Lysosome biogenesis requires Rab9 function and receptor recycling from endosomes to the trans-Golgi network

Rab39, a novel Golgi-associated Rab GTPase from human dendritic cells involved in cellular endocytosis

Rab GTPases are Ras-like small molecular weight GTP binding proteins that are involved in various steps along the exocytic and endocytic pathways. Here we report that Rab39, a novel Rab protein, is a Golgi-associated protein involved in endocytosis of HeLa cells. Full-length cDNA of Rab39 contains 1251bp with an open reading frame (ORF) of 636bp, which is predicted to encode a 211 aa protein. By blast analysis of Rab39 cDNA and protein sequence with homologues, we find that Rab39 may be a short variant of Rab34. Rab39 contains conserved motifs involved in phosphate/guanosine binding and a microbody C-terminal targeting signal. RT-PCR analysis indicates that Rab39 is mainly detected in epithelial cell lines, and Northern blot analysis shows that Rab39 is expressed ubiquitously in human tissues. By using FITC-BSA as an endocytic tracer, we show that Rab39 can facilitate endocytosis in HeLa cells when expressed either transiently or stably. Confocal microscopy examination of Rab39 subcellular localization suggests that Rab39 is associated with Golgi-associated organelles. Our findings demonstrate that Rab39 is a novel Rab GTPase involved in cellular endocytosis.

Mannose 6-phosphate receptors: new twists in the tale

The two mannose 6-phosphate (M6P) receptors were identified because of their ability to bind M6P-containing soluble acid hydrolases in the Golgi and transport them to the endosomal-lysosomal system. During the past decade, we have started to understand the structural features of these receptors that allow them to do this job, and how the receptors themselves are sorted as they pass through various membrane-bound compartments. But trafficking of acid hydrolases is only part of the story. Evidence is emerging that one of the receptors can regulate cell growth and motility, and that it functions as a tumour suppressor.

Dynamics of endosomal sorting

The endocytic pathway receives cargo from the cell surface via endocytosis, biosynthetic cargo from the late Golgi complex, and various molecules from the cytoplasm via autophagy. This review focuses on the dynamics of the endocytic pathway in relationship to these processes and covers new information about the sorting events and molecular complexes involved. The following areas are discussed: dynamics at the plasma membrane, sorting within early endosomes and recycling to the cell surface, the role of the cytoskeleton, transport to late endosomes and sorting into multivesicular bodies, anterograde and retrograde Golgi transport, as well as the autophagic pathway.

Ricin

Ricin is a heterodimeric protein produced in the seeds of the castor oil plant (Ricinus communis). It is exquisitely potent to mammalian cells, being able to fatally disrupt protein synthesis by attacking the Achilles heel of the ribosome. For this enzyme to reach its substrate, it must not only negotiate the endomembrane system but it must also cross an internal membrane and avoid complete degradation without compromising its activity in any way. Cell entry by ricin involves a series of steps: (i) binding, via the ricin B chain (RTB), to a range of cell surface glycolipids or glycoproteins having beta-1,4-linked galactose residues; (ii) uptake into the cell by endocytosis; (iii) entry of the toxin into early endosomes; (iv) transfer, by vesicular transport, of ricin from early endosomes to the trans-Golgi network; (v) retrograde vesicular transport through the Golgi complex to reach the endoplasmic reticulum; (vi) reduction of the disulphide bond connecting the ricin A chain (RTA) and the RTB; (vii) partial unfolding of the RTA to render it translocationally-competent to cross the endoplasmic reticulum (ER) membrane via the Sec61p translocon in a manner similar to that followed by misfolded ER proteins that, once recognised, are targeted to the ER-associated protein degradation (ERAD) machinery; (viii) avoiding, at least in part, ubiquitination that would lead to rapid degradation by cytosolic proteasomes immediately after membrane translocation when it is still partially unfolded; (ix) refolding into its protease-resistant, biologically active conformation; and (x) interaction with the ribosome to catalyse the depurination reaction. It is clear that ricin can take advantage of many target cell molecules, pathways and processes. It has been reported that a single molecule of ricin reaching the cytosol can kill that cell as a consequence of protein synthesis inhibition. The ready availability of ricin, coupled to its extreme potency when administered intravenously or if inhaled, has identified this protein toxin as a potential biological warfare agent. Therapeutically, its cytotoxicity has encouraged the use of ricin in 'magic bullets' to specifically target and destroy cancer cells, and the unusual intracellular trafficking properties of ricin potentially permit its development as a vaccine vector. Combining our understanding of the ricin structure with ways to cripple its unwanted properties (its enzymatic activity and promotion of vascular leak whilst retaining protein stability and important immunodominant epitopes), will also be crucial in the development of a long awaited protective vaccine against this toxin.

Transport of protein toxins into cells: pathways used by ricin, cholera toxin and Shiga toxin

Ricin, cholera, and Shiga toxin belong to a family of protein toxins that enter the cytosol to exert their action. Since all three toxins are routed from the cell surface through the Golgi apparatus and to the endoplasmic reticulum (ER) before translocation to the cytosol, the toxins are used to study different endocytic pathways as well as the retrograde transport to the Golgi and the ER. The toxins can also be used as vectors to carry other proteins into the cells. Studies with protein toxins reveal that there are more pathways along the plasma membrane to ER route than originally believed.

Disruption of Golgi morphology and trafficking in cells expressing mutant prenylated rab acceptor-1

Prenylated Rab acceptor (PRA1) is a protein that binds Rab GTPases and the v-SNARE VAMP2. The protein is localized to the Golgi complex and post-Golgi vesicles. To determine its functional role, we generated a number of point mutations and divided them into three classes based on cellular localization. Class A mutants were retained in the endoplasmic reticulum (ER) and exerted an inhibitory effect on transport of vesicular stomatitis virus envelope glycoprotein (VSVG) from the ER to Golgi as well as to the plasma membrane. Class B mutants exhibited a highly condensed Golgi complex and inhibited exit of anterograde cargo from this organelle. Class C mutants exhibited an intermediate phenotype with Golgi and ER localization along with extensive tubular structures emanating from the Golgi complex. There was a direct correlation between the cellular phenotype and binding to Rab and VAMP2. Class A and C mutants showed a significant decrease in Rab and VAMP2 binding, whereas an increase in binding was observed in the class B mutants. Thus, PRA1 is required for vesicle formation from the Golgi complex and might be involved in recruitment of Rab effectors and SNARE proteins during cargo sequestration.

