Rab proteins

Distinct pathways for the trafficking of angiotensin II and adrenergic receptors from the endoplasmic reticulum to the cell surface: Rab1-independent transport of a G protein-coupled receptor

The molecular mechanism underlying the transport of G protein-coupled receptors from the endoplasmic reticulum (ER) to the cell surface is poorly understood. This issue was addressed by determining the role of Rab1, a Ras-related small GTPase that coordinates vesicular protein transport in the early secretory pathway, in the subcellular distribution and function of the angiotensin II type 1A receptor (AT1R), beta2-adrenergic receptor (AR), and alpha2B-AR in HEK293T cells. Inhibition of endogenous Rab1 function by transient expression of dominant-negative Rab1 mutants or Rab1 small interfering RNA (siRNA) induced a marked perinuclear accumulation and a significant reduction in cell-surface expression of AT1R and beta2-AR. The accumulated receptors were colocalized with calregulin (an ER marker) and GM130 (a Golgi marker), consistent with Rab1 function in regulating protein transport from the ER to the Golgi. In contrast, dominant-negative Rab1 mutants and siRNA had no effect on the subcellular distribution of alpha2B-AR. Similarly, expression of dominant-negative Rab1 mutants and siRNA depletion of Rab1 significantly attenuated AT1R-mediated inositol phosphate accumulation and ERK1/2 activation and beta2-AR-mediated ERK1/2 activation, but not alpha2B-AR-stimulated ERK1/2 activation. These data indicate that Rab1 GTPase selectively regulates intracellular trafficking and signaling of G protein-coupled receptors and suggest a novel, as yet undefined pathway for movement of G protein-coupled receptors from the ER to the cell surface.

Identification and characterization of mouse Rab32 by mRNA and protein expression analysis

Rab proteins, a subfamily of the ras superfamily, are low molecular weight GTPases involved in the regulation of intracellular vesicular transport. Cloning of human RAB32 was recently described. Presently, we report the cloning and characterization of the mouse homologue of Rab32. We show that murine Rab32 exhibits a ubiquitous expression pattern, with tissue-specific variation in expression level. Three cell types with highly specialized organelles, melanocytes, platelets and mast cells, exhibit relatively high level of Rab32. We show that in murine amelanotic in vitro transformed melanocytes as well as in human amelanotic metastatic melanoma cell lines, the expression of Rab32 is markedly reduced or absent, in parallel with the loss of expression of two key enzymes for the production of melanin, tyrosinase and Tyrp1. Therefore, in both mouse and human systems, the expression of Rab32 correlates with the expression of genes involved in pigment production. However, in melanoma samples, amelanotic due to a mutation in the tyrosinase gene, the expression of Rab32 remains at levels comparable to those observed in pigmented melanoma samples. Finally, we observed co-localization of Rab32 and the melanosomal proteins, Tyrp1 and Dct, indicating an association of Rab32 with melanosomes. Based on these data, we propose the inclusion of Rab32 to the so-called melanocyte/platelet family of Rab proteins.

The Rab-binding protein Noc2 is associated with insulin-containing secretory granules and is essential for pancreatic beta-cell exocytosis

The small GTPases Rab3 and Rab27 are associated with secretory granules of pancreatic beta-cells and regulate insulin exocytosis. In this study, we investigated the role of Noc2, a potential partner of these two GTPases, in insulin secretion. In the beta-cell line INS-1E wild-type Noc2, Noc265E, and Noc258A, a mutant capable of interacting with Rab27 but not Rab3, colocalized with insulin-containing vesicles. In contrast, two mutants (Noc2138S,141S and Noc2154A,155A,156A) that bind neither Rab3 nor Rab27 did not associate with secretory granules and were uniformly distributed throughout the cell cytoplasm. Overexpression of wild-type Noc2, Noc265E, or Noc258A inhibited hormone secretion elicited by insulin secretagogues. In contrast, overexpression of the mutants not targeted to secretory granules was without effect. Silencing of the Noc2 gene by RNA interference led to a strong impairment in the capacity of INS-1E cells to respond to insulin secretagogues, indicating that appropriate levels of Noc2 are essential for pancreatic beta-cell exocytosis. The defect was already detectable in the early secretory phase (0-10 min) but was particularly evident during the sustained release phase (10-45 min). Protein-protein binding studies revealed that Noc2 is a potential partner of Munc13, a component of the machinery that controls vesicle priming and insulin exocytosis. These data suggest that Noc2 is involved in the recruitment of secretory granules at the plasma membrane possibly via the interaction with Munc13.

Rabring7, a novel Rab7 target protein with a RING finger motif

Rab7, a member of the Rab family small G proteins, has been shown to regulate intracellular vesicle traffic to late endosome/lysosome and lysosome biogenesis, but the exact roles of Rab7 are still undetermined. Accumulating evidence suggests that each Rab protein has multiple target proteins that function in the exocytic/endocytic pathway. We have isolated a new Rab7 target protein, Rabring7 (Rab7-interacting RING finger protein), using a CytoTrap system. It contains an H2 type RING finger motif at the C termini. Rabring7 shows no homology with RILP, which has been reported as another Rab7 target protein. GST pull-down and coimmunoprecipitation assays demonstrate that Rabring7 specifically binds the GTP-bound form of Rab7 at the N-terminal portion. Rabring7 is found mainly in the cytosol and is recruited efficiently to late endosomes/lysosomes by the GTP-bound form of Rab7 in BHK cells. Overexpression of Rabring7 not only affects epidermal growth factor degradation but also causes the perinuclear aggregation of lysosomes, in which the accumulation of the acidotropic probe LysoTracker is remarkably enhanced. These results suggest that Rabring7 plays crucial roles as a Rab7 target protein in vesicle traffic to late endosome/lysosome and lysosome biogenesis.

Involvement of the Rab27 binding protein Slac2c/MyRIP in insulin exocytosis

Rab27a is a GTPase associated with insulin-containing secretory granules of pancreatic beta-cells. Selective reduction of Rab27a expression by RNA interference did not alter granule distribution and basal secretion but impaired exocytosis triggered by insulin secretagogues. Screening for potential effectors of the GTPase revealed that the Rab27a-binding protein Slac2c/MyRIP is associated with secretory granules of beta-cells. Attenuation of Slac2c/MyRIP expression by RNA interference did not modify basal secretion but severely impaired hormone release in response to secretagogues. Although beta-cells express Myosin-Va, a potential partner of Slac2c/MyRIP, no functional link between the two proteins could be demonstrated. In fact, overexpression of the Myosin-Va binding domain of Slac2c/MyRIP did not affect granule localization and hormone exocytosis. In contrast, overexpression of the actin-binding domain of Slac2c/MyRIP led to a potent inhibition of exocytosis without detectable alteration in granule distribution. This effect was prevented by point mutations that abolish actin binding. Taken together our data suggest that Rab27a and Slac2c/MyRIP are part of a complex mediating the interaction of secretory granules with cortical actin cytoskeleton and participate to the regulation of the final steps of insulin exocytosis.

