Three distinct steps in transport of vesicular stomatitis virus glycoprotein from the ER to the cell surface in vivo with differential sensitivities to GTP gamma S

Comparison of the Cisterna Maturation-Progression Model with the Kiss-and-Run Model of Intra-Golgi Transport: Role of Cisternal Pores and Cargo Domains

The Golgi complex is the central station of the secretory pathway. Knowledge about the mechanisms of intra-Golgi transport is inconsistent. Here, we compared the explanatory power of the cisterna maturation-progression model and the kiss-and-run model. During intra-Golgi transport, conventional cargoes undergo concentration and form cisternal distensions or distinct membrane domains that contain only one membrane cargo. These domains and distension are separated from the rest of the Golgi cisternae by rows of pores. After the arrival of any membrane cargo or a large cargo aggregate at the Golgi complex, the cis-Golgi SNAREs become enriched within the membrane of cargo-containing domains and then replaced by the trans-Golgi SNAREs. During the passage of these domains, the number of cisternal pores decreases. Restoration of the cisternal pores is COPI-dependent. Our observations are more in line with the kiss-and-run model.

Balanced trafficking between the ER and the Golgi apparatus increases protein secretion in yeast

The yeast Saccharomyces cerevisiae is widely used as a cell factory to produce recombinant proteins. However, S. cerevisiae naturally secretes only a few proteins, such as invertase and the mating alpha factor, and its secretory capacity is limited. It has been reported that engineering protein anterograde trafficking from the endoplasmic reticulum to the Golgi apparatus by the moderate overexpression of SEC16 could increase recombinant protein secretion in S. cerevisiae. In this study, the retrograde trafficking in a strain with moderate overexpression of SEC16 was engineered by overexpression of ADP-ribosylation factor GTP activating proteins, Gcs1p and Glo3p, which are involved in the process of COPI-coated vesicle formation. Engineering the retrograde trafficking increased the secretion of α-amylase but did not induce production of reactive oxygen species. An expanded ER membrane was detected in both the GCS1 and GLO3 overexpression strains. Physiological characterizations during batch fermentation showed that GLO3 overexpression had better effect on recombinant protein secretion than GCS1 overexpression. Additionally, the GLO3 overexpression strain had higher secretion of two other recombinant proteins, endoglucanase I from Trichoderma reesei and glucan-1,4-α-glucosidase from Rhizopus oryzae, indicating overexpression of GLO3 in a SEC16 moderate overexpression strain might be a general strategy for improving production of secreted proteins by yeast.

A role for Sar1 and ARF1 GTPases during Golgi biogenesis in the protozoan parasite Trypanosoma brucei

A single Golgi stack is duplicated and partitioned into two daughter cells during the cell cycle of the protozoan parasite Trypanosoma brucei The source of components required to generate the new Golgi and the mechanism by which it forms are poorly understood. Using photoactivatable GFP, we show that the existing Golgi supplies components directly to the newly forming Golgi in both intact and semipermeabilized cells. The movement of a putative glycosyltransferase, GntB, requires the Sar1 and ARF1 GTPases in intact cells. In addition, we show that transfer of GntB from the existing Golgi to the new Golgi can be recapitulated in semipermeabilized cells and is sensitive to the GTP analogue GTPγS. We suggest that the existing Golgi is a key source of components required to form the new Golgi and that this process is regulated by small GTPases.

Combined biochemical and cytological analysis of membrane trafficking using lectins

We have tested the application of high-mannose-binding lectins as analytical reagents to identify N-glycans in the early secretory pathway of HeLa cells during subcellular fractionation and cytochemistry. Post-endoplasmic reticulum (ER) pre-Golgi intermediates were separated from the ER on Nycodenz-sucrose gradients, and the glycan composition of each gradient fraction was profiled using lectin blotting. The fractions containing the post-ER pre-Golgi intermediates are found to contain a subset of N-linked α-mannose glycans that bind the lectins Galanthus nivalis agglutinin (GNA), Pisum sativum agglutinin (PSA), and Lens culinaris agglutinin (LCA) but not lectins binding Golgi-modified glycans. Cytochemical analysis demonstrates that high-mannose-containing glycoproteins are predominantly localized to the ER and the early secretory pathway. Indirect immunofluorescence microscopy revealed that GNA colocalizes with the ER marker protein disulfide isomerase (PDI) and the COPI coat protein β-COP. In situ competition with concanavalin A (ConA), another high-mannose specific lectin, and subsequent GNA lectin histochemistry refined the localization of N-glyans containing nonreducing mannosyl groups, accentuating the GNA vesicular staining. Using GNA and treatments that perturb ER-Golgi transport, we demonstrate that lectins can be used to detect changes in membrane trafficking pathways histochemically. Overall, we find that conjugated plant lectins are effective tools for combinatory biochemical and cytological analysis of membrane trafficking of glycoproteins.

Sedlin controls the ER export of procollagen by regulating the Sar1 cycle

Newly synthesized proteins exit the endoplasmic reticulum (ER) via coat protein complex II (COPII) vesicles. Procollagen (PC), however, forms prefibrils that are too large to fit into typical COPII vesicles; PC thus needs large transport carriers, which we term megacarriers. TANGO1 assists PC packing, but its role in promoting the growth of megacarriers is not known. We found that TANGO1 recruited Sedlin, a TRAPP component that is defective in spondyloepiphyseal dysplasia tarda (SEDT), and that Sedlin was required for the ER export of PC. Sedlin bound and promoted efficient cycling of Sar1, a guanosine triphosphatase that can constrict membranes, and thus allowed nascent carriers to grow and incorporate PC prefibrils. This joint action of TANGO1 and Sedlin sustained the ER export of PC, and its derangement may explain the defective chondrogenesis underlying SEDT.

