Rab9 functions in transport between late endosomes and the trans Golgi network

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.

Live Salmonella modulate expression of Rab proteins to persist in a specialized compartment and escape transport to lysosomes

We investigated the intracellular route of Salmonella in macrophages to determine a plausible mechanism for their survival in phagocytes. Western blot analysis of isolated phagosomes using specific antibodies revealed that by 5 min after internalization dead Salmonella-containing phagosomes acquire transferrin receptors (a marker for early endosomes), whereas by 30 min the dead bacteria are found in vesicles carrying the late endosomal markers cation-dependent mannose 6-phosphate receptors, Rab7 and Rab9. In contrast, live Salmonella-containing phagosomes (LSP) retain a significant amount of Rab5 and transferrin receptor until 30 min, selectively deplete Rab7 and Rab9, and never acquire mannose 6-phosphate receptors even 90 min after internalization. Retention of Rab5 and Rab18 and selective depletion of Rab7 and Rab9 presumably enable the LSP to avoid transport to lysosomes through late endosomes. The presence of immature cathepsin D (48 kDa) and selective depletion of the vacuolar ATPase in LSP presumably contributes to the less acidic pH of LSP. In contrast, proteolytically processed cathepsin D (M(r) 17,000) was detected by 30 min on the dead Salmonella-containing phagosomes. Morphological analysis also revealed that after uptake by macrophages, the dead Salmonella are transported to lysosomes, whereas the live bacteria persist in compartments that avoid fusion with lysosomes, indicating that live Salmonella bypass the normal endocytic route targeted to lysosomes and mature in a specialized compartment.

Niemann-Pick type C mutations cause lipid traffic jam

The Niemann-Pick C protein (NPC1) is required for cholesterol transport from late endosomes and lysosomes to other cellular membranes. Mutations in NPC1 cause lysosomal lipid storage and progressive neurological degeneration. Cloning of the NPC1 gene has given us tools with which to investigate the function of this putative cholesterol transporter. Here, we discuss recent studies indicating that NPC1 is not a cholesterol-specific transport molecule. Instead, NPC1 appears to be required for the vesicular shuttling of both lipids and fluid-phase constituents from multivesicular late endosomes to destinations such as the trans-Golgi network.

Characterization of a Rab11 homologue in Trypanosoma cruzi

Vesicle trafficking between organelles occurs through fusion of donor and specific acceptor membranes. This process is highly regulated and ensures proper direction in sorting and packaging of a number of molecules in eukaryotic cells. Monomeric GTPases of the Rab family play a pivotal role in the control of membrane fusion and vesicle traffic. In this paper, we characterize a Trypanosoma cruzi Rab 11 homologue (TcRab11) that shares at, the amino acid level, 40% similarity with human rab11, Arabdopsis thaliana rab11 and yeast rab11 homologue genes. Western blot analysis, using a polyclonal rabbit antiserum raised against a synthetic peptide derived from the COOH-terminus of predicted the TcRab11 protein, reacted to a 26kDa protein. In immunofluorescence assays, TcRab 11, was shown to be expressed in epimastigote and amastigote forms, but it was absent in trypomastigotes. Interestingly, the TcRab11 product seems to be located at the reservosome complex, a site of active endocytosis and vesicle fusion present only in the epimastigote stage. Therefore, TcRab11 may represent the first molecular marker of this peculiar organelle.

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.

Rab7: a key to lysosome biogenesis

The molecular machinery behind lysosome biogenesis and the maintenance of the perinuclear aggregate of late endocytic structures is not well understood. A likely candidate for being part of this machinery is the small GTPase Rab7, but it is unclear whether this protein is associated with lysosomes or plays any role in the regulation of the perinuclear lysosome compartment. Previously, Rab7 has mainly been implicated in transport from early to late endosomes. We have now used a new approach to analyze the role of Rab7: transient expression of Enhanced Green Fluorescent Protein (EGFP)-tagged Rab7 wt and mutant proteins in HeLa cells. EGFP-Rab7 wt was associated with late endocytic structures, mainly lysosomes, which aggregated and fused in the perinuclear region. The size of the individual lysosomes as well as the degree of perinuclear aggregation increased with the expression levels of EGFP-Rab7 wt and, more dramatically, the active EGFP-Rab7Q67L mutant. In contrast, upon expression of the dominant-negative mutants EGFP-Rab7T22N and EGFP-Rab7N125I, which localized mainly to the cytosol, the perinuclear lysosome aggregate disappeared and lysosomes, identified by colocalization of cathepsin D and lysosome-associated membrane protein-1, became dispersed throughout the cytoplasm, they were inaccessible to endocytosed molecules such as low-density lipoprotein, and their acidity was strongly reduced, as determined by decreased accumulation of the acidotropic probe LysoTracker Red. In contrast, early endosomes associated with Rab5 and the transferrin receptor, late endosomes enriched in the cation-independent mannose 6-phosphate receptor, and the trans-Golgi network, identified by its enrichment in TGN-38, were unchanged. These data demonstrate for the first time that Rab7, controlling aggregation and fusion of late endocytic structures/lysosomes, is essential for maintenance of the perinuclear lysosome compartment.

Facing inward from compartment shores: how many pathways were we looking for?

Protein toxins of the Shiga family have become potent tools in studying a number of intracellular transport events such as endocytosis, the communication between endosomes and the biosynthetic/secretory pathway, and retrograde transport from the Golgi apparatus to the endoplasmic reticulum. It seems clear today that most of these transport events can be explained from the toxins' interactions with cellular factors. This review will primarily focus on the discussion of recent data obtained on Shiga toxin and related toxins. We will point out to what extent the study of these proteins has opened new avenues for the development of intracellular targeting tools.

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.

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.

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.