Mycobacterium's arrest of phagosome maturation in macrophages requires Rab5 activity and accessibility to iron

Many mycobacteria are intramacrophage pathogens that reside within nonacidified phagosomes that fuse with early endosomes but do not mature to phagolysosomes. The mechanism by which mycobacteria block this maturation process remains elusive. To gain insight into whether fusion with early endosomes is required for mycobacteria-mediated inhibition of phagosome maturation, we investigated how perturbing the GTPase cycles of Rab5 and Rab7, GTPases that regulate early and late endosome fusion, respectively, would affect phagosome maturation. Retroviral transduction of the constitutively activated forms of both GTPases into primary murine macrophages had no effect on Mycobacterium avium retention in an early endosomal compartment. Interestingly, expression of dominant negative Rab5, Rab5(S34N), but not dominant negative Rab7, resulted in a significant increase in colocalization of M. avium with markers of late endosomes/lysosomes and increased mycobacterial killing. This colocalization was specific to mycobacteria since Rab5(S34N) expressing cells showed diminished trafficking of endocytic tracers to lysosomes. We further demonstrated that maturation of M. avium phagosomes was halted in Rab5(S34N) expressing macrophages supplemented with exogenous iron. These findings suggest that fusion with early endosomes is required for mycobacterial retention in early phagosomal compartments and that an inadequate supply of iron is one factor in mycobacteria's inability to prevent the normal maturation process in Rab5(S34N)-expressing macrophages.

Protein prenyltransferases: anchor size, pseudogenes and parasites

Lipid modification of eukaryotic proteins by protein prenyltransferases is required for critical signaling pathways, cell cycle progression, cytoskeleton remodeling, induction of apoptosis and vesicular trafficking. This review analyzes the influence of distinct states of sequential posttranslational processing that can be obtained after single or double prenylation, reversible palmitoylation, proteolytic cleavage of the C-terminus and possible reversible carboxymethylation. This series of modifications, as well as the exact length of the prenyl anchor, are determinants in protein-membrane and specific protein-protein interactions of protein prenyltransferase substrates. Furthermore, the occurrence and distribution of pseudogenes of protein prenyltransferase subunits are discussed. Besides being developed as anti-cancer agents, prenyltransferase inhibitors are effective against an increasing number of parasitic diseases. Extensive screens for protein prenyltransferases in genomic data of fungal and protozoan pathogens unveil a series of new pharmacologic targets for prenyltransferase inhibition, including the parasites Brugia malayi, Onchocerca volvulus, Aspergillus nidulans, Pneumocystis carinii, Entamoeba histolytica, Strongyloides stercoralis, Trichinella spiralis and Cryptosporidium parvum.

Transcytosis: crossing cellular barriers

Transcytosis, the vesicular transport of macromolecules from one side of a cell to the other, is a strategy used by multicellular organisms to selectively move material between two environments without altering the unique compositions of those environments. In this review, we summarize our knowledge of the different cell types using transcytosis in vivo, the variety of cargo moved, and the diverse pathways for delivering that cargo. We evaluate in vitro models that are currently being used to study transcytosis. Caveolae-mediated transcytosis by endothelial cells that line the microvasculature and carry circulating plasma proteins to the interstitium is explained in more detail, as is clathrin-mediated transcytosis of IgA by epithelial cells of the digestive tract. The molecular basis of vesicle traffic is discussed, with emphasis on the gaps and uncertainties in our understanding of the molecules and mechanisms that regulate transcytosis. In our view there is still much to be learned about this fundamental process.

Insulin-induced GLUT4 translocation involves protein kinase C-lambda-mediated functional coupling between Rab4 and the motor protein kinesin

Insulin stimulates glucose transport by promoting translocation of GLUT4 proteins from the perinuclear compartment to the cell surface. It has been previously suggested that the microtubule-associated motor protein kinesin, which transports cargo toward the plus end of microtubules, plays a role in translocating GLUT4 vesicles to the cell surface. In this study, we investigated the role of Rab4, a small GTPase-binding protein, and the motor protein KIF3 (kinesin II in mice) in insulin-induced GLUT4 exocytosis in 3T3-L1 adipocytes. Photoaffinity labeling of Rab4 with [gamma-(32)P]GTP-azidoanilide showed that insulin stimulated Rab4 GTP loading and that this insulin effect was inhibited by pretreatment with the phosphatidylinositol 3-kinase (PI3-kinase) inhibitor LY294002 or expression of dominant-negative protein kinase C-lambda (PKC-lambda). Consistent with previous reports, expression of dominant-negative Rab4 (N121I) decreased insulin-induced GLUT4 translocation by 45%. Microinjection of an anti-KIF3 antibody into 3T3-L1 adipocytes decreased insulin-induced GLUT4 exocytosis by 65% but had no effect on endocytosis. Coimmunoprecipitation experiments showed that Rab4, but not Rab5, physically associated with KIF3, and this was confirmed by showing in vitro association using glutathione S-transferase-Rab4. A microtubule capture assay demonstrated that insulin stimulation increased the activity for the binding of KIF3 to microtubules and that this activation was inhibited by pretreatment with the PI3-kinase inhibitor LY294002 or expression of dominant-negative PKC-lambda. Taken together, these data indicate that (i) insulin signaling stimulates Rab4 activity, the association of Rab4 with kinesin, and the interaction of KIF3 with microtubules and (ii) this process is mediated by insulin-induced PI3-kinase-dependent PKC-lambda activation and participates in GLUT4 exocytosis in 3T3-L1 adipocytes.

Biochemical and functional characterization of Rab27a mutations occurring in Griscelli syndrome patients

Rab27a is a member of the Rab family of small GTPase proteins, and thus far is the first member to be associated with a human disease (ie, the Griscelli syndrome type 2). Mutations in the Rab27a gene cause pigment as well as cytotoxic granule transport defects, accounting for the partial albinism and severe immune disorder characteristics of this syndrome. So far, 3 Rab27a missense mutations have been identified. They open a unique opportunity to designate critical structural and functional residues of Rab proteins. We show here that the introduction of a proline residue in the alpha 4 (Ala152Pro) or beta 5 (Leu130Pro) loop, observed in 2 of these spontaneous mutants, dramatically affects both guanosine triphosphate (GTP) and guanosine diphosphate (GDP) nucleotide-binding activity of Rab27a, probably by disrupting protein folding. The third mutant, Trp73Gly, is located within an invariant hydrophobic triad at the switch interface, and was previously shown in active Rab3A to mediate rabphilin3A effector interaction. Trp73Gly is shown to display the same nucleotide-binding and GTPase characteristics as the constitutively active mutant Gln78Leu. However, in contrast to Gln78Leu, Trp73Gly mutant construct neither interacts with the Rab27a effector melanophilin nor modifies melanosome distribution and cytotoxic granule exocytosis. Substitutions introduced at the 73 position, including the leucine residue present in Ras, did not restore Rab27a protein functions. Taken together, our results characterize new critical residues of Rab proteins, and identify the Trp73 residue of Rab27a as a key position for interaction with the specific effectors of Rab27a, both in melanocytes and cytotoxic cells.