L1 syndrome mutations impair neuronal L1 function at different levels by divergent mechanisms

Mutations in the human L1CAM gene cause neurodevelopmental disorders collectively referred to as L1 syndrome. Here, we investigated cellular pathomechanisms underlying two L1 syndrome mutations, R184Q and W1036L. We demonstrate that these mutations cause partial endoplasmic reticulum (ER) retention of L1, reduce L1 cell surface expression, but do not induce ER stress in neuronal NSC-34 cells. We provide evidence that surface trafficking of mutated L1 is affected by defective sorting to ER exit sites and attenuated ER export. However, in differentiated neuronal cultures and long-term cultured hippocampal slices, the L1-R184Q protein is restricted to cell bodies, whereas L1-W1036L also aberrantly localizes to dendrites. These trafficking defects preclude axonal targeting of L1, thereby affecting L1-mediated axon growth and arborization. Our results indicate that L1 syndrome mutations impair neuronal L1 function at different levels, firstly by attenuating ER export and secondly by interfering with polarized neuronal trafficking.

Cholera toxin: A paradigm for multi-functional engagement of cellular mechanisms (Review)

Cholera toxin (Ctx) from Vibrio cholerae and its closely related homologue, heat-labile enterotoxin (Etx) from Escherichia coli have become superb tools for illuminating pathways of cellular trafficking and immune cell function. These bacterial protein toxins should be viewed as conglomerates of highly evolved, multi-functional elements equipped to engage the trafficking and signalling machineries of cells. Ctx and Etx are members of a larger family of A-B toxins of bacterial (and plant) origin that are comprised of structurally and functionally distinct enzymatically active A and receptor-binding B sub-units or domains. Intoxication of mammalian cells by Ctx and Etx involves B pentamer-mediated receptor binding and entry into a vesicular pathway, followed by translocation of the enzymatic A1 domain of the A sub-unit into the target cell cytosol, where covalent modification of intracellular targets leads to activation of adenylate cyclase and a sequence of events culminating in life-threatening diarrhoeal disease. Importantly, Ctx and Etx also have the capacity to induce a wide spectrum of remarkable immunological processes. With respect to the latter, it has been found that these toxins activate signalling pathways that modulate the immune system. This review explores the complexities of the cellular interactions that are engaged by these bacterial protein toxins, and highlights some of the new insights to have recently emerged.

Ankyrin facilitates intracellular trafficking of alpha1-Na+-K+-ATPase in polarized cells

Defects in ankyrin underlie many hereditary disorders involving the mislocalization of membrane proteins. Such phenotypes are usually attributed to ankyrin's role in stabilizing a plasma membrane scaffold, but this assumption may not be accurate. We found in Madin-Darby canine kidney cells and in other cultured cells that the 25-residue ankyrin-binding sequence of alpha(1)-Na(+)-K(+)-ATPase facilitates the entry of alpha(1),beta(1)-Na(+)-K(+)-ATPase into the secretory pathway and that replacement of the cytoplasmic domain of vesicular stomatitis virus G protein (VSV-G) with this ankyrin-binding sequence bestows ankyrin dependency on the endoplasmic reticulum (ER) to Golgi trafficking of VSV-G. Expression of the ankyrin-binding sequence of alpha(1)-Na(+)-K(+)-ATPase alone as a soluble cytosolic peptide acts in trans to selectively block ER to Golgi transport of both wild-type alpha(1)-Na(+)-K(+)-ATPase and a VSV-G fusion protein that includes the ankyrin-binding sequence, whereas the trafficking of other proteins remains unaffected. Similar phenotypes are also generated by small hairpin RNA-mediated knockdown of ankyrin R or the depletion of ankyrin in semipermeabilized cells. These data indicate that the adapter protein ankyrin acts not only at the plasma membrane but also early in the secretory pathway to facilitate the intracellular trafficking of alpha(1)-Na(+)-K(+)-ATPase and presumably other selected proteins. This novel ankyrin-dependent assembly pathway suggests a mechanism whereby hereditary disorders of ankyrin may be manifested as diseases of membrane protein ER retention or mislocalization.

Mouse hepatitis coronavirus RNA replication depends on GBF1-mediated ARF1 activation

Coronaviruses induce in infected cells the formation of double membrane vesicles, which are the sites of RNA replication. Not much is known about the formation of these vesicles, although recent observations indicate an important role for the endoplasmic reticulum in the formation of the mouse hepatitis coronavirus (MHV) replication complexes (RCs). We now show that MHV replication is sensitive to brefeldin A (BFA). Consistently, expression of a dominant-negative mutant of ARF1, known to mimic the action of the drug, inhibited MHV infection profoundly. Immunofluorescence analysis and quantitative electron microscopy demonstrated that BFA did not block the formation of RCs per se, but rather reduced their number. MHV RNA replication was not sensitive to BFA in MDCK cells, which are known to express the BFA-resistant guanine nucleotide exchange factor GBF1. Accordingly, individual knockdown of the Golgi-resident targets of BFA by transfection of small interfering RNAs (siRNAs) showed that GBF1, but not BIG1 or BIG2, was critically involved in MHV RNA replication. ARF1, the cellular effector of GBF1, also appeared to be involved in MHV replication, as siRNAs targeting this small GTPase inhibited MHV infection significantly. Collectively, our results demonstrate that GBF1-mediated ARF1 activation is required for efficient MHV RNA replication and reveal that the early secretory pathway and MHV replication complex formation are closely connected.

Coronaviruses are the causative agents of many respiratory and enteric infections in humans and animals. As with all viruses, virtually all of the steps of their infection cycle depend on host cellular factors. As the first and most crucial step after their entry into cells, coronaviruses assemble their replication complexes (RCs) in association with characteristic, newly induced membranous structures. The cellular pathways hijacked by these plus-strand RNA viruses to create these “factories” have not been elucidated. Here, we study the involvement of the secretory pathway in mouse hepatitis coronavirus (MHV) replication by using the drug brefeldin A (BFA), which is known to interfere with ER–Golgi membrane traffic by inhibiting the activation of ADP-ribosylation factor (ARF) small GTPases. Our observations show that MHV RNA replication is sensitive to BFA. In agreement herewith we demonstrate, by using various techniques, that the BFA-sensitive guanidine nucleotide exchange factor GBF1 and its downstream effector ARF1 are of critical importance for coronavirus replication. From our results we conclude that MHV RNA replication depends on GBF1-mediated ARF1 activation. Our study provides new insights into the close connection between MHV replication and the early secretory pathway.

