Search PubMed⌕ Search

SEARCH · Search PubMed

Results for “rab4 GTP-Binding Proteins”

Search indexed PubMed citations on genomics, clinical trials, systematic reviews and public health. Explore titles, authors and supplied subject terms, then open the PubMed record.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 19 recordsLinked to original sources
PubMed1993

Post-translational processing and membrane association of the two early endosome-associated rab GTP-binding proteins (rab4 and rab5).

The two early endosome-associated rab GTP-binding proteins, rab4 and rab5, are suggested to regulate endocytosis. In this report, we examined post-translational processing and membrane association of the two rab proteins. Human rab4 and rab5 were expressed in chicken embryo fibroblasts using a Sindbis virus expression vector. Cells were labeled with either [35S]methionine or [3H]mevalonolactone. Cell lysates were immunoprecipitated with antisera specific for rab4 and rab5, respectively, and analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. It was found that both rab4 and rab5 contained at least three forms: the precursor form, the isoprenylated intermediate, and the isoprenylated mature form of faster mobility. The rab5 intermediate comigrated with the precursor form, whereas the rab4 intermediate migrated slightly faster than the precursor form. The intermediate form of rab4, but not rab5, accumulated in the presence of a chymotrypsin-like protease inhibitor (N-acetyl Tyr ethyl ester), suggesting that proteolysis was required for generation of the mature form. Furthermore, the intermediate and mature forms of rab4, but not rab5, were carboxyl-methylated as demonstrated by incorporation of alkali-labile counts from [methyl-3H]methionine. Membrane association of the distinct rab4 and rab5 forms was examined by subcellular fractionation and Triton X-114 partitioning. The precursor forms were found in the cytosol and partitioned into the aqueous phase. The mature forms were membrane-associated and partitioned into the detergent phase. Unexpectedly, the isoprenylated intermediate forms of both rab4 and rab5 partitioned exclusively into the aqueous phase. Taken together, the data indicate that the entire post-translational processing, which includes isoprenylation, carboxyl methylation (rab4), and possibly proteolysis, confers the competency for membrane association of rab4 and rab5.

Animals↗
PubMed1991

The small GTP-binding protein rab4 is associated with early endosomes.

Small GTP-binding proteins of the rab family have been implicated as playing important roles in controlling membrane traffic on the biosynthetic and endocytic pathways. We demonstrate that a distinct rab protein, rab4p, is associated with the population of early endosomes involved in transferrin-receptor recycling. An antibody to human rab4p was found to detect a doublet of approximately 24-kDa proteins on immunoblots from various cell types. Seventy-five percent of these proteins were tightly membrane bound and could be released only by detergent treatment. Upon isolation of early endosomes, late endosomes, and lysosomes, by free-flow electrophoresis and Percoll density-gradient centrifugation, most (70%) of the rab4p was found to cofractionate with early endosomes and endocytic vesicles containing 125I-labeled transferrin. The rab proteins previously localized to the endoplasmic reticulum and/or Golgi apparatus were not found in these fractions. We also localized rab4p to transferrin-receptor-containing early endosomes by immunofluorescence after expression of rab4 cDNA. The association of rab4p with early endosomes and other vesicles involved in the intracellular transport of transferrin receptor suggests that rab4p may play a role in regulating the pathway of receptor recycling.

Animals↗
PubMed1992

The small GTP-binding protein rab4 controls an early sorting event on the endocytic pathway.

rab4 is a ras-like GTP-binding protein that associates with early endosomes in a cell cycle-dependent fashion. To determine its role during endocytosis, we generated stable cell lines that overexpressed mutant or wild-type rab4. By measuring endocytosis, transport to lysosomes, and recycling, we found that overexpression of wild-type rab4 had differential effects on the endocytic pathway. Although initial rates of internalization and degradation were not inhibited, the transfectants exhibited a 3-fold decrease in fluid phase endocytosis as well as an alteration in transferrin receptor (Tfn-R) recycling. Wild-type rab4 caused a redistribution of Tfn-R's from endosomes to the plasma membrane. It also blocked iron discharge by preventing the delivery of Tfn to acidic early endosomes, instead causing Tfn accumulation in a population of nonacidic vesicles and tubules. rab4 thus appears to control the function or formation of endosomes involved in recycling.

Animals↗
PubMed1993

The small GTP-binding proteins Rab4 and ARF are associated with released exosomes during reticulocyte maturation.

