Coat proteins in intracellular membrane transport

Characterization of Rabaptin-5 γ isoform

Rab GTPases are key regulators of intracellular membrane traffic acting through their effector molecules. Rabaptin-5 is a Rab5 effector in early endosome fusion and connects Rab5- and Rab4-positive membrane compartments owing to its ability to interact with Rab4 GTPase. Recent studies showed that Rabaptin-5 transcript is subjected to extensive alternative splicing, thus resulting in expression of Rabaptin-5 isoforms mostly bearing short deletions in the polypeptide chain. As interactions of a Rab GTPase with different effectors lead to different responses, functional characterization of Rabaptin-5 isoforms becomes an attractive issue. Indeed, it was shown that Rab GTPase effector properties of Rabaptin-5 and its α and δ isoforms are different. This work focused on another Rabaptin-5 isoform, Rabaptin-5γ. Despite its ability to interact with Rab5, endogenously produced Rabaptin-5γ was absent from early endosomes. Rather, it was found to be tightly associated with trans-Golgi network and partially localized to a Rab4-positive membrane compartment. The revealed intracellular localization of Rabaptin-5γ indicates that it is not involved in Rab5-driven events; rather, it functions in other membrane transport steps. Our study signifies the role of alternative splicing in determination of functional activities of Rab effector molecules.

Meeting of conventional and unconventional pathways at the TGN

The biosynthetic pathway allows the transport of newly synthesized proteins to the TGN via the reticulum endoplasmic and Golgi apparatus. However, many large particles reach the TGN by unconventional means. For instance, Herpes simplex virus type 1(HSV-1) capsids assemble within the nucleus, bud into the perinuclear space, fuse with the outer nuclear membrane and finally travel unenveloped towards the TGN. Given the central role of protein kinase D in the transport of small cargo from the TGN to the cell surface, we probed its potential contribution in HSV-1 egress, as a model for studying large cargo exiting from the TGN. Using a synchronized infection, we show that inactivation of protein kinase D with pharmacological inhibitors, a kinase dead mutant or siRNA all causes the retention of HSV-1 at the TGN. This highlights the role of PKD in viral exit and a dependence of the virus on the classical host cell machinery to leave the TGN, unlike its previous transport steps. Conceptually, this supports a model in which the TGN is a meeting point where conventional and unconventional routes encounter.

Sorting through the cell biology of Alzheimer's disease: intracellular pathways to pathogenesis

During the first 100 years of Alzheimer's disease research, this devastating and intractable disorder has been characterized at the clinical, histological, and molecular levels. Nevertheless, many key mechanistic questions remain unanswered. Here we will emphasize the importance of the cell biology of Alzheimer's disease, reviewing the relevant literature that has expanded our mechanistic understanding, with a particular focus on pathways regulating protein sorting. Accumulated evidence indicates that sorting pathways may be uniquely vulnerable to disease pathogenesis, and recent studies have begun to reveal disease-related defects in the regulation of protein sorting.

Reconstitution of herpes simplex virus type 1 nuclear capsid egress in vitro

Newly assembled herpesvirus capsids travel from the nucleus to the plasma membrane by a mechanism that is poorly understood. Furthermore, the contribution of cellular proteins to this egress has yet to be clarified. To address these issues, an in vitro nuclear egress assay that reproduces the exit of herpes simplex virus type 1 (HSV-1) capsids from nuclei isolated from infected cells was established. As expected, the assay has all the hallmarks of intracellular transport assays, namely, a dependence on time, energy, and temperature. Surprisingly, it is also dependent on cytosol and was slightly enhanced by infected cytosol, suggesting an implication of both host and viral proteins in the process. The capsids escaped these nuclei by budding through the inner nuclear membrane, accumulated as enveloped capsids between the two nuclear membranes, and were released in cytosol exclusively as naked capsids, exactly as in intact cells. This is most consistent with the view that the virus escapes by crossing the two nuclear membranes rather than through nuclear pores. Unexpectedly, nuclei isolated at the nonpermissive temperature from cells infected with a U(L)26 thermosensitive protease mutant (V701) supported capsid egress. Although electron microscopy, biochemical, and PCR analyses hinted at a likely reconstitution of capsid maturation, DNA encapsidation could not be confirmed by a traditional SQ test. This assay should prove very useful for identification of the molecular players involved in HSV-1 nuclear egress.

