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

The rab7 GTPase resides on a vesicular compartment connected to lysosomes

Rab GTPases belong to the Ras GTPase superfamily and are key regulators of membrane traffic. Among them, rab7 has been localized on late endosomes of NRK cells but its function remains unknown. In order to investigate its role, we generated stable HeLa cell lines that express either wild type or a GTPase-defective mutant of rab7 in an inducible manner. A morphological analysis of the intracellular localization of these proteins was performed by confocal laser microscopy. Here we show that, in HeLa cells, rab7 is present on a vesicular compartment that extends from the perinuclear area to the cell periphery and shows only a partial colocalization with the cation-independent mannose 6-phosphate receptor, a marker for late endosomes. The topology of this compartment is dependent on the microtubule network since nocodazole treatment results in its scattering throughout the cytoplasm. In addition, we observed that, in contrast to the wild-type protein, a rab7 mutant with a reduced GTPase activity is in part associated with lysosomal membranes. This observation was confirmed by subcellular fractionation in a Percoll gradient. Our data implicate rab7 as the first GTPase functioning on terminal endocytic structures in mammalian cells.

Rab7 and Rab9 are recruited onto late endosomes by biochemically distinguishable processes

Rab GTPases are localized to the surfaces of distinct membrane-bound organelles and function in transport vesicle docking and/or fusion. Prenylated Rab9, bound to GDP dissociation inhibitor-alpha, can be recruited selectively onto a membrane fraction enriched in late endosomes; this process is accompanied by nucleotide exchange. We used this system to address whether each Rab uses a distinct machinery to associate with its cognate organelle. Purified, prenylated Rab1B, Rab7, and Rab9 proteins were each reconstituted as stoichiometric complexes with purified GDP dissociation inhibitor-alpha, and their recruitment onto endosome- or ER-enriched membrane fractions was quantified. The two late endosomal proteins, Rab9 and Rab7, were each recruited onto endosome membranes with approximate apparent Km values of 9 and 22 nM, respectively. However, while control Rab9.GDP dissociation inhibitor-alpha complexes inhibited the initial rate of myc-tagged Rab9 recruitment with an apparent Ki of approximately 9 nM, Rab7 complexes inhibited this process much less effectively (apparent Ki approximately 112 nM). Similarly, complexes of the endoplasmic reticulum-localized Rab1B protein were even less potent than Rab7 complexes (apparent Ki approximately 405 nm). Rab9 complexes inhibited Rab7 recruitment with the same low efficacy as Rab7 complexes inhibited Rab9 recruitment. These experiments distinguish, biochemically, the recruitment of different Rab proteins onto a single class of organelle. Since Rab7 and Rab9 are both localized at least in large part, to late endosomes, this suggests that a single organelle may bear multiple Rab recruitment machines.

Effect of guanine nucleotide binding on the intrinsic tryptophan fluorescence properties of Rab5

To gain further insight into structural elements involved in Rab5 function, differences in the intrinsic tryptophan fluorescence of the GDP- and guanosine 5'-O-(3-thiotriphosphate) (GTP gamma S)-bound forms of the protein were examined. When excited at 290 nm, Rab5 displays emission maxima at 339.7 nm for the GDP-bound and 336.7 nm for the GTP gamma S-bound forms. The tryptophan fluorescence intensity is quenched by approximately 25% in the GTP gamma S-bound form relative to the GDP-bound conformation. Variant Rab5 molecules were created by site-directed mutagenesis to convert the protein's two tryptophans to phenylalanine residues. Fluorescence studies reveal that the observed changes upon GDP/GTP gamma S exchange are due to a blue shift in the emission spectra for both Trp74 (342.0 to 339.5 nm) and Trp114 (335.3 to 333.7 nm) and fluorescence quenching of Trp114. Consistent with the blue shift in the emission spectra, both tryptophans are more resistant to oxidation by N-bromosuccinimide in the GTP gamma S-bound state. These data indicate that both of Rab5's tryptophans are brought into a more sequestered, hydrophobic environment upon conformational changes promoted by guanine nucleotide exchange. Since Trp74 lies adjacent to Rab5's cognate switch II domain, local conformational changes would be predicted based on the known structure of Ras. However, Trp114 lies within a region of Rab5 potentially related to the switch III domain unique to heterotrimeric G alpha t. Thus, changes in the fluorescence properties of Trp114 upon guanine nucleotide exchange suggest that Rab proteins may have structure-function relationships similar to those described for heterotrimeric GTP-binding proteins.