Selective regulation of the Rab9-independent transport of ricin to the Golgi apparatus by calcium

Transport of ricin from endosomes to the Golgi apparatus occurs, in contrast to the transport of the mannose 6-phosphate receptor, by a Rab9-independent process. To characterize the pathway of ricin transport to the Golgi apparatus, we investigated whether it was regulated by calcium. As shown here, our data indicate that calcium is selectively involved in the regulation of ricin transport to the Golgi apparatus. Thapsigargin, which inhibits calcium transport into the ER, and the calcium ionophore A23187 both increased the transport of ricin to the Golgi apparatus by a factor of 20. By contrast, transport of the mannose 6-phosphate receptor to the Golgi apparatus was unaffected. Ricin and mannose 6-phosphate receptor transport were measured by quantifying the sulfation of modified forms of ricin and the mannose 6-phosphate receptor. The increased transport of ricin was reduced by wortmannin and LY294002, suggesting that phosphoinositide 3-kinase might be involved in transport of ricin to the Golgi apparatus. Together, these findings indicate that the different pathways to the Golgi apparatus utilized by ricin and the mannose 6-phosphate receptor are regulated by different mechanisms.

Rab proteins mediate Golgi transport of caveola-internalized glycosphingolipids and correct lipid trafficking in Niemann-Pick C cells

We recently showed that human skin fibroblasts internalize fluorescent analogues of the glycosphingolipids lactosylceramide and globoside almost exclusively by a clathrin-independent mechanism involving caveolae. In contrast, a sphingomyelin analogue is internalized approximately equally via clathrin-dependent and caveolar routes. Here, we further characterized the caveolar pathway for glycosphingolipids, showing that Golgi targeting of sphingolipids internalized via caveolae required microtubules and phosphoinositol 3-kinases and was inhibited in cells expressing dominant-negative Rab7 and Rab9 constructs. In addition, overexpression of wild-type Rab7 or Rab9 (but not Rab11) in Niemann-Pick type C (NP-C) lipid storage disease fibroblasts resulted in correction of lipid trafficking defects, including restoration of Golgi targeting of fluorescent lactosylceramide and endogenous GM(1) ganglioside, and a dramatic reduction in intracellular cholesterol stores. Our results demonstrate a role for Rab7 and Rab9 in the Golgi targeting of glycosphingolipids and suggest a new therapeutic approach for restoring normal lipid trafficking in NP-C cells.

Identification of residues in TIP47 essential for Rab9 binding

TIP47 (tail-interacting protein of 47 kDa) binds to the cytoplasmic domains of the cation-dependent and cation-independent mannose 6-phosphate receptors (MPRs) and is required for their transport from endosomes to the trans-Golgi network in vitro and in living cells. TIP47 recognizes distinct determinants in the cytoplasmic domains of these two receptors, and its ability to bind to the cation-independent MPR is enhanced by the concomitant binding of the Rab9 GTPase. We show here that TIP47 residues 161-169 are essential, but likely not sufficient, for Rab9 binding. Mutation of these residues led to a significant decrease in Rab9 binding, but did not alter the global folding of the protein. The most impaired mutant was indistinguishable from wild-type TIP47 in its circular dichroism spectrum, and mutant proteins that showed decreased Rab9 binding retained full capacity to bind to MPR cytoplasmic domains. Closely related sequences in a related protein, adipophilin, did not confer Rab9 binding capacity to that protein. Partial proteolysis of TIP47 and TIP47 mutant proteins revealed subtle conformational differences, suggesting that residues 161-169 reside in a portion of TIP47 that is important for its conformation. These experiments reveal distinct binding domains for the Rab9 GTPase and MPR cytoplasmic domains in the cargo selection protein TIP47.

Visualization of Rab9-mediated vesicle transport from endosomes to the trans-Golgi in living cells

Mannose 6-phosphate receptors (MPRs) are transported from endosomes to the trans-Golgi via a transport process that requires the Rab9 GTPase and the cargo adaptor TIP47. We have generated green fluorescent protein variants of Rab9 and determined their localization in cultured cells. Rab9 is localized primarily in late endosomes and is readily distinguished from the trans-Golgi marker galactosyltransferase. Coexpression of fluorescent Rab9 and Rab7 revealed that these two late endosome Rabs occupy distinct domains within late endosome membranes. Cation-independent mannose 6-phosphate receptors are enriched in the Rab9 domain relative to the Rab7 domain. TIP47 is likely to be present in this domain because it colocalizes with the receptors in fixed cells, and a TIP47 mutant disrupted endosome morphology and sequestered MPRs intracellularly. Rab9 is present on endosomes that display bidirectional microtubule-dependent motility. Rab9-positive transport vesicles fuse with the trans-Golgi network as followed by video microscopy of live cells. These data provide the first indication that Rab9-mediated endosome to trans-Golgi transport can use a vesicle (rather than a tubular) intermediate. Our data suggest that Rab9 remains vesicle associated until docking with the Golgi complex and is rapidly removed concomitant with or just after membrane fusion.

A membrane protein enriched in endoplasmic reticulum exit sites interacts with COPII

Although all mammalian COPII components have now been cloned, little is known of their interactions with other regulatory proteins involved in exit from the endoplasmic reticulum (ER). We report here that a mammalian protein (Yip1A) that is about 31% identical to S. cerevisiae and which interacts with and modulates COPII-mediated ER-Golgi transport. Yip1A transcripts are ubiquitously expressed. Transcripts of a related mammalian homologue, Yip1B, are found specifically in the heart. Indirect immunofluorescence microscopy revealed that Yip1A is localized to vesicular structures that are concentrated at the perinuclear region. The structures marked by Yip1A co-localized with Sec31A and Sec13, components of the COPII coat protein complex. Immunoelectron microscopy also showed that Yip1A co-localizes with Sec13 at ER exit sites. Overexpression of the hydrophilic N terminus of Yip1A arrests ER-Golgi transport of the vesicular stomatitis G protein and causes fragmentation and dispersion of the Golgi apparatus. A glutathione S-transferase fusion protein with the hydrophilic N terminus of Yip1A (GST-Yip1A) is able to bind to and deplete vital components from rat liver cytosol that is essential for in vitro vesicular stomatitis G transport. Peptide sequence analysis of cytosolic proteins that are specifically bound to GST-Yip1A revealed, among other proteins, mammalian COPII components Sec23 and Sec24. A highly conserved domain at the N terminus of Yip1A is required for Sec23/Sec24 interaction. Our results suggest that Yip1A is involved in the regulation of ER-Golgi traffic at the level of ER exit sites.

