Evidence for segregation of sphingomyelin and cholesterol during formation of COPI-coated vesicles

In higher eukaryotes, phospholipid and cholesterol synthesis occurs mainly in the endoplasmic reticulum, whereas sphingomyelin and higher glycosphingolipids are synthesized in the Golgi apparatus. Lipids like cholesterol and sphingomyelin are gradually enriched along the secretory pathway, with their highest concentration at the plasma membrane. How a cell succeeds in maintaining organelle-specific lipid compositions, despite a steady flow of incoming and outgoing transport carriers along the secretory pathway, is not yet clear. Transport and sorting along the secretory pathway of both proteins and most lipids are thought to be mediated by vesicular transport, with coat protein I (COPI) vesicles operating in the early secretory pathway. Although the protein constituents of these transport intermediates are characterized in great detail, much less is known about their lipid content. Using nano-electrospray ionization tandem mass spectrometry for quantitative lipid analysis of COPI-coated vesicles and their parental Golgi membranes, we find only low amounts of sphingomyelin and cholesterol in COPI-coated vesicles compared with their donor Golgi membranes, providing evidence for a significant segregation from COPI vesicles of these lipids. In addition, our data indicate a sorting of individual sphingomyelin molecular species. The possible molecular mechanisms underlying this segregation, as well as implications on COPI function, are discussed.     

Segregation of COPI-rich and anterograde-cargo-rich domains in endoplasmic-reticulum-to-Golgi transport complexes.

Membrane traffic between the endoplasmic reticulum (ER) and the Golgi complex is regulated by two vesicular coat complexes, COPII and COPI. COPII has been implicated in the selective packaging of anterograde cargo into coated transport vesicles budding from the ER [1]. In mammalian cells, these vesicles coalesce to form tubulo-vesicular transport complexes (TCs), which shuttle anterograde cargo from the ER to the Golgi complex [2] [3] [4]. In contrast, COPI-coated vesicles are proposed to mediate recycling of proteins from the Golgi complex to the ER [1] [5] [6] [7]. The binding of COPI to COPII-coated TCs [3] [8] [9], however, has led to the proposal that COPI binds to TCs and specifically packages recycling proteins into retrograde vesicles for return to the ER [3] [9]. To test this hypothesis, we tracked fluorescently tagged COPI and anterograde-transport markers simultaneously in living cells. COPI predominated on TCs shuttling anterograde cargo to the Golgi complex and was rarely observed on structures moving in directions consistent with retrograde transport. Furthermore, a progressive segregation of COPI-rich domains and anterograde-cargo-rich domains was observed in the TCs. This segregation and the directed motility of COPI-containing TCs were inhibited by antibodies that blocked COPI function. These observations, which are consistent with previous biochemical data [2] [9], suggest a role for COPI within TCs en route to the Golgi complex. By sequestering retrograde cargo in the anterograde-directed TCs, COPI couples the sorting of ER recycling proteins [10] to the transport of anterograde cargo.     

Sequential tethering of Golgins and catalysis of SNAREpin assembly by the vesicle-tethering protein p115

p115 tethers coat protein (COP)I vesicles to Golgi membranes. The acidic COOH-terminal domain of p115 links the Golgins, Giantin on COPI vesicles, to GM130 on Golgi membranes. We now show that a SNARE motif-related domain within p115 stimulates the specific assembly of endogenous Golgi SNAREpins containing the t-SNARE, syntaxin 5. p115 catalyzes the construction of a cognate GOS-28-syntaxin-5 (v-/t-SNARE) complex by first linking the SNAREs to promote their direct interaction. These events are essential for NSF-catalyzed reassembly of postmitotic Golgi vesicles and tubules into mature cisternae. Staging experiments reveal that the linking of Golgins precedes SNAREpin assembly. Thus, p115 coordinates sequential tethering and docking of COPI vesicles by first using long tethers (Golgins) and then short tethers (SNAREs).     

A Role for Giantin in Docking COPI Vesicles to Golgi Membranes

We have previously shown that p115, a vesicle docking protein, binds to two proteins (p130 and p400) in detergent extracts of Golgi membranes. p130 was identified as GM130, a Golgi matrix protein, and was shown to act as a membrane receptor for p115. p400 has now been identified as giantin, a Golgi membrane protein with most of its mass projecting into the cytoplasm. Giantin is found on COPI vesicles and pretreatment with antibodies inhibits both the binding of p115 and the docking of these vesicles with Golgi membranes. In contrast, GM130 is depleted from COPI vesicles and inhibition of the GM130 on Golgi membranes, using either antibodies or an NH₂-terminal GM130 peptide, inhibits p115 binding and vesicle docking. Together these results suggest that COPI vesicles are docked by giantin on the COPI vesicles and GM130 on Golgi membranes with p115 providing a bridge.     

