Disruptions in Golgi structure and membrane traffic in a conditional lethal mammalian cell mutant are corrected by epsilon-COP

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 (gamma-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 alpha-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.

The mammalian homolog of yeast Sec13p is enriched in the intermediate compartment and is essential for protein transport from the endoplasmic reticulum to the Golgi apparatus

The role of COPII components in endoplasmic reticulum (ER)-Golgi transport, first identified in the yeast Saccharomyces cerevisiae, has yet to be fully characterized in higher eukaryotes. A human cDNA whose predicted amino acid sequence showed 70% similarity to the yeast Sec13p has previously been cloned. Antibodies raised against the human SEC13 protein (mSEC13) recognized a cellular protein of 35 kDa in both the soluble and membrane fractions. Like the yeast Sec13p, mSEC13 exist in the cytosol in both monomeric and higher-molecular-weight forms. Immunofluorescence microscopy localized mSEC13 to the characteristic spotty ER-Golgi intermediate compartment (ERGIC) in cells of all species examined, where it colocalized well with the KDEL receptor, an ERGIC marker, at 15 degrees C. Immunoelectron microscopy also localized mSEC13 to membrane structures close to the Golgi apparatus. mSEC13 is essential for ER-to-Golgi transport, since both the His6-tagged mSEC13 recombinant protein and the affinity-purified mSEC13 antibody inhibited the transport of restrictive temperature-arrested vesicular stomatitis virus G protein from the ER to the Golgi apparatus in a semi-intact cell assay. Moreover, cytosol immunodepleted of mSEC13 could no longer support ER-Golgi transport. Transport could be restored in a dose-dependent manner by a cytosol fraction enriched in the high-molecular-weight mSEC13 complex but not by a fraction enriched in either monomeric mSEC13 or recombinant mSEC13. As a putative component of the mammalian COPII complex, mSEC13 showed partially overlapping but mostly different properties in terms of localization, membrane recruitment, and dynamics compared to that of beta-COP, a component of the COPI complex.

Characterization of the endogenous mono-ADP-ribosylation stimulated by brefeldin A

We have recently described a novel enzymatic mono-ADP-ribosyl transfer reaction induced by brefeldin A, a well characterized inhibitor of vesicular traffic, which selectively modifies two cytosolic proteins of 38 kDa (p38) and 50 kDa (BARS-50). p38 was identified as glyceraldehyde-3-phosphate dehydrogenase (GAPDH), a glycolytic enzyme and a multifunctional protein involved in several cellular processes; BARS-50 might be a novel G protein, since it is able to bind GTP and the beta gamma subunit of G proteins. We have characterized this enzymatic activity and screened in vitro the effects of different drugs belonging to the coumarine (dicumarol, coumermicin A1 and novobiocin) and quinone (ilimaquinones, benzoquinones and naphtoquinones) class. These drugs blocked the BFA-dependent mono-ADP-ribosylation, showed remarkable effects on Golgi morphology in control cells, and antagonized the tubular reticular redistribution of the Golgi complex in brefeldin A treated cells (see papers of Corda and Colanzi in this issue) suggesting a possible role for ADP-ribosylation in both the cellular effects of brefeldin A and the maintenance of the structure/function of the Golgi complex.

Endocytosis and molecular sorting

Endocytosis in eukaryotic cells is characterized by the continuous and regulated formation of prolific numbers of membrane vesicles at the plasma membrane. These vesicles come in several different varieties, ranging from the actin-dependent formation of phagosomes involved in particle uptake, to smaller clathrin-coated vesicles responsible for the internalization of extracellular fluid and receptor-bound ligands. In general, each of these vesicle types results in the delivery of their contents to lysosomes for degradation. The membrane components of endocytic vesicles, on the other hand, are subject to a series of highly complex and iterative molecular sorting events resulting in their targeting to specific destinations. In recent years, much has been learned about the function of the endocytic pathway and the mechanisms responsible for the molecular sorting of proteins and lipids. This review attempts to integrate these new concepts with long-established views of endocytosis to present a more coherent picture of how the endocytic pathway is organized and how the intracellular transport of internalized membrane components is controlled. Of particular importance are emerging concepts concerning the protein-based signals responsible for molecular sorting and the cytosolic complexes responsible for the decoding of these signals.