VAMP-7 mediates vesicular transport from endosomes to lysosomes

A more complete picture of the molecules that are critical for the organization of membrane compartments is beginning to emerge through the characterization of proteins in the vesicle-associated membrane protein (also called synaptobrevin) family of membrane trafficking proteins. To better understand the mechanisms of membrane trafficking within the endocytic pathway, we generated a series of monoclonal and polyclonal antibodies against the cytoplasmic domain of vesicle-associated membrane protein 7 (VAMP-7). The antibodies recognize a 25-kD membrane-associated protein in multiple tissues and cell lines. Immunohistochemical analysis reveals colocalization with a marker of late endosomes and lysosomes, lysosome-associated membrane protein 1 (LAMP-1), but not with other membrane markers, including p115 and transferrin receptor. Treatment with nocodozole or brefeldin A does not disrupt the colocalization of VAMP-7 and LAMP-1. Immunoelectron microscopy analysis shows that VAMP-7 is most concentrated in the trans-Golgi network region of the cell as well as late endosomes and transport vesicles that do not contain the mannose-6 phosphate receptor. In streptolysin- O-permeabilized cells, antibodies against VAMP-7 inhibit the breakdown of epidermal growth factor but not the recycling of transferrin. These data are consistent with a role for VAMP-7 in the vesicular transport of proteins from the early endosome to the lysosome.

Rab15 mediates an early endocytic event in Chinese hamster ovary cells

Rab GTPases comprise a large family of monomeric proteins that regulate a diverse number of membrane trafficking events, including endocytosis. In this paper, we examine the subcellular distribution and function of the GTPase Rab15. Our biochemical and confocal immunofluorescence studies demonstrate that Rab15 associates with the transferrin receptor, a marker for the early endocytic pathway, but not with Rab7 or the cation-independent mannose 6-phosphate receptor, markers for late endosomal membranes. Furthermore, Rab15 colocalizes with Rab4 and -5 on early/sorting endosomes, as well as Rab11 on pericentriolar recycling endosomes. Consistent with its localization to early endosomal membranes, overexpression of the constitutively active mutant HArab15Q67L reduces receptor-mediated and fluid phase endocytosis. Therefore, our functional studies suggest that Rab15 may function as an inhibitory GTPase in early endocytic trafficking.

Comparison of the effects on secretion in chromaffin and PC12 cells of Rab3 family members and mutants. Evidence that inhibitory effects are independent of direct interaction with Rabphilin3

The Rab class of low molecular weight GTPases has been implicated in the regulation of vesicular trafficking between membrane compartments in eukaryotic cells. The Rab3 family consisting of four highly homologous isoforms is associated with secretory granules and synaptic vesicles. Many different types of experiments indicate that Rab3a is a negative regulator of exocytosis and that its GTP-bound form interacts with Rabphilin3, a possible effector. Overexpression of Rabphilin3 in chromaffin cells enhances secretion. We have investigated the expression, localization, and effects on secretion of the various members of the Rab3 family in bovine chromaffin and PC12 cells. We found that Rab3a, Rab3b, Rab3c, and Rab3d are expressed to varying degrees in PC12 cells and in a fraction enriched in chromaffin granule membranes from the adrenal medulla. Immunocytochemistry revealed that all members of the family when overexpressed in PC12 cells localize to secretory granules. Binding constants for the interaction of the GTP-bound forms of Rab3a, Rab3b, Rab3c, and Rab3d with Rabphilin3 were comparable (Kd = 10-20 nM). Overexpression of each of the four members of the Rab3 family inhibited secretion. Mutations in Rab3a were identified that strongly impaired the ability of the GTP-bound form to interact with Rabphilin3. The mutated proteins inhibited secretion similarly to wild type Rab3a. Although Rab3a and Rabphilin3 are located on the same secretory granule or secretory vesicle and interact both in vitro and in situ, it is concluded that the inhibition of secretion by overexpression of Rab3a is unrelated to its ability to interact with Rabphilin3.

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.

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.

Maturation of phagosomes is accompanied by specific patterns of small GTPases

In this study we purified phagosomes of the ciliated protozoan Tetrahymena thermophila to analyze aspects of the maturation pathway of phagocytotic vesicles. Phagosomes were labeled with magnetic microparticles and then purified in high amounts with the help of a permanent magnet. By combining a pulse-chase labeling protocol with the magnetic separation procedure we were able to isolate phagosomes of defined ages, which represent distinct stages of their maturation pathway. GTP-overlay assays showed that a set of small GTPases of the ras superfamily is associated with these phagosomes. Phagosomes isolated at different stages of maturation revealed a change in the pattern of the small GTPases. Some small GTPases identified by the GTPase overlay assays could be aligned to India ink stained protein spots in two-dimensional gels of isolated phagosomes. The results presented here are a first step to identify the members of small GTPases associated with phagosomes during their maturation pathway. Microsequencing of pooled polypeptides by mass-spectrometric techniques will identify the primary structure of these small GTPases.

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.