Localization and membrane topology of coronavirus nonstructural protein 4: involvement of the early secretory pathway in replication

The coronavirus nonstructural proteins (nsp's) derived from the replicase polyproteins collectively constitute the viral replication complexes, which are anchored to double-membrane vesicles. Little is known about the biogenesis of these complexes, the membrane anchoring of which is probably mediated by nsp3, nsp4, and nsp6, as they contain several putative transmembrane domains. As a first step to getting more insight into the formation of the coronavirus replication complex, the membrane topology, processing, and subcellular localization of nsp4 of the mouse hepatitis virus (MHV) and severe acute respiratory syndrome-associated coronavirus (SARS-CoV) were elucidated in this study. Both nsp4 proteins became N glycosylated, while their amino and carboxy termini were localized to the cytoplasm. These observations imply nsp4 to assemble in the membrane as a tetraspanning transmembrane protein with a Nendo/Cendo topology. The amino terminus of SARS-CoV nsp4, but not that of MHV nsp4, was shown to be (partially) processed by signal peptidase. nsp4 localized to the endoplasmic reticulum (ER) when expressed alone but was recruited to the replication complexes in infected cells. nsp4 present in these complexes did not colocalize with markers of the ER or Golgi apparatus, while the susceptibility of its sugars to endoglycosidase H indicated that the protein had also not traveled trough the latter compartment. The important role of the early secretory pathway in formation of the replication complexes was also demonstrated by the inhibition of coronaviral replication when the ER export machinery was blocked by use of the kinase inhibitor H89 or by expression of a mutant, Sar1[H79G].

Differential Requirements for ts-O45-G and Procollagen Biosynthetic Transport

Emerging experimental evidence favours the existence of cargo sorting occurring upon the endoplasmic reticulum (ER) exit. Recent studies revealed that, in contrast to the conventional secretory marker ts-O45-G, procollagen (PC I) exits the ER at sites not coated with coat protein II and is transported to the Golgi complex in carriers devoid of coat protein I. Here, we investigated whether PC I trafficking requires a different molecular machinery in comparison with the ts-O45-G. By combining colocalization of the cargoes with endogenous markers, downregulation of transport machinery by RNA interference and knock-ins by complementary DNA over-expression, we provide strong evidence that PC I and ts-O45-G have common but also different molecular requirements during pre- and post-Golgi trafficking events.

Delivery of the malaria virulence protein PfEMP1 to the erythrocyte surface requires cholesterol-rich domains

The particular virulence of the human malaria parasite Plasmodium falciparum derives from export of parasite-encoded proteins to the surface of the mature erythrocytes in which it resides. The mechanisms and machinery for the export of proteins to the erythrocyte membrane are largely unknown. In other eukaryotic cells, cholesterol-rich membrane microdomains or "rafts" have been shown to play an important role in the export of proteins to the cell surface. Our data suggest that depletion of cholesterol from the erythrocyte membrane with methyl-beta-cyclodextrin significantly inhibits the delivery of the major virulence factor P. falciparum erythrocyte membrane protein 1 (PfEMP1). The trafficking defect appears to lie at the level of transfer of PfEMP1 from parasite-derived membranous structures within the infected erythrocyte cytoplasm, known as the Maurer's clefts, to the erythrocyte membrane. Thus our data suggest that delivery of this key cytoadherence-mediating protein to the host erythrocyte membrane involves insertion of PfEMP1 at cholesterol-rich microdomains. GTP-dependent vesicle budding and fusion events are also involved in many trafficking processes. To determine whether GTP-dependent events are involved in PfEMP1 trafficking, we have incorporated non-membrane-permeating GTP analogs inside resealed erythrocytes. Although these nonhydrolyzable GTP analogs reduced erythrocyte invasion efficiency and partially retarded growth of the intracellular parasite, they appeared to have little direct effect on PfEMP1 trafficking.

Microinjection as a tool to explore small GTPase function

Microinjection overcomes the plasma membrane barrier to the introduction of charged or large nonlipid soluble molecules into cells by the direct insertion of a hollow capillary micropipette into the cell. With the application of pressure, aqueous solution is then directly transferred into either the cytosol or the nucleus. I give specific examples of the application of this approach to the functional study of small GTPases of the Sar1, ARF, and rab family in membrane trafficking between the Golgi apparatus and endoplasmic reticulum (ER). The principles illustrated by these examples should be generally applicable to other small GTPases. Detailed protocols for capillary microinjection using semiautomated equipment are given.

Biogenesis of tubular ER-to-Golgi transport intermediates

Tubular transport intermediates (TTIs) have been described as one class of transport carriers in endoplasmic reticulum (ER)-to-Golgi transport. In contrast to vesicle budding and fusion, little is known about the molecular regulation of TTI synthesis, transport and fusion with target membranes. Here we have used in vivo imaging of various kinds of GFP-tagged proteins to start to address these questions. We demonstrate that under steady-state conditions TTIs represent approximately 20% of all moving transport carriers. They increase in number and length when more transport cargo becomes available at the donor membrane, which we induced by either temperature-related transport blocks or increased expression of the respective GFP-tagged transport markers. The formation and motility of TTIs is strongly dependent on the presence of intact microtubules. Microinjection of GTPgammaS increases the frequency of TTI synthesis and the length of these carriers. When Rab proteins are removed from membranes by microinjection of recombinant Rab-GDI, the synthesis of TTIs is completely blocked. Microinjection of the cytoplasmic tails of the p23 and p24 membrane proteins also abolishes formation of p24-containing TTIs. Our data suggest that TTIs are ER-to-Golgi transport intermediates that form preferentially when transport-competent cargo exists in excess at the donor membrane. We propose a model where the interaction of the cytoplasmic tails of membrane proteins with microtubules are key determinants for TTI synthesis and may also serve as a so far unappreciated model for aspects of transport carrier formation.