During maturation of reticulocytes to erythrocytes, small vesicles, termed exosomes, are released into the extracellular medium. GTP-binding proteins associated with exosomes were identified by separating proteins by sodium dodecyl sulfate polyacrylamide gel electrophoresis, transferring the proteins to nitrocellulose, and probing the blots with [alpha-32P]GTP. At least three GTP-binding proteins were detected with a molecular mass of 27, 26, and 20 kDa. Binding of GTP by exosomes was resistant to trypsin in the absence, but not in the presence of detergent. This indicates that the GTP-binding proteins are within the lumen of exosomes. During reticulocyte maturation, the amount of GTP-binding proteins released from reticulocytes was proportional to the amount of exosomes released. Western blot analysis demonstrated that the 27 kDa, 26 kDa, and 20 kDa proteins were Rab5p, Rab4p and ADP-ribosylation factor (ARF), respectively. In reticulocytes, Rab4p was highly enriched in exosomes and endosomes compared to plasma membrane and cytosol. Although mainly cytosolic, ARF was also found associated with endosomes and exosomes but not with plasma membrane. In contrast to Rab4p and ARF, Rab5p was enriched in the plasma membrane compared to cytosol, exosomes and endosomes. As exosomes are believed to derive from endosomes, Rab4p and ARF may be involved in the mechanism of exosome formation.

ADP-Ribosylation Factors↗
PubMed1993

Differences in the endosomal distributions of the two mannose 6-phosphate receptors.

Multiple immunolabeling of cryosections was performed to compare the subcellular distributions of the two mannose 6-phosphate receptors (MPRs) involved in the intracellular targeting of lysosomal enzymes: the cation-dependent (CD) and cation-independent (CI) MPR. In two cell types, the human hepatoma cell line HepG2 and BHK cells double transfected with cDNA's encoding for the human CD-MPR and CI-MPR, we found the two receptors at the same sites: the trans-Golgi reticulum (TGR), endosomes, electron-dense cytoplasmic vesicles, and the plasma membrane. In the TGR the two receptors colocalized and were concentrated to the same extent in the same HA I-adaptor positive coated buds and vesicles. Endosomes were identified by the presence of exogenous tracers. The two MPR codistributed to the same endosomes, but semiquantitative analysis showed a relative enrichment of the CI-MPR in endosomes containing many internal vesicles. Two endosomal subcompartments were discerned, the central vacuole and the associated tubules and vesicles (ATV). We found an enrichment of CD-MPR over CI-MPR in the ATV. Lateral segregation of the two receptors within the plane of membranes was also detected on isolated organelles. Double immunolabeling for the CD-MPR and the asialoglycoprotein receptor, which mainly recycles between endosomes and the plasma membrane, revealed that these two receptors were concentrated in different subpopulations of endosomal ATV. The small GTP-binding protein rab4, which has been shown to mediate recycling from endosomes to the plasma membrane, was localized at the cytosolic face of many endosomal ATV. Quantitative analysis of double-immunolabeled cells revealed only a limited codistribution of the MPRs and rab4 in ATV. These data suggest that the two MPRs exit the TGR via the same coated vesicles, but that upon arrival in the endosomes CD-MPR is more rapidly than CI-MPR, segregated into ATV which probably are destined to recycle MPRs to TGR.

Animals↗
PubMed1992

Reversible phosphorylation--dephosphorylation determines the localization of rab4 during the cell cycle.

The ras-like GTP binding protein rab4 is the only known rab protein on endosomes that is phosphorylated during mitosis. Since a large fraction of rab4 accumulates in the cytosol in mitotic cells, we investigated the molecular mechanism controlling membrane association of rab4. We first show that human rab4 is phosphorylated by recombinant mammalian p34cdc2 kinase in vitro. Next, the actual site of phosphorylation and its functional significance were determined using stably transfected CHO cell lines producing high levels of wild type rab4 or rab4 mutants bearing alterations at Ser196, which occurs within a consensus site for p34cdc2 kinase phosphorylation (S196PRR). Mutation of Ser196 to glutamine or aspartic acid completely prevented rab4 phosphorylation in mitotic cells and also blocked its appearance in the cytosol. Neither C-terminal isoprenylation nor carboxymethylation of rab4 was affected by the mutations or by phosphorylation. Finally, dephosphorylation and reassociation of soluble rab4 with membranes occurred upon exit of cells from mitosis. Thus, phosphorylation of Ser196 is directly responsible for the reversible translocation of rab4 into the cytosol of mitotic cells.