Hydrolysis of GTP on rab11 is required for the direct delivery of transferrin from the pericentriolar recycling compartment to the cell surface but not from sorting endosomes

Rab11 is a small GTP-binding protein that in cultured mammalian cells has been shown to be concentrated in the pericentriolar endosomal recycling compartment and to play a key role in passage of the recycling transferrin receptor through that compartment [Ullrich, O., Reinsch, S., Urbé, S., Zerial, M. & Parton, R. G. (1996) J. Cell Biol. 135, 913-924]. To obtain insights into the site(s) of action of rab11 within the recycling pathway, we have now compared the effects on recycling at 37 degreesC of overexpression of wild-type rab11 and various mutant forms of this protein in cells that had been loaded with transferrin at either 37 degreesC or 16 degreesC. We show that incubation at 16 degreesC blocks passage of endocytosed transferrin into the recycling compartment and that, whereas the rab11 dominant negative mutant form (S25N) inhibits transferrin recycling after interiorization at either temperature, the wild-type rab11 and constitutively active mutant (Q70L) have no inhibitory effect on the recycling of molecules that were interiorized at 16 degreesC. This differential inhibitory effect shows that two distinct pathways for recycling are followed by the bulk of the transferrin molecules interiorized at the two different temperatures. The incapacity of the constitutively active form of rab11 (Q70L) to inhibit recycling of molecules interiorized at 16 degreesC is consistent with their recycling taking place directly from sorting endosomes, in a process that does not require hydrolysis of GTP on rab11. The fact that the dominant negative (S25N) form of rab11 inhibits recycling of molecules interiorized at both temperatures indicates that activation of rab11 by GTP is required for exit of transferrin from sorting endosomes, regardless of whether this exit is toward the recycling compartment or directly to the plasma membrane.

TIP47: a cargo selection device for mannose 6-phosphate receptor trafficking

Mannose 6-phosphate receptors (MPRs) transport newly synthesized lysosomal hydrolases from the Golgi to prelysosomes and then return to the Golgi for another round of transport. We have identified a 47 kDa protein (TIP47) that binds selectively to the cytoplasmic domains of cation-independent and cation-dependent MPRs. TIP47 is present in cytosol and on endosomes and is required for MPR transport from endosomes to the trans-Golgi network in vitro and in vivo. TIP47 recognizes a phenylalanine/tryptophan signal in the tail of the cation-dependent MPR that is essential for its proper sorting within the endosomal pathway. These data suggest that TIP47 binds MPR cytoplasmic domains and facilitates their collection into transport vesicles destined for the Golgi.

A novel Rab GTPase, Rab33B, is ubiquitously expressed and localized to the medial Golgi cisternae

Small GTP-binding proteins of the Rab family play important roles at defined steps of vesicular transport in protein secretion and the endocytosis pathway. In mammals, more than 30 proteins belonging to the Rab family have been reported to date. We report here the molecular cloning and characterization of a novel Rab protein, Rab33B. The amino acid sequence of Rab33B shows 55.3% identity to the Rab33A protein (previously called S10), and these two proteins share unique amino acid sequences at the effector domain. The genomic organization of rab33B was the same as rab33A: it consists of two exons. Thus, these two proteins make a subclass within the Rab family. Northern blot analysis showed that rab33B is expressed ubiquitously in mouse tissues, in contrast to rab33A whose expression is restricted to the brain and the immune system. A 26 kDa protein was detected by western blotting using a Rab33B-specific monoclonal antibody. Using immunofluorescence studies, Rab33B was shown to co-localize with (alpha)-mannosidase II, a Golgi-specific marker. Immunoelectron microscopy analysis further defined the localization of Rab33B to the medial Golgi cisternae. These results suggest Rab33B plays a role in intra-Golgi transport.

Fusion of lysosomes with late endosomes produces a hybrid organelle of intermediate density and is NSF dependent

Using a cell-free content mixing assay containing rat liver endosomes and lysosomes in the presence of pig brain cytosol, we demonstrated that after incubation at 37 degrees C, late endosome-lysosome hybrid organelles were formed, which could be isolated by density gradient centrifugation. ImmunoEM showed that the hybrids contained both an endocytosed marker and a lysosomal enzyme. Formation of the hybrid organelles appeared not to require vesicular transport between late endosomes and lysosomes but occurred as a result of direct fusion. Hybrid organelles with similar properties were isolated directly from rat liver homogenates and thus were not an artifact of cell-free incubations. Direct fusion between late endosomes and lysosomes was an N-ethylmaleimide-sensitive factor-dependent event and was inhibited by GDP-dissociation inhibitor, indicating a requirement for a rab protein. We suggest that in cells, delivery of endocytosed ligands to an organelle where proteolytic digestion occurs is mediated by direct fusion of late endosomes with lysosomes. The consequences of this fusion to the maintenance and function of lysosomes are discussed.

Expression of mutant dynamin inhibits toxicity and transport of endocytosed ricin to the Golgi apparatus

Endocytosis and intracellular transport of ricin were studied in stable transfected HeLa cells where overexpression of wild-type (WT) or mutant dynamin is regulated by tetracycline. Overexpression of the temperature-sensitive mutant dynG273D at the nonpermissive temperature or the dynK44A mutant inhibits clathrin-dependent endocytosis (Damke, H., T. Baba, A.M. van der Blieck, and S.L. Schmid. 1995. J. Cell Biol. 131: 69-80; Damke, H., T. Baba, D.E. Warnock, and S.L. Schmid. 1994. J. Cell Biol. 127:915-934). Under these conditions, ricin was endocytosed at a normal level. Surprisingly, overexpression of both mutants made the cells less sensitive to ricin. Butyric acid and trichostatin A treatment enhanced dynamin overexpression and increased the difference in toxin sensitivity between cells with normal and mutant dynamin. Intoxication with ricin seems to require toxin transport to the Golgi apparatus (Sandirg, K., and B. van Deurs. 1996. Physiol. Rev. 76:949-966), and this process was monitored by measuring the incorporation of radioactive sulfate into a modified ricin molecule containing a tyrosine sulfation site. The sulfation of ricin was much greater in cells expressing dynWT than in cells expressing dynK44A. Ultrastructural analysis using a ricin-HRP conjugate confirmed that transport to the Golgi apparatus was severely inhibited in cells expressing dynK44A. In contrast, ricin transport to lysosomes as measured by degradation of 125I-ricin was essentially unchanged in cells expressing dynK44A. These data demonstrate that although ricin is internalized by clathrin-independent endocytosis in cells expressing mutant dynamin, there is a strong and apparently selective inhibition of ricin transport to the Golgi apparatus. Also, in cells with mutant dynamin, there is a redistribution of the mannose-6-phosphate receptor.