Characterization of myosin-II binding to Golgi stacks in vitro

In addition to important roles near the actin-rich cell cortex, ample evidence indicates that multiple myosins are also involved in membrane movements in the endomembrane system. Nonmuscle myosin-II has been shown to have roles in anterograde and retrograde trafficking at the Golgi. Myosin-II is present on Golgi stacks isolated from intestinal epithelial cells and has been localized to the Golgi in several polarized and unpolarized cell lines. An understanding of roles of myosin-II in Golgi physiology will be facilitated by understanding the molecular arrangement of myosin-II at the Golgi. Salt-washing removes endogenous myosin-II from isolated Golgi and purified brush border myosin-II can bind in vitro. Brush border myosin-II binds to a tightly bound Golgi peripheral membrane protein with a K(1/2) of 75 nM and binding is saturated at 0.7 pmol myosin/microg Golgi. Binding studies using papain cleavage fragments of brush border myosin-II show that the 120-kDa rod domain, but not the head domain, of myosin heavy chain can bind directly to Golgi stacks. The 120-kDa domain does not bind to Golgi membranes when phosphorylated in vitro with casein kinase-II. These results suggest that phosphorylation in the rod domain may regulate the binding and/or release of myosin-II from the Golgi. These data support a model in which myosin-II is tethered to the Golgi membrane by its tail and actin filaments by its head. Thus, translocation along actin filaments may extend Golgi membrane tubules and/or vesicles away from the Golgi complex.

Differential effects of a GTP-restricted mutant of Sar1p on segregation of cargo during export from the endoplasmic reticulum

Export of cargo from the endoplasmic reticulum (ER) is the first membrane trafficking step in the secretory pathway. To date, all cargo proteins appear to use a common set of machinery for the initial stages of export, namely the COPII coat complex. Recent data from both yeast and mammalian systems have emerged suggesting that specific cargoes could be sorted from one another at the point of exit from the endoplasmic reticulum or immediately afterwards. Here, we have examined the mechanisms used for export of different types of cargo molecule from the endoplasmic reticulum. All cargoes examined utilise the COPII machinery, but specific differences are seen in the accumulation of cargo into ER-derived pre-budding complexes following expression of a GTP-restricted mutant of the Sar1p GTPase. Glycosylphosphatidylinositol (GPI)-anchored GFP is seen to be restricted to the ER under these conditions whereas other cargoes, including ts045-G and lumFP accumulate in pre-budding complexes. Following exit, GPI-FP, lumFP and ts045-G-FP all travel to the Golgi in the same vesicular tubular clusters (VTCs). These data show a differential requirement for efficient GTP hydrolysis by the Sar1p GTPase in export of cargo from the ER.

High-content screening microscopy identifies novel proteins with a putative role in secretory membrane traffic

Here we describe the establishment of microscope-based functional screening assays in intact cells that allow us to systematically identify new proteins involved in secretory membrane traffic, and proteins that can influence the integrity of the Golgi complex. We were able to identify 20 new proteins that affected either secretory transport, Golgi morphology, or both, when overexpressed in cells. Control experiments with human orthologs to yeast proteins with a role in membrane traffic, or already well characterized mammalian regulators of the secretory pathway, confirmed the specificity and significance of our results. Proteins localized to the Golgi complex or endoplasmic reticulum (ER) showed preferential interference in our assays. Bioinformatic analysis of the new proteins interfering with membrane traffic and/or Golgi integrity revealed broad functional variety, but demonstrated a bias towards proteins with predicted coiled-coil domains and repeat structures. Extending our approach to a much larger set of novel proteins in the future will be an important step toward a more comprehensive understanding of the molecular basis of the secretory pathway. It will also serve as an example for similar microscope-based screens addressing different biological questions.

Arf GAPs: multifunctional proteins that regulate membrane traffic and actin remodelling

The ADP-ribosylation factor (Arf) Arf GTPase-activating proteins (GAPs) are a family of proteins that induce hydrolysis of GTP bound to Arf. A conserved domain containing a zinc finger motif mediates catalysis. The substrate, Arf.GTP, affects membrane trafficking and actin remodelling. Consistent with activity as an Arf regulator, the Arf GAPs affect both of these pathways. However, the Arf GAPs are likely to have Arf-independent activities that contribute to their cellular functions. Structures of the Arf GAPs are diverse containing catalytic, protein-protein interaction and lipid interaction domains in addition to the Arf GAP domain. Some Arf GAPs have been identified and characterized on the basis of activities other than Arf GAP. Here, we describe the Arf GAP family, enzymology of some members of the Arf GAP family and known functions of the proteins. The results discussed illustrate roles for both Arf-dependent and -independent activities in the regulation of cellular architecture.

Evidence against an essential role of COPII-mediated cargo transport to the endoplasmic reticulum-Golgi intermediate compartment in the formation of the primary membrane of vaccinia virus

Vaccinia virus assembles two distinct lipoprotein membranes. The primary membrane contains nonglycosylated proteins, appears as crescents in the cytoplasm, and delimits immature and mature intracellular virions. The secondary or wrapping membrane contains glycoproteins, is derived from virus-modified trans-Golgi or endosomal cisternae, forms a loose coat around some intracellular mature virions, and becomes the envelope of extracellular virions. Although the mode of formation of the wrapping membrane is partially understood, we know less about the primary membrane. Recent reports posit that the primary membrane originates from the endoplasmic reticulum-Golgi intermediate compartment (ERGIC). According to this model, viral primary membrane proteins are cotranslationally inserted into the ER and accumulate in the ERGIC. To test the ERGIC model, we employed Sar1(H79G), a dominant negative form of the Sar1 protein, which is an essential component of coatomer protein II (COPII)-mediated cargo transport from the ER to the ERGIC and other post-ER compartments. Overexpression of Sar1(H79G) by transfection or by a novel recombinant vaccinia virus with an inducible Sar1(H79G) gene resulted in retention of ERGIC 53 in the ER but did not interfere with localization of viral primary membrane proteins in factory regions or with formation of viral crescent membranes and infectious intracellular mature virions. Wrapping of intracellular mature virions and formation of extracellular virions did not occur, however, because some proteins that are essential for the secondary membrane were retained in the ER as a consequence of Sar1(H79G) overexpression. Our data argue against an essential role of COPII-mediated cargo transport and the ERGIC in the formation of the viral primary membrane. Instead, viral membranes may be derived directly from the ER or by a novel mechanism.