Aspergillus nidulans sod(VI)C1 mutation causes defects in cell wall biogenesis and protein secretion

Growth at the restrictive temperature (42 degrees C) of Aspergillus nidulans B120, carrying the conditional-lethal mutation sod(VI)C1, was partially improved by the addition of 1.0 M sorbitol to the medium. The mutant grown at 42 degrees C, with osmotic stabilizer, showed abnormal hyphal morphology, a decrease in beta-1,3-glucan synthase activity as well as cell wall sugar content, but an increase in chitin synthase activity and N-acetyl-glucosamine content. The mutation also affected the secretion of extracellular protease. The temperature-dependent osmo-sensitive phenotype of a Saccharomyces cerevisiae alpha-COP mutation can be rescued by the A. nidulans sod(VI)C(+) gene. These results indicate that the sod(VI)C1 mutation affects proper processing of secretory proteins destined for the surface of cells or beyond.

Autoantibodies in childhood post-varicella acute cerebellar ataxia

BACKGROUND

Anti-Purkinje cell antibodies have been reported in cerebellar ataxia following Epstein-Barr virus (EBV) infection. We investigated autoantibody responses, including anti-Purkinje cell antibodies, and the clinical course in eight children who developed post-varicella ataxia, five of their siblings with uncomplicated varicella, one child with post-EBV ataxia, two children with acute disseminated encephalomyelitis (ADEM) and one with neuroblastoma associated ataxia, and in age and gender matched controls.

METHODS

Autoantibodies were tested by indirect immunofluorescence (IIF) on cryopreserved cerebrum and cerebellum sections. Other autoantibodies were measured by conventional IIF protocols using HEp-2 cells as a substrate. Antibodies to myelin associated glycoprotein (MAG), asialo-GM1, beta2 glycoprotein 1, cardiolipin and myelin basic protein (MBP) were measured by ELISA.

RESULTS

Three of eight children with acute post-varicella ataxia, one child with post-EBV ataxia, one child with ADEM and one child with uncomplicated varicella, had high titer autoantibodies (>1/160) that reacted with cerebrum and cerebellar tissue. This reactivity was not seen in one child with ADEM, in one with neuroblastoma and ataxia, in the remainder of the children with uncomplicated varicella or age and gender matched controls. Autoantibodies were not seen in CSF from two children with post-varicella ataxia. The punctate staining seen on cerebrum and cerebellum sections co-localized with rabbit antibodies to the centrosome protein pericentrin. All patients with strong reactivity with cerebrum and cerebellar tissue by IIF had elevated levels of anti-MAG that was not confirmed by absorption assay. No reactivity was seen with asialo-GM1, MBP, beta2 glycoprotein 1 or cardiolipin. None of the sera had autoantibodies directed against endosomes, the Golgi complex, or the paraneoplastic autoantigens Hu and Yo.

CONCLUSION

Some children with post-viral ataxia develop antibodies that have strong reactivity with cerebral and cerebellar tissue. Some of the antigenic reactivity co-localized with the centrosome protein pericentrin.

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.

Inhibition of secretion by 1,3-Cyclohexanebis(methylamine), a dibasic compound that interferes with coatomer function

We noted previously that certain aminoglycoside antibiotics inhibit the binding of coatomer to Golgi membranes in vitro. The inhibition is mediated in part by two primary amino groups present at the 1 and 3 positions of the 2-deoxystreptamine moiety of the antibiotics. These two amines appear to mimic the epsilon-amino groups present in the two lysine residues of the KKXX motif that is known to bind coatomer. Here we report the effects of 1, 3-cyclohexanebis(methylamine) (CBM) on secretion in vivo, a compound chosen for study because it contains primary amino groups that resemble those in 2-deoxystreptamine and it should penetrate lipid bilayers more readily than antibiotics. CBM inhibited coatomer binding to Golgi membranes in vitro and in vivo and inhibited secretion by intact cells. Despite depressed binding of coatomer in vivo, the Golgi complex retained its characteristic perinuclear location in the presence of CBM and did not fuse with the endoplasmic reticulum (ER). Transport from the ER to the Golgi was also not blocked by CBM. These data suggest that a full complement of coat protein I (COPI) on membranes is not critical for maintenance of Golgi integrity or for traffic from the ER to the Golgi but is necessary for transport through the Golgi to the plasma membrane.

A widely expressed betaIII spectrin associated with Golgi and cytoplasmic vesicles