Ypt/rab gtpases: regulators of protein trafficking

Ypt/Rab guanosine triphosphatases (GTPases) have emerged in the last decade as key regulators of protein transport in all eukaryotic cells. They seem to be involved in all aspects of vesicle trafficking: vesicle formation, motility, and docking, and membrane remodeling and fusion. The functions of Ypt/Rabs are themselves controlled by upstream regulators that stimulate both their nucleotide cycling and their cycling between membranes. Ypt/Rabs transmit signals to downstream effectors in a guanosine triphosphate (GTP)-dependent manner. The identity of upstream regulators and downstream effectors is known for a number of Ypt/Rabs, and models for their mechanisms of action are emerging. In at least two cases, Ypt/Rab upstream regulators and downstream effectors are found together in a single complex. In agreement with the idea that Ypt/Rabs function in all aspects of vesicular transport, their diverse effectors have recently been shown to function in all identified aspects of vesicle transport. Activators and effectors for individual Ypt/Rabs share no similarity, but are conserved between yeast and mammalian cells. Finally, cross talk demonstrated among the various Ypt/Rabs, and between Ypt/Rabs and other signaling factors, suggests possible coordination among secretory steps, as well as between protein transport and other cellular processes.

Role of phosphatidylinositol 3-kinase and Rab5 effectors in phagosomal biogenesis and mycobacterial phagosome maturation arrest

Phagosomal biogenesis is a fundamental biological process of particular significance for the function of phagocytic and antigen-presenting cells. The precise mechanisms governing maturation of phagosomes into phagolysosomes are not completely understood. Here, we applied the property of pathogenic mycobacteria to cause phagosome maturation arrest in infected macrophages as a tool to dissect critical steps in phagosomal biogenesis. We report the requirement for 3-phosphoinositides and acquisition of Rab5 effector early endosome autoantigen (EEA1) as essential molecular events necessary for phagosomal maturation. Unlike the model phagosomes containing latex beads, which transiently recruited EEA1, mycobacterial phagosomes excluded this regulator of vesicular trafficking that controls membrane tethering and fusion processes within the endosomal pathway and is recruited to endosomal membranes via binding to phosphatidylinositol 3-phosphate (PtdIns[3]P). Inhibitors of phosphatidylinositol 3'(OH)-kinase (PI-3K) activity diminished EEA1 recruitment to newly formed latex bead phagosomes and blocked phagosomal acquisition of late endocytic properties, indicating that generation of PtdIns(3)P plays a role in phagosomal maturation. Microinjection into macrophages of antibodies against EEA1 and the PI-3K hVPS34 reduced acquisition of late endocytic markers by latex bead phagosomes, demonstrating an essential role of these Rab5 effectors in phagosomal biogenesis. The mechanism of EEA1 exclusion from mycobacterial phagosomes was investigated using mycobacterial products. Coating of latex beads with the major mycobacterial cell envelope glycosylated phosphatidylinositol lipoarabinomannan isolated from the virulent Mycobacterium tuberculosis H37Rv, inhibited recruitment of EEA1 to latex bead phagosomes, and diminished their maturation. These findings define the generation of phosphatidylinositol 3-phosphate and EEA1 recruitment as: (a) important regulatory events in phagosomal maturation and (b) critical molecular targets affected by M. tuberculosis. This study also identifies mycobacterial phosphoinositides as products with specialized toxic properties, interfering with discrete trafficking stages in phagosomal maturation.

Endosome to Golgi transport of ricin is independent of clathrin and of the Rab9- and Rab11-GTPases

The plant toxin ricin is transported to the Golgi and the endoplasmic reticulum before translocation to the cytosol where it inhibits protein synthesis. The toxin can therefore be used to investigate pathways leading to the Golgi apparatus. Except for the Rab9-mediated transport of mannose 6-phosphate receptors from endosomes to the trans-Golgi network (TGN), transport routes between endosomes and the Golgi apparatus are still poorly characterized. To investigate endosome to Golgi transport, we have used here a modified ricin molecule containing a tyrosine sulfation site and quantified incorporation of radioactive sulfate, a TGN modification. A tetracycline-inducible mutant Rab9S21N HeLa cell line was constructed and characterized to study whether Rab9 was involved in transport of ricin to the TGN and, if not, to further investigate the route used by ricin. Induced expression of Rab9S21N inhibited Golgi transport of mannose 6-phosphate receptors but did not affect the sulfation of ricin, suggesting that ricin is transported to the TGN via a Rab9-independent pathway. Moreover, because Rab11 is present in the endosomal recycling compartment and the TGN, studies of transient transfections with mutant Rab11 were performed. The results indicated that routing of ricin from endosomes to the TGN occurs by a Rab11-independent pathway. Finally, because clathrin has been implicated in early endosome to TGN transport, ricin transport was investigated in cells with inducible expression of antisense to clathrin heavy chain. Importantly, endosome to TGN transport (sulfation of endocytosed ricin) was unchanged when clathrin function was abolished. In conclusion, ricin is transported from endosomes to the Golgi apparatus by a Rab9-, Rab11-, and clathrin-independent pathway.

Endocytosis of NBD-sphingolipids in neurons: exclusion from degradative compartments and transport to the Golgi complex

Sphingolipids are abundant constituents of neuronal membranes that have been implicated in intracellular signaling, neurite outgrowth and differentiation. Differential localization and trafficking of lipids to membrane domains contribute to the specialized functions. In non-neuronal cultured cell lines, plasma membrane short-chain sphingomyelin and glucosylceramide are recycled via endosomes or sorted to degradative compartments. However, depending on cell type and lipid membrane composition, short-chain glucosylceramide can also be diverted to the Golgi complex. Here, we show that NBD-labeled glucosylceramide and sphingomyelin are transported from the plasma membrane to the Golgi complex in cultured rat hippocampal neurons irrespective of the stage of neuronal differentiation. Golgi complex localization was confirmed by colocalization and Golgi disruption studies, and importantly did not result from conversion of NBD-glucosylceramide or NBD-sphingomyelin to NBD-ceramide. Double-labeling experiments with transferrin or wheat-germ agglutinin showed that NBD-sphingolipids are first internalized to early/recycling endosomes, and subsequently transported to the Golgi complex. The internalization of these two sphingolipid analogs was energy and temperature dependent, and their intracellular transport was insensitive to the NBD fluorescence quencher sodium dithionite. These results indicate that vesicles mediate the transport of internalized NBD-glucosylceramide and NBD-sphingomyelin to the Golgi complex.