Animals↗
PubMed1995

Wortmannin inhibits the action of insulin but not that of okadaic acid in skeletal muscle: comparison with fat cells.

To look for the possible involvement of phosphatidylinositol-3-kinase (PI3-kinase) in insulin action in muscle, we have used wortmannin, described as a specific inhibitor of the enzyme, and compared its effect in muscle and in adipose cells. Both in intact mouse soleus muscle and in isolated rat adipocytes, wortmannin blocked insulin effect on glucose uptake, without markedly altering basal glucose uptake. In adipocyte, this effect results from a blockade of the translocation process because wortmannin inhibited the stimulatory action of insulin on both the Glut 4 movement from the internal compartment to the plasma membranes and the Rab4 departure from the microsomes. In a similar fashion, two other insulin effects, the activation of glycogen synthase and the stimulation of amino acid uptake, were blocked by wortmannin in skeletal muscle. Lipogenesis from acetate was also inhibited by wortmannin in adipocytes. By contrast, wortmannin did not affect muscle deoxglucose uptake when it was stimulated either by okadaic acid or by the protein kinase C activator tumor promoting agent. These results suggest that, in muscle and adipocyte, PI3-kinase inhibition causes a blockade of all insulin effects studied. By contrast, wortmannin did not affect the same responses elicited in muscle by okadaic acid or tumor promoting agent.

Adipocytes↗
PubMed1994

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.

Amino Acid Sequence↗
PubMed1994

Rab4 is phosphorylated by the insulin-activated extracellular-signal-regulated kinase ERK1.

Rab4, a low-molecular-mass GTP-binding protein, is associated with vesicles containing Glut 4 in adipocytes. Following insulin stimulation, the translocation of Glut 4 to the plasma membrane is associated with the movement of Rab4 to the cytosol. The same modifications are induced by the phosphatase inhibitor, okadaic acid. To establish a possible role for phosphorylation in Rab4 cycling, we searched for insulin-stimulated cytosolic kinase(s) which could phosphorylate Rab4. In 3T3-L1 adipocytes, insulin induced a rapid and transient activation of cytosolic kinase(s), which phosphorylated Rab4 in vitro. At least part of the Rab4 phosphorylation can be accounted for by ERK (extracellular-signal-regulated kinases) since immunopurified ERK1 from insulin-stimulated cells phosphorylated Rab4 with a comparable time-course. Both with cytosolic extracts and immunopurified ERK1, only serine residues were phosphorylated on Rab4. The phosphorylation site was localized in the C-terminus of the molecule, and occurred very probably on Ser196. These results indicate that Rab4 is an in vitro substrate for ERK, and suggest that the insulin-induced movement of Rab4 from the Glut-4-containing vesicles to the cytosol could result from phosphorylation of Rab4 by ERK.

3T3 Cells↗
PubMed1991

ADP-ribosylation of p21ras and related proteins by Pseudomonas aeruginosa exoenzyme S.

Pseudomonas aeruginosa exoenzyme S ADP-ribosylates p21ras and several related proteins. ADP-ribosylation of p21ras does not alter interactions with guanine nucleotides. The ras-related GTP-binding proteins, including Rab3, Rab4, Ral, Rap1A, and Rap2, are also substrates; given these results, we propose a model for the role of exoenzyme S in pathogenesis.

ADP Ribose Transferases↗
PubMed1994

Parallel changes in Glut 4 and Rab4 movements in two insulin-resistant states.

Insulin-induced Glut 4 and Rab4 movements were studied in two insulin-resistant states. In adipocytes from streptozotocin diabetic rats, the amount of Glut 4 was decreased by 60%. The remaining Glut 4 molecules were translocated in response to insulin, and in parallel, Rab4 left the intracellular compartment. In contrast, in 3T3-L1 adipocytes rendered insulin-resistant by a prolonged insulin treatment, both Rab4 and Glut 4 remained in the intracellular compartment following an acute insulin stimulation. Those results illustrate a similar behavior of Glut 4 and Rab4 in two situations where insulin resistance results from different mechanisms, and add further support for a role of Rab4 in Glut 4 translocation.

Adipocytes↗
PubMed1995

Rab4, but not the transferrin receptor, is colocalized with GLUT4 in an insulin-sensitive intracellular compartment in rat skeletal muscle.