Control of membrane fusion during spermiogenesis and the acrosome reaction

Membrane fusion is important to reproduction because it occurs in several steps during the process of fertilization. Many events of intracellular trafficking occur during both spermiogenesis and oogenesis. The acrosome reaction, a key feature during mammalian fertilization, is a secretory event involving the specific fusion of the outer acrosomal membrane and the sperm plasma membrane overlaying the principal piece of the acrosome. Once the sperm has crossed the zona pellucida, the gametes fuse, but in the case of the sperm this process takes place through a specific membrane domain in the head, the equatorial segment. The cortical reaction, a process that prevents polyspermy, involves the exocytosis of the cortical granules to the extracellular milieu. In lower vertebrates, the formation of the zygotic nucleus involves the fusion (syngamia) of the male pronucleus with the female pronucleus. Other undiscovered membrane trafficking processes may also be relevant for the formation of the zygotic centrosome or other zygotic structures. In this review, we focus on the recent discovery of molecular machinery components involved in intracellular trafficking during mammalian spermiogenesis, notably related to acrosome biogenesis. We also extend our discussion to the molecular mechanism of membrane fusion during the acrosome reaction. The data available so far suggest that proteins participating in the intracellular trafficking events leading to the formation of the acrosome during mammalian spermiogenesis are also involved in controlling the acrosome reaction during fertilization.

Co-localization of rab4 with endocytosis-related proteins in the rat parotid glands

Small GTP-binding proteins have been implicated in the regulation of vesicular traffic. We investigated the localization of Rab4 in the rat parotid glands by Western blotting and light-microscopic immunohistochemistry. Rab4 was localized mainly on the intracellular membranes in the subapical-actin terminal web, but was not present in the basolateral region both in acinar and ductal cells. Actin, alpha-adaptin, Rab5A and aquaporin5 were detected in the Rab4-containing intracellular membrane fraction prepared using anti-Rab4 antibody covalently coupled to magnetic beads. Detection of actin indicated that the Rab4-containing intracellular membranes were attached to the actin filaments. Although alpha-adaptin was immunohistochemically distributed along the plasma membrane, this protein coexisted with Rab4 only at the apical region. Rab5A immunoreactivity was distributed all around the cytoplasm. These findings suggested that Rab4 participates in endocytosis at the apical membrane of parotid glands.

Hierarchy of protein assembly at the vertex ring domain for yeast vacuole docking and fusion

Vacuole tethering, docking, and fusion proteins assemble into a "vertex ring" around the apposed membranes of tethered vacuoles before catalyzing fusion. Inhibitors of the fusion reaction selectively interrupt protein assembly into the vertex ring, establishing a causal assembly hierarchy: (a) The Rab GTPase Ypt7p mediates vacuole tethering and forms the initial vertex ring, independent of t-SNAREs or actin; (b) F-actin disassembly and GTP-bound Ypt7p direct the localization of other fusion factors; (c) The t-SNAREs Vam3p and Vam7p regulate each other's vertex enrichment, but do not affect Ypt7p localization. The v-SNARE Vti1p is enriched at vertices by a distinct pathway that is independent of the t-SNAREs, whereas both t-SNAREs will localize to vertices when trans-pairing of SNAREs is blocked. Thus, trans-SNARE pairing is not required for SNARE vertex enrichment; and (d) The t-SNAREs regulate the vertex enrichment of both G-actin and the Ypt7p effector complex for homotypic fusion and vacuole protein sorting (HOPS). In accord with this hierarchy concept, the HOPS complex, at the end of the vertex assembly hierarchy, is most enriched at those vertices with abundant Ypt7p, which is at the start of the hierarchy. Our findings provide a unique view of the functional relationships between GTPases, SNAREs, and actin in membrane fusion.

Neuronal p38 MAPK signalling: an emerging regulator of cell fate and function in the nervous system

p38 mitogen-activated protein kinases (MAPKs), together with extracellular signal-regulated kinases (ERKs) and c-Jun N-terminal kinases (JNKs), constitute the MAPK family. Multiple intracellular signalling pathways that converge on MAPKs exist in all eukaryotic cells and play pivotal roles in a wide variety of cellular functions. p38 MAPKs and JNKs, also termed stress-activated protein kinases (SAPKs), are preferentially activated by various cytotoxic stresses and cytokines and appear to be potent regulators of stress-induced apoptosis. Whereas JNKs have been shown to play pivotal roles in the regulation of neuronal apoptosis, the role of p38 MAPKs in the nervous system is poorly understood. However, accumulating evidence from mammalian cell culture systems and the strong genetic tool C. elegans suggests that neuronal p38 signalling has diverse functions beyond the control of cell death and survival. This review focuses on possible roles for the p38 pathway in the nervous system, with principal emphasis placed on the roles in neuronal cell fate decision and function.

Stimulation of PrP(C) retrograde transport toward the endoplasmic reticulum increases accumulation of PrP(Sc) in prion-infected cells

Prion diseases are fatal and transmissible neurodegenerative disorders characterized by the accumulation of an abnormally folded isoform of the cellular prion protein (PrP(C)) denoted PrP(Sc). To identify intracellular organelles involved in PrP(Sc) formation, we studied the role of the Ras-related GTP-binding proteins Rab4 and Rab6a in intracellular trafficking of the prion protein and production of PrP(Sc). When a dominant-negative Rab4 mutant or a constitutively active GTP-bound Rab6a protein was overexpressed in prion-infected neuroblastoma N2a cells, there was a marked increase of PrP(Sc) formation. By immunofluorescence and cell fractionation studies, we have shown that expression of Rab6a-GTP delocalizes PrP within intracellular compartments, leading to an accumulation in the endoplasmic reticulum. These results suggest that prion protein can be subjected to retrograde transport toward the endoplasmic reticulum and that this compartment may play a significant role in PrP(Sc) conversion.

The mammalian Golgi--complex debates

Since the first description of the Golgi in 1898, key issues regarding this organelle have remained contentious among cell biologists. Resolving these complex debates, which revolve around Golgi structure-function relationships, is prerequisite to understanding how the Golgi fulfils its role as the central organelle and sorting station of the mammalian secretory pathway.

Mutant Rab24 GTPase is targeted to nuclear inclusions

BACKGROUND

Members of the Rab GTPase family regulate intracellular protein trafficking, but the specific function of Rab24 remains unknown. Several attributes distinguish this protein from other members of the Rab family, including a low intrinsic GTPase activity.

RESULTS

The functions of other Rab proteins have been defined through the use of dominant-negative mutants with amino acid substitutions in the conserved N(T)KxD nucleotide binding motif. Surprisingly, when such Rab24 constructs were expressed in cultured cells, they accumulated in nuclear inclusions which disrupted the integrity of the nuclear envelope. The inclusions reacted positively with antibodies against ubiquitin and Hsp70, similar to protein aggregates observed in polyglutamine disorders. They also appeared to sequester importin-beta and GFP-coupled glucocorticoid receptor. Other Rab GTPases with similar mutations in the N(T)KxD motif were never found in inclusions, suggesting that the unusual localization of Rab24 is not related solely to misfolding of its nucleotide-free form. Studies with Rab24/Rab1B chimeras indicated that targeting of the mutant protein to inclusions requires the unique C-terminal domain of Rab24.