Role of Rab9 GTPase in facilitating receptor recruitment by TIP47

Mannose 6-phosphate receptors (MPRs) deliver lysosomal hydrolases from the Golgi to endosomes and then return to the Golgi complex. TIP47 recognizes the cytoplasmic domains of MPRs and is required for endosome-to-Golgi transport. Here we show that TIP47 also bound directly to the Rab9 guanosine triphosphatase (GTPase) in its active, GTP-bound conformation. Moreover, Rab9 increased the affinity of TIP47 for its cargo. A functional Rab9 binding site was required for TIP47 stimulation of MPR transport in vivo. Thus, a cytosolic cargo selection device may be selectively recruited onto a specific organelle, and vesicle budding might be coupled to the presence of an active Rab GTPase.

The molecular machinery for lysosome biogenesis

The lysosome serves as a site for delivery of materials targeted for removal from the eukaryotic cell. The mechanisms underlying the biogenesis of this organelle are currently the subject of renewed interest due to advances in our understanding of the protein sorting machinery. Genetic model systems such as yeast and Drosophila have been instrumental in identifying both protein and lipid components of this machinery. Importantly, many of these components, as well as the processes in which they are involved, are proving conserved in mammals. Other recently identified components, however, appear to be unique to higher eukaryotes. BioEssays 23:333-343, 2001. Published 2001 John Wiley & Sons, Inc.

Rab proteins as membrane organizers

Cellular organelles in the exocytic and endocytic pathways have a distinctive spatial distribution and communicate through an elaborate system of vesiculo-tubular transport. Rab proteins and their effectors coordinate consecutive stages of transport, such as vesicle formation, vesicle and organelle motility, and tethering of vesicles to their target compartment. These molecules are highly compartmentalized in organelle membranes, making them excellent candidates for determining transport specificity and organelle identity.

Small GTP-binding proteins

Small GTP-binding proteins (G proteins) exist in eukaryotes from yeast to human and constitute a superfamily consisting of more than 100 members. This superfamily is structurally classified into at least five families: the Ras, Rho, Rab, Sar1/Arf, and Ran families. They regulate a wide variety of cell functions as biological timers (biotimers) that initiate and terminate specific cell functions and determine the periods of time for the continuation of the specific cell functions. They furthermore play key roles in not only temporal but also spatial determination of specific cell functions. The Ras family regulates gene expression, the Rho family regulates cytoskeletal reorganization and gene expression, the Rab and Sar1/Arf families regulate vesicle trafficking, and the Ran family regulates nucleocytoplasmic transport and microtubule organization. Many upstream regulators and downstream effectors of small G proteins have been isolated, and their modes of activation and action have gradually been elucidated. Cascades and cross-talks of small G proteins have also been clarified. In this review, functions of small G proteins and their modes of activation and action are described.

Rab11 regulates the compartmentalization of early endosomes required for efficient transport from early endosomes to the trans-golgi network

Several GTPases of the Rab family, known to be regulators of membrane traffic between organelles, have been described and localized to various intracellular compartments. Rab11 has previously been reported to be associated with the pericentriolar recycling compartment, post-Golgi vesicles, and the trans-Golgi network (TGN). We compared the effect of overexpression of wild-type and mutant forms of Rab11 on the different intracellular transport steps in the endocytic/degradative and the biosynthetic/exocytic pathways in HeLa cells. We also studied transport from endosomes to the Golgi apparatus using the Shiga toxin B subunit (STxB) and TGN38 as reporter molecules. Overexpression of both Rab11 wild-type (Rab11wt) and mutants altered the localization of the transferrrin receptor (TfR), internalized Tf, the STxB, and TGN38. In cells overexpressing Rab11wt and in a GTPase-deficient Rab11 mutant (Rab11Q70L), these proteins were found in vesicles showing characteristics of sorting endosomes lacking cellubrevin (Cb). In contrast, they were redistributed into an extended tubular network, together with Cb, in cells overexpressing a dominant negative mutant of Rab11 (Rab11S25N). This tubularized compartment was not accessible to Tf internalized at temperatures <20 degrees C, suggesting that it is of recycling endosomal origin. Overexpression of Rab11wt, Rab11Q70L, and Rab11S25N also inhibited STxB and TGN38 transport from endosomes to the TGN. These results suggest that Rab11 influences endosome to TGN trafficking primarily by regulating membrane distribution inside the early endosomal pathway.

Endosome to Golgi transport of ricin is regulated by cholesterol

We have here studied the role of cholesterol in transport of ricin from endosomes to the Golgi apparatus. Ricin is endocytosed even when cells are depleted for cholesterol by using methyl-beta-cyclodextrin (m beta CD). However, as here shown, the intracellular transport of ricin from endosomes to the Golgi apparatus, measured by quantifying sulfation of a modified ricin molecule, is strongly inhibited when the cholesterol content of the cell is reduced. On the other hand, increasing the level of cholesterol by treating cells with mbetaCD saturated with cholesterol (m beta CD/chol) reduced the intracellular transport of ricin to the Golgi apparatus even more strongly. The intracellular transport routes affected include both Rab9-independent and Rab9-dependent pathways to the Golgi apparatus, since both sulfation of ricin after induced expression of mutant Rab9 (mRab9) to inhibit late endosome to Golgi transport and sulfation of a modified mannose 6-phosphate receptor (M6PR) were inhibited after removal or addition of cholesterol. Furthermore, the structure of the Golgi apparatus was affected by increased levels of cholesterol, as visualized by pronounced vesiculation and formation of smaller stacks. Thus, our results indicate that transport of ricin from endosomes to the Golgi apparatus is influenced by the cholesterol content of the cell.