Role of Rab GTPases in membrane traffic

Small GTPases of the Rab subfamily have been known to be key regulators of intracellular membrane traffic since the late 1980s. Today this protein group amounts to more than 40 members in mammalian cells which localize to distinct membrane compartments and exert functions in different trafficking steps on the biosynthetic and endocytic pathways. Recent studies indicate that cycles of GTP binding and hydrolysis by the Rab proteins are linked to the recruitment of specific effector molecules on cellular membranes, which in turn impact on membrane docking/fusion processes. Different Rabs may, nevertheless, have slightly different principles of action. Studies performed in yeast suggest that connections between the Rabs and the SNARE machinery play a central role in membrane docking/fusion. Further elucidation of this linkage is required in order to fully understand the functional mechanisms of Rab GTPases in membrane traffic.

Flow cytometric sorting and biochemical characterization of the late endosomal rab7-containing compartment

Rab7 is a small molecular weight GTPase that is known to be associated with late endocytic compartments. Studies in which wild-type or mutant forms of this protein have been overexpressed in mammalian cells have indicated that rab7 plays a role in controlling membrane transport between late endocytic compartments. However, both the precise site(s) of action and localization of rab7 remain unclear. In the present study, we have used density-gradient centrifugation in combination with a new epitope-specific flow cytometric sorting method to isolate rab7-containing vesicles from baby hamster kidney (BHK) cells. Electron-micrographs of sorted elements showed a homogeneous population of vesicles that resembles late endosomes. The polypeptide composition of rab7-containing vesicles was then analyzed by two-dimensional (2-D) gel electrophoresis. Rab7-containing vesicles were enriched in the cation-independent mannose 6-phosphate receptor and especially in the precursor forms of cathepsin D. Taken together, these results show that the rab7-containing vesicles are a component of the endocytic pathway that connects late endosomes and lysosomes and in which precursor forms of lysosomal hydrolases, segregated from their receptor, might be included.

Vesicular transport: how many Ypt/Rab-GTPases make a eukaryotic cell?

In eukaryotic cells, protein transport through the secretory and endocytic pathways is mediated by vesicular intermediates. Individual transport steps are regulated by Ras-like guanine nucleotide-binding proteins, termed Ypt in yeast or Rab in mammals. The complete sequencing of the Saccharomyces cerevisiae genome has revealed the total number of Ypt GTPases in this organism. There is some redundancy among the 11 Ypt proteins, and only those involved in the biosynthetic pathway are essential for cell viability.

Homotypic lysosome fusion in macrophages: analysis using an in vitro assay

Lysosomes are dynamic structures capable of fusing with endosomes as well as other lysosomes. We examined the biochemical requirements for homotypic lysosome fusion in vitro using lysosomes obtained from rabbit alveolar macrophages or the cultured macrophage-like cell line, J774E. The in vitro assay measures the formation of a biotinylated HRP-avidin conjugate, in which biotinylated HRP and avidin were accumulated in lysosomes by receptor-mediated endocytosis. We determined that lysosome fusion in vitro was time- and temperature-dependent and required ATP and an N-ethylmaleimide (NEM)-sensitive factor from cytosol. The NEM-sensitive factor was NSF as purified recombinant NSF could completely replace cytosol in the fusion assay whereas a dominant-negative mutant NSF inhibited fusion. Fusion in vitro was extensive; up to 30% of purified macrophage lysosomes were capable of self-fusion. Addition of GTPgammas to the in vitro assay inhibited fusion in a concentration-dependent manner. Purified GDP-dissociation inhibitor inhibited homotypic lysosome fusion suggesting the involvement of rabs. Fusion was also inhibited by the heterotrimeric G protein activator mastoparan, but not by its inactive analogue Mas-17. Pertussis toxin, a Galphai activator, inhibited in vitro lysosome fusion whereas cholera toxin, a Galphas activator did not inhibit the fusion reaction. Addition of agents that either promoted or disrupted microtubule function had little effect on either the extent or rate of lysosome fusion. The high value of homotypic fusion was supported by in vivo experiments examining lysosome fusion in heterokaryons formed between cells containing fluorescently labeled lysosomes. In both macrophages and J774E cells, almost complete mixing of the lysosome labels was observed within 1-3 h of UV sendai-mediated cell fusion. These studies provide a model system for identifying the components required for lysosome fusion.

A novel assay reveals a role for soluble N-ethylmaleimide-sensitive fusion attachment protein in mannose 6-phosphate receptor transport from endosomes to the trans Golgi network

Soluble N-ethylmaleimide-sensitive fusion protein (NSF) attachment protein (alpha-SNAP) is a soluble protein that enables the NSF ATPase to associate with membranes and facilitate membrane trafficking events. Although NSF and alpha-SNAP have been shown to be required for many membrane transport processes, their role in the transport of mannose 6-phosphate receptors from endosomes to the trans Golgi network was not established. We present here a novel in vitro assay that monitors the transport of cation-dependent mannose 6-phosphate receptors between endosomes and the trans Golgi network. The assay relies on the trans Golgi network localization of tyrosine sulfotransferase and monitors transport of mannose 6-phosphate receptors engineered to contain a consensus sequence for modification by this enzyme. Using this new assay we show that alpha-SNAP strongly stimulates transport in reactions containing limiting amounts of cytosol. Together with alpha-SNAP, NSF can increase the extent of transport. These data show that alpha-SNAP, a soluble component of the SNAP receptor machinery, facilitates transport from endosomes to the trans Golgi network.