CONCLUSION

These studies demonstrate that mutations in Rab24 can trigger a cytopathic cellular response involving accumulation of nuclear inclusions. If the N(T)KxD mutants of Rab24 function as dominant suppressors, these studies may point to a unique role for Rab24 in degradation of misfolded cellular proteins or trafficking of proteins to the nuclear envelope. However, we cannot yet eliminate the possibility that these phenomena are related to unusual non-physiological protein interactions with the mutant form of Rab24.

Novel and nondetected human signaling protein polymorphisms

The frequency of single nucleotide polymorphisms (SNPs) in downstream signaling proteins was determined by combination heteroduplex HPLC and double-stranded sequencing of genomic DNA from 96-144 congestive heart failure (CHF) patients. Analysis of 56 coding exons in 9 signaling genes revealed 17 novel and 8 previously reported synonymous (no change in amino acid) SNPs, as well as one novel nonsynonymous SNP in the Rad small G protein. Because this initial analysis failed to detect numerous SNPs reported in the NCBI and Celera databases, double-strand sequencing of relevant exons from 74-91 CHF patients was used to confirm the absence of 10 previously reported nonsynonymous SNPs. Our results show that synonymous SNPs are frequent in signaling protein genes, whereas nonsynonymous SNPs are rare, suggesting a high degree of evolutionary conservation among these downstream signaling molecules. Comparisons of our results to the NCBI and Celera databases indicates that 56% of their SNP entries are not detected in our cohort. Importantly, while 31% of database SNPs were verified, 69% of SNPs detected in our cohort are not included in these databases. These findings indicate that caution may be warranted in relying exclusively on SNP databases as catalogs for polymorphic signaling protein genes.

Induction of autophagy causes dramatic changes in the subcellular distribution of GFP-Rab24

Rab GTPases comprises a large family of proteins, with more than 50 gene products localized in distinct subcellular compartments. Rab24 is a member of this family whose function is not presently known. In order to elucidate the role of this protein we have generated a GFP-tagged Rab24 and studied the distribution of this chimera by fluorescence microscopy. GFP-Rab24 showed a perinuclear reticular localization that often encircled the nucleus. This reticular pattern partially overlapped with ER markers, cis-Golgi, and the ER-Golgi intermediate compartment. Surprisingly, when GFP-Rab24-transfected cells were starved to induce autophagy the distribution of the protein changed dramatically. GFP-Rab24 localized in large dots, cup-shaped structures and ring-shaped vesicles. Some of these vesicles were labeled with monodansylcadaverine, a specific autophagosome marker. In the presence of vinblastine, an agent that induces the formation of very large autophagic vesicles, GFP-Rab24 accumulated in the large vacuoles that were also labeled by monodansylcadaverine. Furthermore, Rab24 colocalized with LC3, a mammalian homolog of the yeast protein Apg8/Aut7, an essential gene for autophagy. This is the first report indicating that Rab24 localizes on autophagosomes, suggesting that this Rab protein is involved in the autophagic pathway.

Intracellular transport mechanisms of signal transducers

Recent discoveries have revolutionized our conceptions of enzyme-substrate specificity in signal transduction pathways. Protein kinases A and C are localized to discreet subcellular regions, and this localization changes in an isozyme-specific manner upon activation, a process referred to as translocation. The mechanisms for translocation involve interactions of soluble kinases with membrane-bound anchor proteins that recognize individual kinase isoenzymes and their state of activation. Recently, modulation of kinase-anchor protein interactions has been used to specifically regulate, positively or negatively, the activity of C kinase isozymes. Also described in this review is a role for the Rab family of small G proteins in regulating subcellular protein trafficking. The pathophysiological significance of disrupted subcellular protein transport in cell signaling and the potential therapeutic utility of targeted regulation of these events are in the process of being characterized.

Cell-specific abnormal prenylation of Rab proteins in platelets and melanocytes of the gunmetal mouse

The mutant gunmetal mouse exhibits reduced rates of platelet synthesis, abnormalities of platelet alpha and dense granules and hypopigmentation. Several of these features resemble those of human alpha/delta platelet storage pool disease, grey platelet syndrome and Hermansky-Pudlak syndrome. Gunmetal mice have reduced levels of Rab geranylgeranyltransferase (RabGGTase), which adds lipophilic prenyl groups to the carboxyl terminus of Rab proteins. The degree of prenylation and the subcellular distribution of several Rab proteins were evaluated in mutant platelets, melanocytes and other tissues. Significant deficits in prenylation and membrane binding of most Rabs were observed in platelets and melanocytes. In contrast, minimal alterations in Rab prenylation were apparent in several other gunmetal tissues despite the fact that RabGGTase activity was equally diminished in these tissues. The mutant tissue-specific effects are probably due to increased concentrations of Rab proteins in platelets and melanocytes. These experiments show that Rab proteins are differentially sensitive to levels of RabGGTase activity and that normal platelet synthesis, platelet organelle function and normal pigmentation are highly sensitive to the degree of prenylation and membrane association of Rab proteins. Further, the tissue-specific effects of the gunmetal mutation suggest that RabGGTase is a potential target for therapy of thrombocytosis.

Male infertility, impaired spermatogenesis, and azoospermia in mice deficient for the pseudophosphatase Sbf1

Pseudophosphatases display extensive sequence similarities to phosphatases but harbor amino acid alterations in their active-site consensus motifs that render them catalytically inactive. A potential role in substrate trapping or docking has been proposed, but the specific requirements for pseudophosphatases during development and differentiation are unknown. We demonstrate here that Sbf1, a pseudophosphatase of the myotubularin family, is expressed at high levels in seminiferous tubules of the testis, specifically in Sertoli's cells, spermatogonia, and pachytene spermatocytes, but not in postmeiotic round spermatids. Mice that are nullizygous for Sbf1 exhibit male infertility characterized by azoospermia. The onset of the spermatogenic defect occurs in the first wave of spermatogenesis at 17 days after birth during the synchronized progression of pachytene spermatocytes to haploid spermatids. Vacuolation of the Sertoli's cells is the earliest observed phenotype and is followed by reduced formation of spermatids and eventual depletion of the germ cell compartment in older mice. The nullizygous phenotype in conjunction with high-level expression of Sbf1 in premeiotic germ cells and Sertoli's cells is consistent with a crucial role for Sbf1 in transition from diploid to haploid spermatocytes. These studies demonstrate an essential role for a pseudophosphatase and implicate signaling pathways regulated by myotubularin family proteins in spermatogenesis and germ cell differentiation.

Vesicular protein transport

Protein transport and sorting in the secretory and endocytic pathways via vesicles is required for organelle biogenesis, constitutive and regulated secretion and constitutive and regulated endocytosis. It is essential for a multicellular organism and the function of its specialised cell types that the multiple transport and sorting events are highly accurate. They determine the protein and lipid composition of specialised compartments, receptor protein function and membrane homeostasis. This review describes the individual events involved in the process of vesicle mediated protein transport and sorting and summarizes the knowledge about the function of proteins and lipids orchestrating the process.