Overexpression of a novel sorting nexin, SNX15, affects endosome morphology and protein trafficking

Sorting nexin (SNX) 15 is a novel member of the SNX family of proteins. Although the functions of most SNXs have not yet been determined, several family members (e.g., SNX1, SNX2, SNX3, and SNX8) are orthologs of yeast proteins involved in protein trafficking. Overexpression of myc-tagged SNX15 in COS-7 cells altered the morphology of several endosomal compartments. In transient transfection experiments, myc-SNX15 was first seen in small punctate spots and small ring structures. Later, myc-SNX15 was found in larger rings. Finally, myc-SNX15 was observed in large, amorphous membrane-limited structures. These structures contained proteins from lysosomes, late endosomes, early endosomes, and the trans-Golgi network. However, the morphology of the endoplasmic reticulum and Golgi was not affected by overexpression of myc-SNX15. In myc-SNX15-overexpressing cells, the endocytosis of transferrin was severely inhibited and endocytosis of tac-trans-Golgi network (TGN) 38 and tac-furin was slowed. In addition, the recycling of internalized tac-TGN38 and tac-furin was also inhibited. Both the morphological and biochemical data indicate that SNX15 plays a crucial role in trafficking through the endocytic pathway. This is the first demonstration that a mammalian SNX protein is involved in protein trafficking.

Sorting of yeast membrane proteins into an endosome-to-Golgi pathway involves direct interaction of their cytosolic domains with Vps35p

Resident late-Golgi membrane proteins in Saccharomyces cerevisiae are selectively retrieved from a prevacuolar-endosomal compartment, a process dependent on aromatic amino acid-based sorting determinants on their cytosolic domains. The formation of retrograde vesicles from the prevacuolar compartment and the selective recruitment of vesicular cargo are thought to be mediated by a peripheral membrane retromer protein complex. We previously described mutations in one of the retromer subunit proteins, Vps35p, which caused cargo-specific defects in retrieval. By genetic and biochemical means we now show that Vps35p directly associates with the cytosolic domains of cargo proteins. Chemical cross-linking, followed by coimmunoprecipitation, demonstrated that Vps35p interacts with the cytosolic domain of A-ALP, a model late-Golgi membrane protein, in a retrieval signal-dependent manner. Furthermore, mutations in the cytosolic domains of A-ALP and another cargo protein, Vps10p, were identified that suppressed cargo-specific mutations in Vps35p but did not suppress the retrieval defects of a vps35 null mutation. Suppression was shown to be due to an improvement in protein sorting at the prevacuolar compartment. These data strongly support a model in which Vps35p acts as a "receptor" protein for recognition of the retrieval signal domains of cargo proteins during their recruitment into retrograde vesicles.

Regulators of membrane trafficking and Mycobacterium tuberculosis phagosome maturation block

The biogenesis and maturation of phagosomes is an area of study which has been employing aspects of proteomic analyses and variations on that theme by identifying components on isolated organelles and following their dynamic changes and interactions with the endocytic pathway. In the case of Mycobacterium tuberculosis phagosome, the arrest of its maturation in infected macrophages, referred to in classical texts as the inhibition of phagosome-lysosome fusion, represents a phenomenon that is used to functionally dissect the phagosomal maturation pathway. In this review, we summarize the recent studies on regulators of membrane trafficking and other organelle components in the context of phagosomal biogenesis and mycobacterial phagosome maturation arrest.

Impairment of bile salt-dependent lipase secretion in human pancreatic tumoral SOJ-6 cells

Bile salt-dependent lipase (BSDL) was detected in human SOJ-6 and rat AR4-2J pancreatic cells. Whereas AR4-2J cells actively secreted the enzyme, BSDL was retained within the Golgi compartment of SOJ-6 cells. Because Rab6 is involved in vesicle transport in the Golgi apparatus and the trans-Golgi network, we confirmed the presence of Rab6 in these cells. In rat AR4-2J cells, Rab6 as well as Rab1A/B and Rab2, partitioned between the cytosol and microsomes. In SOJ-6 cells Rab1A/B and Rab2 also partitioned between the cytosol and microsomes, but Rab6 was strictly associated with microsome membranes, suggesting a specific defect of Rab6 cycling in human SOJ-6 cells. The apparent defect of cycling in these cells is not due to the expression of a defective Rab6 since its correct sequence was confirmed. We further demonstrated that AR4-2J and SOJ-6 cells express the Rab-GDIbeta and Rab-GDIalpha isoforms, respectively. However, the sequence of Rab-GDIbeta, which may be the main form expressed by SOJ-6 cells, identified a few substitutions located in regions that are essential for Rab-GDI function. We conclude that the deficient secretion of BSDL by SOJ-6 cells could be due to the expression of defective Rab-GDIbeta. In spite of the alterations in Rab-GDIbeta, membrane proteins such as CD71 and NHE3 were correctly localized to the cell plasma membrane of SOJ-6 cells, suggesting that two functional distinct secretory pathway coexist in pancreatic cells.

Rab11b is essential for recycling of transferrin to the plasma membrane

Members of the Rab family of small GTPases play important roles in membrane trafficking along the exocytic and endocytic pathways. The Rab11 subfamily consists of two highly conserved members, Rab11a and Rab11b. Rab11a has been localized both to the pericentriolar recycling endosome and to the trans-Golgi network and functions in recycling of transferrin. However, the localization and function of Rab11b are completely unknown. In this study green fluorescent protein (GFP)-tagged Rab11b was used to determine its subcellular localization. GFP-Rab11b colocalized with internalized transferrin, and using different mutants of Rab11b, the role of this protein in transferrin uptake and recycling was examined. Two of these mutants, Rab11b-Q/L (constitutively active) and Rab11b-S/N (constitutively inactive), strongly inhibited the recycling of transferrin. Interestingly, both of them had no effect on transferrin uptake. In contrast, the C-terminally altered mutant Rab11b-DeltaC, which cannot be prenylated and therefore cannot interact with membranes, did not interfere with wild-type Rab11b function. From these data we concluded that functional Rab11b is essential for the transport of internalized transferrin from the recycling compartment to the plasma membrane.

