A Rab1 mutant affecting guanine nucleotide exchange promotes disassembly of the Golgi apparatus

The Golgi apparatus is a dynamic organelle whose structure is sensitive to vesicular traffic and to cell cycle control. We have examined the potential role for rab1a, a GTPase previously associated with ER to Golgi and intra-Golgi transport, in the formation and maintenance of Golgi structure. Bacterially expressed, recombinant rab1a protein was microinjected into rat embryonic fibroblasts, followed by analysis of Golgi morphology by fluorescence and electron microscopy. Three recombinant proteins were tested: wild-type rab, mutant rab1a(S25N), a constitutively GDP-bound form (Nuoffer, C., H. W. Davidson, J. Matteson, J. Meinkoth, and W. E. Balch, 1994. J. Cell Biol. 125: 225- 237), and mutant rab1a(N124I) defective in guanine nucleotide binding. Microinjection of wild-type rab1a protein or a variety of negative controls (injection buffer alone or activated ras protein) did not affect the appearance of the Golgi, as visualized by immunofluorescence of alpha-mannosidase II (Man II), used as a Golgi marker. In contrast, microinjection of the mutant forms promoted the disassembly of the Golgi stacks into dispersed vesicular structures visualized by immunofluorescence. When S25N-injected cells were analyzed by EM after immunoperoxidase labeling, Man II was found in isolated ministacks and large vesicular elements that were often surrounded by numerous smaller unlabeled vesicles resembling carrier vesicles. Golgi disassembly caused by rab1a mutants differs from BFA-induced disruption, since beta- COP remains membrane associated, and Man II does not redistribute to the ER. BFA can still cause these residual Golgi elements to fuse and disperse, albeit at a slower rate. Moreover, BFA recovery is incomplete in the presence of rab1 mutants or GTP gamma S. We conclude that GTP exchange and hydrolysis by GTPases, specifically rab1a, are required to form and maintain normal Golgi stacks. The similarity of Golgi disassembly seen with rab1a mutants to that occurring during mitosis, may point to a molecular basis involving rab1a for fragmentation of the Golgi apparatus during cell division.     

Rab1 and Ca2+ are required for the fusion of carrier vesicles mediating endoplasmic reticulum to Golgi transport

Members of the rab/YPT1/SEC4 gene family of small molecular weight GTPases play key roles in the regulation of vesicular traffic between compartments of the exocytic pathway. Using immunoelectron microscopy, we demonstrate that a dominant negative rab1a mutant, rab1a(N124I), defective for guanine nucleotide binding in vitro, leads to the accumulation of vesicular stomatitis virus glycoprotein (VSV-G) in numerous pre-cis-Golgi vesicles and vesicular-tubular clusters containing rab1 and beta-COP, a subunit of the coatomer complex. Similar to previous observations (Balch et al. 1994. Cell. 76:841-852), VSV-G was concentrated nearly 5-10-fold in vesicular carriers that accumulate in the presence of the rab1a(N124I) mutant. VSV-G containing vesicles and vesicular-tubular clusters were also found to accumulate in the presence of a rab1a effector domain peptide mimetic that inhibits endoplasmic reticulum to Golgi transport, as well as in the absence of Ca2+. These results suggest that the combined action of a Ca(2+)-dependent protein and conformational changes associated with the GTPase cycle of rab1 are essential for a late targeting/fusion step controlling the delivery of vesicles to Golgi compartments.     

Rab11 Is Required for Trans-Golgi Network–to–Plasma Membrane Transport and a Preferential Target for GDP Dissociation Inhibitor