Expression and allocation of proteins of the exo-endocytotic machinery in U373 glioma cells: similarities to long-term cultured astrocytes

1. Cultured astrocytes cells release a variety of low and high molecular weight messenger substances and express proteins of the exocytotic pathway including synaptic SNARE proteins. For analyzing the molecular mechanisms of astrocytic messenger release, permanent cell lines with astrocytic properties would provide useful tools. 2. We analyzed the potential of the human malignant astrocytoma-derived cell line U373 MG to express proteins involved in regulated exo- and endocytosis. An immunoblot analysis identified the astrocyte marker glial fibrillary acidic protein, microtubule-associated protein 2, the v-SNAREs VAMP I, VAMP II, and cellubrevin and the t-SNAREs syntaxin I, SNAP-23, and SNAP-25. 3. The cells also express the secretory granule protein secretogranin II. Although secretogranin II immunofluorescence reveals larger fluorescence spots, the majority of the SNARE proteins is associated with smaller organelles. The immunofluorescence is distributed throughout the cytoplasm and accumulates at processes and the growing edges of cells. 4. The organellar association of SNARE proteins was confirmed by heterologous expression of recombinant fusion proteins. Following subcellular fractionation organelles of lower buoyant density carried the majority of VAMP 11. Secretogranin II was associated with organelles of high buoyant density containing a small contribution of VAMP II. 5. The results suggest that U373 MG cells have in common a considerable number of properties with long-term cultured astrocytes rather than with cultured oligodendrocytes or neurons. They contain two types of organelles that can be physically separated and may be employed in the differential release of messengers.

Non-conventional trafficking of the cystic fibrosis transmembrane conductance regulator through the early secretory pathway

The mechanism(s) of cystic fibrosis transmembrane conductance regulator (CFTR) trafficking from the endoplasmic reticulum (ER) through the Golgi apparatus, the step impaired in individuals afflicted with the prevalent CFTR-DeltaF508 mutation leading to cystic fibrosis, is largely unknown. Recent morphological observations suggested that CFTR is largely absent from the Golgi in situ (Bannykh, S. I., Bannykh, G. I., Fish, K. N., Moyer, B. D., Riordan, J. R., and Balch, W. E. (2000) Traffic 1, 852-870), raising the possibility of a novel trafficking pathway through the early secretory pathway. We now report that export of CFTR from the ER is regulated by the conventional coat protein complex II (COPII) in all cell types tested. Remarkably, in a cell type-specific manner, processing of CFTR from the core-glycosylated (band B) ER form to the complex-glycosylated (band C) isoform followed a non-conventional pathway that was insensitive to dominant negative Arf1, Rab1a/Rab2 GTPases, or the SNAp REceptor (SNARE) component syntaxin 5, all of which block the conventional trafficking pathway from the ER to the Golgi. Moreover, CFTR transport through the non-conventional pathway was potently blocked by overexpression of the late endosomal target-SNARE syntaxin 13, suggesting that recycling through a late Golgi/endosomal system was a prerequisite for CFTR maturation. We conclude that CFTR transport in the early secretory pathway can involve a novel pathway between the ER and late Golgi/endosomal compartments that may influence developmental expression of CFTR on the cell surface in polarized epithelial cells.

The biology of cortical granules

An egg-that took weeks to months to make in the adult-can be extraordinarily transformed within minutes during its fertilization. This review will focus on the molecular biology of the specialized secretory vesicles of fertilization, the cortical granules. We will discuss their role in the fertilization process, their contents, how they are made, and the molecular mechanisms that regulate their secretion at fertilization. This population of secretory vesicles has inherent interest for our understanding of the fertilization process. In addition, they have import because they enhance our understanding of the basic processes of secretory vesicle construction and regulation, since oocytes across species utilize this vesicle type. Here, we examine diverse animals in a comparative approach to help us understand how these vesicles function throughout phylogeny and to establish conserved themes of function.

The death domain of Rab3 guanine nucleotide exchange protein in GDP/GTP exchange activity in living cells

Rab3 GTPases regulate exocytosis of neurons, endocrine and exocrine cells. In the present paper, we report a system to measure the guanine nucleotide status of Rab3 proteins in living cells. The assay is based on the ability of the Rab3 interacting molecule RIM to extract selectively the GTP-bound form of Rab3. Using this system, we found that approx. 20% of wild-type Rab3A, -B, -C or -D transfected in the insulin-secreting cell line HIT-T15 is in the GTP-bound conformation. The pool of activated Rab3 is decreased under conditions that stimulate exocytosis or by co-expression of the Rab3 GTPase-activating protein. In contrast, co-expression of Mss4 or Rab3-GEP (guanine nucleotide exchange protein) increases by approx. 3-fold the GTP-bound pool of Rab3 isoforms. Rab3-GEP is very similar to MADD, a death domain-containing protein that associates with the type 1 tumour necrosis factor receptor. We observed that the death domain of Rab3-GEP is involved in intramolecular interactions and that deletions or mutations that affect this domain of the protein impair the nucleotide exchange activity towards Rab3. We propose that the death domain of Rab3-GEP acts as a molecular switch and co-ordinates multiple functions of the protein by exchanging its binding partners.

Sly1 binds to Golgi and ER syntaxins via a conserved N-terminal peptide motif

Sec1/munc18-like proteins (SM proteins) and SNARE complexes are probably universally required for membrane fusion. However, the molecular mechanism by which they interact has only been defined for synaptic vesicle fusion where munc18 binds to syntaxin in a closed conformation that is incompatible with SNARE complex assembly. We now show that Sly1, an SM protein involved in Golgi and ER fusion, binds to a short, evolutionarily conserved N-terminal peptide of Sed5p and Ufe1p in yeast and of syntaxins 5 and 18 in vertebrates. In these syntaxins, the Sly1 binding peptide is upstream of a separate, autonomously folded N-terminal domain. These data suggest a potentially general mechanism by which SM proteins could interact with peptides in target proteins independent of core complex assembly and suggest that munc18 binding to syntaxin is an exception.

Glyceraldehyde-3-phosphate dehydrogenase is phosphorylated by protein kinase Ciota /lambda and plays a role in microtubule dynamics in the early secretory pathway

The small GTPase Rab2 immunolocalizes to vesicular tubular clusters (VTCs) that function as transport complexes carrying cargo between the endoplasmic reticulum and the Golgi complex. Our previous studies showed that Rab2 promotes vesicle formation from VTCs and that the released vesicles are enriched in beta-coat protein, protein kinase C iota/lambda (PKCiota/lambda), glyceraldehyde-3-phosphate dehydrogenase (GAPDH), and the recycling protein p53/gp58. Because PKCiota/lambda kinase activity was necessary for vesicle formation, a search was initiated to identify the substrate(s) that potentiate Rab2 function within VTCs. In this study, we found that PKCiota/lambda phosphorylates GAPDH. Moreover, GAPDH interacts directly with the PKCiota/lambda regulatory domain. Based on numerous observations that show (beta-COP) GAPDH associates with cytoskeletal elements, we examined the role of phospho-GAPDH in promoting microtubule (MT) binding to membrane. Using a quantitative microsomal binding assay, we found that membrane association of beta-tubulin was dependent on phospho-GAPDH and was blocked by reagents that interfere with Rab2-dependent GAPDH membrane recruitment or with PKCiota/lambda kinase activity. Furthermore, normal rat kidney cells transfected with a constitutively activated form of Rab2 (Q65L) or with our anti-GAPDH polyclonal antibody displayed a dramatic change in MT organization. These combined results suggest that Rab2 stimulated PKCiota/lambda and GAPDH recruitment to VTCs, and the subsequent PKCiota/lambda phosphorylation of GAPDH ultimately influences MT dynamics in the early secretory pathway.