Quantitative analysis of TIP47-receptor cytoplasmic domain interactions: implications for endosome-to-trans Golgi network trafficking

TIP47 (tail-interacting protein of 47 kDa) binds to the cytoplasmic domains of the cation-independent and cation-dependent mannose 6-phosphate receptors and is required for their transport from late endosomes to the trans Golgi network in vitro and in vivo. We report here a quantitative analysis of the interaction of recombinant TIP47 with mannose 6-phosphate receptor cytoplasmic domains. Recombinant TIP47 binds more tightly to the cation-independent mannose 6-phosphate receptor (K(D) = 1 microm) than to the cation-dependent mannose 6-phosphate receptor (K(D) = 3 microm). In addition, TIP47 fails to interact with the cytoplasmic domains of the hormone-processing enzymes, furin, phosphorylated furin, and metallocarboxypeptidase D, as well as the cytoplasmic domain of TGN38, proteins that are also transported from endosomes to the trans Golgi network. Although these proteins failed to bind TIP47, furin and TGN38 were readily recognized by the clathrin adaptor, AP-2. These data suggest that TIP47 recognizes a very select set of cargo molecules. Moreover, our data suggest unexpectedly that furin, TGN38, and carboxypeptidase D may use a distinct vesicular carrier and perhaps a distinct route for transport between endosomes and the trans Golgi network.

Small G-protein networks: their crosstalk and signal cascades

Small GTP-binding proteins (G-proteins) exist in eukaryotes from yeast to human and constitute a superfamily consisting of more than 100 members. This superfamily is structurally classified into at least five families: the Ras, Rho/Rac/Cdc42, Rab, Sar1/Arf, and Ran families. They play key roles not only in temporal but also in spatial determination of specific cell functions. It has become clear that multiple small G-proteins form signalling cascades that are involved in various cellular functions, such as budding processes of the yeast and regulation of the actin cytoskeleton in fibroblasts. In addition, two distinct small G-proteins regulate specific cellular functions in a cooperative or antagonistic manner. A single small G-protein exerts various biological responses through different downstream effectors. Moreover, some of these downstream effectors sequentially activate further downstream effector proteins. Thus, small G-proteins appear to exert their functions through their mutual crosstalk and multiple downstream effectors in a variety of cellular functions.

Sorting in the endosomal system in yeast and animal cells

The endosomal system is a major membrane-sorting apparatus. New evidence reveals that novel coat proteins assist specific sorting steps and docking factors ensure the vectorial nature of trafficking in the endosomal compartment. There is also good evidence for ubiquitin regulating passage of certain proteins into multivesicular late endosomes, which mature by accumulating invaginated membrane. Lipids play a central role in this involution process, as do the class E vacuolar protein-sorting proteins.

Role for dynamin in late endosome dynamics and trafficking of the cation-independent mannose 6-phosphate receptor

It is well established that dynamin is involved in clathrin-dependent endocytosis, but relatively little is known about possible intracellular functions of this GTPase. Using confocal imaging, we found that endogenous dynamin was associated with the plasma membrane, the trans-Golgi network, and a perinuclear cluster of cation-independent mannose 6-phosphate receptor (CI-MPR)-containing structures. By electron microscopy (EM), it was shown that these structures were late endosomes and that the endogenous dynamin was preferentially localized to tubulo-vesicular appendices on these late endosomes. Upon induction of the dominant-negative dynK44A mutant, confocal microscopy demonstrated a redistribution of the CI-MPR in mutant-expressing cells. Quantitative EM analysis of the ratio of CI-MPR to lysosome-associated membrane protein-1 in endosome profiles revealed a higher colocalization of the two markers in dynK44A-expressing cells than in control cells. Western blot analysis showed that dynK44A-expressing cells had an increased cellular procathepsin D content. Finally, EM revealed that in dynK44A-expressing cells, endosomal tubules containing CI-MPR were formed. These results are in contrast to recent reports that dynamin-2 is exclusively associated with endocytic structures at the plasma membrane. They suggest instead that endogenous dynamin also plays an important role in the molecular machinery behind the recycling of the CI-MPR from endosomes to the trans-Golgi network, and we propose that dynamin is required for the final scission of vesicles budding from endosome tubules.

Rab GTPases coordinate endocytosis

Endocytosis is characterized by vesicular transport along numerous pathways. Common steps in each pathway include membrane budding to form vesicles, transport to a particular destination, and ultimately docking and fusion with the target membrane. Specificity of vesicle targeting is rendered in part by associated Rab GTPases. This review summarizes current knowledge about Rab GTPase functions in the endocytic pathways and provides insight into the regulation of Rab GTPase activity and mechanisms of Rab protein function. Functional assays have identified some Rab proteins that operate on individual pathways, but Rab proteins in several pathways remain controversial or have not been identified. Control of Rab GTPase activity is exerted through multiple levels of regulation. Significant new information pertaining to Rab protein function in regulating transport has emerged. Remarkably, Rab5 GTPase links budding, cytoskeletal transport and docking/fusion activities. This paradigm will most likely be generally applicable to other Rab GTPase pathways. Together with the cross-talk between different Rab proteins and their effectors, this may provide an integrated system for the general coordination of endocytic pathways to maintain organelle homeostasis.

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.

Asparagine-linked oligosaccharides protect Lamp-1 and Lamp-2 from intracellular proteolysis

Lysosomes contain several integral membrane proteins (termed Lamps and Limps) that are extensively glycosylated with asparagine-linked oligosaccharides. It has been postulated that these glycans protect the underlying polypeptides from the proteolytic environment of the lysosome. Previous attempts to test this hypothesis have been inconclusive because they utilized approaches that prevent initial glycosylation and thereby impair protein folding. We have used endoglycosidase H to remove the Asn-linked glycans from fully folded lysosomal membrane proteins in living cells. Deglycosylation of Lamp-1 and Lamp-2 resulted in their rapid degradation, whereas Limp-2 was relatively stable in the lysosome in the absence of high mannose Asn-linked oligosaccharides. Depletion of Lamp-1 and Lamp-2 had no measurable effect on endosomal/lysosomal pH, osmotic stability, or density, and cell viability was maintained. Transport of endocytosed material to dense lysosomes was delayed in endoglycosidase H treated cells, but the rate of degradation of internalized bovine serum albumin was unchanged. These data provide direct evidence that Asn-linked oligosaccharides protect a subset of lysosomal membrane proteins from proteolytic digestion in intact cells.