Signal-mediated sorting of membrane proteins between the endoplasmic reticulum and the golgi apparatus

Each organelle of the secretory pathway is required to selectively allow transit of newly synthesized secretory and plasma membrane proteins and also to maintain a unique set of resident proteins that define its structural and functional properties. In the case of the endoplasmic reticulum (ER), residency is achieved in two ways: (a) prevention of residents from entering newly forming transport vesicles and (b) retrieval of those residents that escape. The latter mechanism is directed by discrete retrieval motifs: Soluble proteins have a H/KDEL sequence at their carboxy-terminus; membrane proteins have a dibasic motif, either di-lysine or di-arginine, located close to the terminus of their cytoplasmic domain. Recently it was found that di-lysine motifs bind the complex of cytosolic coat proteins, COP I, and that this interaction functions in the retrieval of proteins from the Golgi to the ER. Also discussed are the potential roles this interaction may have in vesicular trafficking.

Bimodal interaction of coatomer with the p24 family of putative cargo receptors

Cytoplasmic domains of members of the p24 family of putative cargo receptors were shown to bind to coatomer, the coat protein of COPI-coated transport vesicles. Domains that contained dilysine endoplasmic reticulum retrieval signals bound the alpha-, beta'-, and epsilon-COP subunits of coatomer, whereas other p24 domains bound the beta-, gamma-, and zeta-COP subunits and required a phenylalanine-containing motif. Transit of a CD8-p24 chimera from the endoplasmic reticulum through the Golgi complex was slowed when the phenylalanine motif was mutated, suggesting that this motif may function as an anterograde transport signal. The either-or bimodal binding of coatomer to p24 tails suggests models for how coatomer can potentially package retrograde-directed and anterograde-directed cargo into distinct COPI-coated vesicles.

Targeting of nitric oxide synthase to endothelial cell caveolae via palmitoylation: implications for nitric oxide signaling

The membrane association of endothelial nitric oxide synthase (eNOS) plays an important role in the biosynthesis of nitric oxide (NO) in vascular endothelium. Previously, we have shown that in cultured endothelial cells and in intact blood vessels, eNOS is found primarily in the perinuclear region of the cells and in discrete regions of the plasma membrane, suggesting trafficking of the protein from the Golgi to specialized plasma membrane structures. Here, we show that eNOS is found in Triton X-100-insoluble membranes prepared from cultured bovine aortic endothelial cells and colocalizes with caveolin, a coat protein of caveolae, in cultured bovine lung microvascular endothelial cells as determined by confocal microscopy. To examine if eNOS is indeed in caveolae, we purified luminal endothelial cell plasma membranes and their caveolae directly from intact, perfused rat lungs. eNOS is found in the luminal plasma membranes and is markedly enriched in the purified caveolae. Because palmitoylation of eNOS does not significantly influence its membrane association, we next examined whether this modification can affect eNOS targeting to caveolae. Wild-type eNOS, but not the palmitoylation mutant form of the enzyme, colocalizes with caveolin on the cell surface in transfected NIH 3T3 cells, demonstrating that palmitoylation of eNOS is necessary for its targeting into caveolae. These data suggest that the subcellular targeting of eNOS to caveolae can restrict NO signaling to specific targets within a limited microenvironment at the cell surface and may influence signal transduction through caveolae.

A single point mutation in epsilon-COP results in temperature-sensitive, lethal defects in membrane transport in a Chinese hamster ovary cell mutant