The role of Rab4, a small molecular weight GTP binding protein implicated in endosomal/plasma membrane (PM) recycling, in the translocation of the GLUT4 transporter in rat skeletal muscle was studied. Muscle membranes, prepared by subcellular fractionation of control and insulin treated rat skeletal muscle, were subjected to SDS/PAGE and immunoblot analyses. Insulin treatment caused an increase in GLUT4 in a plasma membrane (PM) enriched fraction from an intracellular membrane (IM) fraction. Immunoprecipitation of GLUT4 vesicles from the IM compartment revealed that Rab4 could be coprecipitated. Importantly, however, and unlike in adipocytes, immunoisolated GLUT4 vesicles from rat skeletal muscle contained no detectable immunoreactivity towards the transferrin receptor, suggesting that Rab4 was present in a GLUT4 IM pool that was not characteristic of early endosomes. The involvement of Rab4 in GLUT4 translocation was further supported by the finding that insulin treatment resulted in a significant (43%) reduction in Rab4 in the IM compartment. Our results suggest (i) that insulin induces the loss of both GLUT4 and Rab4 from the same IM compartment, (ii) that Rab4 may be involved in GLUT4 translocation based on its coprecipitation with the transporter from the insulin-sensitive pool and (iii) that Rab4 can be localized to intracellular membranes which appear not to be of early endosome origin.

Animals↗
PubMed1994

Mapping small GTP-binding proteins on high-resolution two-dimensional gels by a combination of GTP binding and labeling with in situ periodate-oxidized GTP.

We compared two approaches to identify and map small GTP-binding proteins in combination with high-resolution two-dimensional (2-D) gel electrophoresis. The first approach involved direct GTP ligand binding after a renaturing transfer onto nitrocellulose. In the second, affinity labeling with in situ periodate-oxidized GTP was used in permeabilized cells (Peter, M. E., She, J., Huber, L. A. and Terhorst, C. Anal. Biochem. 1993, 210, 77-85). Analysis by 2-D gel electrophoresis revealed a number of distinct intracellular small GTP-binding proteins in Madine-darby canine kidney strain II cells (MDCKII). Using specific antibodies the electrophoretic coordinates of rab4, rap1a/b, and rap2 were identified for native as well as for crosslinked GTPases. These methods allow the identification of small GTP-binding proteins in total cell lysates and purified subcellular fractions, providing excellent markers throughout the course of differentiation and development.

Animals↗
PubMed1989

The human Rab genes encode a family of GTP-binding proteins related to yeast YPT1 and SEC4 products involved in secretion.

Seven cDNA clones corresponding to the rab1, rab2, rab3A, rab3B, rab4, rab5, and rab6 genes were isolated from a human pheochromocytoma cDNA library. They encode 23-25 kDa polypeptides which share approximately 30-50% homology and belong to the ras superfamily. The rab1, rab2, rab3A, and rab4 proteins are the human counterparts of the rat rab gene products that we have previously characterized. Comparison of the seven human rab proteins with the yeast YPT1 (YPT1p) and SEC4 (SEC4p) proteins reveals highly significant sequence similarities. H-rab1p shows 75% amino acid identity with YPT1p and may be therefore considered as its human counterpart. The other proteins share approximately 40% homology with YPT1p and SEC4p. The homology (approximately 30%) between these rab proteins and p21ras is restricted to the four conserved domains involved in the GTP/GDP binding. Human rab proteins were produced in Escherichia coli. Large amounts of rab proteins in soluble form can be extracted and purified without the use of detergents. All six proteins bind GTP and exhibit GTPase activities. A possible involvement of the rab proteins in secretion is discussed.

Adrenal Gland Neoplasms↗
PubMed1992

rab GTP-binding proteins with three different carboxyl-terminal cysteine motifs are modified in vivo by 20-carbon isoprenoids.