Spectrin is an important structural component of the plasma membrane skeleton. Heretofore-unidentified isoforms of spectrin also associate with Golgi and other organelles. We have discovered another member of the beta-spectrin gene family by homology searches of the GenBank databases and by 5' rapid amplification of cDNA ends of human brain cDNAs. Collectively, 7,938 nucleotides of contiguous clones are predicted to encode a 271,294-Da protein, called betaIII spectrin, with conserved actin-, protein 4.1-, and ankyrin-binding domains, membrane association domains 1 and 2, a spectrin dimer self-association site, and a pleckstrin-homology domain. betaIII spectrin transcripts are concentrated in the brain and present in the kidneys, liver, and testes and the prostate, pituitary, adrenal, and salivary glands. All of the tested tissues contain major 9.0-kb and minor 11.3-kb transcripts. The human betaIII spectrin gene (SPTBN2) maps to chromosome 11q13 and the mouse gene (Spnb3) maps to a syntenic region close to the centromere on chromosome 19. Indirect immunofluorescence studies of cultured cells using antisera specific to human betaIII spectrin reveal a Golgi-associated and punctate cytoplasmic vesicle-like distribution, suggesting that betaIII spectrin associates with intracellular organelles. This distribution overlaps that of several Golgi and vesicle markers, including mannosidase II, p58, trans-Golgi network (TGN)38, and beta-COP and is distinct from the endoplasmic reticulum markers calnexin and Bip. Liver Golgi membranes and other vesicular compartment markers cosediment in vitro with betaIII spectrin. betaIII spectrin thus constitutes a major component of the Golgi and vesicular membrane skeletons.

A spectrin membrane skeleton of the Golgi complex

The existence of a Golgi-localized membrane cytoskeleton has been revealed by the identification of two major components of the spectrin membrane skeleton, spectrin and ankyrin, that associate with the Golgi complex. Golgi spectrin was identified with an antibody specific for the beta-subunit of the erythroid isoform of spectrin (beta1Sigma1). This antibody recognizes a 220 kDa polypeptide that localizes to discrete regions of the Golgi complex and associates with Golgi membranes in a Brefeldin A sensitive manner. Two isoforms of Golgi ankyrin have been identified: a 119 kDa form (AnkG119) which represents a truncated, alternatively spliced isoform of a recently cloned novel ankyrin of the nervous system AnkG, and a larger 195 kDa ankyrin (Ank195) that cross-reacts with antibodies to erythrocyte ankyrin. A Golgi localized membrane skeleton composed of these unique membrane skeleton isoforms could serve a variety of important functions, including the maintenance of Golgi structural organization and the formation of discrete membrane domains within Golgi compartments.

In polarized MDCK cells basolateral vesicles arise from clathrin-gamma-adaptin-coated domains on endosomal tubules

Human transferrin receptors (TR) and receptors for polymeric immunoglobulins (pIgR) expressed in polarized MDCK cells maintain steady-state, asymmetric distributions on the separate basolateral and apical surfaces even though they are trafficking continuously into and across these cells. The intracellular mechanisms required to maintain these asymmetric distributions have not been located. Here we show that TR and pIgR internalize from both surfaces to a common interconnected endosome compartment that includes tubules with buds coated with clathrin lattices. These buds generate vesicles that carry TR to the basolateral border. The lattices contain gamma-adaptin and are dispersed by treatment with brefeldin A (BFA). Since BFA treatment abrogates the vectorial trafficking of TR in polarized MDCK cells, we propose that the clathrin-coated domains of the endosome tubules contain the polarized sorting mechanism responsible for their preferential basolateral distribution.

gp25L/emp24/p24 protein family members of the cis-Golgi network bind both COP I and II coatomer

Abstract. Five mammalian members of the gp25L/ emp24/p24 family have been identified as major constituents of the cis-Golgi network of rat liver and HeLa cells. Two of these were also found in membranes of higher density (corresponding to the ER), and this correlated with their ability to bind COP I in vitro. This binding was mediated by a K(X)KXX-like retrieval motif present in the cytoplasmic domain of these two members. A second motif, double phenylalanine (FF), present in the cytoplasmic domain of all five members, was shown to participate in the binding of Sec23 (COP II). This motif is part of a larger one, similar to the F/YXXXXF/Y strong endocytosis and putative AP2 binding motif. In vivo mutational analysis confirmed the roles of both motifs so that when COP I binding was expected to be impaired, cell surface expression was observed, whereas mutation of the Sec23 binding motif resulted in a redistribution to the ER. Surprisingly, upon expression of mutated members, steady-state distribution of unmutated ones shifted as well, presumably as a consequence of their observed oligomeric properties.

Okadaic acid induces selective arrest of protein transport in the rough endoplasmic reticulum and prevents export into COPII-coated structures

Quantitative immunoelectron microscopy and subcellular fractionation established the site of endoplasmic reticulum (ER)-Golgi transport arrest induced by the phosphatase inhibitor okadaic acid (OA). OA induced the disappearance of transitional element tubules and accumulation of the anterograde-transported Chandipura (CHP) virus G protein only in the rough ER (RER) and not at more distal sites. The block was specific to the early part of the anterograde pathway, because CHP virus G protein that accumulated in the intermediate compartment (IC) at 15 degrees C could gain access to Golgi stack enzymes. OA also induced RER accumulation of the IC protein p53/p58 via an IC-RER recycling pathway which was resistant to OA and inhibited by the G protein activator aluminium fluoride. The role of COPII coats in OA transport block was investigated by using immunofluorescence and cell fractionation. In untreated cells the COPII coat protein sec 13p colocalized with p53/p58 in Golgi-IC structures of the juxtanuclear region and peripheral cytoplasm. During OA treatment, p53/p58 accumulated in the RER but was excluded from sec 13p-containing membrane structures. Taken together our data indicate that OA induces an early defect in RER export which acts to prevent entry into COPII-coated structures of the IC region.