Deficient geranylgeranylation of Ram/Rab27 in choroideremia

Choroideremia, an X-linked form of retinal degeneration, results from defects in the Rab escort protein-1 (REP-1) gene. REP-1 and REP-2 assist in the attachment of geranylgeranyl groups to Rab GTPases, a modification essential for their action as molecular switches regulating intracellular vesicular transport. If Rabs that depend preferentially on REP-1 for prenylation exist, they will accumulate unprenylated in choroideremia cells. Using recombinant Rab geranylgeranyl transferase and REPs to label unprenylated cytosolic proteins, we identified one unprenylated protein in choroideremia lymphoblasts that was prenylated in vitro more efficiently by REP-1 than by REP-2. This protein was purified and identified as Ram (renamed Rab27), a previously cloned Rab of unknown function. Immunohistochemistry of rat retina showed that Ram/Rab27 is expressed in the pigment epithelium and choriocapillaris, the two retinal cell layers that degenerate earliest in choroideremia. These results raise the possibility that the retinal degeneration in choroideremia results from the deficient geranylgeranylation of Ram/Rab27 or a closely related protein.

Intracellular transport and maturation of nascent low density lipoprotein receptor is blocked by mutation in the Ras-related GTP-binding protein, RAB1B

The relationship between the Ras-related GTP-binding protein, Rab1B, and intracellular transport of nascent low density lipoprotein (LDL) receptor was studied in cultured human embryonic kidney cells (line 293) cotransfected with plasmids encoding the LDL-receptor and either wild-type Rab1B or a Rab1B mutant (N121I) known to act as a dominant suppressor of endogenous Rab1B function. [35S]Methionine pulse-chase analysis of immunoprecipitated LDL-receptor indicated that coexpression with Rab1BN121I, but not Rab1BWT, impaired its conversion from the Endo-H-sensitive 120-125 kDa form to the O-glycosylated 160-170 kDa form, consistent with a block in ER-->Golgi trafficking of the nascent receptor. In cells expressing Rab1BN121I, the newly synthesized LDL-receptor was unable to reach the cell surface as evidenced by its inaccessibility to sulfo-NHS-biotin added to the cultures. These observations provide a direct demonstration of Rab protein involvement in LDL receptor trafficking and lend support to the concept of Rab1B as a universal mediator of ER-->Golgi transport of membrane glycoproteins in mammalian cells.

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

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

Membrane transport in the endocytic pathway

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

Evidence that the Rab3a-binding protein, rabphilin3a, enhances regulated secretion. Studies in adrenal chromaffin cells

Rabphilin3a had been identified in brain as a Rab3a-binding protein and may serve as an effector for Rab3a function. We have cloned a splice variant of brain-Rabphilin3a from a bovine adrenal chromaffin cell cDNA library and investigated the function of the protein in regulated exocytosis in bovine chromaffin cells. The predicted amino acid sequence of chromaffin cell (c-) Rabphilin3a was identical with that of brain (b-) Rabphilin3a except for a 6-amino-acid insert VFSLSA in the amino-terminal half of the protein. An antibody directed against a carboxyl-terminal peptide recognized an 85-kDa protein in COS7 cells transfected with the cDNA in a mammalian expression vector. A band of similar mobility was enriched in a fraction of highly purified chromaffin granule membranes, consistent with the Rabphilin3a being associated with chromaffin granule membranes. Overexpression of either chromaffin cell or brain Rabphilin3a by transfection with the corresponding cDNAs in mammalian expression vectors enhanced DMPP-induced secretion of co-expressed human growth hormone (GH) approximately 30%. Chromaffin cells transfected with a plasmid with the entire coding sequence of c-Rabphilin3a inserted in the antisense orientation inhibited secretion of co-expressed GH by approximately 30%. Rabphilin3a mutants lacking one or both of the carboxyl-terminal C2 domains strongly inhibited DMPP-stimulated exocytosis. The single C2 domain deletion also strongly inhibited Ca(2+)-dependent secretion from digitonin-permeabilized cells. These data indicate that Rabphilin3a is a positive regulator of exocytosis. Because the C2 deletion mutants contain the amino-terminal Rab3a-GTP binding domain, they may inhibit secretion by competing with endogenous Rabphilin3a for interaction with Rab3a-GTP without being able to mimic the functional effects of full-length Rabphilin3a.