p21ras and several other ras-related GTP-binding proteins are modified post-translationally by addition of 15-carbon farnesyl or 20-carbon geranylgeranyl isoprenoids to cysteines within a conserved carboxyl-terminal sequence motif, Caa(M/S/L), where a is an aliphatic amino acid. Proteins ending with M or S are substrates for farnesyltransferase, whereas those ending with L are modified preferentially by geranylgeranyltransferase. We recently reported that GTP-binding proteins encoded by rab1B (GGCC), rab2 (GGCC), and rab5 (CCSN) are modified by 20-carbon isoprenyl derivatives of [3H]mevalonate when translated in vitro, despite having carboxyl-terminal sequences distinct from the Caa(M/S/L) motif. We now show that these proteins function as specific acceptors for geranylgeranyl in vitro and are modified by 20-carbon isoprenyl groups in COS cells metabolically labeled with [3H]mevalonate. Proteins encoded by rab4 and rab6, with yet another distinct carboxyl-terminal motif (xCxC), are similarly modified by 20-carbon isoprenoids in vitro and in vivo. The geranylgeranyl modification of rab5 protein (CCSN) is catalyzed by an enzyme in brain cytosol but not by a purified geranylgeranyltransferase that modifies GTP-binding proteins with the CaaL motif. Unlike the prenylation of proteins with Caa(M/S/L) termini, the prenylation of rab5 protein is not inhibited by a synthetic peptide based on its carboxyl-terminal sequence (TRNQCCSN). When cellular isoprenoid synthesis is blocked by treatment of cells with lovastatin, rab proteins that are normally localized in membranes of the endoplasmic reticulum, Golgi apparatus, and endosomes accumulate in the cytosol. This change in rab protein localization is reversed by providing cells with mevalonate. These findings suggest that geranylgeranyl modification underlies the ability of rab GTP-binding proteins to associate with intracellular membranes, where they are postulated to function as mediators of vesicular traffic.

Animals↗
PubMed1994

Cloning, characterization, and expression of a novel GDP dissociation inhibitor isoform from skeletal muscle.

Cellular mechanisms for controlling membrane trafficking appear to involve small GTP-binding proteins such as the Rab proteins. Rab function is regulated by GDP dissociation inhibitor (GDI), which releases Rab proteins from membranes and inhibits GDP dissociation. Here we report the isolation of a full-length cDNA encoding a novel GDI isoform of 445 amino acids (GDI-2) with a deduced molecular weight of 50,649 from mouse skeletal muscle. Full-length and partial cDNA clones encoding a previously reported GDI protein (GDI-1) were also isolated from cDNA libraries prepared from rat brain and mouse skeletal muscle, respectively. The degree of deduced amino acid sequence identity between mouse GDI-2 and our mouse GDI-1 cDNA clone is 86%. Northern (RNA blot) analysis revealed that in human tissues, both GDI-1 and GDI-2 transcripts were abundant in brain, skeletal muscle, and pancreas but were weakly expressed in heart and liver. GDI-1 mRNA was expressed in kidney, whereas GDI-2 was almost absent, while in lung the relative amounts of these mRNA species were reversed. Specific antibodies against mouse GDI-1 and GDI-2 based on unique peptide sequences in the proteins were raised. Differentiation of 3T3-L1 fibroblasts into highly insulin-responsive adipocytes was accompanied by large increases in both mRNA and protein levels of GDI-1 and GDI-2. GDI-1 and GDI-2 expressed as glutathione S-transferase fusion proteins were both able to solubilize the membrane-bound forms of Rab4 and Rab5 in a GDP/GTP-dependent manner. Taken together, these data demonstrate that the protein products of at least two genes regulate the membrane dynamics of Rab proteins in mice.

Amino Acid Sequence↗
PubMed1995

Two-hybrid system screen with the small GTP-binding protein Rab6. Identification of a novel mouse GDP dissociation inhibitor isoform and two other potential partners of Rab6.

Rab6 is a small GTP-binding protein that belongs to the Ras superfamily and is involved in intra-Golgi transport. Using a two-hybrid system screen of a mouse brain cDNA library, we have isolated several clones encoding proteins that interact with Rab6. Approximately 60% of the clones identified encoded a new mouse Rab GDP dissociation inhibitor (GDI) isoform. This GDI isoform is distinct from mouse mGDI-1 and mGDI-2, which have been characterized previously, and most likely represents the mouse counterpart of the rat Rab GDI beta isoform. In the two-hybrid system, GDI beta interacts with wild-type Rab6 and Rab5, but not with a GTP-bound Rab6 mutant, or a Rab6 mutant that cannot be post-translationally processed. We further examined whether mouse GDI beta is functional; we show that recombinant mouse GDI beta is able to remove several Rab proteins, including Rab1, Rab2, Rab4, and Rab6, from membranes. The identification of a third GDI isoform in mouse raised the question whether GDI genes belong to a larger multigenic family. We have shown, by Southern blot analysis of genomic DNA, that at least five GDI gene copies exist in both the mouse and rat genomes. In our two-hybrid screen, we have also characterized another clone that specifically interacts with Rab6. This clone was partially sequenced but shows no homology to known sequences. Finally, a third clone, interacting with both Rab5 and Rab6, also appears to encode a novel protein.

Amino Acid Sequence↗