At the nonpermissive temperature of 39.5 degrees C, the Chinese hamster ovary cell conditionally lethal, temperature-sensitive (ts) mutant ldlF exhibits the following defects: rapid degradation of low density lipoprotein receptors, disruption of ER-through Golgi transport, and disintegration of the Golgi apparatus. All of these are corrected by transfection with an expression vector for wild-type epsilon-COP, a subunit of coatomers (Guo, Q., Vasile, E., and Krieger, M. (1994) J. Cell Biol. 125, 1213-1224). We now report the identification in ldlF cells of a point mutation in the epsilon-COP gene, Glu251 to Lys251, which prevents the corresponding cDNA from correcting the defects in transfected ldlF cells and the immunochemical analysis of the synthesis, structure, and stability of epsilon-COP. At the permissive temperature (34 degrees C), the steady state level of ts-epsilon-COP in ldlF cells was about half that of epsilon-COP in wild-type Chinese hamster ovary cells and the isoelectric point of ts-epsilon-COP was 0.14 pH units higher than that of the wild-type protein. The stability but not the biosynthesis of ts-epsilon-COP was temperature-sensitive (t1/2 > 6 h at 34 degrees C and approximately 1-2 h at 39.5 degrees C), and this accounts for the virtual absence of detectable ts-epsilon-COP protein in ldlF cells after incubation at 39.5 degrees C for > 6h. The steady state levels in ldlF cells of another coatomer subunit, beta-COP, and the peripheral Golgi protein ldlCp were not temperature-sensitive. Thus, a mutation in epsilon-COP that causes instability at 39.5 degrees C is responsible for all of the temperature-sensitive defects in ldlF cells, and the stability of beta-COP is not linked directly to that of epsilon-COP. ldlF cells should be useful for the future analysis of the structure and function of epsilon-COP, the assembly of COPs into coatomers, and the participation of coatomers in intracellular membrane transport.

Protein sorting by transport vesicles

Eukaryotic life depends on the spatial and temporal organization of cellular membrane systems. Recent advances in understanding the machinery of vesicle transport have established general principles that underlie a broad variety of physiological processes, including cell surface growth, the biogenesis of distinct intracellular organelles, endocytosis, and the controlled release of hormones and neurotransmitters.

An endosomal beta COP is involved in the pH-dependent formation of transport vesicles destined for late endosomes

In this paper, we show that beta COP is present on endosomes and is required for the formation of vesicles which mediate transport from early to late endosomes. Both the association of beta COP to endosomal membranes as well as transport vesicle formation depend on the lumenal pH. We find that epsilon COP, but not gamma COP, is also associated to endosomes, and that this association is also lumenal pH dependent. Our data, thus, indicate that a subset of COPs is part of the mechanism regulating endosomal membrane transport, and that membrane association of these COPs is controlled by the acidic properties of early endosomes, presumably via a trans-membrane pH sensor.

Biochemical heterogeneity and phosphorylation of coatomer subunits

The coat protomer complex I (COPI) family of coat proteins are involved in the assembly of membrane-associated coats thought to mediate vesicular transport between the endoplasmic reticulum and the Golgi complex, between adjacent Golgi cisternae, and possibly in the endocytic pathway. We investigated whether this heterogeneity in the sites of COPI action might be reflected in biochemical heterogeneity of one or more COPI subunits. A simplified method was devised to purify the cytosolic COPI precursor complex, coatomer, from rat liver cytosol. The individual subunits were analyzed by high resolution two dimensional gel electrophoresis and mass spectroscopic analysis of tryptic peptides. Considerable charge heterogeneity was observed, particularly for the beta-COP and delta-COP subunits. The multiple species detected, however, did not appear to reflect the presence of distinct translation products but rather a significant degree of protein phosphorylation. The observed pI of beta-COP was sensitive to alkaline phosphatase digestion. Moreover, isolation of coatomer from metabolically labeled tissue culture cells demonstrated directly that both beta-COP and delta-COP, but no other coatomer subunits, were serine-phosphorylated. COPI phosphorylation may regulate coatomer assembly, membrane recruitment, or the specificity of coatomer-organelle interaction.

Coat proteins and vesicle budding

The trafficking of proteins within eukaryotic cells is achieved by the capture of cargo and targeting molecules into vesicles that bud from a donor membrane and deliver their contents to a receiving department. This process is bidirectional and may involve multiple organelles within a cell. Distinct coat proteins mediate each budding event, serving both to shape the transport vesicle and to select by direct or indirect interaction the desired set of cargo molecules. Secretion, which has been viewed as a default pathway, may require sorting and packaging signals on transported molecules to ensure their rapid delivery to the cell surface.

COPI- and COPII-coated vesicles bud directly from the endoplasmic reticulum in yeast

The cytosolic yeast proteins Sec13p-Sec31p, Sec23p-Sec24p, and the small GTP-binding protein Sar1p generate protein transport vesicles by forming the membrane coat termed COPII. We demonstrate by thin section and immunoelectron microscopy that purified COPII components form transport vesicles directly from the outer membrane of isolated yeast nuclei. Another set of yeast cytosolic proteins, coatomer and Arf1p (COPI), also form coated buds and vesicles from the nuclear envelope. Formation of COPI-coated, but not COPII-coated, buds and vesicles on the nuclear envelope is inhibited by the fungal metabolite brefeldin A. The two vesicle populations are distinct. However, both vesicle types are devoid of endoplasmic reticulum (ER) resident proteins, and each contains targeting proteins necessary for docking at the Golgi complex. Our data suggest that COPI and COPII mediate separate vesicular transport pathways from the ER.