Analysis of a family of ypt genes and their products from Chlamydomonas reinhardtii

Small G-proteins encoded by the ras-like ypt genes are ubiquitous in eukaryotic cells. They have been shown to play an essential role in membrane vesicle transport. We have isolated four ypt genes, yptC1, yptC4, yptC5 and yptC6, from Chlamydomonas reinhardtii (Cr) genomic and cDNA libraries. Three of them, yptC1, yptC4 and yptC5, are close homologues of ypt genes previously found in the multicellular alga Volvox carteri (Vc), the fourth, yptC6, is new. Each yptC gene is present as a single copy in the genome. Comparisons of genomic and cDNA sequences revealed that the coding regions are interrupted by five (yptC5), six (yptC6), seven (yptC4) and eight (yptC1) introns, respectively. Cr ypt genes and the closely related Vc ypt genes have identical exon-intron structures, but the corresponding intron sequences are completely different. Polyadenylation is signalled by UAUAA, UGUAG and UGUAA. The deduced amino acid (aa) sequence of YptC6 exhibited 79% identity with HRab2; YptC1, YptC4 and YptC5 exhibited over 90% identity with their Vc homologues. Primary structures of the 9-aa 'effector domain' and the contiguous 'helix3-loop7' motif (approx. 30 aa) are 'diagnostic' features for functional assignment. Recombinant YptC proteins, overproduced in Escherichia coli and purified to near homogeneity, displayed strong and specific binding of GTP, but not of GMP or ATP. The four Cr Ypt proteins showed immunochemical cross reactions to their Vc counterparts. Moreover, Western blots demonstrated at least six types of Ypt in both Cr and Vc, suggesting that these Ypt are used for household functions responsible for vesicle transport rather than for cellular differentiation.

Quantitative analysis of the interactions between prenyl Rab9, GDP dissociation inhibitor-alpha, and guanine nucleotides

Rab9 is a Ras-like GTPase required for the transport of mannose 6-phosphate receptors between late endosomes and the trans Golgi network. Rab9 occurs in the cytosol as a complex with GDP dissociation inhibitor (GDI), which we have shown delivers prenyl Rab9 to late endosomes in a functional form. We report here basal rate constants for guanine nucleotide dissociation and GTP hydrolysis for prenyl Rab9. Both rate constants were influenced in part by the hydrophobic environment of the prenyl group. Guanine nucleotide dissociation and GTP hydrolysis rates were lower in the presence of lipid; detergent stimulated intrinsic nucleotide exchange. GDI-alpha inhibited GDP dissociation from prenyl Rab9 by 2.4-fold. GDI-alpha associated with prenyl Rab9 with a KD of 60 nM in 0.1% Lubrol and 23 nM in 0.02% Lubrol. In 0.1% Lubrol, GDI-alpha inhibited GDP dissociation half maximally at 72 +/- 18 nM, consistent with the KD determinations. These data suggest that GDI-alpha associates with prenyl Rab9 with a KD of < or = 23 nM under physiological conditions. Finally, a previously uncharacterized minor form of GDI-alpha inhibited GDP dissociation from prenyl Rab9 by 1.9-fold and bound prenyl Rab9 with a KD of 67 nM in 0.1% Lubrol.

The Ras-related GTP-binding protein, Rab1B, regulates early steps in exocytic transport and processing of beta-amyloid precursor protein

The role of the Ras-related GTP-binding protein, Rab1B, in intracellular trafficking of beta-amyloid precursor protein (beta APP) was studied in cultured 293 cells. beta APP is processed via one of two alternative routes. In the major secretory pathway, beta APP is cleaved by alpha-secretase within the region comprising the beta-amyloid peptide (A beta), resulting in release of a soluble NH2-terminal exodomain (APP alpha) and a 3-kDa peptide (p3) derived from the carboxyl-terminal tail. In the alternative amyloidogenic pathway, beta APP is cleaved by beta-secretase, with the release of a truncated exodomain (APP beta) and an intact A beta peptide. When beta APP751 was coexpressed with Rab1B(wt) or dominant-negative Rab1B mutants (Rab1BN121I or Rab1BS22N) there was a marked decrease in conversion of the immature Endo-H sensitive form of beta APP751 (108 kDa) to the mature O-glycosylated form of beta APP751 (130 kDa) in cells expressing the mutant forms of Rab1B. The block in Golgi-dependent processing of beta APP was accompanied by inhibition of secretion of APPS (APP alpha). A similar decrease in secretion of APPS (APP alpha+APP beta) was observed in cells that were coexpressing Rab1BN121I with the "Swedish" variant of beta APP751 (i.e. beta APPSW751), which undergoes increased amyloidogenic processing. Coincident with the decline in APPS secretion, the cells coexpressing beta APPSW751 with Rab1BN121I showed a 90% decrease in A beta secretion. The data indicate that Rab1B plays a key role in endoplasmic reticulum-->Golgi transport of beta APP, and that beta APP must pass through a late Golgi compartment before entering either the alpha-secretase or the amyloidogenic beta-secretase pathway. The results also suggest that mutant versions of other Rab proteins that function in different parts of the exocytic and endocytic pathways may be useful in defining the specific routes of beta APP transport involved in the biogenesis of A beta.