The rab11 GTPase has been localized to both the Golgi and recycling endosomes; however, its Golgi-associated function has remained obscure. In this study, rab11 function in exocytic transport was analyzed by using two independent means to perturb its activity. First, expression of the dominant interfering rab11S25N mutant protein led to a significant inhibition of the cell surface transport of vesicular stomatitis virus (VSV) G protein and caused VSV G protein to accumulate in the Golgi. On the other hand, the expression of wild-type rab11 or the activating rab11Q70L mutant had no adverse effect on VSV G transport. Next, the membrane association of rab11, which is crucial for its function, was perturbed by modest increases in GDP dissociation inhibitor (GDI) levels. This led to selective inhibition of the trans-Golgi network to cell surface delivery, whereas endoplasmic reticulum–to–Golgi and intra-Golgi transport were largely unaffected. The transport inhibition was reversed specifically by coexpression of wild-type rab11 with GDI. Under the same conditions two other exocytic rab proteins, rab2 and rab8, remained membrane bound, and the transport steps regulated by these rab proteins were unaffected. Neither mutant rab11S25N nor GDI overexpression had any impact on the cell surface delivery of influenza hemagglutinin. These data show that functional rab11 is critical for the export of a basolateral marker but not an apical marker from the trans-Golgi network and pinpoint rab11 as a sensitive target for inhibition by excess GDI.     

GTP-binding mutants of rab1 and rab2 are potent inhibitors of vesicular transport from the endoplasmic reticulum to the Golgi complex

We have examined the role of ras-related rab proteins in transport from the ER to the Golgi complex in vivo using a vaccinia recombinant T7 RNA polymerase virus to express site-directed rab mutants. These mutations are within highly conserved domains involved in guanine nucleotide binding and hydrolysis found in ras and all members of the ras superfamily. Substitutions in the GTP-binding domains of rab1a and rab1b (equivalent to the ras 17N and 116I mutants) resulted in proteins which were potent trans dominant inhibitors of vesicular stomatitis virus glycoprotein (VSV-G protein) transport between the ER and cis Golgi complex. Immunofluorescence analysis indicated that expression of rab1b121I prevented delivery of VSV-G protein to the Golgi stack, which resulted in VSV-G protein accumulation in pre-Golgi punctate structures. Mutants in guanine nucleotide exchange or hydrolysis of the rab2 protein were also strong trans dominant transport inhibitors. Analogous mutations in rab3a, rab5, rab6, and H-ras did not inhibit processing of VSV-G to the complex, sialic acid containing form diagnostic of transport to the trans Golgi compartment. We suggest that at least three members of the rab family (rab1a, rab1b, and rab2) use GTP hydrolysis to regulate components of the transport machinery involved in vesicle traffic between early compartments of the secretory pathway.     

Sar1 promotes vesicle budding from the endoplasmic reticulum but not Golgi compartments

Two new members (Sar1a and Sar1b) of the SAR1 gene family have been identified in mammalian cells. Using immunoelectron microscopy, Sar1 was found to be restricted to the transitional region where the protein was enriched 20-40-fold in vesicular carriers mediating ER to Golgi traffic. Biochemical analysis revealed that Sar1 was essential for an early step in vesicle budding. A Sar1-specific antibody potently inhibited export of vesicular stomatitis virus glycoprotein (VSV-G) from the ER in vitro. Consistent with the role of guanine nucleotide exchange in Sar1 function, a trans-dominant mutant (Sar1a[T39N]) with a preferential affinity for GDP also strongly inhibited vesicle budding from the ER. In contrast, Sar1 was not found to be required for the transport of VSV-G between sequential Golgi compartments, suggesting that components active in formation of vesicular carriers mediating ER to Golgi traffic may differ, at least in part, from those involved in intra-Golgi transport. The requirement for novel components at different stages of the secretory pathway may reflect the recently recognized differences in protein transport between the Golgi stacks as opposed to the selective sorting and concentration of protein during export from the ER.     

Rab1b regulates vesicular transport between the endoplasmic reticulum and successive Golgi compartments

We report an essential role for the ras-related small GTP-binding protein rab1b in vesicular transport in mammalian cells. mAbs detect rab1b in both the ER and Golgi compartments. Using an assay which reconstitutes transport between the ER and the cis-Golgi compartment, we find that rab1b is required during an initial step in export of protein from the ER. In addition, it is also required for transport of protein between successive cis- and medial-Golgi compartments. We suggest that rab1b may provide a common link between upstream and downstream components of the vesicular fission and fusion machinery functioning in early compartments of the secretory pathway.     