Envelope glycoprotein cytoplasmic domains from diverse lentiviruses interact with the prenylated Rab acceptor

Lentivirus envelope glycoproteins have unusually long cytoplasmic domains compared to those of other retroviruses. To identify cellular binding partners of the simian immunodeficiency virus (SIV) envelope transmembrane protein (gp41) cytoplasmic domain (CD), we performed a yeast two-hybrid screen of a phytohemagglutinin-activated human T-cell cDNA library with the SIV gp41 CD. The majority of positive clones (50 of 54) encoded the prenylated Rab acceptor (PRA1). PRA1 is a 21-kDa protein associated with Golgi membranes that binds to prenylated Rab proteins in their GTP-bound state. While the cellular function of PRA1 is presently unknown, this protein appears to participate in intracellular vesicular trafficking, based on its cellular localization and ability to bind multiple members of the Rab protein family. Mammalian two-hybrid assays confirmed the interaction between the SIV gp41 CD and PRA1. Furthermore, gp41 sequences important for PRA1 binding were mapped to a central leucine-rich, amphipathic alpha-helix in the SIV gp41 cytoplasmic tail. Although the human immunodeficiency virus (HIV-1) gp41 CD failed to interact with PRA1 in the yeast two-hybrid system, its interaction with PRA1 was significantly better than that of the SIV gp41 CD in mammalian two-hybrid assays. Interestingly, PRA1 also interacted with the Env CDs of HIV-2, bovine immunodeficiency virus, equine infectious anemia virus, and feline immunodeficiency virus. Thus, PRA1 associates with envelope glycoproteins from widely divergent lentiviruses.

Rab3B in human platelet is membrane bound and interacts with Ca(2+)/calmodulin

The subcellular distribution of Rab3B in fresh and aged platelets was determined and majority of the protein was localized with the particulate fraction with only a minor amount detected in the cytosol. Rab3B was pulled out from platelet particulate fraction with GST-RabGDI-alpha fusion protein. Using GST-Rab3B in in vitro pull-down experiments, the binding of calmodulin from platelet cytosol to Rab3B was demonstrated. In the reverse experiment, binding of Rab3B from platelet particulate and cytosolic fractions to Sepharose-CaM beads was also observed. The interaction between Rab3B and calmodulin was Ca(2+)-dependent but independent of the guanine nucleotide status of Rab3B. These findings provide evidence that Rab3B is primarily localized with the particulate fraction and that Ca(2+)/calmodulin could regulate function of this GTPase in the platelet.

Increased myocardial Rab GTPase expression: a consequence and cause of cardiomyopathy

The Ras-like Rab GTPases regulate vesicle transport in endocytosis and exocytosis. We found that cardiac Rabs1, 4, and 6 are upregulated in a dilated cardiomyopathy model overexpressing beta(2)-adrenergic receptors. To determine if increased Rab GTPase expression can contribute to cardiomyopathy, we transgenically overexpressed in mouse hearts prototypical Rab1a, the small G protein that regulates vesicle transport from endoplasmic reticulum to and through Golgi. In multiple independent mouse lines, Rab1a overexpression caused cardiac hypertrophy that progressed in a time- and transgene dose-dependent manner to heart failure. Isolated cardiac myocytes were hypertrophied and exhibited contractile depression with impaired calcium reuptake. Ultrastructural analysis revealed enlarged Golgi stacks and increased transitional vesicles in ventricular myocytes, with increased secretory atrial natriuretic peptide granules and degenerative myelin figures in atrial myocytes; immunogold studies localized Rab1a to these abnormal vesicular structures. A survey of hypertrophy signaling molecules revealed increased protein kinase C (PKC) alpha and delta, and confocal microscopy showed abnormal subcellular distribution of PKCalpha in Rab1a transgenics. These results indicate that increased expression of Rab1 GTPase in myocardium distorts subcellular localization of proteins and is sufficient to cause cardiac hypertrophy and failure.

Identification of rabaptin-5, rabex-5, and GM130 as putative effectors of rab33b, a regulator of retrograde traffic between the Golgi apparatus and ER

The role of rab33b, a Golgi-specific rab protein, was investigated. Microinjection of rab33b mutants stabilised in the GTP-specific state resulted in a marked inhibition of anterograde transport within the Golgi and in the recycling of glycosyltransferases from the Golgi to the ER, respectively. A GST-rab33b fusion protein stabilised in its GTP form was found to interact by Western blotting or mass spectroscopy with Golgi protein GM130 and rabaptin-5 and rabex-5, two rab effector molecules thought to function exclusively in the endocytic pathway. A similar binding was seen to rab1 but not to rab6, both Golgi rabs. In contrast, rab5 was as expected, shown to bind rabaptin-5 and rabex-5 as well as the endosomal effector protein EEA1 but not GM130. No binding of EEA1 was seen to any of the Golgi rabs.

Fragile X mental retardation protein targets G quartet mRNAs important for neuronal function

Loss of fragile X mental retardation protein (FMRP) function causes the fragile X mental retardation syndrome. FMRP harbors three RNA binding domains, associates with polysomes, and is thought to regulate mRNA translation and/or localization, but the RNAs to which it binds are unknown. We have used RNA selection to demonstrate that the FMRP RGG box binds intramolecular G quartets. This data allowed us to identify mRNAs encoding proteins involved in synaptic or developmental neurobiology that harbor FMRP binding elements. The majority of these mRNAs have an altered polysome association in fragile X patient cells. These data demonstrate that G quartets serve as physiologically relevant targets for FMRP and identify mRNAs whose dysregulation may underlie human mental retardation.

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

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

The endocytic pathway: a mosaic of domains

Organelles in the endocytic pathway are composed of a mosaic of structural and functional regions. These regions consist, at least in part, of specialized protein-lipid domains within the plane of the membrane, or of protein complexes associated with specific membrane lipids. Whereas some of these molecular assemblies can be found in more than one compartment, a given combination seems to be unique to each compartment, indicating that membrane organization might be modular.

Role of Rab3 GDP/GTP exchange protein in synaptic vesicle trafficking at the mouse neuromuscular junction

The Rab3 small G protein family consists of four members, Rab3A, -3B, -3C, and -3D. Of these members, Rab3A regulates Ca(2+)-dependent neurotransmitter release. These small G proteins are activated by Rab3 GDP/GTP exchange protein (Rab3 GEP). To determine the function of Rab3 GEP during neurotransmitter release, we have knocked out Rab3 GEP in mice. Rab3 GEP-/- mice developed normally but died immediately after birth. Embryos at E18.5 showed no evoked action potentials of the diaphragm and gastrocnemius muscles in response to electrical stimulation of the phrenic and sciatic nerves, respectively. In contrast, axonal conduction of the spinal cord and the phrenic nerve was not impaired. Total numbers of synaptic vesicles, especially those docked at the presynaptic plasma membrane, were reduced at the neuromuscular junction approximately 10-fold compared with controls, whereas postsynaptic structures and functions appeared normal. Thus, Rab3 GEP is essential for neurotransmitter release and probably for formation and trafficking of the synaptic vesicles.