Characterization of membrane-bound small GTP-binding proteins from Nicotiana tabacum

We have cloned nine cDNAs encoding small GTP-binding proteins from leaf cDNA libraries of tobacco (Nicotiana tabacum). These cDNAs encode distinct proteins (22-25 kD) that display different levels of identity with members of the mammalian Rab family: Nt-Rab6 with Rab6 (83%), Nt-Rab7a-c with Rab7 (63-70%), and Nt-Rab11a-e with Rab11 (53-69%). Functionally important regions of these proteins, including the "effector binding" domain, the C-terminal Cys residues for membrane attachment, and the four regions involved in GTP-binding and hydrolysis, are highly conserved. Northern and western blot analyses show that these genes are expressed, although at slightly different levels, in all plant tissues examined. We demonstrate that the plant Rab5, Rab6, and Rab11 proteins, similar to their mammalian and yeast counterparts, are tightly bound to membranes and that they exhibit different solubilization characteristics. Furthermore, we show that the yeast GTPase-activating protein Gyp6, shown to be specifically required to control the GTP hydrolysis of the yeast Ypt6 protein, could interact with tobacco GTP-binding proteins. It increases in vitro the GTP hydrolysis rate of the wild-type Nt-Rab7 protein. In addition, it also increases, at different levels, the GTP hydrolysis rates of a Nt-Rab7m protein with a Rab6 effector domain and of two other chimaeric Nt-Rab6/Nt-Rab7 proteins. However, it does not interact with the wild-type Nt-Rab6 protein, which is most similar to the yeast Ypt6 protein.

Localization of the small GTP-binding protein rab1p to early compartments of the secretory pathway

We have studied the localization of the small GTPase rab1p in different cell types using polyclonal antibodies prepared against the rab1A isoform of the protein. Immunofluorescence microscopy of normal rat kidney (NRK) and mouse myeloma cells showed the association of the protein with the Golgi complex and peripheral sites where it colocalized with p58, a pre- and cis-Golgi marker protein. Rab1p and p58 also had similar distributions in membrane fractions derived from rat pancreas microsomes. Both were concentrated in two intermediate density subfractions between the rough endoplasmic reticulum and trans-Golgi, whereas rab6p, previously localized to middle and trans-Golgi, was enriched in the light density trans-Golgi fraction. Immunoperoxidase electron microscopy of NRK and myeloma cells revealed the association of rab1p with 1–2 cisternae, vacuolar, and tubulovesicular membranes in the cis-Golgi region. The rab1p-specific staining typically covered the entire lateral surface of the cisternae but, in weakly stained cells, local labeling between closely opposed membranes could also be seen. The rab1p-positive pre-Golgi compartment had a predominantly tubulovesicular appearance in NRK cells whereas in myeloma cells it consisted of vacuoles surrounded by rab1p-positive vesicles and tubules of heterogeneous size. In both cell types the rough ER cisternae and the nuclear envelope contained negligible labeling and no continuities between these and the rab1p-positive membranes were observed. In addition, in myeloma cells the smooth ER subcompartment, containing endogenous retrovirus particles, was devoid of rab1p-labeling. These results indicate that the pre-Golgi (intermediate) compartment consists of different membrane domains and its morphology can vary considerably between different cell types. Further, they suggest that the recruitment of rab1p to membranes occurs predominantly in a post-ER location and that the protein functions in targeting/fusion events within the pre- and cis-Golgi membranes.