Requirement of nucleotide exchange factor for Ypt1 GTPase mediated protein transport

Small GTPases of the rab family are involved in the regulation of vesicular transport. It is believed that cycling between the GTP- and GDP-bound forms, and accessory factors regulating this cycling are crucial for rab function. However, an essential role for rab nucleotide exchange factors has not yet been demonstrated. In this report we show the requirement of nucleotide exchange factor activity for Ypt1 GTPase mediated protein transport. The Ypt1 protein, a member of the rab family, plays a role in targeting vesicles to the acceptor compartment and is essential for the first two steps of the yeast secretory pathway. We use two YPT1 dominant mutations that contain alterations in a highly conserved GTP-binding domain, N121I and D124N. YPT1-D124N is a novel mutation that encodes a protein with nucleotide specificity modified from guanine to xanthine. This provides a tool for the study of an individual rab GTPase in crude extracts: a xanthosine triphosphate (XTP)-dependent conditional dominant mutation. Both mutations confer growth inhibition and a block in protein secretion when expressed in vivo. The purified mutant proteins do not bind either GDP or GTP. Moreover, they completely inhibit the ability of the exchange factor to stimulate nucleotide exchange for wild type Ypt1 protein, and are potent inhibitors of ER to Golgi transport in vitro at the vesicle targeting step. The inhibitory effects of the Ypt1-D124N mutant protein on both nucleotide exchange activity and protein transport in vitro can be relieved by XTP, indicating that it is the nucleotide-free form of the mutant protein that is inhibitory. These results suggest that the dominant mutant proteins inhibit protein transport by sequestering the exchange factor from the wild type Ypt1 protein, and that this factor has an essential role in vesicular transport.     

Rab8, a small GTPase involved in vesicular traffic between the TGN and the basolateral plasma membrane

Small GTP-binding proteins of the rab family have been implicated as regulators of membrane traffic along the biosynthetic and endocytic pathways in eukaryotic cells. We have investigated the localization and function of rab8, closely related to the yeast YPT1/SEC4 gene products. Confocal immunofluorescence microscopy and immunoelectron microscopy on filter-grown MDCK cells demonstrated that, rab8 was localized to the Golgi region, vesicular structures, and to the basolateral plasma membrane. Two-dimensional gel electrophoresis showed that rab8p was highly enriched in immuno-isolated basolateral vesicles carrying vesicular stomatitis virus-glycoprotein (VSV-G) but was absent from vesicles transporting the hemagglutinin protein (HA) of influenza virus to the apical cell surface. Using a cytosol dependent in vitro transport assay in permeabilized MDCK cells we studied the functional role of rab8 in biosynthetic membrane traffic. Transport of VSV-G from the TGN to the basolateral plasma membrane was found to be significantly inhibited by a peptide derived from the hypervariable COOH-terminal region of rab8, while transport of the influenza HA from the TGN to the apical surface and ER to Golgi transport were unaffected. We conclude that rab8 plays a role in membrane traffic from the TGN to the basolateral plasma membrane in MDCK cells.     

rab GTP-binding proteins with three different carboxyl-terminal cysteine motifs are modified in vivo by 20-carbon isoprenoids.