The yeast p24 complex is required for the formation of COPI retrograde transport vesicles from the Golgi apparatus

The p24 family members are transmembrane proteins assembled into heteromeric complexes that continuously cycle between the ER and the Golgi apparatus. These cargo proteins were assumed to play a structural role in COPI budding because of their major presence in mammalian COPI vesicles. However, this putative function has not been proved conclusively so far. Furthermore, deletion of all eight yeast p24 family members does not produce severe transport phenotypes, suggesting that the p24 complex is not essential for COPI function. In this paper we provide direct evidence that the yeast p24 complex plays an active role in retrograde transport from Golgi to ER by facilitating the formation of COPI-coated vesicles. Therefore, our results demonstrate that p24 proteins are important for vesicle formation instead of simply being a passive traveler, supporting the model in which cargo together with a small GTPase of the ARF superfamily and coat subunits act as primer for vesicle formation.     

Mutations in a highly conserved region of the Arf1p activator GEA2 block anterograde Golgi transport but not COPI recruitment to membranes

We have identified an important functional region of the yeast Arf1 activator Gea2p upstream of the catalytic Sec7 domain and characterized a set of temperature-sensitive (ts) mutants with amino acid substitutions in this region. These gea2-ts mutants block or slow transport of proteins traversing the secretory pathway at exit from the endoplasmic reticulum (ER) and the early Golgi, and accumulate both ER and early Golgi membranes. No defects in two types of retrograde trafficking/sorting assays were observed. We find that a substantial amount of COPI is associated with Golgi membranes in the gea2-ts mutants, even after prolonged incubation at the nonpermissive temperature. COPI in these mutants is released from Golgi membranes by brefeldin A, a drug that binds directly to Gea2p and blocks Arf1 activation. Our results demonstrate that COPI function in sorting of at least three retrograde cargo proteins within the Golgi is not perturbed in these mutants, but that forward transport is severely inhibited. Hence this region of Gea2p upstream of the Sec7 domain plays a role in anterograde transport that is independent of its role in recruiting COPI for retrograde transport, at least of a subset of Golgi-ER cargo.     

Biosynthetic protein transport through the early secretory pathway

 Newly synthesized proteins destined for delivery to the cell surface are inserted cotranslationally into the endoplasmic reticulum (ER) and, after their correct folding, are transported out of the ER. During their transport to the cell surface, cargo proteins pass through the various cisternae of the Golgi apparatus and, in the trans-most cisternae of the stack, are sorted into constitutive secretory vesicles that fuse with the plasma membrane. Simultaneously with anterograde protein transport, retrograde protein transport occurs within the Golgi complex as well as from the Golgi back to the ER. Vesicular transport within the early secretory pathway is mediated by two types of non-clathrin coated vesicles: COPI- and COPII-coated vesicles. The formation of these carrier vesicles depends on the recruitment of cytosolic coat proteins that are thought to act as a mechanical device to shape a flattened donor membrane into a spherical vesicle. A general molecular machinery that mediates targeting and fusion of carrier vesicles has been identified as well. Beside a general overview of the various coat structures known today, we will discuss issues specifically related to the biogenesis of COPI-coated vesicles: (1) a possible role of phospholipase D in the formation of COPI-coated vesicles; (2) a functional role of a novel family of transmembrane proteins, the p24 family, in the initiation of COPI assembly; and (3) the direction COPI-coated vesicles may take within the early secretory pathway. Moreover, we will consider two alternative mechanisms of protein transport through the Golgi stack: vesicular transport versus cisternal maturation. Accepted: 24 October 1997     

Arabidopsis Sec21p and Sec23p homologs. Probable coat proteins of plant COP-coated vesicles