Functional dissection of COP-I subunits in the biogenesis of multivesicular endosomes

In the present paper, we show that transport from early to late endosomes is inhibited at the restrictive temperature in a mutant CHO cell line (ldlF) with a ts-defect in epsilon coatomer protein (epsilonCOP), although internalization and recycling continue. Early endosomes then appear like clusters of thin tubules devoid of the typical multivesicular regions, which are normally destined to become vesicular intermediates during transport to late endosomes. We also find that the in vitro formation of these vesicles from BHK donor endosomes is inhibited in cytosol prepared from ldlF cells incubated at the restrictive temperature. Although epsilonCOP is rapidly degraded in ldlF cells at the restrictive temperature, cellular amounts of the other COP-I subunits are not affected. Despite the absence of epsilonCOP, we find that a subcomplex of beta, beta', and zetaCOP is still recruited onto BHK endosomes in vitro, and this binding exhibits the characteristic properties of endosomal COPs with respect to stimulation by GTPgammaS and sensitivity to the endosomal pH. Previous studies showed that gamma and deltaCOP are not found on endosomes. However, alphaCOP, which is normally present on endosomes, is no longer recruited when epsilonCOP is missing. In contrast, all COP subunits, except obviously epsilonCOP itself, still bind BHK biosynthetic membranes in a pH-independent manner in vitro. Our observations thus indicate that the biogenesis of multivesicular endosomes is coupled to early endosome organization and depends on COP-I proteins. Our data also show that membrane association and function of endosomal COPs can be dissected: whereas beta, beta', and zetaCOP retain the capacity to bind endosomal membranes, COP function in transport appears to depend on the presence of alpha and/or epsilonCOP.

Interaction of coatomer with aminoglycoside antibiotics: evidence that coatomer has at least two dilysine binding sites

Coatomer is the soluble precursor of the COPI coat (coat protein I) involved in traffic among membranes of the endoplasmic reticulum and the Golgi apparatus. We report herein that neomycin precipitates coatomer from cell extracts and from purified coatomer preparations. Precipitation first increased and then decreased as the neomycin concentration increased, analogous to the precipitation of a polyvalent antigen by divalent antibodies. This suggested that neomycin cross-linked coatomer into large aggregates and implies that coatomer has two or more binding sites for neomycin. A variety of other aminoglycoside antibiotics precipitated coatomer, or if they did not precipitate, they interfered with the ability of neomycin to precipitate. Coatomer is know to interact with a motif (KKXX) containing adjacent lysine residues at the carboxyl terminus of the cytoplasmic domains of some membrane proteins resident in the endoplasmic reticulum. All of the antibiotics that interacted with coatomer contain at least two close amino groups, suggesting that the antibiotics might be interacting with the di-lysine binding site of coatomer. Consistent with this idea, di-lysine itself blocked the interaction of antibiotics with coatomer. Moreover, di-lysine and antibiotics each blocked the coating of Golgi membranes by coatomer. These data suggest that certain aminoglycoside antibiotics interact with di-lysine binding sites on coatomer and that coatomer contains at least two of these di-lysine binding sites.

Na,K-ATPase transport from endoplasmic reticulum to Golgi requires the Golgi spectrin-ankyrin G119 skeleton in Madin Darby canine kidney cells

Spectrin (betaISigma*) and ankyrin (AnkG119) associate with Golgi membranes and the dynactin complex, but their role in vesicle trafficking remains uncertain. We find that the actin-binding domain and membrane-association domain 1 (MAD1) of betaI spectrin together form a constitutive Golgi targeting signal in transfected MDCK cells. Expression of this signal in transfected cells disrupts the endogenous Golgi spectrin skeleton and blocks transport of alpha- and beta-Na,K-ATPase and vesicular stomatitis virus-G protein from the endoplasmic reticulum (ER) but does not disrupt the formation of Golgi stacks, the distribution of beta-COP, or the transport and surface display of E-cadherin. The Golgi spectrin skeleton is thus required for the transport of a subset of membrane proteins from the ER to the Golgi. We postulate that together with polyfunctional adapter proteins such as AnkG119, Golgi spectrin forms a docking complex that acts prior to the cis-Golgi, presumably with vesicular-tubular clusters (VTCs or ERGIC), to sequester specific membrane proteins into vesicles transiting between the ER and Golgi, and subsequently (probably involving other isoforms of spectrin and ankyrin) to mediate cargo transport within the Golgi and to other membrane compartments. We hypothesize that this vesicular spectrin-ankyrin adapter-protein trafficking (or tethering) system (SAATS) mediates the capture and transport of many membrane proteins and acts in conjunction with vesicle-targeting molecules to effect the efficient transport of cargo proteins.