p21ras and several other ras-related GTP-binding proteins are modified post-translationally by addition of 15-carbon farnesyl or 20-carbon geranylgeranyl isoprenoids to cysteines within a conserved carboxyl-terminal sequence motif, Caa(M/S/L), where a is an aliphatic amino acid. Proteins ending with M or S are substrates for farnesyltransferase, whereas those ending with L are modified preferentially by geranylgeranyltransferase. We recently reported that GTP-binding proteins encoded by rab1B (GGCC), rab2 (GGCC), and rab5 (CCSN) are modified by 20-carbon isoprenyl derivatives of [3H]mevalonate when translated in vitro, despite having carboxyl-terminal sequences distinct from the Caa(M/S/L) motif. We now show that these proteins function as specific acceptors for geranylgeranyl in vitro and are modified by 20-carbon isoprenyl groups in COS cells metabolically labeled with [3H]mevalonate. Proteins encoded by rab4 and rab6, with yet another distinct carboxyl-terminal motif (xCxC), are similarly modified by 20-carbon isoprenoids in vitro and in vivo. The geranylgeranyl modification of rab5 protein (CCSN) is catalyzed by an enzyme in brain cytosol but not by a purified geranylgeranyltransferase that modifies GTP-binding proteins with the CaaL motif. Unlike the prenylation of proteins with Caa(M/S/L) termini, the prenylation of rab5 protein is not inhibited by a synthetic peptide based on its carboxyl-terminal sequence (TRNQCCSN). When cellular isoprenoid synthesis is blocked by treatment of cells with lovastatin, rab proteins that are normally localized in membranes of the endoplasmic reticulum, Golgi apparatus, and endosomes accumulate in the cytosol. This change in rab protein localization is reversed by providing cells with mevalonate. These findings suggest that geranylgeranyl modification underlies the ability of rab GTP-binding proteins to associate with intracellular membranes, where they are postulated to function as mediators of vesicular traffic.     

Depletion of beta-COP reveals a role for COP-I in compartmentalization of secretory compartments and in biosynthetic transport of caveolin-1

We have utilized small interfering RNA (siRNA)-mediated depletion of the beta-COP subunit of COP-I to explore COP-I function in organellar compartmentalization and protein traffic. Reduction in beta-COP levels causes the colocalization of markers for the endoplasmic reticulum (ER)-Golgi intermediate compartment (ERGIC), Golgi, trans-Golgi network (TGN), and recycling endosomes in large, globular compartments. The lack of spatial differentiation of these compartments is not due to a general collapse of all cellular organelles since markers for the early endosomes and lysosomes do not redistribute to the common structures. Anterograde trafficking of the transmembrane cargo vesicular stomatitis virus membrane glycoprotein and of a subset of soluble cargoes is arrested within the common globular compartments. Similarly, recycling traffic of transferrin through the common compartment is perturbed. Furthermore, the trafficking of caveolin-1 (Cav1), a structural protein of caveolae, is arrested within the globular structures. Importantly, Cav1 coprecipitates with the gamma-subunit of COP-I, suggesting that Cav1 is a COP-I cargo. Our findings suggest that COP-I is required for the compartmentalization of the ERGIC, Golgi, TGN, and recycling endosomes and that COP-I plays a novel role in the biosynthetic transport of Cav1.     

The small GTP-binding protein rab6 functions in intra-Golgi transport

Rab6 is a ubiquitous ras-like GTP-binding protein associated with the membranes of the Golgi complex (Goud, B., A. Zahraoui, A. Tavitian, and J. Saraste. 1990. Nature (Lond.). 345:553-556; Antony, C., C. Cibert, G. Geraud, A. Santa Maria, B. Maro, V. Mayau, and B. Goud. 1992. J. Cell Sci. 103: 785-796). We have transiently overexpressed in mouse L cells and human HeLa cells wild-type rab6, GTP (rab6 Q72L), and GDP (rab6 T27N) -bound mutants of rab6 and analyzed the intracellular transport of a soluble secreted form of alkaline phosphatase (SEAP) and of a plasma membrane protein, the hemagglutinin protein (HA) of influenza virus. Over-expression of wild-type rab6 and rab6 Q72L greatly reduced transport of both markers between cis/medial (alpha- mannosidase II positive) and late (sialyl-transferase positive) Golgi compartments, without affecting transport from the endoplasmic reticulum (ER) to cis/medial-Golgi or from the trans-Golgi network (TGN) to the plasma membrane. Whereas overexpression of rab6 T27N did not affect the individual steps of transport between ER and the plasma membrane, it caused an apparent delay in secretion, most likely due to the accumulation of the transport markers in late Golgi compartments. Overexpression of both rab6 Q72L and rab6 T27N altered the morphology of the Golgi apparatus as well as that of the TGN, as assessed at the immunofluorescence level with several markers. We interpret these results as indicating that rab6 controls intra-Golgi transport, either acting as an inhibitor in anterograde transport or as a positive regulator of retrograde transport.     

