The mammalian Rab family of small GTPases: definition of family and subfamily sequence motifs suggests a mechanism for functional specificity in the Ras superfamily

Rab5-mediated endosome-endosome fusion regulates hemoglobin endocytosis in Leishmania donovani

To understand the trafficking of endocytosed hemoglobin (Hb) in Leishmania, we investigated the characteristics of in vitro fusion between endosomes containing biotinylated Hb (BHb) and avidin-horseradish peroxidase (AHRP). We showed that early endosome fusion in Leishmania is temperature and cytosol dependent and is inhibited by ATP depletion, ATPgammaS, GTPgammaS and N-ethylmaleimide treatment. The Rab5 homolog from Leishmania donovani, LdRab5, was cloned and expressed. Our results showed that homotypic fusion between the early endosomes in Leishmania is Rab5 dependent. Early endosomes containing BHb fused efficiently with late endosomes in a process regulated by Rab7, whereas no fusion between early and late endosomes was detected using fluid phase markers. Pre-treatment of early endosomes containing BHb with monoclonal antibody specific for the C-terminus of the Hb receptor (HbR) or the addition of the C-terminal cytoplasmic fragment of the HbR specifically inhibited the fusion with late endosomes, suggesting that signal(s) mediated through the HbR cytoplasmic tail promotes the fusion of early endosomes containing Hb with late endosomes.

Rab GTPases are recruited to chlamydial inclusions in both a species-dependent and species-independent manner

Chlamydiae are obligate intracellular bacteria that replicate within an inclusion that is trafficked to the peri-Golgi region where it fuses with exocytic vesicles. The host and chlamydial proteins that regulate the trafficking of the inclusion have not been identified. Since Rab GTPases are key regulators of membrane trafficking, we examined the intracellular localization of several green fluorescent protein (GFP)-tagged Rab GTPases in chlamydia-infected HeLa cells. GFP-Rab4 and GFP-Rab11, which function in receptor recycling, and GFP-Rab1, which functions in endoplasmic reticulum (ER)-to-Golgi trafficking, are recruited to Chlamydia trachomatis, Chlamydia muridarum, and Chlamydia pneumoniae inclusions, whereas GFP-Rab5, GFP-Rab7, and GFP-Rab9, markers of early and late endosomes, are not. In contrast, GFP-Rab6, which functions in Golgi-to-ER and endosome-to-Golgi trafficking, is associated with C. trachomatis inclusions but not with C. pneumoniae or C. muridarum inclusions, while the opposite was observed for the Golgi-localized GFP-Rab10. Colocalization studies between transferrin and GFP-Rab11 demonstrate that a portion of GFP-Rab11 that localizes to inclusions does not colocalize with transferrin, which suggests that GFP-Rab11's association with the inclusion is not mediated solely through Rab11's association with transferrin-containing recycling endosomes. Finally, GFP-Rab GTPases remain associated with the inclusion even after disassembly of microtubules, which disperses recycling endosomes and the Golgi apparatus within the cytoplasm, suggesting a specific interaction with the inclusion membrane. Consistent with this, GFP-Rab11 colocalizes with C. trachomatis IncG at the inclusion membrane. Therefore, chlamydiae recruit key regulators of membrane trafficking to the inclusion, which may function to regulate the trafficking or fusogenic properties of the inclusion.

Molecular identification of Rab7 (ApRab7) in Aiptasia pulchella and its exclusion from phagosomes harboring zooxanthellae

The establishment and maintenance of the intracellular association between marine cnidarians and their symbiotic microalgae is essential to the well being of coral reef ecosystems; however, little is known concerning its underlying molecular mechanisms. In light of the critical roles of the small GTPase, Rab7, as a key regulator of vesicular trafficking, we cloned and characterized the Rab7 protein in the endosymbiosis system between the sea anemone, Aiptasia pulchella and its algal symbiont, Symbiodinium spp. The Aiptasia homologue of Rab7 proteins, ApRab7 is 88% identical to human Rab7 protein and contains all Rab-specific signature motifs. Results of EGFP reporter analysis, protein fractionation, and immunocytochemistry support that ApRab7 is located in late endocytic and phagocytic compartments and is able to promote their fusion. Significantly, the majority of phagosomes containing live symbionts that either have taken long residency in, or were newly internalized by Aiptasia digestive cells did not contain detectable levels of ApRab7, while most phagosomes containing either heat-killed or photosynthesis-impaired symbionts were positive for ApRab7 staining. Overall, our data suggest that live algal symbionts persist inside their host cells by actively excluding ApRab7 from their phagosomes, and thereby, establish and/or maintain an endosymbiotic relationship with their cnidarian hosts.