Cytoplasmic coat proteins involved in endosome function

Endosomes are intermediates for a complex series of sorting and transport events that occur during receptor-mediated endocytosis. These involve the recognition of targeting determinants on the cytoplasmic domains of many membrane proteins as well as the formations of specific transport vesicles. Accordingly, endosome function is likely to be governed by the regulated assembly of cytoplasmic coat complexes. We have found that, in vitro, endosomes recruit a characteristic set of cytoplasmic proteins in a GTP gamma S-stimulated and brefeldin A-sensitive fashion. Among these are members of the COP-I and ARF families of coat proteins. In addition, endosomes were also found to assemble distinct, clathrin-like coats. Since microinjection of antibodies to beta-COP inhibits the entry of enveloped viruses via the endocytic pathway, it is apparent that the recruitment of COP-I or COP-I-related proteins plays an important role in the function of endosomes in intact cells.

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.

Sorting and retrieval between the endoplasmic reticulum and Golgi apparatus

The highlight of the past year was the demonstration that retrieval of endoplasmic reticulum membrane proteins containing the di-lysine motif involves COPI coat proteins. Other findings contributed to the debate about the nature of the 'intermediate compartment' between the endoplasmic reticulum and the Golgi apparatus, and the mechanism by which transported proteins are concentrated at this step.

Membrane transport in the endocytic pathway

Despite controversies and debates, some fundamental properties of endosomes become apparent when comparing results from in vivo and in vitro strategies used to study endosomal membrane traffic. In addition, recent studies are starting to unravel the complex organization of early endosomes, in particular along the route followed by recycling receptors.

Anterograde transport through the Golgi complex: do Golgi tubules hold the key?

Biochemical studies have suggested that anterograde protein transport through the Golgi complex is mediated by coatomer-coated vesicles that bud from one compartment and then transfer to, and fuse with, the next. However, recent genetic studies have shown that coatomer mutations block retrograde, but not anterograde, transport in yeast, calling into question the role of coatomer vesicles in anterograde transport. Peggy Weidman proposes that these findings might be explained if anterograde transport occurs by transient fusion of Golgi tubules and if coatomers have related, but separable, functions in tubule and vesicle dynamics.

The role of coat proteins in the biosynthesis of secretory proteins

The biosynthesis of secretory proteins requires vesicle-mediated transport between the organelles of the secretory pathway. Biochemical and genetic analysis of the secretory pathway has identified two non-clathrin coats--COPI and COPII--that drive the formation of vesicles that mediate transport between the endoplasmic reticulum and the Golgi apparatus, and through the compartments of the Golgi. Recently, a molecular description of the subunits of these coats and the development of biochemical reagents to study their function has yielded new information on how these proteins share the task of organizing vesicle traffic early in the secretory pathway.

Non-clathrin-coat protein alpha is a conserved subunit of coatomer and in Saccharomyces cerevisiae is essential for growth

To complete the molecular characterization of coatomer, the preformed cytosolic complex that is involved in the formation of biosynthetic transport vesicles, we have cloned and characterized the gene for non-clathrin-coat protein alpha (alpha-COP) from Saccharomyces cerevisiae. The derived protein, molecular weight of 135,500, contains four WD-40 repeated motifs (Trp/Asp-containing motifs of approximately 40 amino acids). Disruption of the yeast alpha-COP gene is lethal. Comparison of the DNA-derived primary structure with peptides from bovine alpha-COP shows a striking homology. alpha-COP is localized to coated transport vesicles and coated buds of Golgi membranes derived from CHO cells.