Rab 7: an important regulator of late endocytic membrane traffic

Rab5 and rab7 proteins belong to a superfamily of small molecular weight GTPases known to be associated with early and late endosomes, respectively. The rab5 protein plays an important regulatory role in early endocytosis, yet the function of rab7 protein was previously uncharacterized. This question was addressed by comparing the kinetics of vesicular stomatitis virus (VSV) G protein internalization in baby hamster kidney cells overexpressing wild-type or dominant negative mutant forms of the rab7 protein (rab7N125I and rab7T22N). Overexpression of wild-type rab7 protein allowed normal transport to late endosomes (mannose 6-phosphate receptor positive), while the rab7N125I mutant caused the VSV G protein to accumulate specifically in early (transferrin receptor positive) endosomes. Horseradish peroxidase and paramyxovirus SV5 hemagglutinin-neuraminidase (HN) were used in quantitative biochemical assays to further demonstrate that rab7 function was not required for early internalization events, but was crucial in downstream degradative events. The characteristic cleavage of SV5 HN in the late endosome distinguishes internalization from transport to later stages of the endocytic pathway. Mutant rab7N125I or rab7T22N proteins had no effect on the internalization of either horseradish peroxidase or SV5 HN protein. In contrast, the mutant proteins markedly inhibited the subsequent cleavage of the SV5 HN protein. Taken together, these data support a key role for rab7, downstream of rab5, in regulating membrane transport leading from early to late endosomes. We compare our findings to those obtained for the yeast homologues Ypt51p, Ypt52p, Ypt53p, and Ypt7p.     

Sequential transport of protein between the endoplasmic reticulum and successive Golgi compartments in semi-intact cells.

The vectorial transport of vesicular stomatitis virus (VSV) G protein between the ER and the cis and medial Golgi compartments has been reconstituted using semi-intact (perforated) cells. The transport of VSV-G protein between successive compartments is measured by the sequential processing of the two N-linked oligosaccharide chains present on VSV-G protein to the endoglycosidase (endo) H-resistant structures which have unique electrophoretic mobilities during sodium dodecyl sulfate-gel electrophoresis. The appearance of a form of VSV-G which contains only one endo H-resistant oligosaccharide chain (GH1) is kinetically and biochemically indistinguishable from the appearance of the Man5, endo D-sensitive form (GD), the latter being a processing reaction diagnostic of transport from the ER to the cis Golgi compartment. These results provide evidence that the cis Golgi compartment may contain in addition to alpha-1,2-mannosidase I, both N-acetylglueosamine transferase I and alpha-1,2-mannosidase II. VSV-G protein is subsequently processed to the form which contains two endo H-resistant oligosaccharides (GH2) after a second wave of vesicular transport. Processing of GH1 to GH2 in vitro occurs only after a lag period following the appearance of GH1; processing is sensitive to N-ethylmaleimide, guanosine-5'-O-(3-thiotriphosphate), and a synthetic peptide homologous to the rab1 protein effector domain, and processing is inhibited in the absence of free Ca2+ (in the presence of EGTA), reagents which potently inhibit ER to cis Golgi transport. These results suggest that VSV-G protein proceeds through at least two rounds of vesicular transport from the ER to the medial Golgi compartment for processing to the GH2 form, providing a model system to study the regulation of the vectorial membrane fission and fusion events involved in vesicular trafficking and organelle dynamics in the early stages of the secretory pathway.     