A di-acidic signal required for selective export from the endoplasmic reticulum

Transport of membrane proteins between intracellular compartments requires specific sequences in the protein cytoplasmic domain to direct packaging into vesicle shuttles. A sequence that mediates export from the endoplasmic reticulum (ER) has proved elusive. A di-acidic signal (Asp-X-Glu, where X represents any amino acid) on the cytoplasmic tail of vesicular stomatitis virus glycoprotein (VSV-G) and other cargo molecules was required for efficient recruitment to vesicles mediating export from the ER in baby hamster kidney cells. The existence of such a signal provides evidence that export from the ER occurs through a selective mechanism.

Elevation of vacuolar pH inhibits the cytotoxic activity of furin-cleaved exotoxin A

Exotoxin A (ETA) inhibits protein synthesis in cells by a process that involves receptor-mediated endocytosis and the transport of a 37-kDa proteolytic fragment across a membrane into the cytoplasm. The fragment is apparently generated by the endoprotease furin after the toxin has been endocytosed. Cleavage of ETA by furin requires a low pH in vitro, and presumably also in vivo. Drugs that raise the pH of intracellular compartments are known to protect cells from ETA. The simplest hypothesis to explain this protection has been that the drugs interfere with furin cleavage. To test this idea, we measured the effect of pH-elevating drugs on the action of ETA that had been precleaved with recombinant furin before addition to cells. Surprisingly, we found that pH-elevating drugs protected cells from precleaved ETA as well as intact ETA. These results suggest that the process by which ETA intoxicates cells requires a low vacuolar pH for another event in addition to proteolysis by furin.

Golgi membrane skeleton: identification, localization and oligomerization of a 195 kDa ankyrin isoform associated with the Golgi complex

To extend our finding of a Golgi-localized form of the membrane skeleton protein spectrin, we have identified an isoform of ankyrin that associates at steady state with the Golgi complex. Immuno-light and -electron microscopy show that this ankyrin isoform localizes to the perinuclear cytoplasm on tubular vesicular structures that co-stain with Golgi marker proteins. An antiserum raised against erythrocyte ankyrin, which was used to identify the Golgi ankyrin, recognized three prominent polypeptides of 220, 213 and 195 kDa in MDCK cells. Affinity purification of this antiserum against each of these MDCK cell ankyrins revealed that only an antibody specific for the 195 kDa form retained the ability to stain the Golgi complex; affinity purified antibody preparations specific for both the 220 and 213 kDa forms stained punctate and reticular cytoplasmic structures distinct from the Golgi complex. Antibody specific for the 195 kDa ankyrin did not recognize a recently identified 119 kDa ankyrin that is also localized to the Golgi. The 195 kDa Golgi ankyrin binds purified erythrocyte spectrin, and rapidly co-sediments with Golgi beta-spectrin during brief, low speed centrifugation of Triton X-100 extracts of MDCK cells. Golgi ankyrin and beta-spectrin are retained on tubular vesicular ‘Golgi ghosts’ following extraction of cultured cells with Triton X-100. Significantly, Golgi ghost tubules containing ankyrin/spectrin are co-linear with individual microtubules, suggesting a role for both Golgi membrane skeleton and microtubules in spatial localization of the Golgi. Golgi ankyrin dissociates from Golgi membranes during mitosis and in cells treated with brefeldin A, indicating that Golgi ankyrin has a dynamic assembly state similar to that of Golgi spectrin and other Golgi membrane coat proteins.

Coats and vesicle budding

Transport vesicles need coat proteins in order to form. The coat proteins are recruited from the cytosol onto a particular membrane, where they drive vesicle budding and select the vesicle cargo. So far, three types of coated transport vesicles have been purified and characterized, and candidates for components of other types of coats have been identified. This review gives a brief overview of what is known about the various coats and their role in transport vesicle formation.

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.