Ara6, a plant-unique novel type Rab GTPase, functions in the endocytic pathway of Arabidopsis thaliana

Ara6 of Arabidopsis thaliana is a novel member of the Rab/Ypt GTPase family with unique structural features. It resembles Rab5 GTPases best, but lacks a large part of the C-terminal hypervariable region and the cysteine motif, and instead harbors an extra stretch of amino acid residues containing myristoylation and palmitoylation sites at the N-terminus. Ara6 is tightly associated with membranes and is expressed constitutively. In contrast, the conventional Rab5 ortholog, Ara7, is highly expressed only in actively dividing cells. Examination of green fluorescent protein (GFP)-tagged proteins indicates that both Ara6 and Ara7 are distributed on a subpopulation of endosomes and suggests their roles in endosomal fusion. The endosomal localization of Ara6 requires N-terminal fatty acylation, nucleotide binding and the C-terminal amino acid sequence coordinately. Proteins similar to Ara6 are found only in higher plants and thus represent a novel class of Rab GTPases regulating endocytic function in a plant- specific manner.

Direct interaction of the Rab3 effector RIM with Ca2+ channels, SNAP-25, and synaptotagmin

To define the role of the Rab3-interacting molecule RIM in exocytosis we searched for additional binding partners of the protein. We found that the two C(2) domains of RIM display properties analogous to those of the C(2)B domain of synaptotagmin-I. Thus, RIM-C(2)A and RIM-C(2)B bind in a Ca(2+)-independent manner to alpha1B, the pore-forming subunit of N-type Ca(2+) channels (EC(50) = approximately 20 nm). They also weakly interact with the alpha1C but not the alpha1D subunit of L-type Ca(2+) channels. In addition, the C(2) domains of RIM associate with SNAP-25 and synaptotagmin-I. The binding affinities for these two proteins are 203 and 24 nm, respectively, for RIM-C(2)A and 224 and 16 nm for RIM-C(2)B. The interactions of the C(2) domains of RIM with SNAP-25 and synaptotagmin-I are modulated by Ca(2+). Thus, in the presence of Ca(2+) (EC(50) = approximately 75 microm) the interaction with synaptotagmin-I is increased, whereas SNAP-25 binding is reduced. Synaptotagmin-I binding is abolished by mutations in two positively charged amino acids in the C(2) domains of RIM and by the addition of inositol polyphosphates. We propose that the Rab3 effector RIM is a scaffold protein that participates through its multiple binding partners in the docking and fusion of secretory vesicles at the release sites.

Pseudo-phosphatase Sbf1 contains an N-terminal GEF homology domain that modulates its growth regulatory properties

Sbf1 (SET binding factor 1) is a pseudo-phosphatase related to the myotubularin family of dual specificity phosphatases, some of which have been implicated in cellular growth and differentiation by virtue of their mutation in human genetic disorders. Sbf1 contains germline-encoded alterations of its myotubularin homology domain that render it non-functional as a phosphatase. We report here the complete structure of Sbf1 and further characterization of its growth regulatory properties. In addition to its similarity to myotubularin, the predicted full-length Sbf1 protein contains pleckstrin (PH)and GEF homology domains that are conserved in several proteins implicated in signaling and growth control. Forced expression of wild-type Sbf1 in NIH 3T3 cells inhibited their proliferation and altered their morphology. These effects required intact PH, GEF and myotubularin homology domains, implying that growth inhibition may be an intrinsic property of wild-type Sbf1. Conversely, deletion of its conserved N-terminal 44 amino acids alone was sufficient to convert Sbf1 from an inhibitor of cellular growth to a transforming protein in NIH 3T3 cells. Oncogenic forms of Sbf1 partially localized to the nucleus, in contrast to the exclusively cytoplasmic subcellular localization of endogenous Sbf1 in all cell lines and mammalian tissues tested. These data show that the N-terminal GEF homology domain serves to inhibit the transforming effects of Sbf1, possibly sequestering the protein to the cytoplasm, and suggest that this region may be a modulatory domain that relays growth control signals.

Organization of the Rab-GDI/CHM superfamily: the functional basis for choroideremia disease

Choroideremia is an X-chromosome-linked disease that leads to the degeneration of the choriocapillaris, the retinal pigment epithelium and the photoreceptor layer in the eye. The gene product defective in choroideremia, CHM, is identical to Rab escort protein 1 (REP1). CHM/REP1 is an essential component of the catalytic geranylgeranyltransferase II complex (GGTrII) that delivers newly synthesized small GTPases belonging to the RAB gene family to the catalytic complex for post-translational modification. CHM/REP family members are evolutionarily related to members of the guanine nucleotide dissociation inhibitor (GDI) family, proteins involved in the recycling of Rab proteins required for vesicular membrane trafficking through the exocytic and endocytic pathways, forming the GDI/CHM superfamily. Biochemical and structural analyses have now revealed a striking parallel in the organization and function of these two families allowing us to generate a general model for GDI/CHM superfamily function in health and disease.

Array analysis of the genes regulated during neuronal differentiation of human embryonal cells

Recent advances in genetic technology have provided a new platform on which the simultaneous analysis of a large number of genes is possible in a rapid and efficient fashion. To assess the differential expression of human genes during neuronal differentiation, we compared the transcript profiles of undifferentiated, partially differentiated, and fully differentiated NT2/D1 cultures with cDNA expression arrays. Approximately 75 genes (13% of the gene array pool) were differentially expressed during neuronal development of NT2/D1 cells. Genes coding for pyruvate kinase M2 isozyme, clathrin assembly proteins, calmodulin, fibronectin, laminin, thymosin β-10, and many others were upregulated as NT2/D1 cells differentiated into neurons. In contrast, several kinases, phosphatases, and G-protein coupled receptor genes showed downregulation upon neuronal differentiation. The information provided here is an invaluable reference for characterizing the phenotype of these cells. This information can also be used in cell therapy and transplantation in which the graft microenvironment and interaction with the host tissue is crucial.Key words: Atlas cDNA expression arrays, differentiation, neurodevelopment, neuron, NT2/D1 cells.