Membrane targeting and activation of the Lowe syndrome protein OCRL1 by rab GTPases

The X-linked disorder oculocerebrorenal syndrome of Lowe is caused by mutation of the OCRL1 protein, an inositol polyphosphate 5-phosphatase. OCRL1 is localised to the Golgi apparatus and early endosomes, and can translocate to lamellipodia upon growth factor stimulation. We show here that OCRL1 interacts with several members of the rab family of small GTPases. Strongest interaction is seen with Golgi-associated rab1 and rab6 and endosomal rab5. Point mutants defective in rab binding fail to target to the Golgi apparatus and endosomes, strongly suggesting rab interaction is required for targeting of OCRL1 to these compartments. Membrane recruitment via rab binding is required for changes in Golgi and endosomal dynamics induced by overexpression of catalytically inactive OCRL1. In vitro experiments demonstrate that rab5 and rab6 directly stimulate the 5-phosphatase activity of OCRL1. We conclude that rabs play a dual role in regulation of OCRL1, firstly targeting it to the Golgi apparatus and endosomes, and secondly, directly stimulating the 5-phosphatase activity of OCRL1 after membrane recruitment.     

Traffic of Kv4 K+ channels mediated by KChIP1 is via a novel post-ER vesicular pathway

The traffic of Kv4 K+ channels is regulated by the potassium channel interacting proteins (KChIPs). Kv4.2 expressed alone was not retained within the ER, but reached the Golgi complex. Coexpression of KChIP1 resulted in traffic of the channel to the plasma membrane, and traffic was abolished when mutations were introduced into the EF-hands with channel captured on vesicular structures that colocalized with KChIP1(2-4)-EYFP. The EF-hand mutant had no effect on general exocytic traffic. Traffic of Kv4.2 was coat protein complex I (COPI)-dependent, but KChIP1-containing vesicles were not COPII-coated, and expression of a GTP-loaded Sar1 mutant to block COPII function more effectively inhibited traffic of vesicular stomatitis virus glycoprotein (VSVG) than did KChIP1/Kv4.2 through the secretory pathway. Therefore, KChIP1seems to be targeted to post-ER transport vesicles, different from COPII-coated vesicles and those involved in traffic of VSVG. When expressed in hippocampal neurons, KChIP1 co-distributed with dendritic Golgi outposts; therefore, the KChIP1 pathway could play an important role in local vesicular traffic in neurons.     

The role of ARF1 and rab GTPases in polarization of the Golgi stack

The organization and sorting of proteins within the Golgi stack to establish and maintain its cis to trans polarization remains an enigma. The function of Golgi compartments involves coat assemblages that facilitate vesicle traffic, Rab-tether-SNAP receptor (SNARE) machineries that dictate membrane identity, as well as matrix components that maintain structure. We have investigated how the Golgi complex achieves compartmentalization in response to a key component of the coat complex I (COPI) coat assembly pathway, the ARF1 GTPase, in relationship to GTPases-regulating endoplasmic reticulum (ER) exit (Sar1) and targeting fusion (Rab1). Following collapse of the Golgi into the ER in response to inhibition of activation of ARF1 by Brefeldin A, we found that Sar1- and Rab1-dependent Golgi reformation took place at multiple peripheral and perinuclear ER exit sites. These rapidly converged into immature Golgi that appeared as onion-like structures composed of multiple concentrically arrayed cisternae of mixed enzyme composition. During clustering to the perinuclear region, Golgi enzymes were sorted to achieve the degree of polarization within the stack found in mature Golgi. Surprisingly, we found that sorting of Golgi enzymes into their subcompartments was insensitive to the dominant negative GTP-restricted ARF1 mutant, a potent inhibitor of COPI coat disassembly and vesicular traffic. We suggest that a COPI-independent, Rab-dependent mechanism is involved in the rapid reorganization of resident enzymes within the Golgi stack following synchronized release from the ER, suggesting an important role for Rab hubs in directing Golgi polarization.     

Phosphatidylinositol 4-kinasebeta is critical for functional association of rab11 with the Golgi complex

Phosphatidylinositol 4-kinasebeta (PI4Kbeta) plays an essential role in maintaining the structural integrity of the Golgi complex. In a search for PI4Kbeta-interacting proteins, we found that PI4Kbeta specifically interacts with the GTP-bound form of the small GTPase rab11. The PI4Kbeta-rab11 interaction is of functional significance because inhibition of rab11 binding to PI4Kbeta abolished the localization of rab11 to the Golgi complex and significantly inhibited transport of vesicular stomatitis virus G protein from the Golgi complex to the plasma membrane. We propose that a novel function of PI4Kbeta is to act as a docking protein for rab11 in the Golgi complex, which is important for biosynthetic membrane transport from the Golgi complex to the plasma membrane.     