A novel Rab9 effector required for endosome-to-TGN transport

Rab9 GTPase is required for the transport of mannose 6-phosphate receptors from endosomes to the trans-Golgi network in living cells, and in an in vitro system that reconstitutes this process. We have used the yeast two-hybrid system to identify proteins that interact preferentially with the active form of Rab9. We report here the discovery of a 40-kD protein (p40) that binds Rab9-GTP with roughly fourfold preference to Rab9-GDP. p40 does not interact with Rab7 or K-Ras; it also fails to bind Rab9 when it is bound to GDI. The protein is found in cytosol, yet a significant fraction (approximately 30%) is associated with cellular membranes. Upon sucrose density gradient flotation, membrane- associated p40 cofractionates with endosomes containing mannose 6-phosphate receptors and the Rab9 GTPase. p40 is a very potent transport factor in that the pure, recombinant protein can stimulate, significantly, an in vitro transport assay that measures transport of mannose 6-phosphate receptors from endosomes to the trans-Golgi network. The functional importance of p40 is confirmed by the finding that anti-p40 antibodies inhibit in vitro transport. Finally, p40 shows synergy with Rab9 in terms of its ability to stimulate mannose 6-phosphate receptor transport. These data are consistent with a model in which p40 and Rab9 act together to drive the process of transport vesicle docking.

A Rab GTPase is required for homotypic assembly of the endoplasmic reticulum

To define the requirements for the homotypic fusion of mammalian endoplasmic reticulum (ER) membranes, we have developed a quantitative in vitro enzyme-linked immunosorbent assay. This assay measures the formation of IgG (H2L2) following the fusion of ER microsomes containing either the heavy or light chain subunits. Guanine nucleotide dissociation inhibitor (GDI), a protein that extracts Rab GTPases in the GDP-bound form from membranes, potently inhibits fusion. Inhibition was not observed using GDI mutants defective in Rab binding. Kinetic analysis of the inhibitory effects of GDI revealed that Rab activation is required immediately preceding or coincident with fusion and that this step is preceded by a priming event requiring a member of the AAA ATPase family. Our results suggest that homotypic fusion of ER membranes requires Rab and that Rab activation is a transient event necessary for the formation of a fusion pore leading to the mixing of luminal contents of ER microsomes.

Rab7 regulates transport from early to late endocytic compartments in Xenopus oocytes

Rab7 has been shown to localize to late endosomes and to mediate transport from early to late endosome/lysosome in mammalian cells and in yeast. We developed a novel assay to quantify transport from early to late endosomes using the Xenopus oocyte. Oocytes were pulsed with avidin after which the oocytes were incubated to allow avidin transport to a late compartment. The oocytes were then allowed to internalize biotin-horseradish peroxidase (HRP). The oocytes were then injected with test proteins and incubated further to allow transport of biotin-HRP from early endosomes to late endosomal/lysosomal compartments. Transport was quantified by assessing the formation of HRP-biotin-avidin complexes. Injection of Rab7:wild-type (WT) and Rab7:Q67L, a GTPase defective mutant, stimulated transport. Rab5:WT had no effect. Rab7:WT-stimulated transport was inhibited by nocodazole, suggesting a role for intact microtubules. Wortmannin, a phosphatidylinositol 3-kinase inhibitor, blocked Rab7:WT-stimulated transport, but Rab7:Q67L-stimulated transport was unaffected by the drug. Rab7:Q67L is constitutively activated and may not require phosphatidylinositol 3-kinase activity for activation. Rab7-stimulated transport requires N-ethylmaleimide-sensitive factor (NSF) activity as transport was blocked by N-ethylmaleimide and ATPase defective NSF mutants. Our results indicate that sequentially acting endocytic Rab GTPases utilize similar factors although their modes of action may be different.

Small GTP-binding protein, Rab6, is associated with secretory granules in atrial myocytes

Rab proteins, a subfamily of small GTP-binding proteins, have been shown to play key roles in regulation of vesicular traffic in eukaryotic cells. In this study, we have intended to identify, the atrial granule-associated Rab proteins that seem to be required for formation or intracellular transport of the granules. Atrial granules contained at least four small GTP-binding proteins, and we have demonstrated by biochemical analysis that one of the small GTP-binding proteins associated with the atrial granules is a Rab6 protein (Rab6p). Rab6p was also detected in highly purified zymogen granules of pancreatic exocrine gland. Immunogold electron microscopy performed on ultrathin cryosections of rat auricle revealed that Rab6p was associated with the atrial granule membranes. Association of Rab6p with the atrial granule membranes was also confirmed by immunodiffusion electron microscopy in agarose-embedded atrial granules. These data indicate that Rab6p is associated with the atrial granules and that it might function in the intracellular traffic of the secretory granules in the atrial myocytes.

Cloning and mapping of human Rab7 and Rab9 cDNA sequences and identification of a Rab9 pseudogene

Rab GTPases reside in specific intracellular compartments and are key regulators of vesicular transport. To facilitate studies of the mechanism of lysosomal integral membrane protein (LAMP-1) transport, cDNAs for human Rab7 and Rab9 were isolated, and their nucleotide sequences were determined. During isolation and characterization of these cDNAs a Rab9 pseudogene was identified. The sequences are highly homologous to other mammalian Rab proteins and also share homology with proteins of the Rab GTPase family. Rab7 and the Rab9 pseudogene were mapped to chromosomes 3 and 5, respectively, by amplification of their sequences from human monochromosomal somatic cell hybrids. In addition, preliminary studies using antisense expression indicate that down-regulation of either Rab7 or Rab9 proteins induces severe cell vacuolation that resembles the phenotype seen in fibroblasts from patients with Chediak-Higashi syndrome.