Intracellular protein transport between the endoplasmic reticulum (ER) and the Golgi apparatus and within the Golgi apparatus is facilitated by COP (coat protein)-coated vesicles. Their existence in plant cells has not yet been demonstrated, although the GTP-binding proteins required for coat formation have been identified. We have generated antisera against glutathione-S-transferase-fusion proteins prepared with cDNAs encoding the Arabidopsis Sec21p and Sec23p homologs (AtSec21p and AtSec23p, respectively). The former is a constituent of the COPI vesicle coatomer, and the latter is part of the Sec23/24p dimeric complex of the COPII vesicle coat. Cauliflower (Brassica oleracea) inflorescence homogenates were probed with these antibodies and demonstrated the presence of AtSec21p and AtSec23p antigens in both the cytosol and membrane fractions of the cell. The membrane-associated forms of both antigens can be solubilized by treatments typical for extrinsic proteins. The amounts of the cytosolic antigens relative to the membrane-bound forms increase after cold treatment, and the two antigens belong to different protein complexes with molecular sizes comparable to the corresponding nonplant coat proteins. Sucrose-density-gradient centrifugation of microsomal cell membranes from cauliflower suggests that, although AtSec23p seems to be preferentially associated with ER membranes, AtSec21p appears to be bound to both the ER and the Golgi membranes. This could be in agreement with the notion that COPII vesicles are formed at the ER, whereas COPI vesicles can be made by both Golgi and ER membranes. Both AtSec21p and AtSec23p antigens were detected on membranes equilibrating at sucrose densities equivalent to those typical for in vitro-induced COP vesicles from animal and yeast systems. Therefore, a further purification of the putative plant COP vesicles was undertaken.     

In situ localization and in vitro induction of plant COPI-coated vesicles

Coat protein (COP)-coated vesicles have been shown to mediate protein transport through early steps of the secretory pathway in yeast and mammalian cells. Here, we attempt to elucidate their role in vesicular trafficking of plant cells, using a combined biochemical and ultrastructural approach. Immunogold labeling of cryosections revealed that COPI proteins are localized to microvesicles surrounding or budding from the Golgi apparatus. COPI-coated buds primarily reside on the cis-face of the Golgi stack. In addition, COPI and Arf1p show predominant labeling of the cis-Golgi stack, gradually diminishing toward the trans-Golgi stack. In vitro COPI-coated vesicle induction experiments demonstrated that Arf1p as well as coatomer could be recruited from cauliflower cytosol onto mixed endoplasmic reticulum (ER)/Golgi membranes. Binding of Arf1p and coatomer is inhibited by brefeldin A, underlining the specificity of the recruitment mechanism. In vitro vesicle budding was confirmed by identification of COPI-coated vesicles through immunogold negative staining in a fraction purified from isopycnic sucrose gradient centrifugation. Similar in vitro induction experiments with tobacco ER/Golgi membranes prepared from transgenic plants overproducing barley alpha-amylase-HDEL yielded a COPI-coated vesicle fraction that contained alpha-amylase as well as calreticulin.     

Traffic COPs and the formation of vesicle coats

Forward and retrograde trafficking of secretory proteins between the endoplasmic reticulum and the Golgi apparatus is driven by two biochemically distinct vesicle coats, COPI and COPII. Assembly of the coats on their target membranes is thought to provide the driving force for membrane deformation and the selective packaging of cargo and targeting molecules into nascent transport vesicles. This review describes our current knowledge on these issues and discusses how the two coats may be differentially targeted and assembled to achieve protein sorting and transport within the early secretory pathway.     

ArfGAP1 Activity and COPI Vesicle Biogenesis

Golgi-derived coat protein I (COPI) vesicles mediate transport in the early secretory pathway. The minimal machinery required for COPI vesicle formation from Golgi membranes in vitro consists of (i) the hetero-heptameric protein complex coatomer, (ii) the small guanosine triphosphatase ADP-ribosylation factor 1 (Arf1) and (iii) transmembrane proteins that function as coat receptors, such as p24 proteins. Various and opposing reports exist on a role of ArfGAP1 in COPI vesicle biogenesis. In this study, we show that, in contrast to data in the literature, ArfGAP1 is not required for COPI vesicle formation. To investigate roles of ArfGAP1 in vesicle formation, we titrated the enzyme into a defined reconstitution assay to form and purify COPI vesicles. We find that catalytic amounts of Arf1GAP1 significantly reduce the yield of purified COPI vesicles and that Arf1 rather than ArfGAP1 constitutes a stoichiometric component of the COPI coat. Combining the controversial reports with the results presented in this study, we suggest a novel role for ArfGAP1 in membrane trafficking.     