GDI1 encodes a GDP dissociation inhibitor that plays an essential role in the yeast secretory pathway.

GTP binding proteins of the Sec4/Ypt/rab family regulate distinct vesicular traffic events in eukaryotic cells. We have cloned GDI1, an essential homolog of bovine rab GDI (GDP dissociation inhibitor) from the yeast Saccharomyces cerevisiae. Analogous to the bovine protein, purified Gdi1p slows the dissociation of GDP from Sec4p and releases the GDP-bound form from yeast membranes. Depletion of Gdi1p in vivo leads to loss of the soluble pool of Sec4p and inhibition of protein transport at multiple stages of the secretory pathway. Complementation analysis indicates that GDI1 is allelic to sec19-1. These results establish that Gdi1p plays an essential function in membrane traffic and are consistent with a role for Gdi1p in the recycling of proteins of the Sec4/Ypt/rab family from their target membranes back to their vesicular pools.     

Multiple GTP-binding proteins regulate vesicular transport from the ER to Golgi membranes

Using indirect immunofluorescence we have examined the effects of reagents which inhibit the function of ras-related rab small GTP-binding proteins and heterotrimeric G alpha beta gamma proteins in ER to Golgi transport. Export from the ER was inhibited by an antibody towards rab1B and an NH2-terminal peptide which inhibits ARF function (Balch, W. E., R. A. Kahn, and R. Schwaninger. 1992. J. Biol. Chem. 267:13053-13061), suggesting that both of these small GTP-binding proteins are essential for the transport vesicle formation. Export from the ER was also potently inhibited by mastoparan, a peptide which mimics G protein binding regions of seven transmembrane spanning receptors activating and uncoupling heterotrimeric G proteins from their cognate receptors. Consistent with this result, purified beta gamma subunits inhibited the export of VSV-G from the ER suggesting an initial event in transport vesicle assembly was regulated by a heterotrimeric G protein. In contrast, incubation in the presence of GTP gamma S or AIF(3-5) resulted in the accumulation of transported protein in different populations of punctate pre-Golgi intermediates distributed throughout the cytoplasm of the cell. Finally, a peptide which is believed to antagonize the interaction of rab proteins with putative downstream effector molecules inhibited transport at a later step preceding delivery to the cis Golgi compartment, similar to the site of accumulation of transported protein in the absence of NSF or calcium (Plutner, H., H. W. Davidson, J. Saraste, and W. E. Balch. 1992. J. Cell Biol. 119:1097-1116). These results are consistent with the hypothesis that multiple GTP-binding proteins including a heterotrimeric G protein(s), ARF and rab1 differentially regulate steps in the transport of protein between early compartments of the secretory pathway. The concept that G protein-coupled receptors gate the export of protein from the ER is discussed.     

A GRASP55-rab2 effector complex linking Golgi structure to membrane traffic.

Membrane traffic between the endoplasmic reticulum (ER) and Golgi apparatus and through the Golgi apparatus is a highly regulated process controlled by members of the rab GTPase family. The GTP form of rab1 regulates ER to Golgi transport by interaction with the vesicle tethering factor p115 and the cis-Golgi matrix protein GM130, also part of a complex with GRASP65 important for the organization of cis-Golgi cisternae. Here, we find that a novel coiled-coil protein golgin-45 interacts with the medial-Golgi matrix protein GRASP55 and the GTP form of rab2 but not other Golgi rab proteins. Depletion of golgin-45 disrupts the Golgi apparatus and causes a block in secretory protein transport. These results demonstrate that GRASP55 and golgin-45 form a rab2 effector complex on medial-Golgi essential for normal protein transport and Golgi structure.