MAL, an integral element of the apical sorting machinery, is an itinerant protein that cycles between the trans-Golgi network and the plasma membrane

The MAL proteolipid is a nonglycosylated integral membrane protein found in glycolipid-enriched membrane microdomains. In polarized epithelial Madin-Darby canine kidney cells, MAL is necessary for normal apical transport and accurate sorting of the influenza virus hemagglutinin. MAL is thus part of the integral machinery for glycolipid-enriched membrane-mediated apical transport. At steady state, MAL is predominantly located in perinuclear vesicles that probably arise from the trans-Golgi network (TGN). To act on membrane traffic and to prevent their accumulation in the target compartment, integral membrane elements of the protein-sorting machinery should be itinerant proteins that cycle between the donor and target compartments. To establish whether MAL is an itinerant protein, we engineered the last extracellular loop of MAL by insertion of sequences containing the FLAG epitope or with sequences containing residues that became O-glycosylated within the cells or that displayed biotinylatable groups. The ectopic expression of these modified MAL proteins allowed us to investigate the surface expression of MAL and its movement through different compartments after internalization with the use of a combination of assays, including surface biotinylation, surface binding of anti-FLAG antibodies, neuraminidase sensitivity, and drug treatments. Immunofluorescence and flow cytometric analyses indicated that, in addition to its Golgi localization, MAL was also expressed on the cell surface, from which it was rapidly internalized. This retrieval implies transport through the endosomal pathway and requires endosomal acidification, because it can be inhibited by drugs such as chloroquine, monensin, and NH(4)Cl. Resialylation experiments of surface MAL treated with neuraminidase indicated that approximately 30% of the internalized MAL molecules were delivered to the TGN, probably to start a new cycle of cargo transport. Together, these observations suggest that, as predicted for integral membrane members of the late protein transport machinery, MAL is an itinerant protein cycling between the TGN and the plasma membrane.

Niemann-Pick C1 is a late endosome-resident protein that transiently associates with lysosomes and the trans-Golgi network

Niemann-Pick type C (NPC) disease is a severe cell lipidosis characterized by the accumulation of unesterified cholesterol in the endosomal/lysosomal system. Recently the primary disease-causing gene, NPC1, was identified, but few clues regarding its potential function(s) could be derived from its predicted amino acid sequence. Therefore, efforts were directed at characterizing the subcellular location of the NPC1 protein. Initial studies with a FLAG-tagged NPC1 cDNA demonstrated that NPC1 is a glycoprotein that associates with the membranes of a population of cytoplasmic vesicles. Immunofluorescence microscopy using anti-NPC1 polyclonal antibodies confirmed this analysis. Double-label immunofluorescence microscopy and subcellular fractionation studies indicated that NPC1 associates predominantly with late endosomes (Rab9 GTPase-positive vesicles) and, to a lesser extent, with lysosomes and the trans-Golgi network. When cholesterol egress from lysosomes was blocked by treatment of cells with U18666A, the NPC1 location shifted from late endosomes to the trans-Golgi network and lysosomes. Subcellular fractionation of liver homogenates from U18666A-treated mice confirmed these observations. These data suggest that U18666A may inhibit the retrograde transport of NPC1 from lysosomes to late endosomes for subsequent transfer to the trans-Golgi network.

Endocytosis and intracellular transport of ricin: recent discoveries

The plant toxin ricin has proven valuable as a membrane marker in studies of endocytosis as well as studies of different intracellular transport steps. The toxin, which consists of two polypeptide chains, binds by one chain (the B-chain) to both glycolipids and glycoproteins with terminal galactose at the cell surface. The other chain (the A-chain) enters the cytosol and inhibits protein synthesis enzymatically. After binding the toxin is endocytosed by different mechanisms, and it is transported via endosomes to the Golgi apparatus and the endoplasmic reticulum before translocation of the A-chain to the cytosol. The different transport steps have been analyzed by studying trafficking of ricin as well as modified ricin molecules.

Distinct domains within Vps35p mediate the retrieval of two different cargo proteins from the yeast prevacuolar/endosomal compartment

Resident membrane proteins of the trans-Golgi network (TGN) of Saccharomyces cerevisiae are selectively retrieved from a prevacuolar/late endosomal compartment. Proper cycling of the carboxypeptidase Y receptor Vps10p between the TGN and prevacuolar compartment depends on Vps35p, a hydrophilic peripheral membrane protein. In this study we use a temperature-sensitive vps35 allele to show that loss of Vps35p function rapidly leads to mislocalization of A-ALP, a model TGN membrane protein, to the vacuole. Vps35p is required for the prevacuolar compartment-to-TGN transport of both A-ALP and Vps10p. This was demonstrated by phenotypic analysis of vps35 mutant strains expressing A-ALP mutants lacking either the retrieval or static retention signals and by an assay for prevacuolar compartment-to-TGN transport. A novel vps35 allele was identified that was defective for retrieval of A-ALP but functional for retrieval of Vps10p. Moreover, several other vps35 alleles were identified with the opposite characteristics: they were defective for Vps10p retrieval but near normal for A-ALP localization. These data suggest a model in which distinct structural features within Vps35p are required for associating with the cytosolic domains of each cargo protein during the retrieval process.

Regulation of membrane transport through the endocytic pathway by rabGTPases

Small GTP binding proteins of the rab family are associated with the cytoplasmic surface of compartments of the central vacuolar system. Several of them, including rab5, rab4 and rab11, are localized to early endocytic organelles where they regulate distinct events in the transferrin receptor pathway. Whereas rab5 is controlling transport to early endosomes, rab4 and rab11 are involved in the regulation of recycling back to the plasma membrane. How GTP-hydrolysis of rab bound GTP is related to the role of these proteins in endocytosis is not yet known, but quick progress is being made towards this goal through the identification of proteins regulating the activity of these rab proteins.

Recycling cell surface glycoproteins undergo limited oligosaccharide reprocessing in LEC1 mutant Chinese hamster ovary cells

The ability of particular cell surface glycoproteins to recycle and become exposed to individual Golgi enzymes has been demonstrated. This study was designed to determine whether endocytic trafficking includes significant reentry into the overall oligosaccharide processing pathway. The Lec1 mutant of Chinese hamster ovary (CHO) cells lack N -acetylglucosaminyltransferase I (GlcNAc-TI) activity resulting in surface expression of incompletely processed Man5GlcNAc2 N -linked oligosaccharides. An oligosaccharide tracer was created by exoglycosylation of cell surface glycoproteins with purified porcine GlcNAc-TI and UDP-[3H]GlcNAc. Upon reculturing, all cell surface glycoproteins that acquired [3H]GlcNAc were acted upon by intracellular mannosidase II, the next enzyme in the Golgi processing pathway of complex N -linked oligosaccharides (t1/2= 3-4 h). That all radiolabeled cell surface glycoproteins were included in this endocytic pathway indicates a common intracellular compartment into which endocytosed cell surface glycoproteins return. Significantly, no evidence was found for continued oligosaccharide processing consistent with transit through the latter cisternae of the Golgi apparatus. These data indicate that, although recycling plasma membrane glycoproteins can be reexposed to individual Golgi-derived enzymes, significant reentry into the overall contiguous processing pathway is not evident.