In vitro generation from the trans-Golgi network of coatomer-coated vesicles containing sialylated vesicular stomatitis virus-G protein

We describe an in vitro system in which post-Golgi vesicles containing metabolically labeled, sialylated, vesicular stomatitis virus (VSV) G protein molecules (VSV-G) are produced from the trans-Golgi network (TGN) of an isolated Golgi membrane fraction. This fraction is prepared from VSV-infected Madin-Darby canine kidney (MDCK) cells in which the (35)S-labeled viral envelope glycoprotein was allowed to accumulate in the trans-Golgi network during a prolonged incubation at 20 degrees C. The vesicles produced in this system are separated from the remnant Golgi membranes by differential centrifugation or by velocity sedimentation in a sucrose gradient. Vesicle production, quantified as the percentage of labeled VSV-G released from the Golgi membranes, is optimal at 37 degrees C and does not occur below 20 degrees C. It requires GTP and the small GTP-binding protein Arf (ADP-ribosylation factor), as well as coat protein type I (COPI) coat components (coatomer) and vesicle scission factors-one of which corresponds to the phosphatidylinositol transfer protein (PITP). Formation of the vesicles does not require GTP hydrolysis which, however, is necessary for their uncoating. Thus, vesicles generated in the presence of the nonhydrolyzable GTP analogs, GTPgammaS or GMP-PNP, retain a coatomer coat visible in the electron microscope, sediment more rapidly in sucrose density gradients than those generated with ATP or GTP, and can be captured with anticoatomerantibodies. The process of coatomer-coated vesicle formation from the TGN can be dissected into two distinct sequential phases, corresponding to coat assembly/bud formation and vesicle scission. The first phase is completed when Golgi fractions are incubated with cytosolic proteins and nonhydrolyzable GTP analogs at 20 degrees C. The scission phase, which leads to vesicle release, takes place when coated Golgi membranes, recovered after phase I, are incubated at higher temperatures in the presence of cytosolic proteins. The scission phase does not take place if protein kinase C inhibitors are added during the first phase, even though these inhibitors do not prevent membrane coating and bud formation. The phosphorylating activity of a protein kinase C, however, plays no role in vesicle formation, since this process does not require ATP.     

Segregation of the Qb‐SNAREs GS27 and GS28 into Golgi Vesicles Regulates Intra‐Golgi Transport

The Golgi apparatus is the main glycosylation and sorting station along the secretory pathway. Its structure includes the Golgi vesicles, which are depleted of anterograde cargo, and also of at least some Golgi‐resident proteins. The role of Golgi vesicles remains unclear. Here, we show that Golgi vesicles are enriched in the Qb‐SNAREs GS27 (membrin) and GS28 (GOS‐28), and depleted of nucleotide sugar transporters. A block of intra‐Golgi transport leads to accumulation of Golgi vesicles and partitioning of GS27 and GS28 into these vesicles. Conversely, active intra‐Golgi transport induces fusion of these vesicles with the Golgi cisternae, delivering GS27 and GS28 to these cisternae. In an in vitro assay based on a donor compartment that lacks UDP‐galactose translocase (a sugar transporter), the segregation of Golgi vesicles from isolated Golgi membranes inhibits intra‐Golgi transport; re‐addition of isolated Golgi vesicles devoid of UDP‐galactose translocase obtained from normal cells restores intra‐Golgi transport. We conclude that this activity is due to the presence of GS27 and GS28 in the Golgi vesicles, rather than the sugar transporter. Furthermore, there is an inverse correlation between the number of Golgi vesicles and the number of inter‐cisternal connections under different experimental conditions. Finally, a rapid block of the formation of vesicles via COPI through degradation of ϵCOP accelerates the cis‐to‐trans delivery of VSVG. These data suggest that Golgi vesicles, presumably with COPI, serve to inhibit intra‐Golgi transport by the extraction of GS27 and GS28 from the Golgi cisternae, which blocks the formation of inter‐cisternal connections .     