Sequential actions of Rab5 and Rab7 regulate endocytosis in the Xenopus oocyte

To explore the role of GTPases in endocytosis, we developed an assay using Xenopus oocytes injected with recombinant proteins to follow the uptake of the fluid phase marker HRP. HRP uptake was inhibited in cells injected with GTPgammaS or incubated with aluminum fluoride, suggesting a general role for GTPases in endocytosis. Injection of Rab5 into oocytes, as well as Rab5:Q79L, a mutant with decreased GTPase activity, increased HRP uptake. Injection of Rab5:S34N, the dominant-negative mutant, inhibited HRP uptake. Injection of N-ethylmaleimide-sensitive factor (NSF) stimulated HRP uptake, and ATPase-defective NSF mutants inhibited HRP uptake when coinjected with Rab5:Q79L, confirming a requirement for NSF in endocytosis. Surprisingly, injection of Rab7:WT stimulated both uptake and degradation/activation of HRP. The latter appears to be due to enhanced transport to a late endosomal/prelysosomal degradative compartment that is monensin sensitive. Enhancement of uptake by Rab7 appears to function via an Rab5-sensitive pathway in oocytes since the stimulatory effect of Rab7 was blocked by coinjection of Rab5:S34N. Stimulation of uptake by Rab5 was blocked by Rab5:S34N but not by Rab7:T22N. Our results suggest that Rab7, while functioning downstream of Rab5, may be rate limiting for endocytosis in oocytes.

Isoforms of ankyrin-3 that lack the NH2-terminal repeats associate with mouse macrophage lysosomes

We have recently cloned and characterized ankyrin-3 (also called ankyrin(G)), a new ankyrin that is widely distributed, especially in epithelial tissues, muscle, and neuronal axons (Peters, L.L., K.M. John, F.M. Lu, E.M. Eicher, A. Higgins, M. Yialamas, L.C. Turtzo, A.J. Otsuka, and S.E. Lux. 1995. J. Cell Biol. 130: 313-330). Here we show that in mouse macrophages, ankyrin-3 is expressed exclusively as two small isoforms (120 and 100 kD) that lack the NH2-terminal repeats. Sequence analysis of isolated Ank3 cDNA clones, obtained by reverse transcription and amplification of mouse macrophage RNA (GenBank Nos. U89274 and U89275), reveals spectrin-binding and regulatory domains identical to those in kidney ankyrin-3 (GenBank No. L40631) preceded by a 29-amino acid segment of the membrane ("repeat") domain, beginning near the end of the last repeat. Antibodies specific for the regulatory and spectrin-binding domains of ankyrin-3 localize the protein to the surface of intracellular vesicles throughout the macrophage cytoplasm. It is not found on the plasma membrane. Also, epitope-tagged mouse macrophage ankyrin-3, transiently expressed in COS cells, associates with intracellular, not plasma, membranes. In contrast, ankyrin-1 (erythrocyte ankyrin, ankyrin(R)), which is also expressed in mouse macrophages, is located exclusively on the plasma membrane. The ankyrin-3-positive vesicles appear dark on phase-contrast microscopy. Two observations suggest that they are lysosomes. First, they are a late compartment in the endocytic pathway. They are only accessible to a fluorescent endocytic tracer (FITC-dextran) after a 24-h incubation, at which time all of the FITC-dextran-containing vesicles contain ankyrin-3 and vice versa. Second, the ankyrin-3-positive vesicles contain lysosomal-associated membrane glycoprotein (LAMP-1), a recognized lysosomal marker. This is the first evidence for the association of an ankyrin with lysosomes and is an example of two ankyrins present in the same cell that segregate to different locations.

Adaptation of intestinal nutrient transport in health and disease. Part I

Why is it important to understand the mechanisms controlling intestinal adaptation? There are two major answers to this question. Firstly, in establishing the cellular and molecular events associated with intestinal adaptation, we will formulate a general framework that may be applied to the understanding of adaptation of other cell membranes. For example, alterations in the synthesis of glucose carriers and their subsequent insertion into membranes may alter sugar entry across the intestinal brush border membrane (BBM) using the sodium-dependent D-glucose transporter, SGLT1, or the BBM sodium-independent facultative fructose transporter, GLUT5, and may alter facilitated sugar exit across the basolateral membrane (BLM) using GLUT2. The precise role of transcriptional and translational processes in the up- or down-regulation of sugar transport requires further definition. Alterations in enterocyte microsomal lipid metabolic enzyme expression occurring during the course of intestinal adaptation will direct the synthesis of lipids destined for trafficking to the BBM and BLM domains of the enterocyte. This will subsequently alter the passive permeability properties of these membranes and ultimately influence lipid absorption. Therefore, establishing the physiological, cellular and molecular mechanisms of adaptation in the intestine will define principles that may be applied to other epithelia. Secondly, enterocyte membrane adaptation is subject to dietary modification, and these may be exploited as a means to enhance a beneficial or to reduce a detrimental aspect of the intestinal adaptive process in disease states. Alterations in membrane function occur in association with changes in dietary lipids, and these are observed in a variety of cells and tissues including lymphocytes, testes, liver, adipocytes, nerve tissue, nuclear envelope and mitochondria. Therefore, the elucidation of the mechanisms of intestinal adaptation and the manner whereby dietary manipulation modulates these processes affords the future possibility of dietary engineering aimed at using food as a therapeutic agent. It is hoped this approach will form the centerpiece for future investigation that would focus on disease prevention, as well as on the development of better therapeutic strategies to prevent the development or to treat the complications of conditions such as diabetes mellitus, obesity, hyperlipidemia and inflammatory bowel diseases. This review deals with the physiology of glucose transport with specific emphasis on transporters of the brush border membrane (BBM) and the basolateral membrane (BLM). On the BBM the sodium (Na)/glucose transporters (SGLT1 and SGLT2), the Na-independent transporter (GLUT5), and on the BLM the hexose transporter (GLUT2) are discussed. The molecular biology of these transporters is also reviewed.