Sorting by COP I-coated vesicles under interphase and mitotic conditions

COP I-coated vesicles were analyzed for their content of resident Golgi enzymes (N-acetylgalactosaminyltransferase; N-acetylglucosaminyltransferase I; mannosidase II; galactosyltransferase), cargo (rat serum albumin; polyimmunoglobulin receptor), and recycling proteins (-KDEL receptor; ERGIC-53/p58) using biochemical and morphological techniques. The levels of these proteins were similar when the vesicles were prepared under interphase or mitotic conditions showing that sorting was unaffected. The average density relative to starting membranes for resident enzymes (14-30%), cargo (16-23%), and recycling proteins (81-125%) provides clues to the function of COP I vesicles in transport through the Golgi apparatus.

The association of annexin I with early endosomes is regulated by Ca2+ and requires an intact N-terminal domain

Annexin I is a member of a multigene family of Ca2+/phospholipid-binding proteins and a major substrate for the epidermal growth factor (EGF) receptor kinase, which has been implicated in membrane-related events along the endocytotic pathway, in particular in the sorting of internalized EGF receptors occurring in the multivesicular body. We analyzed in detail the intracellular distribution of this annexin by cell fractionation and immunoelectron microscopy. These studies used polyclonal as well as a set of species-specific monoclonal antibodies, whose epitopes were mapped to the lateral surface of the molecule next to a region thought to be involved in vesicle aggregation. Unexpectedly, the majority of annexin I was identified on early and not on multivesicular endosomes in a form that required micromolar levels of Ca2+ for the association. The specific cofractionation with early endosomes was also observed in transfected baby hamster kidney cells when the intracellular fate of ectopically expressed porcine annexin I was analyzed by using the species-specific monoclonal antibodies in Western blots of subcellular fractions. Interestingly, a truncation of the N-terminal 26, but not the N-terminal 13 residues of annexin I altered its intracellular distribution, shifting it from fractions containing early to those containing late and multivesicular endosomes. These findings underscore the regulatory importance of the N-terminal domain and provide evidence for an involvement of annexin I in early endocytotic processes.

The in vitro generation of post-Golgi vesicles carrying viral envelope glycoproteins requires an ARF-like GTP-binding protein and a protein kinase C associated with the Golgi apparatus

We have developed a system that recreates in vitro the generation of post-Golgi vesicles from an isolated Golgi fraction prepared from vesicular stomatitis virus- or influenza virus-infected Madin-Darby canine kidney or HepG2 cells. In this system, vesicle generation is temperature- and ATP-dependent and requires a supply of cytosolic proteins, including an N-ethylmaleimide-sensitive factor distinct from NSF. Cytosolic proteins obtained from yeast were as effective as mammalian cytosolic proteins in supporting vesicle formation and had the same requirements. The vesicles produced (50-80 nm in diameter) are depleted of the trans Golgi marker sialyltransferase, contain the viral glycoprotein molecules with their cytoplasmic tails exposed, and do not show an easily recognizable protein coat. Vesicle generation was inhibited by brefeldin A, which indicates that it requires the activation of an Arf-like GTP-binding protein that promotes assembly of a vesicle coat. Vesicles formed in the presence of the nonhydrolyzable GTP analogue guanosine 5'-3-O-(thio)triphosphate retained a nonclathrin protein coat resembling that of COP-coated vesicles, and sedimented more rapidly in a sucrose gradient than the uncoated ones generated in its absence. This indicates that GTP hydrolysis is not required for vesicle generation but that it is for vesicle uncoating. The activity of a Golgi-associated protein kinase C (PKC) was found to be necessary for the release of post-Golgi vesicles, as indicated by the capacity of a variety of inhibitors and antibodies to PKC to suppress it, as well as by the stimulatory effect of the PKC activator 12-O-tetradecanoylphorbol-13-acetate.

Differential distribution of alpha subunits and beta gamma subunits of heterotrimeric G proteins on Golgi membranes of the exocrine pancreas

Heterotrimeric G proteins are well known to be involved in signaling via plasma membrane (PM) receptors. Recent data indicate that heterotrimeric G proteins are also present on intracellular membranes and may regulate vesicular transport along the exocytic pathway. We have used subcellular fractionation and immunocytochemical localization to investigate the distribution of G alpha and G beta gamma subunits in the rat exocrine pancreas which is highly specialized for protein secretion. We show that G alpha s, G alpha i3 and G alpha q/11 are present in Golgi fractions which are > 95% devoid of PM. Removal of residual PM by absorption on wheat germ agglutinin (WGA) did not deplete G alpha subunits. G alpha s was largely restricted to TGN-enriched fractions by immunoblotting, whereas G alpha i3 and G alpha q/11 were broadly distributed across Golgi fractions. G alpha s did not colocalize with TGN38 or caveolin, suggesting that G alpha s is associated with a distinct population of membranes. G beta subunits were barely detectable in purified Golgi fractions. By immunofluorescence and immunogold labeling, G beta subunits were detected on PM but not on Golgi membranes, whereas G alpha s and G alpha i3 were readily detected on both Golgi and PM. G alpha and G beta subunits were not found on membranes of zymogen granules. These data indicate that G alpha s, G alpha q/11, and G alpha i3 associate with Golgi membranes independent of G beta subunits and have distinctive distributions within the Golgi stack. G beta subunits are thought to lock G alpha in the GDP-bound form, prevent it from activating its effector, and assist in anchoring it to the PM. Therefore the presence of free G alpha subunits on Golgi membranes has several important functional implications: it suggests that G alpha subunits associated with Golgi membranes are in the active, GTP-bound form or are bound to some other unidentified protein(s) which can substitute for G beta gamma subunits. It further implies that G alpha subunits are tethered to Golgi membranes by posttranslational modifications (e.g., palmitoylation) or by binding to another protein(s).