Reconstitution of COPII vesicle fusion to generate a pre-Golgi intermediate compartment

What is the first membrane fusion step in the secretory pathway? In mammals, transport vesicles coated with coat complex (COP) II deliver secretory cargo to vesicular tubular clusters (VTCs) that ferry cargo from endoplasmic reticulum exit sites to the Golgi stack. However, the precise origin of VTCs and the membrane fusion step(s) involved have remained experimentally intractable. Here, we document in vitro direct tethering and SNARE-dependent fusion of endoplasmic reticulum–derived COPII transport vesicles to form larger cargo containers. The assembly did not require detectable Golgi membranes, preexisting VTCs, or COPI function. Therefore, COPII vesicles appear to contain all of the machinery to initiate VTC biogenesis via homotypic fusion. However, COPI function enhanced VTC assembly, and early VTCs acquired specific Golgi components by heterotypic fusion with Golgi-derived COPI vesicles.     

COPI-independent Anterograde Transport: Cargo-selective ER to Golgi Protein Transport in Yeast COPI Mutants

The coatomer (COPI) complex mediates Golgi to ER recycling of membrane proteins containing a dilysine retrieval motif. However, COPI was initially characterized as an anterograde-acting coat complex. To investigate the direct and primary role(s) of COPI in ER/Golgi transport and in the secretory pathway in general, we used PCR-based mutagenesis to generate new temperature-conditional mutant alleles of one COPI gene in Saccharomyces cerevisiae, SEC21 (γ-COP). Unexpectedly, all of the new sec21 ts mutants exhibited striking, cargo-selective ER to Golgi transport defects. In these mutants, several proteins (i.e., CPY and α-factor) were completely blocked in the ER at nonpermissive temperature; however, other proteins (i.e., invertase and HSP150) in these and other COPI mutants were secreted normally. Nearly identical cargo-specific ER to Golgi transport defects were also induced by Brefeldin A. In contrast, all proteins tested required COPII (ER to Golgi coat complex), Sec18p (NSF), and Sec22p (v-SNARE) for ER to Golgi transport. Together, these data suggest that COPI plays a critical but indirect role in anterograde transport, perhaps by directing retrieval of transport factors required for packaging of certain cargo into ER to Golgi COPII vesicles. Interestingly, CPY–invertase hybrid proteins, like invertase but unlike CPY, escaped the sec21 ts mutant ER block, suggesting that packaging into COPII vesicles may be mediated by cis-acting sorting determinants in the cargo proteins themselves. These hybrid proteins were efficiently targeted to the vacuole, indicating that COPI is also not directly required for regulated Golgi to vacuole transport. Additionally, the sec21 mutants exhibited early Golgi-specific glycosylation defects and structural aberrations in early but not late Golgi compartments at nonpermissive temperature. Together, these studies demonstrate that although COPI plays an important and most likely direct role both in Golgi–ER retrieval and in maintenance/function of the cis-Golgi, COPI does not appear to be directly required for anterograde transport through the secretory pathway.     

Identification and characterization of GIV, a novel Galpha i/s-interacting protein found on COPI, endoplasmic reticulum-Golgi transport vesicles

In this report, we characterize GIV (Galpha-interacting vesicle-associated protein), a novel protein that binds members of the Galpha(i) and Galpha subfamilies of heterotrimeric G proteins. The Galpha(s) interaction site was mapped to an 83-amino acid region of GIV that is enriched in highly charged amino acids. BLAST searches revealed two additional mammalian family members, Daple and an uncharacterized protein, FLJ00354. These family members share the highest homology at the Galpha binding domain, are homologous at the N terminus and central coiled coil domain but diverge at the C terminus. Using affinity-purified IgG made against two different regions of the protein, we localized GIV to COPI, endoplasmic reticulum (ER)-Golgi transport vesicles concentrated in the Golgi region in GH3 pituitary cells and COS7 cells. Identification as COPI vesicles was based on colocalization with beta-COP, a marker for these vesicles. GIV also codistributes in the Golgi region with endogenous calnuc and the KDEL receptor, which are cis Golgi markers and with Galpha(i3)-yellow fluorescent protein expressed in COS7 cells. By immunoelectron microscopy, GIV colocalizes with beta-COP and Galpha(i3) on vesicles found in close proximity to ER exit sites and to cis Golgi cisternae. In cell fractions prepared from rat liver, GIV is concentrated in a carrier vesicle fraction (CV2) enriched in ER-Golgi transport vesicles. beta-COP and several Galpha subunits (Galpha(i1-3), Galpha(s)) are also most enriched in CV2. Our results demonstrate the existence of a novel Galpha-interacting protein associated with COPI transport vesicles that may play a role in Galpha-mediated effects on vesicle trafficking within the Golgi and/or between the ER and the Golgi.     