Direct pathway from early/recycling endosomes to the Golgi apparatus revealed through the study of shiga toxin B-fragment transport

Shiga toxin and other toxins of this family can escape the endocytic pathway and reach the Golgi apparatus. To synchronize endosome to Golgi transport, Shiga toxin B-fragment was internalized into HeLa cells at low temperatures. Under these conditions, the protein partitioned away from markers destined for the late endocytic pathway and colocalized extensively with cointernalized transferrin. Upon subsequent incubation at 37 degreesC, ultrastructural studies on cryosections failed to detect B-fragment-specific label in multivesicular or multilamellar late endosomes, suggesting that the protein bypassed the late endocytic pathway on its way to the Golgi apparatus. This hypothesis was further supported by the rapid kinetics of B-fragment transport, as determined by quantitative confocal microscopy on living cells and by B-fragment sulfation analysis, and by the observation that actin- depolymerizing and pH-neutralizing drugs that modulate vesicular transport in the late endocytic pathway had no effect on B-fragment accumulation in the Golgi apparatus. B-fragment sorting at the level of early/recycling endosomes seemed to involve vesicular coats, since brefeldin A treatment led to B-fragment accumulation in transferrin receptor-containing membrane tubules, and since B-fragment colocalized with adaptor protein type 1 clathrin coat components on early/recycling endosomes. Thus, we hypothesize that Shiga toxin B-fragment is transported directly from early/recycling endosomes to the Golgi apparatus. This pathway may also be used by cellular proteins, as deduced from our finding that TGN38 colocalized with the B-fragment on its transport from the plasma membrane to the TGN.

Rab11 is required for trans-golgi network-to-plasma membrane transport and a preferential target for GDP dissociation inhibitor

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

Identification of regulators for Ypt1 GTPase nucleotide cycling

Small GTPases of the Ypt/Rab family are involved in the regulation of vesicular transport. Cycling between the GDP- and GTP-bound forms and the accessory proteins that regulate this cycling are thought to be crucial for Ypt/Rab function. Guanine nucleotide exchange factors (GEFs) stimulate both GDP loss and GTP uptake, and GTPase-activating proteins (GAPs) stimulate GTP hydrolysis. Little is known about GEFs and GAPs for Ypt/Rab proteins. In this article we report the identification and initial characterization of two factors that regulate nucleotide cycling by Ypt1p, which is essential for the first two steps of the yeast secretory pathway. The Ypt1p-GEF stimulates GDP release and GTP uptake at least 10-fold and is specific for Ypt1p. Partially purified Ypt1p-GEF can rescue the inhibition caused by the dominant-negative Ypt1p-D124N mutant of in vitro endoplasmic reticulum-to-Golgi transport. This mutant probably blocks transport by inhibiting the GEF, suggesting that we have identified the physiological GEF for Ypt1p. The Ypt1p-GAP stimulates GTP hydrolysis by Ypt1p up to 54-fold, has a higher affinity for the GTP-bound form of Ypt1p than for the GDP-bound form, and is specific to a subgroup of exocytic Ypt proteins. The Ypt1p-GAP activity is not affected by deletion of two genes that encode known Ypt GAPs, GYP7 and GYP1, nor is it influenced by mutations in SEC18, SEC17, or SEC22, genes whose products are involved in vesicle fusion. The GEF and GAP activities for Ypt1p localize to particulate cellular fractions. However, contrary to the predictions of current models, the GEF activity localizes to the fraction that functions as the acceptor in an endoplasmic reticulum-to-Golgi transport assay, whereas the GAP activity cofractionates with markers for the donor. On the basis of our current and previous results, we propose a new model for the role of Ypt/Rab nucleotide cycling and the factors that regulate this process.

Rab proteins

Rab proteins form the largest branch of the Ras superfamily of GTPases. They are localized to the cytoplasmic face of organelles and vesicles involved in the biosynthetic/secretory and endocytic pathways in eukaryotic cells. It is now well established that Rab proteins play an essential role in the processes that underlie the targeting and fusion of transport vesicles with their appropriate acceptor membranes. However, the recent discovery of several putative Rab effectors, which are not related to each other and which fulfil diverse functions, suggests a more complex role for Rab proteins. At least two Rab proteins act at the level of the Golgi apparatus. Rab1 and its yeast counterpart Ypt1 control transport events through early Golgi compartments. Work from our laboratory points out a role for Rab6 in intra-Golgi transport, likely in a retrograde direction.

A membrane coat complex essential for endosome-to-Golgi retrograde transport in yeast

We have recently characterized three yeast gene products (Vps35p, Vps29p, and Vps30p) as candidate components of the sorting machinery required for the endosome-to-Golgi retrieval of the vacuolar protein sorting receptor Vps10p (Seaman, M.N.J., E.G. Marcusson, J.-L. Cereghino, and S.D. Emr. 1997. J. Cell Biol. 137:79-92). By genetic and biochemical means we now show that Vps35p and Vps29p interact and form part of a multimeric membrane-associated complex that also contains Vps26p, Vps17p, and Vps5p. This complex, designated here as the retromer complex, assembles from two distinct subcomplexes comprising (a) Vps35p, Vps29p, and Vps26p; and (b) Vps5p and Vps17p. Density gradient fractionation of Golgi/endosomal/vesicular membranes reveals that Vps35p cofractionates with Vps5p/Vps17p in a vesicle-enriched dense membrane fraction. Furthermore, gel filtration analysis indicates that Vps35p and Vps5p are present on a population of vesicles and tubules slightly larger than COPI/coatomer-coated vesicles. We also show by immunogold EM that Vps5p is localized to discrete regions at the rims of the prevacuolar endosome where vesicles appear to be budding. Size fractionation of cytosolic and recombinant Vps5p reveals that Vps5p can self-assemble in vitro, suggesting that Vps5p may provide the mechanical impetus to drive vesicle formation. Based on these findings we propose a model in which Vps35p/Vps29p/Vps26p function to select cargo for retrieval, and Vps5p/Vps17p assemble onto the membrane to promote vesicle formation. Conservation of the yeast retromer complex components in higher eukaryotes suggests an important general role for this complex in endosome-to-Golgi retrieval.