Role of clathrin-coated vesicles in glycoprotein transport from the cell surface to the Golgi complex

Plasma membrane glycoproteins recycle to the Golgi complex, but the route followed by these proteins is not known. To elucidate the pathway of transport, the involvement of clathrin-coated vesicles was tested. This was accomplished by comparing the traffic of wild type low density lipoprotein receptor (LDLR) and FH 683, a mutant receptor whose endocytosis from the cell surface in coated vesicles is reduced by 90-95%. Wild type LDLR traveled from the cell surface to the sialyltransferase compartment of the Golgi with a half-time of 2.5 h in K562 human leukemia cells expressing receptor from a transfected cDNA. In contrast, FH 683 LDLR recycled to the Golgi at 33% of the wild type rate, suggesting that wild type LDLR is largely transported to the Golgi by a pathway that involves clathrin-coated vesicles. Moreover, because clathrin-coated vesicles that bud from the plasma membrane are transported to endosomes, surface-to-Golgi transport probably involves an endosomal intermediate. Finally, because there was substantial transport of mutant LDLR to the Golgi even though its endocytosis in coated vesicles was greatly reduced, there may be a second pathway of surface-to-Golgi traffic. Our results suggest that wild type LDLR may move from plasma membrane to Golgi by two routes. Two-thirds of the traffic proceeds via a coated vesicle-mediated pathway while the remainder may follow a clathrin-independent pathway.

Rab escort protein-1 is a multifunctional protein that accompanies newly prenylated rab proteins to their target membranes

Rab proteins comprise a family of small GTPases that serve a regulatory role in vesicular membrane traffic. Geranylgeranylation of these proteins on C-terminal cysteine motifs is crucial for their membrane association and function. This post-translational modification is catalysed by rab geranylgeranyl transferase (Rab-GGTase), a multisubunit enzyme consisting of a catalytic heterodimer and an accessory component, named rab escort protein (REP)-1. Previous in vitro studies have suggested that REP-1 presents newly synthesized rab proteins to the catalytic component of the enzyme, and forms a stable complex with the prenylated proteins following the transfer reaction. According to this model, a cellular factor would be required to dissociate the rab protein from REP-1 and to allow it to recycle in the prenylation reaction. RabGDP dissociation inhibitor (RabGDI) was considered an ideal candidate for this role, given its established function in mediating membrane association of prenylated rab proteins. Here we demonstrate that dissociation from REP-1 and binding of rab proteins to the membrane do not require RabGDI or other cytosolic factors. The mechanism of REP-1-mediated membrane association of rab5 appears to be very similar to that mediated by RabGDI. Furthermore, REP-1 and RabGDI share several other functional properties, the ability to inhibit the release of GDP and to remove rab proteins from membranes; however, RabGDI cannot assist in the prenylation reaction. These data suggest that REP-1 is per se sufficient to chaperone newly prenylated rab proteins to their target membranes.

Lamellar bodies of rat alveolar type 2 cells have late endosomal marker proteins on their limiting membranes

Alveolar type 2 cells are known to take up surfactant phospholipids and proteins from the alveolar space and recycle them into secretory organelles via a receptor-mediated endocytic pathway. To clarify the intracellular route(s) through which materials ingested by the cells are processed, we examined the immunocytochemical localization of late endosomal and lysosomal membrane markers, rab 7 and lamp 1 proteins, within rat alveolar type 2 cells. The limiting membranes of lamellar bodies (LBs) showed positive immunoreactivity for both proteins, whereas multivesicular bodies (MVBs) exhibited positive immunoreactivity only for lamp 1 protein on free vesicles in the MVB lumen. From these findings, it is suggested that LBs are not only secretory granules, but also constitute one of the late endosomal compartments of the cells and that MVBs of this cell type may be targeted to cell organelle(s) other than lysosomes.

A Rab4-like GTPase in Dictyostelium discoideum colocalizes with V-H(+)-ATPases in reticular membranes of the contractile vacuole complex and in lysosomes