Carboxyl-methylation of Rab3D in the rat pancreatic acinar tumor cell line AR42J

Rab3D is a small GTPase implicated in regulated exocytosis, and is a marker of secretory granules in exocrine cells. We have previously shown that rab3D undergoes reversible carboxyl-methylation in adult rat pancreatic acinar cells, and that carboxyl-methylation of rab3D is developmentally regulated concomitantly with the maturation of the regulated secretory apparatus in rat pancreas. We also observed that dexamethasone treatment of the rat pancreatic acinar tumor cell line, AR42J, led to a significant increase in the size of the unmethylated pool of a rab3-like protein. The current study was designed to further characterize this rab3-like protein. Here we show that AR42J cells express rab3D, and that the protein focuses on 2D gels as two spots with pI values of 4.9 and 5.0. Treatment of AR42J cells with N-acetyl-S-geranylgeranyl-l-cysteine, an inhibitor of carboxyl-methylation, led to a decrease in the basic form of rab3D and a proportional increase in the acidic form. In contrast, N-acetyl-S-farnesyl-l-cysteine, which inhibits carboxyl-methylation of farnesylated proteins, had no effect. Lovastatin, an inhibitor of geranylgeranylation, also induced an accumulation of the acidic form of rab3D. Taken together, these data indicate that rab3D can undergo reversible carboxyl-methylation in AR42J cells by a geranylgeranyl-specific methyltransferase. The 2D gel and immunoblotting analyses indicated that dexamethasone treatment of AR42J cells led to an increase in the proportion of the unmethylated form of rab3D concurrent to inducing a regulated secretory pathway, similar to the rab3D profile change in developing rat pancreas. Our data, along with previous studies done on developing rat pancreas, indicate that the tumor cell line AR42J represents a good model system for studying the regulated secretory pathway, and that carboxyl-methylation of rab3D may play a role in the acquisition of stimulus-secretion coupling.

Identification and characterization of a family of secretion-related small GTPase-encoding genes from the filamentous fungus Aspergillus niger: a putative SEC4 homologue is not essential for growth

DNA fragments containing genetic information for five secretion-related small GTPases of Aspergillus niger (srgA-E) were isolated and identified as members of different Rab/Ypt subfamilies. This isolation and the search for similar sequences in fungal genomic and EST databases showed that, in contrast to Saccharomyces cerevisiae, filamentous fungi also possess homologues of mammalian Rab2 GTPases. Multiple transcripts with unusually long 5' and 3' untranslated regions were found for all srg genes. Their level of expression was independent of the type of carbon source used for growth. Although the transcripts of srgA and srgB were abundant to the same extent throughout the cultivation, that of the other genes peaked during the early growth phase and then declined. Two genes, srgA and srgB, were characterized further. The protein encoded by srgA exhibited relatively low identity (58%) to its closest S. cerevisiae homologue SEC4, whereas the protein encoded by srgB showed 73% identity with S. cerevisiae YPT1. In contrast to other SEC4 homologues, srgA was unable to complement an S. cerevisiae sec4 mutant, and its disruption was not lethal in A. niger. SrgA mutants displayed a twofold increase in their hyphal diameter, unusual apical branching and strongly reduced protein secretion during growth on glucose.

Membrane trafficking machinery components associated with the mammalian acrosome during spermiogenesis

Active trafficking from the Golgi apparatus is involved in acrosome formation, both by delivering acrosomal contents to the nascent secretory vesicle and by controlling organelle growth and shaping. During murine spermiogenesis, Golgi antigens (giantin, beta-COP, golgin 97, mannosidase II) are detected in the acrosome until the late cap-phase spermatids, but are not found in testicular spermatozoa (maturation-phase spermatids). This suggests that Golgi-acrosome flow may be relatively unselective, with Golgi residents retrieved before spermiation is complete. Treatment of spermatogenic cells with brefeldin A, a drug that causes the Golgi apparatus to collapse into the endoplasmic reticulum, disrupted the Golgi in both pachytene spermatocytes and round spermatids. However, this treatment did not affect the acrosomal granule, and some beta-COP labeling on the acrosome of elongating spermatids was maintained. Additionally, N-ethylmaleimide sensitive factor, soluble NSF attachment proteins, and homologues of the t-SNARE syntaxin and of the v-SNARE VAMP/synaptobrevin, as well as members of the rab family of small GTPases, are associated with the acrosome (but not the acrosomal granule) in round and elongated spermatids. This suggests that rab proteins and the SNARE machinery for membrane recognition/docking/fusion may be involved in trafficking during mammalian acrosome biogenesis.

Vps9, Rabex-5 and DSS4: proteins with weak but distinct nucleotide-exchange activities for Rab proteins

The activities of three Rab-specific factors with GDP/GTP exchange activity, Vps9p, Rabex-5 and DSS4, with their cognate GTPases, Ypt51p, Rab5 and Ypt1p, have been analysed quantitatively. In contrast to other exchange factors examined and to DSS4, Vps9p, and by analogy probably Rabex-5, have considerably lower affinity than GDP to the respective GTPases. In keeping with this, they are relatively weak exchangers, with a maximal rate constant for GDP release from the ternary complex between exchange factor, GTPase and GDP of ca 0.01 s(-1), which is several orders of magnitude lower than for other exchange factors examined. If interaction with these proteins is a mandatory aspect of the Rab cycle, this suggests that the overall rate of cycling might be controlled at this point of the cycle. Surprisingly, DSS4, which has the thermodynamic potential to displace GDP effectively from Ypt1p, also does this very slowly, again with a maximal rate constant of ca 0.01 s(-1). An additional, and based on present knowledge, unique, feature of the Ypt1p.DSS4 complex, is that the association of GTP (or GDP) is more than 10(3)-fold slower than to Ypt1p, thus leading to a long life-time of the binary complex between the two proteins, even at the high nucleotide concentrations that prevail in the cell. This leads to the conclusion that the protein-protein complex is likely to have an important biological significance in addition to its probable role in GTP/GDP exchange.

Membrane targeting of a Rab GTPase that fails to associate with Rab escort protein (REP) or guanine nucleotide dissociation inhibitor (GDI)

The targeting of various Rab proteins to different subcellular compartments appears to be determined by variable amino acid sequences located upstream from geranylgeranylated cysteine residues in the C-terminal tail. All nascent Rab proteins are prenylated by geranylgeranyltransferase II, which recognizes the Rab substrate only when it is bound to Rab escort protein (REP). After prenylation, REP remains associated with the modified Rab until it is delivered to the appropriate subcellular membrane. It remains unclear whether docking of the Rab with the correct membrane is solely a function of features contained within the prenylated Rab itself (with REP serving as a "passive" carrier) or whether REP actively participates in the targeting process. To address this issue, we took advantage of a mutation in the alpha2 helix of Rab1B (i.e. Y78D) that abolishes REP and GDI interaction without disrupting nucleotide binding or hydrolysis. These studies demonstrate that replacing the C-terminal GGCC residues of Rab1B(Y78D) with a CLLL motif permits this protein to be prenylated by geranylgeranyltransferase I but not II both in cell-free enzyme assays and in transfected cells. Subcellular fractionation and immunofluorescence studies reveal that the prenylated Rab1B(Y78D)CLLL, which remains deficient in REP and GDI association is, nonetheless, delivered to the Golgi and endoplasmic reticulum (ER) membranes. When the dominant-negative S22N mutation was inserted into Rab1B-CLLL, the resulting monoprenylated construct suppressed ER --> Golgi protein transport. However, when the Y78D mutation was added to the latter construct, its inhibitory effect on protein trafficking was lost despite the fact that it was localized to the ER/Golgi membrane. Therefore, protein interactions mediated by the alpha2 helical domain of Rab1B(S22N) appear to be essential for its functional interaction with components of the ER --> Golgi transport machinery.