A microscope-based screening platform for large-scale functional protein analysis in intact cells

A modular microscope-based screening platform, with applications in large-scale analysis of protein function in intact cells is described. It includes automated sample preparation, image acquisition, data management and analysis, and the genome-wide automated retrieval of bioinformatic information. The modular nature of the system ensures that it is rapidly adaptable to new biological questions or sets of proteins. Two automated functional assays addressing protein secretion and the integrity of the Golgi complex were developed and tested. This shows the potential of the system in large-scale, cell-based functional proteomic projects.

Biochemical sub-fractionation of the mammalian Golgi apparatus

We have exploited the breakdown of the Golgi apparatus that occurs during mitosis to isolate subfractions using immuno-affinity methods. Rat liver Golgi stacks were treated with mitotic cytosol from HeLa cells, and the fragments were then incubated with antibodies immobilized on magnetic beads. Antibodies against the cis-Golgi marker, GM130, bound membranes that were depleted in the trans-Golgi network marker, TGN38, whereas antibodies against the cytoplasmic tail of TGN38 did the reverse. A range of other Golgi enzymes, SNAREs and tethers were also tested and were found to bind to anti-GM130 antibodies to an extent that reflected their proximity to cis-cisternae as determined by other techniques. This method should provide a useful complement to the immuno-EM methods presently used to map the Golgi apparatus.

Inhibitors of COP-mediated transport and cholera toxin action inhibit simian virus 40 infection

Simian virus 40 (SV40) is a nonenveloped virus that has been shown to pass from surface caveolae to the endoplasmic reticulum in an apparently novel infectious entry pathway. We now show that the initial entry step is blocked by brefeldin A and by incubation at 20 degrees C. Subsequent to the entry step, the virus reaches a domain of the rough endoplasmic reticulum by an unknown pathway. This intracellular trafficking pathway is also brefeldin A sensitive. Infection is strongly inhibited by expression of GTP-restricted ADP-ribosylation factor 1 (Arf1) and Sar1 mutants and by microinjection of antibodies to betaCOP. In addition, we demonstrate a potent inhibition of SV40 infection by the dipeptide N-benzoyl-oxycarbonyl-Gly-Phe-amide, which also inhibits late events in cholera toxin action. Our results identify novel inhibitors of SV40 infection and show that SV40 requires COPI- and COPII-dependent transport steps for successful infection.

Imaging of procollagen transport reveals COPI-dependent cargo sorting during ER-to-Golgi transport in mammalian cells

We have examined the ER-to-Golgi transport of procollagen, which, when assembled in the lumen of the ER, is thought to be physically too large to fit in classically described 60-80 nm COPI- and COPII-coated transport vesicles. We found that procollagen exits the ER via COPII- coated ER exit sites and is transported to the Golgi along microtubules in defined transport complexes. These procollagen-containing transport complexes are, however, distinct from those containing other cargo proteins like ERGIC-53 and ts-045-G. Furthermore,they do not label for the COPI coat complex in contrast to those containing ts-045-G. Inhibition of COPII or COPI function before addition of ascorbate,which is required for the folding of procollagen, inhibits export of procollagen from the ER. Inactivation of COPI coat function after addition of ascorbate results in the localisation of procollagen to transport complexes that now also contain ERGIC-53 and are inhibited in their transport to the Golgi complex. These data reveal the existence of an early COPI-dependent,pre-Golgi cargo sorting step in mammalian cells.

Maintenance of Golgi structure and function depends on the integrity of ER export

The Golgi apparatus comprises an enormous array of components that generate its unique architecture and function within cells. Here, we use quantitative fluorescence imaging techniques and ultrastructural analysis to address whether the Golgi apparatus is a steady-state or a stable organelle. We found that all classes of Golgi components are dynamically associated with this organelle, contrary to the prediction of the stable organelle model. Enzymes and recycling components are continuously exiting and reentering the Golgi apparatus by membrane trafficking pathways to and from the ER, whereas Golgi matrix proteins and coatomer undergo constant, rapid exchange between membrane and cytoplasm. When ER to Golgi transport is inhibited without disrupting COPII-dependent ER export machinery (by brefeldin A treatment or expression of Arf1[T31N]), the Golgi structure disassembles, leaving no residual Golgi membranes. Rather, all Golgi components redistribute into the ER, the cytoplasm, or to ER exit sites still active for recruitment of selective membrane-bound and peripherally associated cargos. A similar phenomenon is induced by the constitutively active Sar1[H79G] mutant, which has the additional effect of causing COPII-associated membranes to cluster to a juxtanuclear region. In cells expressing Sar1[T39N], a constitutively inactive form of Sar1 that completely disrupts ER exit sites, Golgi glycosylation enzymes, matrix, and itinerant proteins all redistribute to the ER. These results argue against the hypothesis that the Golgi apparatus contains stable components that can serve as a template for its biogenesis. Instead, they suggest that the Golgi complex is a dynamic, steady-state system, whose membranes can be nucleated and are maintained by the activities of the Sar1-COPII and Arf1-coatomer systems.

Illuminating the secretory pathway: when do we need vesicles?

Recent studies using GFP-tagged markers and time-lapse microscopy have allowed direct visualisation of membrane traffic in the secretory pathway in living mammalian cells. This work shows that larger membrane structures, 300–500 nm in size, are the vehicles responsible for long distance, microtubule-dependent ER-to-Golgi and trans-Golgi to plasma membrane transport of secretory markers. At least two retrograde transport pathways from the Golgi to the ER exist, both of which are proposed to involve a further class of long, tubular membrane carrier that forms from the Golgi and fuses with the ER. Together, this has challenged established transport models, raising the question of whether larger pleiomorphic structures, rather than small 60–80 nm transport vesicles, mediate long-range transport between the ER and Golgi and between the Golgi and plasma membrane. http://www.biologists.com/JCS/movies/jcs2220.html

Matrix proteins can generate the higher order architecture of the Golgi apparatus

The Golgi apparatus in animal cells comprises a reticulum of linked stacks in the pericentriolar and often in the juxtanuclear regions of the cell. The unique architecture of this organelle is thought to depend on the cytoskeleton and cytoplasmic matrix proteins--the best characterized being the golgin family of fibrous, coiled-coil proteins and the GRASP family of stacking proteins. Here we show that these matrix proteins can be separated from oligosaccharide-modifying enzymes in the Golgi stack without affecting their ability to form a ribbon-like reticulum in the correct location near to the nucleus. Our data suggest that the Golgi is a structural scaffold that can exist independently of, but is normally populated by, the enzyme-containing membranes that modify transiting cargo. This new concept of the Golgi further indicates that the Golgi may be an autonomous organelle rather than one that is in simple dynamic equilibrium with the endoplasmic reticulum.