In the course of screening a cDNA library for ras-related Dictyostelium discoideum genes, we cloned a 0.7 kb cDNA (rabD) encoding a putative protein that was 70% identical at the amino acid level to human Rab4. Rab4 is a small M(r) GTPase, which belongs to the Ras superfamily and functions to regulate endocytosis in mammalian cells. Southern blot analysis indicated that the rabD cDNA was encoded by a single copy gene while Northern blot analysis revealed that the rabD gene was expressed at relatively constant levels during growth and differentiation. Affinity-purified antibodies were prepared against a RabD fusion protein expressed in bacteria; the antibodies recognized a single 23 kDa polypeptide on western blots of cell extracts. Density gradient fractionation revealed that the RabD antigen co-distributed primarily with buoyant membranes rich in vacuolar protons pumps (V-H(+)-ATPases) and, to a lesser extent, with lysosomes. This result was confirmed by examining cell lines expressing an epitope-tagged version of RabD. Magnetically purified early endocytic vesicles and post-lysosomal vacuoles reacted more weakly with anti-RabD antibodies than did lysosomes. Other organelles were negative for RabD. Double-label indirect immunofluorescence microscopy revealed that RabD and the 100 kDa V-H(+)-ATPase subunit colocalized in a fine reticular network throughout the cytoplasm. This network was reminiscent of spongiomes, the tubular elements of the contractile vacuole system. Immunoelectron microscopy confirmed the presence of RabD in lysosome fractions and in the membranes rich in V-H(+)-ATPase. We conclude that a Rab4-like GTPase in D. discoideum is principally associated with the spongiomes of contractile vacuole complex.

Membrane targeting of the small GTPase Rab9 is accompanied by nucleotide exchange

The Rab GTPases are key regulators of vesicular transport. A fraction of Rab proteins is present in the cytosol, bound with GDP, complexed to a protein termed GDI. Rab9 is localized primarily to late endosomes, where it aids the transport of mannose 6-phosphate receptors to the trans-Golgi network. It has been proposed that Rab proteins are delivered to specific membranes by GDI, and that this process is accompanied by the exchange of bound GDP for GTP. In addition, Rab localization requires carboxy-terminal prenylation and specific structural determinants. Here we describe the reconstitution of the selective targeting of prenylated Rab9 protein onto late endosome membranes and show that this process is accompanied by endosome-triggered nucleotide exchange.

Distinct structural elements of rab5 define its functional specificity

Members of the rab family of small GTPases are localized to distinct cellular compartments and function as specific regulators of vesicle transport between organelles. Overexpression of rab5, which is associated with early endosomes and the plasma membrane, increases the rate of endocytosis [Bucci et al. (1992) Cell, 70, 715-728]. From sequence alignments and molecular modelling we identified structural elements that might contribute to the definition of the functional specificity of rab5. To test the role of these elements experimentally, we transplanted them onto rab6, which is associated with the Golgi complex. The chimeric proteins were assayed for intracellular localization and stimulation of endocytosis. First, we found that the C-terminus of rab5 could target rab6 to the plasma membrane and early endosomes but it did not confer rab5-like stimulation of endocytosis. Further replacement of other regions revealed that the N-terminus, helix alpha 2/loop 5 and helix alpha 2/loop 7 were all required to functionally convert rab6 into rab5. Reciprocal hybrids of rab5 containing these regions replaced with those of rab6 were inactive, demonstrating that each region is essential for rab5 function. These results indicate that distinct structural elements specify the localization, membrane association and regulatory function of rab5.

Membrane association of cathepsin B can be induced by transfection of human breast epithelial cells with c-Ha-ras oncogene

Alterations in trafficking and increases in expression of the lysosomal proteases cathepsins B, D and L have been observed in transformed cells and malignant tumors, including human breast carcinoma. ras and the related rab proteins participate in the vesicular transport processes required for normal trafficking of lysosomal enzymes. In addition, transfection of murine fibroblasts with the ras oncogene has been shown to increase the expression of cathepsins L and B. As human cancers are primarily epithelial in origin, we have investigated whether there are alterations in the trafficking and expression of cathepsin B in MCF-10 human breast epithelial cells transfected with wild-type and mutated ras. In all cells examined, i.e. mortal MCF-10M cells, immortal MCF-10A or MCF-10F cells, and transfected MCF-10A cells (transfected with the neomycin resistance gene (MCF-10Aneo) or cotransfected with wild-type proto-oncogenic ras (MCF-10AneoN) or mutated oncogenic ras (MCF-10AneoT)), levels of mRNA transcripts for cathepsin B were similar. However, alterations in trafficking of cathepsin B were observed in the cells transfected with oncogenic ras. In these cells there was an increased association of cathepsin B activity and cathepsin B protein with plasma membrane/endosomal fractions and a more peripheral distribution of immunofluorescent staining for cathepsin B. At the electron microscopic level, immunogold labeling for cathepsin B was localized to the cell membrane as well as to vesicles in the microvilli and adjacent to the cell membrane. In the parental MCF-10A cells, in contrast, cathepsin B was localized to vesicles in the perinuclear region. The cathepsin B associated with plasma membrane/endosomal fractions in the cells transfected with oncogenic ras was mature cathepsin B as demonstrated by immunoblot analysis. This was confirmed further by showing an absence of peripheral immunofluorescent staining in these cells using an antibody specific for the propeptide of cathepsin B. Thus, we have demonstrated by multiple techniques that transfection of human breast epithelial cells with oncogenic ras results in alterations in the trafficking of cathepsin B similar to those observed previously in human and animal tumors of both epithelial and mesenchymal origin.