Quantitative proteomics analysis of the secretory pathway

We report more than 1400 proteins of the secretory-pathway proteome and provide spatial information on the relative presence of each protein in the rough and smooth ER Golgi cisternae and Golgi-derived COPI vesicles. The data support a role for COPI vesicles in recycling and cisternal maturation, showing that Golgi-resident proteins are present at a higher concentration than secretory cargo. Of the 1400 proteins, 345 were identified as previously uncharacterized. Of these, 230 had their subcellular location deduced by proteomics. This study provides a comprehensive catalog of the ER and Golgi proteomes with insight into their identity and function.     

Novel isotypic gamma/zeta subunits reveal three coatomer complexes in mammals

In early secretory transport, coat recruitment for the formation of coat protein I (COPI) vesicles involves binding to donor Golgi membranes of the small GTPase ADP-ribosylation factor 1 and subsequent attachment of the cytoplasmic heptameric complex coatomer. Various hypotheses exist as to the precise role of and possible routes taken by COPI vesicles in the mammalian cell. Here we report the ubiquitous expression of two novel isotypes of coatomer subunits gamma- and zeta-COP that are incorporated into coatomer, and show that three isotypes exist of the complex defined by the subunit combinations gamma 1/zeta 1, gamma 1/zeta 2, and gamma 2/zeta 1. In a liver cytosol, these forms make up the total coatomer in a ratio of about 2:1:2, respectively. The coatomer isotypes are located differentially within the early secretory pathway, and the gamma 2/zeta 1 isotype is preferentially incorporated into COPI vesicles. A population of COPI vesicles was characterized that almost exclusively contains gamma 2/zeta 1 coatomer. This existence of three structurally different forms of coatomer will need to be considered in future models of COPI-mediated transport.     

Sequential coupling between COPII and COPI vesicle coats in endoplasmic reticulum to Golgi transport

COPI and COPII are vesicle coat complexes whose assembly is regulated by the ARF1 and Sar1 GTPases, respectively. We show that COPI and COPII coat complexes are recruited separately and independently to ER (COPII), pre-Golgi (COPI, COPII), and Golgi (COPI) membranes of mammalian cells. To address their individual roles in ER to Golgi transport, we used stage specific in vitro transport assays to synchronize movement of cargo to and from pre-Golgi intermediates, and GDP- and GTP-restricted forms of Sar1 and ARF1 proteins to control coat recruitment. We find that COPII is solely responsible for export from the ER, is lost rapidly following vesicle budding and mediates a vesicular step required for the build-up of pre-Golgi intermediates composed of clusters of vesicles and small tubular elements. COPI is recruited onto pre-Golgi intermediates where it initiates segregation of the anterograde transported protein vesicular stomatitis virus glycoprotein (VSV-G) from the retrograde transported protein p58, a protein which actively recycles between the ER and pre-Golgi intermediates. We propose that sequential coupling between COPII and COPI coats is essential to coordinate and direct bi-directional vesicular traffic between the ER and pre-Golgi intermediates involved in transport of protein to the Golgi complex.     

COPII vesicles derived from mammalian endoplasmic reticulum microsomes recruit COPI

ER to Golgi transport requires the function of two distinct vesicle coat complexes, termed COPI (coatomer) and COPII, whose assembly is regulated by the small GTPases ADP-ribosylation factor 1 (ARF1) and Sar1, respectively. To address their individual roles in transport, we have developed a new assay using mammalian microsomes that reconstitute the formation of ER-derived vesicular carriers. Vesicles released from the ER were found to contain the cargo molecule vesicular stomatitis virus glycoprotein (VSV-G) and p58, an endogenous protein that continuously recycles between the ER and pre-Golgi intermediates. Cargo was efficiently sorted from resident ER proteins during vesicle formation in vitro. Export of VSV-G and p58 were found to be exclusively mediated by COPII. Subsequent movement of ER-derived carriers to the Golgi stack was blocked by a trans-dominant ARF1 mutant restricted to the GDP-bound state, which is known to prevent COPI recruitment. To establish the initial site of coatomer assembly after export from the ER, we immunoisolated the vesicular intermediates and tested their ability to recruit COPI. Vesicles bound coatomer in a physiological fashion requiring an ARF1-guanine nucleotide exchange activity. These results suggest that coat exchange is an early event preceding the targeting of ER-derived vesicles to pre-Golgi intermediates.