Breaking the COPI monopoly on Golgi recycling

The unexpected discovery of a transport pathway from the Golgi to the endoplasmic reticulum (ER) independent of COPI coat proteins sheds light on how Golgi resident enzymes and protein toxins gain access to the ER from as far as the trans Golgi network. This new pathway provides an explanation for how membrane is recycled to allow for an apparent concentration of anterograde cargo at distinct stages of the secretory pathway. As signal-mediated COPI-dependent recycling also involves the concentration of resident proteins into retrograde COPI vesicles, the main bulk of lipids must be recycled, possibly through a COPI-independent pathway.

ADPKD: a human disease altering Golgi function and basolateral exocytosis in renal epithelia

Epithelial cells explanted from autosomal dominant polycystic kidney disease (ADPKD) tissue exhibit impaired exocytosis, specifically between the Golgi and basolateral membrane (Charron A, Nakamura B, Bacallo R, Wandinger-Ness A. Compromised cytoarchitecture and polarized trafficking in autosomal dominant polycystic kidney disease cells. J Cell Biol 2000; 148: 111-124.). Here the defect is shown to result in the accumulation of the basolateral transport marker vesicular stomatitis virus (VSV) G protein in the Golgi complex. Golgi complex morphology is consequently altered in the disease cells, evident in the noticeable fenestration and dilation of the cisternae. Further detailed microscopic evaluation of normal kidney and ADPKD cells revealed that ineffective basolateral exocytosis correlated with modulations in the localization of select post-Golgi transport effectors. The cytosolic coat proteins p200/myosin II and caveolin exhibited enhanced association with the cytoskeleton or the Golgi of the disease cells, respectively. Most cytoskeletal components with known roles in vesicle translocation or formation were normally arrayed with the exception of Golgi beta-spectrin, which was less prevalent on vesicles. The rab8 GTPase, important for basolateral vesicle targeting, was redistributed from the perinuclear Golgi region to disperse vesicles in ADPKD cells. At the basolateral membrane of ADPKD cells, there was a notable loss of the exocyst components sec6/sec8 and an unidentified syntaxin. It is postulated that dysregulated basolateral transport effector function precipitates the disruption of basolateral exocytosis and dilation of the ADPKD cell Golgi as basolateral cargo accumulates within the cisternae.

Coupled transport of p24 family members

Recent studies show that small trans-membrane proteins of approximately 22–24 kDa (the p24 family), which are grouped into 4 sub-families by sequence homology (p23, p24, p25 and p26), are involved in the early secretory pathway. In this study, we have investigated the mutual requirements of ectopically expressed members of the p24 family for targeting to their proper cellular destination. We find that coexpression of p23 and p24 is both necessary and sufficient for each protein to be transported to the cis-Golgi network/Golgi complex. Proteins from other subfamilies did not substitute for either p23 or p24, even after multiple coexpression. However, trafficking of the p23/p24 couple was facilitated by coexpression of proteins from other sub-families. In addition, we find that the sequence resembling an endoplasmic reticulum retrieval signal present in the cytoplasmic domain of p23 (but not p24) is dispensable. In contrast, the conserved coiled-coil region in the lumenal domain is absolutely required in both p23 and p24 for proper targeting of the p23/p24 couple. These data demonstrate that p23 and p24 must interact with each other to reach their destination, but that this strict requirement is combined with a mutual dependence amongst p24 proteins. We speculate that p24 proteins can form different oligomeric complexes, which contribute to confer specialized sorting/trafficking properties to membranes of the early secretory pathway, perhaps serving as membrane organizers.

COPI-coated ER-to-Golgi transport complexes segregate from COPII in close proximity to ER exit sites

Transport of proteins between the endoplasmic reticulum and Golgi apparatus is mediated by two distinct membrane coat complexes, COPI and COPII. Genetic, biochemical and morphological data have accumulated into a model which suggests a sequential mode of action with COPII mediating the selection of cargo and formation of transport vesicles at the ER membrane for ER-to-Golgi transport and COPI mediating recycling of the transport machinery from post-ER membranes. To test this transport model directly in vivo, and to study the precise temporal sequence of COPI and COPII action in ER-to-Golgi transport, we have used time lapse microscopy of living cells to visualise simultaneously the dynamics of COPII and COPI, as well as COPII and GFP tagged secretory markers in living cells. The majority of COPII labelling appears tightly associated with ER membranes that move only within a limited area (less than 2 microm). Secretory cargo segregates from these sites and is then transported to the Golgi apparatus without any apparent association with COPII. COPI-coated transport complexes are seen to form adjacent to the COPII sites on the ER before segregating and moving directionally towards the Golgi apparatus. COPII is not present on these transport complexes and remains associated with the ER. These data demonstrate for the first time directly in vivo that ER-to-Golgi transport is organised in two steps characterised by a sequential mode of action of COPII and COPI.

The role of the tethering proteins p115 and GM130 in transport through the Golgi apparatus in vivo

Biochemical data have shown that COPI-coated vesicles are tethered to Golgi membranes by a complex of at least three proteins: p115, giantin, and GM130. p115 binds to giantin on the vesicles and to GM130 on the membrane. We now examine the function of this tethering complex in vivo. Microinjection of an N-terminal peptide of GM130 or overexpression of GM130 lacking this N-terminal peptide inhibits the binding of p115 to Golgi membranes. Electron microscopic analysis of single microinjected cells shows that the number of COP-sized transport vesicles in the Golgi region increases substantially, suggesting that transport vesicles continue to bud but are less able to fuse. This was corroborated by quantitative immunofluorescence analysis, which showed that the intracellular transport of the VSV-G protein was significantly inhibited. Together, these data suggest that this tethering complex increases the efficiency with which transport vesicles fuse with their target membrane. They also provide support for a model of mitotic Golgi fragmentation in which the tethering complex is disrupted by mitotic phosphorylation of GM130.