Intracellular trafficking of lysosomal membrane proteins

Lysosomes are the site of degradation of obsolete intracellular material during autophagy and of extracellular macromolecules following endocytosis and phagocytosis. The membrane of lysosomes and late endosomes is enriched in highly glycosylated transmembrane proteins of largely unknown function. Significant progress has been made in recent years towards elucidating the pathways by which these lysosomal membrane proteins are delivered to late endosomes and lysosomes. While some lysosomal membrane proteins follow the constitutive secretory pathway and reach lysosomes indirectly via the cell surface and endocytosis, others exit the trans-Golgi network in clathrin-coated vesicles for direct delivery to endosomes and lysosomes. Sorting from the Golgi or the plasma membrane into the endosomal system is mediated by signals encoded by the short cytosolic domain of these proteins. This review will discuss the role of lysosomal membrane proteins in the biogenesis of the late endosomal and lysosomal membranes, with particular emphasis on the structural features and molecular mechanisms underlying the intracellular trafficking of these proteins.

The spectrin-based membrane skeleton as a membrane protein-sorting machine

Normal cell function is dependent on the existence of membrane compartments that have unique populations of membrane proteins. Sorting of membrane proteins forms the basis for the biogenesis of distinct membrane compartments. There are many examples of membrane protein-sorting events in cells, but the molecular machinery involved is poorly understood. We discuss characteristics of a putative membrane protein-sorting machine and show that the spectrin-based membrane skeleton conforms to these characteristics. The spectrin-based membrane skeleton is a submembranous, spatially limited, two-dimensional lattice that binds a subset of membrane proteins. These properties allow the membrane skeleton to facilitate the formation of distinct membrane domains and thus reveal its potential as a membrane protein-sorting machine.

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.

HEP-COP, a novel human gene whose product is highly homologous to the alpha-subunit of the yeast coatomer protein complex

A 4333-bp novel human cDNA sequence designated HEP-COP was isolated from the Hep3B hepatocellular carcinoma cell line by the RACE technique. Within HEP-COP was identified an ORF of 3672 bp encoding a deduced 1224-amino-acid (aa) sequence which exhibited striking homology with the 1201-aa sequence of RET1P, the alpha-subunit of the coatomer complex (alpha-COP) in Saccharomyces cerevisiae which participates in membrane transport between the endoplasmic reticulum and Golgi apparatus. The aa homology was highest in their N-terminal regions which each contained six WD-40 repeat motifs [Van der Voorn and Ploegh, FEBS Lett. 307 (1992) 131-134], and both proteins were predicted to be hydrophilic with similar estimated molecular masses of 138 324 and 135 599 Da, respectively. Northern blot hybridization demonstrated that HEP-COP was expressed in a wide range of human adult and fetal tissues. RT-PCR analysis revealed no differential expression of HEP-COP in 14 human cancer cell lines, as compared with normal control cells. Considering the close similarities between HEP-COP and yeast alpha-COP, and the ubiquitous expression of HEP-COP implying an essential cellular role, it is likely that HEP-COP is the human homologue of alpha-COP.

In vitro assembly and disassembly of coatomer

Coatomer, a complex of seven proteins, is the major component of the non-clathrin (COP I) membrane coat. We report here the first system to reversibly disassemble and reassemble this complex in vitro. Coatomer disassembles at high salt concentrations and reassembles when returned to a more physiological buffer. Using this system, we show that alpha-, beta'-, and epsilon-COP interact directly and that gamma-COP interacts with zeta-COP. A partial complex comprising alpha-, beta'-, and epsilon-COP, obtained after coatomer disassembly, can bind to membranes in vitro. This binding is, at least in part, mediated by interactions with cytoplasmic KKXX motifs of proteins normally retained in or retrieved to the endoplasmic reticulum. Using coatomer disassembly and epitope-specific antibodies, we also demonstrate that the N- and C-terminal domains of beta-COP are buried within the native coatomer complex. These results provide the first insights into how the coatomer is structured.