Molecular cloning and subcellular localization of three GTP-binding proteins of the rab subfamily

Small GTPases of the rab subfamily are involved in regulation of intracellular membrane transport events. We recently used a PCR approach to isolate short cDNA fragments of a number of novel rab sequences. These PCR fragments have not been used with cDNA library screening and PCR-based techniques to clone the cDNAs encoding three of these proteins, rab12, rab22, and rab24. By northern blot analysis, the messages were found to be present in a wide variety of mouse tissues. However, quantitative differences in the mRNA levels between the tissues were detected. We determined the subcellular localization of the GTPases by expressing the c-myc epitope-tagged proteins with the Semliki Forest virus and the vaccinia T7 vector systems. Transiently expressed rab12 was localized to the Golgi complex. This localization was confirmed using a polyclonal anti-peptide antibody detecting the endogenous protein in BHK cells. rab22 expressed from the cDNA was localized to endosomal compartments and to the plasma membrane. After longer periods of expression, the protein was found on abnormally large perinuclear endosomal structures, suggesting that it is a potent regulator of events in the endocytic pathway. Finally, rab24 was found in the endoplasmic reticulum/cis-Golgi region and on late endosomal structures. The localization of rab24 may indicate its involvement in autophagy-related processes.

Rab11, a small GTPase associated with both constitutive and regulated secretory pathways in PC12 cells

A specific polyclonal antibody was used to investigate the subcellular distribution of the small GTPase, rab11p, in the neuroendocrine cell line, PC12. We took advantage of a previously described pulse-chase protocol based on sulfation to examine the distribution of rab11 along the secretory pathway. Using the rab11 antiserum, but not serum depleted of rab11 antibodies, we were able to specifically immunoisolate markers of the constitutive and the regulated secretory pathway in the trans-Golgi network (TGN) as well as after their exit from this compartment (constitutive secretory vesicles, immature, and mature secretory granules). We therefore conclude that rab11p is associated with the TGN and with TGN-derived vesicles of both the constitutive and the regulated secretory pathway in PC12 cells.

The small GTP-binding protein rab6p is redistributed in the cytosol by brefeldin A

Rab6 protein belongs to the Sec4/Ypt/rab subfamily of small GTP-binding proteins involved in intracellular membrane trafficking in yeast and mammalian cells. Its localization both in medial and trans-Golgi network prompted us to study the effects of brefeldin A (BFA) on rab6p redistribution. By two techniques, indirect immunofluorescence and cell fractionation, we investigated the fate of rab6p and compared it to other Golgi or trans-Golgi network markers in BHK-21 and NIH-3T3 cells. BFA, at 5 micrograms/ml, induced redistribution of rab6p according to a biphasic process: during the first 10–15 minutes, tubulo-vesicular structures--colabelled with a bona fide medial Golgi marker called CTR 433--were observed; these structures were then replaced by punctate diffuse staining, which was stable for up to 3 hours. The 110 kDa peripheral membrane protein beta-COP was released much more rapidly from the Golgi membranes, whereas the trans-Golgi network marker TGN 38 relocated to the microtubule organizing center. The kinetics of reversion of BFA action on these antigens was also followed by immunofluorescence. Consistent with these results, rab6 antigen, originally found as 40% in the cytosolic versus 60% in the particulate (P 150,000 g) fraction, became almost entirely cytosolic; moreover, it partitioned in the aqueous phase of Triton X-114 whereas the membrane fraction was detergent-soluble. Rab6p did not become part of the coatomers after its BFA-induced release from Golgi structures. Three requirements seemed to be necessary for such a release: integrity of the microtubules, presence of energy, and a hypothetical trimeric G protein, as revealed by the respective roles of nocodazole, ATP depletion, and sensitivity to aluminium fluoride. Finally, we have shown that BFA does not prevent attachment of newly synthesized rab6p to membranes.