COPI vesicles accumulating in the presence of a GTP restricted arf1 mutant are depleted of anterograde and retrograde cargo

Microinjection of the slowly hydrolyzable GTP analogue GTP(gamma)S or the ectopic expression of a GTP restricted mutant of the small GTPase arf1 (arf1[Q71L]) leads to the rapid accumulation of COPI coated vesicles and buds in living cells. This effect is blocked at 15 degrees C and by microinjection of antibodies against (beta)-COP. Anterograde and retrograde membrane protein transport markers, which have been previously shown to be incorporated into COPI vesicles between the endoplasmic reticulum and Golgi complex, are depleted from the GTP(gamma)S or arf1[Q71L] induced COPI coated vesicles and buds. In contrast, in control cells 30 to 60% of the COPI carriers co-localize with these markers. These in vivo data corroborate recent in vitro work, suggesting that GTP(gamma)S and arf1[Q71L] interfere with the sorting of membrane proteins into Golgi derived COPI vesicles, and provide the first in vivo evidence for a role of GTP hydrolysis by arf1 in the sorting of cargo into COPI coated vesicles and buds.

Rab6 coordinates a novel Golgi to ER retrograde transport pathway in live cells

We visualized a fluorescent-protein (FP) fusion to Rab6, a Golgi-associated GTPase, in conjunction with fluorescent secretory pathway markers. FP-Rab6 defined highly dynamic transport carriers (TCs) translocating from the Golgi to the cell periphery. FP-Rab6 TCs specifically accumulated a retrograde cargo, the wild-type Shiga toxin B-fragment (STB), during STB transport from the Golgi to the endoplasmic reticulum (ER). FP-Rab6 TCs associated intimately with the ER, and STB entered the ER via specialized peripheral regions that accumulated FP-Rab6. Microinjection of antibodies that block coatomer protein I (COPI) function inhibited trafficking of a KDEL-receptor FP-fusion, but not FP-Rab6. Additionally, markers of COPI-dependent recycling were excluded from FP-Rab6/STB TCs. Overexpression of Rab6:GDP (T27N mutant) using T7 vaccinia inhibited toxicity of Shiga holotoxin, but did not alter STB transport to the Golgi or Golgi morphology. Taken together, our results indicate Rab6 regulates a novel Golgi to ER transport pathway.

Evidence for a COP-I-independent transport route from the Golgi complex to the endoplasmic reticulum

The cytosolic coat-protein complex COP-I interacts with cytoplasmic 'retrieval' signals present in membrane proteins that cycle between the endoplasmic reticulum (ER) and the Golgi complex, and is required for both anterograde and retrograde transport in the secretory pathway. Here we study the role of COP-I in Golgi-to-ER transport of several distinct marker molecules. Microinjection of anti-COP-I antibodies inhibits retrieval of the lectin-like molecule ERGIC-53 and of the KDEL receptor from the Golgi to the ER. Transport to the ER of protein toxins, which contain a sequence that is recognized by the KDEL receptor, is also inhibited. In contrast, microinjection of anti-COP-I antibodies or expression of a GTP-restricted Arf-1 mutant does not interfere with Golgi-to-ER transport of Shiga toxin/Shiga-like toxin-1 or with the apparent recycling to the ER of Golgi-resident glycosylation enzymes. Overexpression of a GDP-restricted mutant of Rab6 blocks transport to the ER of Shiga toxin/Shiga-like toxin-1 and glycosylation enzymes, but not of ERGIC-53, the KDEL receptor or KDEL-containing toxins. These data indicate the existence of at least two distinct pathways for Golgi-to-ER transport, one COP-I dependent and the other COP-I independent. The COP-I-independent pathway is specifically regulated by Rab6 and is used by Golgi glycosylation enzymes and Shiga toxin/Shiga-like toxin-1.

The KDEL retrieval system is exploited by Pseudomonas exotoxin A, but not by Shiga-like toxin-1, during retrograde transport from the Golgi complex to the endoplasmic reticulum

To investigate the role of the KDEL receptor in the retrieval of protein toxins to the mammalian cell endoplasmic reticulum (ER), lysozyme variants containing AARL or KDEL C-terminal tags, or the human KDEL receptor, have been expressed in toxin-treated COS 7 and HeLa cells. Expression of the lysozyme variants and the KDEL receptor was confirmed by immunofluorescence. When such cells were challenged with diphtheria toxin (DT) or Escherichia coli Shiga-like toxin 1 (SLT-1), there was no observable difference in their sensitivities as compared to cells which did not express these exogenous proteins. By contrast, the cytotoxicity of Pseudomonas exotoxin A (PE) is reduced by expressing lysozyme-KDEL, which causes a redistribution of the KDEL receptor from the Golgi complex to the ER, and cells are sensitised to this toxin when they express additional KDEL receptors. These data suggest that, in contrast to SLT-1, PE can exploit the KDEL receptor in order to reach the ER lumen where it is believed that membrane transfer to the cytosol occurs. This contention was confirmed by microinjecting into Vero cells antibodies raised against the cytoplasmically exposed tail of the KDEL receptor. Immunofluorescence confirmed that these antibodies prevented the retrograde transport of the KDEL receptor from the Golgi complex to the ER, and this in turn reduced the cytotoxicity of PE, but not that of SLT-1, to these cells.