Heterotrimeric G proteins interact with the small GTPase ARF. Possibilities for the regulation of vesicular traffic

Trimeric G proteins have emerged as important regulators of membrane trafficking. To explore a role for G beta gamma in endosome fusion, we have taken advantage of beta-adrenergic receptor kinase (beta ARK), an enzyme translocated to membranes by interaction with G beta gamma. The COOH terminus of beta ARK (beta ARKct) has a G beta gamma-binding domain which blocks some G beta gamma-mediated processes. We found that beta ARKct and peptide G, a peptide derived from beta ARKct, inhibit in vitro endosome fusion. Interestingly, peptide G and ARF share sequence similarity. Peptide G and beta ARKct reversed ARF-mediated inhibition of endosome fusion and blocked ARF binding to membranes. Using an ARF fusion protein, we show that both G beta gamma and G alpha s interact with the small GTPase ARF, an interaction that is regulated by nucleotide binding. We conclude that G proteins may participate in the regulation of vesicular trafficking by directly interacting with ARF, a cytosolic factor required for transport.

Ricin cytotoxicity is sensitive to recycling between the endoplasmic reticulum and the Golgi complex

Cytotoxic proteins that kill mammalian cells by catalytically inhibiting protein synthesis must enter the cytosol in order to reach their substrates. With the exception of diphtheria toxin, which enters the cytosol from acidified endosomes, the intracellular site of translocation of other toxins including ricin, Escherichia coli Shiga-like toxin-1, and Pseudomonas exotoxin A is likely to involve early compartments of the secretory pathway. We have used a molecular approach to identify the site and mechanism of toxin delivery to the cytosol by transiently expressing mutant GTPases that inhibit the assembly of biochemical complexes mediating anterograde and retrograde transport in the exocytic and endocytic pathways. The results provide evidence to suggest that receptors actively recycling between the endoplasmic reticulum and terminal Golgi compartments are essential for toxin translocation to the cytosol from the endoplasmic reticulum. The rapid kinetics of intoxication demonstrate a substantial level of bidirectional membrane flow and sorting through the early secretory pathway.

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.

Cytotoxicity of brefeldin A correlates with its inhibitory effect on membrane binding of COP coat proteins

The fungal metabolite brefeldin A (BFA) causes the inhibition of protein secretion and the disruption of the structure and function of organelles along the exocytic and endocytic pathways including the Golgi complex. Such effects of BFA have been ascribed in large part to its ability to prevent recruitment of cytosolic coat proteins onto organelle membranes. Here we show that mammalian cell lines differ from one another with respect to sensitivity to this drug. The BFA sensitivity of a given cell line appears to be dependent on the species or the order from which the cell line originates, rather than on the cell line itself. In each cell line, the dose of BFA required for inhibition of cell growth and of protein secretion correlates with the dose required for inhibition of binding of beta-COP, a coat protein of COP-coated vesicles, but not that for inhibition of binding of gamma-adaptin, a component of HA-I/AP-1 adaptor of clathrin-coated vesicles. These observations suggest that: (i) there are at least two targets for BFA that differ from each other in sensitivity to this drug, (ii) the difference in the sensitivity to BFA of the beta-COP binding is determined by the difference in the structure of a target protein for this drug, and (iii) the cytotoxicity of BFA is ascribed mainly to its inhibitory effect on the membrane binding of COP-coat proteins.

Developmental induction of Golgi structure and function in the primitive eukaryote Giardia lamblia

A fundamental characteristic of eukaryotic cells is the presence of membrane-bound compartments and membrane transport pathways in which the Golgi complex plays a central role in the selective processing, sorting, and secretion of proteins. The parasitic protozoan Giardia lamblia belongs to the earliest identified lineage among eukaryotes and therefore offers unique insight into the progression from primitive to more complex eukaryotic cells. Here, we report that Giardia trophozoites undergo a developmental induction of Golgi enzyme activities, which correlates with the appearance of a morphologically identifiable Golgi complex, as they differentiate to cysts. Prior to this induction, no morphologically or biochemically identifiable Golgi complex exists within nonencysting cells. Remarkably, protein secretion in both nonencysting and encysting trophozoites is inhibited by brefeldin A, and brefeldin A-sensitive membrane association of ADP-ribosylation factor and beta-COP is observed. These results suggest that the secretory machinery of Giardia resembles that of higher eukaryotes despite the absence of a Golgi complex in nonencysting trophozoites. These findings have implications both for defining the minimal machinery for protein secretion in eukaryotes and for examining the biogenesis of Golgi structure and function.