A fluorescent lipid analogue can be used to monitor secretory activity and for isolation of mammalian secretion mutants

The use of reporter proteins to study the regulation of secretion has often been complicated by posttranslational processing events that influence the secretion of certain proteins, but are not part of the cellular mechanisms that specifically regulate secretion. This has been a particular limitation for the isolation of mammalian secretion mutants, which has typically been a slow process. To provide a reporter of secretory activity independent of protein processing events, cells were labeled with the fluorescent lipid analogue C5-DMB-ceramide (ceramide coupled to the fluorophore boron dipyrromethene difluoride) and its secretion was followed by fluorescence microscopy and fluorescence-activated cell sorting. Brefeldin A, which severely inhibits secretion in Chinese hamster ovary cells, blocked secretion of C5-DMB-ceramide. At high temperature, export of C5-DMB-ceramide was inhibited in HRP-1 cells, which have a conditional defect in secretion. Using C5-DMB-ceramide as a reporter of secretory activity, several different pulse-chase protocols were designed that selected mutant Chinese hamster ovary cells that were resistant to the drug brefeldin A and others that were defective in the transport of glycoproteins to the cell surface. Mutant cells of either type were identified in a mutagenized population at a frequency of 10(-6). Thus, the fluorescent lipid C5-DMB-ceramide can be used as a specific marker of secretory activity, providing an efficient, general approach for isolating mammalian cells with defects in the secretory pathway.

Coatomer is essential for retrieval of dilysine-tagged proteins to the endoplasmic reticulum

Dilysine motifs in cytoplasmic domains of transmembrane proteins are signals for their continuous retrieval from the Golgi back to the endoplasmic reticulum (ER). We describe a system to assess retrieval to the ER in yeast cells making use of a dilysine-tagged Ste2 protein. Whereas retrieval was unaffected in most sec mutants tested (sec7, sec12, sec13, sec16, sec17, sec18, sec19, sec22, and sec23), a defect in retrieval was observed in previously characterized coatomer mutants (sec21-1, sec27-1), as well as in newly isolated retrieval mutants (sec21-2, ret1-1). RET1 was cloned by complementation and found to encode the alpha subunit of coatomer. While temperature-sensitive for growth, the newly isolated coatomer mutants exhibited a very modest defect in secretion at the nonpermissive temperature. Coatomer from beta'-COP (sec27-1) and alpha-COP (ret1-1) mutants, but not from gamma-COP (sec21) mutants, had lost the ability to bind dilysine motifs in vitro. Together, these results suggest that coatomer plays an essential role in retrograde Golgi-to-ER transport and retrieval of dilysine-tagged proteins back to the ER.

LDLC encodes a brefeldin A-sensitive, peripheral Golgi protein required for normal Golgi function

Two genetically distinct classes of low density lipoprotein (LDL) receptor-deficient Chinese hamster ovary cell mutants, ldlB and ldlC, exhibit nearly identical pleiotropic defects in multiple medial and trans Golgi-associated processes (Kingsley, D., K. F. Kozarsky, M. Segal, and M. Krieger. 1986. J. Cell Biol. 102:1576-1585). In these mutants, the synthesis of virtually all N- and O-linked glycoproteins and of the major lipid-linked oligosaccharides is abnormal. The abnormal glycosylation of LDL receptors in ldlB and ldlC cells results in their dramatically reduced stability and thus very low LDL receptor activity. We have cloned and sequenced a human cDNA (LDLC) which corrects the mutant phenotypes of ldlC, but not ldlB, cells. Unlike wild-type CHO or ldlB cells, ldlC cells had virtually no detectable endogenous LDLC mRNA, indicating that LDLC is likely to be the normal human homologue of the defective gene in ldlC cells. The predicted sequence of the human LDLC protein (ldlCp, approximately 83 kD) is not similar to that of any known proteins, and contains no major common structural motifs such as transmembrane domains or an ER translocation signal sequence. We have also determined the sequence of the Caenorhabditis elegans ldlCp by cDNA cloning and sequencing. Its similarity to that of human ldlCp suggests that ldlCp mediates a well-conserved cellular function. Immunofluorescence studies with anti-ldlCp antibodies in mammalian cells established that ldlCp is a peripheral Golgi protein whose association with the Golgi is brefeldin A sensitive. In ldlB cells, ldlCp was expressed at normal levels; however, it was not associated with the Golgi. Thus, a combination of somatic cell and molecular genetics has identified a previously unrecognized protein, ldlCp, which is required for multiple Golgi functions and whose peripheral association with the Golgi is both LDLB dependent and brefeldin A sensitive.