Role of microtubules in the organization of the Golgi complex

The Golgi complex of mammalian cells is composed of cisternal stacks that function in processing and sorting of membrane and luminal proteins during transport from the site of synthesis in the endoplasmic reticulum to lysosomes, secretory vacuoles, and the cell surface. Even though exceptions are found, the Golgi stacks are usually arranged as an interconnected network in the region around the centrosome, the major organizing center for cytoplasmic microtubules. A close relation thus exists between Golgi elements and microtubules (especially the stable subpopulation enriched in detyrosinated and acetylated tubulin). After drug-induced disruption of microtubules, the Golgi stacks are disconnected from each other, partly broken up, dispersed in the cytoplasm, and redistributed to endoplasmic reticulum exit sites. Despite this, intracellular protein traffic is only moderately disturbed. Following removal of the drugs, scattered Golgi elements move along reassembling microtubules back to the centrosomal region and reunite into a continuous system. The microtubule-dependent motor proteins cytoplasmic dynein and kinesin bind to Golgi membranes and have been implicated in vesicular transport to and from the Golgi complex. Microinjection of dynein heavy chain antibodies causes dispersal of the Golgi complex, and the Golgi complex of cells lacking cytoplasmic dynein is likewise spread throughout the cytoplasm. In a similar manner, kinesin antibodies have been found to inhibit Golgi-to-endoplasmic reticulum transport in brefeldin A-treated cells and scattering of Golgi elements along remaining microtubules in cells exposed to a low concentration of nocodazole. The molecular mechanisms in the interaction between microtubules and membranes are, however, incompletely understood. During mitosis, the Golgi complex is extensively reorganized in order to ensure an equal partitioning of this single-copy organelle between the daughter cells. Mitosis-promoting factor, a complex of cdc2 kinase and cyclin B, is a key regulator of this and other events in the induction of cell division. Cytoplasmic microtubules depolymerize in prophase and as a result thereof, the Golgi stacks become smaller, disengage from each other, and take up a perinuclear distribution. The mitotic spindle is thereafter put together, aligns the chromosomes in the metaphase plate, and eventually pulls the sister chromatids apart in anaphase. In parallel, the Golgi stacks are broken down into clusters of vesicles and tubules and movement of protein along the exocytic and endocytic pathways is inhibited. Using a cell-free system, it has been established that the fragmentation of the Golgi stacks is due to a continued budding of transport vesicles and a concomitant inhibition of the fusion of the vesicles with their target membranes. In telophase and after cytokinesis, a Golgi complex made up of interconnected cisternal stacks is recreated in each daughter cell and intracellular protein traffic is resumed. This restoration of a normal interphase morphology and function is dependent on reassembly of a radiating array of cytoplasmic microtubules along which vesicles can be carried and on reactivation of the machinery for membrane fusion.

Dual-color visualization of trans-Golgi network to plasma membrane traffic along microtubules in living cells

The mechanisms and carriers responsible for exocytic protein trafficking between the trans-Golgi network (TGN) and the plasma membrane remain unclear. To investigate the dynamics of TGN-to-plasma membrane traffic and role of the cytoskeleton in these processes we transfected cells with a GFP-fusion protein, vesicular stomatitis virus G protein tagged with GFP (VSVG3-GFP). After using temperature shifts to block VSVG3-GFP in the endoplasmic reticulum and subsequently accumulate it in the TGN, dynamics of TGN-to-plasma membrane transport were visualized in real time by confocal and video microscopy. Both small vesicles (<250 nm) and larger vesicular-tubular structures (>1.5 microm long) are used as transport containers (TCs). These TCs rapidly moved out of the Golgi along curvilinear paths with average speeds of approximately 0.7 micrometer/second. Automatic computer tracking objectively determined the dynamics of different carriers. Fission and fusion of TCs were observed, suggesting that these late exocytic processes are highly interactive. To directly determine the role of microtubules in post-Golgi traffic, rhodamine-tubulin was microinjected and both labeled cargo and microtubules were simultaneously visualized in living cells. These studies demonstrated that exocytic cargo moves along microtubule tracks and reveals that carriers are capable of switching between tracks.

Recycling of golgi-resident glycosyltransferases through the ER reveals a novel pathway and provides an explanation for nocodazole-induced Golgi scattering

During microtubule depolymerization, the central, juxtanuclear Golgi apparatus scatters to multiple peripheral sites. We have tested here whether such scattering is due to a fragmentation process and subsequent outward tracking of Golgi units or if peripheral Golgi elements reform through a novel recycling pathway. To mark the Golgi in HeLa cells, we stably expressed the Golgi stack enzyme N-acetylgalactosaminyltransferase-2 (GalNAc-T2) fused to the green fluorescent protein (GFP) or to an 11-amino acid epitope, VSV-G (VSV), and the trans/TGN enzyme beta1,4-galactosyltransferase (GalT) fused to GFP. After nocodazole addition, time-lapse microscopy of GalNAc-T2-GFP and GalT-GFP revealed that scattered Golgi elements appeared abruptly and that no Golgi fragments tracked outward from the compact, juxtanuclear Golgi complex. Once formed, the scattered structures were relatively stable in fluorescence intensity for tens of minutes. During the entire process of dispersal, immunogold labeling for GalNAc-T2-VSV and GalT showed that these were continuously concentrated over stacked Golgi cisternae and tubulovesicular Golgi structures similar to untreated cells, suggesting that polarized Golgi stacks reform rapidly at scattered sites. In fluorescence recovery after photobleaching over a narrow (FRAP) or wide area (FRAP-W) experiments, peripheral Golgi stacks continuously exchanged resident proteins with each other through what appeared to be an ER intermediate. That Golgi enzymes cycle through the ER was confirmed by microinjecting the dominant-negative mutant of Sar1 (Sar1pdn) blocking ER export. Sar1pdn was either microinjected into untreated or nocodazole-treated cells in the presence of protein synthesis inhibitors. In both cases, this caused a gradual accumulation of GalNAc-T2-VSV in the ER. Few to no peripheral Golgi elements were seen in the nocodazole-treated cells microinjected with Sar1pdn. In conclusion, we have shown that Golgi-resident glycosylation enzymes recycle through the ER and that this novel pathway is the likely explanation for the nocodazole-induced Golgi scattering observed in interphase cells.

Regulation of membrane traffic in animal cells by COPI

Intracellular membrane transport is mediated predominantly by vesicles which bud from one compartment and fuse specifically with the next compartment in the pathway, resulting in delivery of cargo. COPI-coated vesicles were first identified as intermediates in intra-Golgi transport and subsequent work has shown that they are also involved in transport between the endoplasmic reticulum and the Golgi complex. The COPI coat components have been characterised in detail at the molecular level and a role for membrane proteins and lipids in membrane recruitment of COPI has been uncovered. However, precisely how these distinct membrane components regulate coat recruitment is still unclear and is currently a matter for debate. Furthermore, it is still not clear at exactly how many transport steps COPI is involved and whether it mediates secretory transport in the anterograde or retrograde direction or both. This review focuses on our understanding of COPI structure and function and describes recent findings on the sites of action of COPI in animal cells.