Involvement of Ypt7p, a small GTPase, in traffic from late endosome to the vacuole in yeast

The YPT7 gene encodes the Saccharomyces cerevisiae homolog of mammalian rab7 protein. Data obtained from studies on a delta ypt7 mutant suggested that Ypt7p is involved in the endocytic pathway in yeast (Wichmann et al., Cell 71, 1131–1142, 1992). We report here that endocytosed pheromone alpha-factor accumulates in late endosomes in delta ypt7 cells, indicating that Ypt7p is involved in the regulation of transport steps from late endosomes to the vacuole. We also show that alpha-factor can be degraded in a PEP4-dependent manner in a prevacuolar/endosomal compartment in delta ypt7 cells, providing independent evidence that the pathways of vacuole biogenesis and endocytosis in yeast may intersect in the endosomal membrane system.

Rab6 is associated with a compartment that transports rhodopsin from the trans-Golgi to the site of rod outer segment disk formation in frog retinal photoreceptors

The biogenesis of light sensitive membranes in retinal rod photoreceptors involves polarized sorting and targeting of newly synthesized rhodopsin to a specialized domain, the rod outer segment (ROS). We have isolated and characterized the population of post-Golgi membranes that mediate intracellular transport of rhodopsin. In the present study we have examined the association of small (20-25 kDa) GTP-binding (G) proteins with these membranes. We found that one of the small G proteins, rab6, behaves like an integral membrane protein of the post-Golgi vesicles, although approximately 30% of rab6 is soluble. The distribution of the membrane-associated and the soluble forms is highly polarized. By confocal and EM immunocytochemistry it can be seen that most of rab6 is associated with the photoreceptor trans-Golgi cisternae, trans-Golgi network (TGN) and post-Golgi vesicles. The photoreceptor axon and synaptic terminal are unlabeled, but dendrites of deeper retinal layers are labeled. The distribution of rab6 across sucrose density gradient fractions parallels the distribution of sialyltransferase (a TGN marker) activity. About 9% of membrane-bound rab6 is associated, however, with the rhodopsin-bearing sialyltransferase-free post-Golgi vesicles, which represent a very small fraction (&lt; 1%) of the total retinal membranes. Rab6 is absent from the mature ROS disk membranes but it is present at the sites of new ROS disk formation and in the ROS cytoplasm. This suggests that rab6 becomes soluble upon disk membrane formation. Therefore, rab6 may function not only as a component of the sorting machinery of photoreceptors that delivers rhodopsin to its appropriate subcellular domain but may also participate in some aspects of ROS disk morphogenesis.

Interaction of Sec4 with GDI proteins from bovine brain, Drosophila melanogaster and Saccharomyces cerevisiae. Conservation of GDI membrane dissociation activity

Rab GDP dissociation inhibitor (Rab GDI), will induce the dissociation of GDP-bound rab3A from synaptic membranes and will inhibit GDP dissociation from Sec4, a member of the Rab subgroup of the Ras GTPase superfamily which is required for exocytosis in Saccharomyces cerevisiae. We report that Rab GDI releases GDP-bound Sec4 from yeast membranes. dGDI, a Drosophila homologue can similarly inhibit GDP dissociation from Sec4 and release GDP-bound Sec4 from yeast membranes. An activity partially purified from yeast cytosol dissociates GDP-bound Sec4 from yeast membranes, suggesting that yeast also possess a GDI protein that functions to recycle Sec4 from its target membrane.

An immunologist's look at the Rho and Rab GTP-binding proteins

The ras superfamily of small GTP-binding proteins contains three major branches: the Ras, Rho and Rab protein subfamilies. Recent advances in the field of ras-related small GTP-binding proteins suggest that it may be worthwhile to look at this superfamily from the standpoint of immunology. The subject of this review is to outline briefly the areas of lymphocyte function which may implicate small G proteins, with special emphasis on the established or possible roles of proteins of the Rho and Rab subfamilies in cytoskeleton organization and antigen presentation.

Rab GTPases in vesicular transport

Specificity and directionality are two features shared by the numerous steps of membrane transport that connect cellular organelles. By shuttling between specific membrane compartments and the cytoplasm, small GTPases of the Rab family appear to regulate membrane traffic in a cyclical manner. The restriction of certain Rab proteins to differentiated cell types supports a role for these GTPases in defining the specificity of membrane trafficking.