Evidence for a functional link between Rab3 and the SNARE complex

Rab3 is a monomeric GTP-binding protein associated with secretory vesicles which has been implicated in the control of regulated exocytosis. We have exploited Rab3 mutant proteins to investigate the function of Rab3 in the process of neurotransmitter release from Aplysia neurons. A GTPase-deficient Rab3 mutant protein was found to inhibit acetylcholine release suggesting that GTP hydrolysis by Rab3 is rate-limiting in the exocytosis process. This effect was abolished by a mutation in the effector domain, and required the association of Rab3 with membranes. In order to determine the step at which Rab3 interferes with the secretory process, tetanus and botulinum type A neurotoxins were applied to Aplysia neurons pre-injected with the GTPase-deficient Rab3 mutant protein. These neurotoxins are Zn(2+)-dependent proteases that cleave VAMP/synaptobrevin and SNAP-25, two proteins which can form a ternary complex (termed the SNARE complex) with syntaxin and have been implicated in the docking of synaptic vesicles at the plasma membrane. The onset of toxin-induced inhibition of neurotransmitter release was strongly delayed in these cells, indicating that the mutant Rab3 protein led to the accumulation of a toxin-insensitive component of release. Since tetanus and botulinum type A neurotoxins cannot attack their targets, VAMP/synaptobrevin and SNAP-25, when the latter are engaged in the SNARE complex, we propose that Rab3 modulates the activity of the fusion machinery by controlling the formation or the stability of the SNARE complex.

Rab1a and multiple other Rab proteins are associated with the transcytotic pathway in rat liver

To better understand the function of Rab1a, we have immunoisolated Rab1a-associated transport vesicles from rat liver using affinity-purified anti-Rab1a-coated magnetic beads. A fraction enriched in endoplasmic reticulum (ER) to Golgi transport vesicles (CV2, rho = 1.158) was subjected to immunoisolation, and proteins of the bound and non-bound subfractions were analyzed by Western blotting. To our surprise, we found that immunoisolated vesicles contained not only ER markers (105-kDa form of the polymeric IgA receptor (pIgAR)) but also transcytotic markers (dIgA and the 120-kDa form of pIgAR), suggesting that Rab1a is associated with transcytotic vesicles in rat liver. To investigate this possibility, we used an antibody to the cytoplasmic domain of pIgAR to immunoisolate transcytotic vesicles from a fraction (CV1, rho = 1. 146) known to be enriched in these vesicles. Rab1a was detected in the immunoadsorbed subfractions. The composition of the vesicles immunoisolated from the CV1 fraction on anti-Rab1a was similar to that of transcytotic vesicles immunoisolated from the same fraction on pIgAR. Both were enriched in transcytotic markers and depleted in ER and Golgi markers. The main difference between the two was that those isolated on anti-Rab1a appeared to be enriched in postendosomal transcytotic vesicles, whereas those isolated on pIgAR contained both pre- and postendosomal elements. Analysis of anti-Rab1a isolated vesicles using [alpha-32P]GTP overlay demonstrated the presence of multiple GTP-binding proteins. Some of these were identified by immunoblotting as epithelia-specific Rab17 and ubiquitous Rabs1b, -2, and -6. Taken together, these results indicate that: 1) Rab1a is associated with both ER to Golgi and postendosomal transcytotic vesicles, and 2) multiple GTP-binding proteins are associated with each class of isolated vesicle.

Rab4 and Rab7 define distinct nonoverlapping endosomal compartments

Several Rab GTPases have been localized to distinct compartments of the endocytic pathway. Rab4 is associated with early endosomes and recycling vesicles and regulates membrane recycling from early endosomes. Rab7 is localized to late endosomes and is involved in the regulation of membrane transport between late endosomes and lysosomes. Although Rab4 and Rab7 appear to regulate distinct transport events in endocytosis, it is not clear whether they perform their activities in related or entirely distinct intracellular compartments. To address this question, we generated stable cell lines expressing Rab4 tagged with a novel X31 influenza hemagglutinin (NH) epitope tag. These antibodies are characterized in this paper and were used to immunoisolate endocytic vesicles with cytoplasmically exposed NHRab4. Immunoisolated membranes contain internalized 125I-transferrin, but are devoid of Rab7. Confocal immunofluorescence microscopy showed that the early endosomal GTPases Rab4 and Rab5 both do not codistribute with Rab7 within the same cell. These observations suggest that each of the three Rab GTPases operationally defines a distinct station of the endocytic pathway.

Endocytosis and molecular sorting

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

Regulation of cellular signalling by fatty acid acylation and prenylation of signal transduction proteins

Covalent modification by fatty acylation and prenylation occurs on a wide variety of cellular signalling proteins. The enzymes that catalyze attachment of these lipophilic moieties to proteins have recently been identified and characterized. Each lipophilic group confers unique properties to the modified protein, resulting in alterations in protein/protein interactions, membrane binding and targeting, and intracellular signalling. The biochemistry and cell biology of protein myristoylation, farnesylation and geranylgeranylation is reviewed here, with emphasis on the Src family of tyrosine kinases, Ras proteins and G protein coupled signalling systems.

