Lowe M Rabouille C Nakamura N Watson R Jackman M Jamsa E Rahman D Pappin D.J Warren G Cdc2 kinase directly phosphorylates the cis-Golgi matrix protein GM130 and is required for Golgi fragmentation in mitosis.Cell. 1998; 94: 783-793Abstract Full Text Full Text PDF PubMed Scopus (248) Google Scholar). It has generally been thought that Golgi cisternae fragment into vesicles and/or tubuloreticular elements, which then fuse homotypically to regenerate cisternae (51Rabouille C Warren G Changes in the architecture of the Golgi apparatus during mitosis.in: Berger E.G Roth J The Golgi Apparatus. Birkhäuser, Basel1997Google Scholar, 60Shima D.T Cabrera-Poch N Pepperkok R Warren G An ordered inheritance strategy for the Golgi apparatus visualization of mitotic disassembly reveals a role for the mitotic spindle.J. Cell Biol. 1998; 141: 955-966Crossref PubMed Scopus (160) Google Scholar). An alternative suggestion is that Golgi components transiently return to the ER during mitosis (35Lippincott-Schwartz J Smith C Insights into secretory and endocytic membrane traffic using green fluorescent protein chimeras.Curr. Opin. Neurobiol. 1997; 7: 631-639Crossref PubMed Scopus (51) Google Scholar). Consistent with this idea, anterograde traffic through the secretory pathway ceases during vertebrate mitosis (68Warren G Membrane partitioning during cell division.Annu. Rev. Biochem. 1993; 62: 323-348Crossref PubMed Scopus (224) Google Scholar); if retrograde traffic persists during mitosis, Golgi components should return to the ER. During mitosis in plants and fungi, the secretory pathway continues to operate, and Golgi structures do not break down (68Warren G Membrane partitioning during cell division.Annu. Rev. Biochem. 1993; 62: 323-348Crossref PubMed Scopus (224) Google Scholar, 11Driouich A Staehelin L.A The plant Golgi apparatus structural organization and functional properties.in: Berger E.G Roth J The Golgi Apparatus. Birkhäuser, Basel1997Google Scholar). Hence, Golgi organization and reorganization seem to reflect the patterns of membrane flux through the secretory pathway. This view poses a challenge for the biochemical analysis of Golgi dynamics, because purified Golgi structures may not undergo the same transformations as Golgi structures in whole cells. The available evidence suggests the following working hypotheses. (1) COPII vesicles fuse to form ERGIC clusters, which fuse in turn to form a new Golgi cisterna. (2) Each cisterna progresses through the stack and ultimately fragments into transport vesicles. (3) All resident Golgi proteins are continually recycled via retrograde vesicles. This vesicular transport drives the maturation of Golgi cisternae.
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