Post-translational modifications of Ras and Ral are important for the action of Ral GDP dissociation stimulator

Ral GDP dissociation stimulator (RalGDS) is a GDP/GTP exchange protein of Ral and a new effector protein of Ras. Therefore, there may be a new signaling pathway from Ras to Ral. In this paper, we examined the roles of the post-translational modifications of Ras and Ral on this new signal transduction pathway. The post-translationally modified form of Ras bound to RalGDS more effectively than the unmodified form. The modification of Ras was required to regulate the distribution of RalGDS between the cytosol and membrane fractions in COS cells. The post-translational modification of Ral enhanced the activities of RalGDS to stimulate the dissociation of GDP from and the binding of GTP to Ral. Furthermore, the modified form of Ral bound to Ral-binding protein 1 (RalBP1), a putative effector protein of Ral, more effectively than the unmodified form. Taken together with the observations that Ras and Ral are localized to the membranes, these results suggest that the post-translational modifications of Ras and Ral play a role for transmitting the signal effectively on the membranes in the signal transduction pathway of Ras/RalGDS/Ral/RalBP1.

Transport between cis and medial Golgi cisternae requires the function of the Ras-related protein Rab6

The small GTP-binding protein Rab6, a member of the Ras superfamily, is localized on the membranes of the Golgi apparatus and the trans Golgi network. Recent studies revealed that the Rab6 protein might be involved in the transit of proteins through the Golgi complex. In this report we demonstrate the essential function of the Rab6 protein in a distinct step of reconstituted Golgi transport. Polyclonal antibodies and Fab fragments directed against the C-terminal part of the Rab6 protein inhibit transport between the cis and the medial Golgi cisternae. Inhibition also occurred when a trans-dominant mutant form of the Rab6 protein (N126I) was added to the reconstituted transport. Furthermore, Rab6 antibodies inhibit uncoupled fusion of Golgi membranes. From these data we conclude that Rab6 is involved in a process related to membrane fusion at the cisternal membranes of the Golgi apparatus and therefore is needed for the consumption of Golgi-derived vesicles by their target membranes.

The vacuolar ATPase proton pump is required for the cytotoxicity of Bacillus anthracis lethal toxin

The nature of the cytopathic effect exerted by the lethal factor toxin (LF) of Bacillus anthracis on sensitive cells is unknown. The toxin requires the passage through acidic vesicles in order to exert its effect within the cytosol. Here, we show that bafilomycins and concanamycin A, selective inhibitors of the vacuolar ATPase proton pump, are the most powerful known inhibitors of LF macrophage toxicity. These inhibitors are fully active long after LF addition to macrophages, suggesting that LF enters the cytosol after having reached a late endosomal compartment.

Rab3 reversibly recruits rabphilin to synaptic vesicles by a mechanism analogous to raf recruitment by ras

GTP activates the interaction between the synaptic vesicle proteins rabphilin and rab3. This raises the question of whether rabphilin is a resident vesicle protein that recruits rab3 in a stage-dependent fashion, or if it is instead an effector protein recruited by rab3. We now show that rabphilin, like rab3, dissociates from synaptic vesicles after exocytosis in a manner requiring both Ca2+ and membrane fusion. Rabphilin interacts with GTP-rab3 via a N-terminal domain comprising a novel Zn2+(-)finger motif, and this interaction is essential for rabphilin binding to synaptic vesicles. Thus, in the same way that ras recruits raf to the plasma membrane, rab3 reversibly recruits rabphilin to synaptic vesicles in a stage-dependent manner. These results reveal an unexpected similarity between the molecular mechanisms by which small G protein function in recruiting effector proteins to membranes during membrane traffic and signal transduction.

The effects of ribavirin on the GTP level and the VIP receptor dynamic of human IGR39 cells

GTP is one of the major cellular molecules involved in fundamental functions of cell life. Ribavirin, and antiviral and antitumoral agent, the primary site of action of which is the IMP deshydrogenase, was used in order to depress the intracellular GTP level. Consequential effects were tested on the property and dynamic of the VIP receptor on human melanoma IGR 39 cells. A concentration of 100 microM of Ribavirin reduced the intracelluar GTP level by more than 60% and induced a reversible growth arrest. Nevertheless this drug displayed no effect on: i) the VIP binding parameters (Kd and Bmax) of both high and low affinity receptors; ii) the cycling of the VIP receptor; iii) the based and VIP-stimulated cAMP production and iv) the subcellular GTP distribution. We show that Ribavirin, in the range of concentrations used, is very efficient to inhibit GTP synthesis in the human melanoma cell line IGR 39 and its growth, without affecting VIP receptor functions.

GDP dissociation inhibitor serves as a cytosolic acceptor for newly synthesized and prenylated Rab5

In vitro synthesis and post-translational prenylation of Rab5 is accomplished using reticulocyte lysate supplemented with prenyl precursors (Sanford, J. C., Pan, Y., and Wessling-Resnick, M. (1993) J. Biol. Chem. 268, 23773-23776). When Rab5 is translated in the presence of biotin-lysine-tRNA, it incorporates biotin-lysine into its peptide backbone and is efficiently prenylated; since this modification is dependent on guanine nucleotide binding, biotin-Rab5's functional integrity must be maintained. Prenylated biotin-Rab5 associates with a 45-kDa reticulocyte GDP dissociation inhibitor (GDI), sedimenting as a approximately 70-kDa particle on 5-20% sucrose density gradients. The GDI-Rab5 complex can be captured using streptavidin-linked agarose beads. Only Rab5 peptides that are substrates for prenylation are found to cosediment with the lysate GDI on sucrose gradients. Post-translational association of Rab5 and GDI is a novel finding, since previous reports suggested Rab5 remains associated with Rab escort protein (REP) after prenylation (Alexandrov, K., Horiuchi, H., Steele-Mortimer, O., Seabra, M. C., and Zerial, M. (1994) EMBO J. 13, 5262-5273). Since post-translational prenylation is catalytically mediated by REP, our study suggests that a complex between Rab5 and this factor is transient in nature. Thus, newly synthesized and prenylated Rab5 is most likely escorted to its target membrane by a GDI acceptor molecule. Biotin-Rab5 provides a novel tool for future efforts to capture and characterize additional accessory factors required for Rab protein function in vesicle transport.