Dual prenylation is required for Rab protein localization and function

The majority of Rab proteins are posttranslationally modified with two geranylgeranyl lipid moieties that enable their stable association with membranes. In this study, we present evidence to demonstrate that there is a specific lipid requirement for Rab protein localization and function. Substitution of different prenyl anchors on Rab GTPases does not lead to correct function. In the case of YPT1 and SEC4, two essential Rab genes in Saccharomyces cerevisiae, alternative lipid tails cannot support life when present as the sole source of YPT1 and SEC4. Furthermore, our data suggest that double geranyl-geranyl groups are required for Rab proteins to correctly localize to their characteristic organelle membrane. We have identified a factor, Yip1p that specifically binds the di-geranylgeranylated Rab and does not interact with mono-prenylated Rab proteins. This is the first demonstration that the double prenylation modification of Rab proteins is an important feature in the function of this small GTPase family and adds specific prenylation to the already known determinants of Rab localization.

COPI recruitment is modulated by a Rab1b-dependent mechanism

The small GTPase Rab1b is essential for endoplasmic reticulum (ER) to Golgi transport, but its exact function remains unclear. We have examined the effects of wild-type and three mutant forms of Rab1b in vivo. We show that the inactive form of Rab1b (the N121I mutant with impaired guanine nucleotide binding) blocks forward transport of cargo and induces Golgi disruption. The phenotype is analogous to that induced by brefeldin A (BFA): it causes resident Golgi proteins to relocate to the ER and induces redistribution of ER-Golgi intermediate compartment proteins to punctate structures. The COPII exit machinery seems to be functional in cells expressing the N121I mutant, but COPI is compromised, as shown by the release of beta-COP into the cytosol. Our results suggest that Rab1b function influences COPI recruitment. In support of this, we show that the disruptive effects of N121I can be reversed by expressing known mediators of COPI recruitment, the GTPase ARF1 and its guanine nucleotide exchange factor GBF1. Further evidence is provided by the finding that cells expressing the active form of Rab1b (the Q67L mutant with impaired GTPase activity) are resistant to BFA. Our data suggest a novel role for Rab1b in ARF1- and GBF1-mediated COPI recruitment pathway.

Membrane targeting of Rab GTPases is influenced by the prenylation motif

Rab GTPases are regulators of membrane traffic. Rabs specifically associate with target membranes via the attachment of (usually) two geranylgeranyl groups in a reaction involving Rab escort protein and Rab geranylgeranyl transferase. In contrast, related GTPases are singly prenylated by CAAX prenyl transferases. We report that di-geranylgeranyl modification is important for targeting of Rab5a and Rab27a to endosomes and melanosomes, respectively. Transient expression of EGFP-Rab5 mutants containing two prenylatable cysteines (CGC, CC, CCQNI, and CCA) in HeLa cells did not affect endosomal targeting or function, whereas mono-cysteine mutants (CSLG, CVLL, or CVIM) were mistargeted to the endoplasmic reticulum (ER) and were nonfunctional. Similarly, Rab27aCVLL mutant is also mistargeted to the ER and transgenic expression on a Rab27a null background (Rab27aash) did not rescue the coat color phenotype, suggesting that Rab27aCVLL is not functional in vivo. CAAX prenyl transferase inhibition and temperature-shift experiments further suggest that Rabs, singly or doubly modified are recruited to membranes via a Rab escort protein/Rab geranylgeranyl transferase-dependent mechanism that is distinct from the insertion of CAAX-containing GTPases. Finally, we show that both singly and doubly modified Rabs are extracted from membranes by RabGDIalpha and propose that the mistargeting of Rabs to the ER results from loss of targeting information.

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.

GAPs galore! A survey of putative Ras superfamily GTPase activating proteins in man and Drosophila

Typical members of the Ras superfamily of small monomeric GTP-binding proteins function as regulators of diverse processes by cycling between biologically active GTP- and inactive GDP-bound conformations. Proteins that control this cycling include guanine nucleotide exchange factors or GEFs, which activate Ras superfamily members by catalyzing GTP for GDP exchange, and GTPase activating proteins or GAPs, which accelerate the low intrinsic GTP hydrolysis rate of typical Ras superfamily members, thus causing their inactivation. Two among the latter class of proteins have been implicated in common genetic disorders associated with an increased cancer risk, neurofibromatosis-1, and tuberous sclerosis. To facilitate genetic analysis, I surveyed Drosophila and human sequence databases for genes predicting proteins related to GAPs for Ras superfamily members. Remarkably, close to 0.5% of genes in both species (173 human and 64 Drosophila genes) predict proteins related to GAPs for Arf, Rab, Ran, Rap, Ras, Rho, and Sar family GTPases. Information on these genes has been entered into a pair of relational databases, which can be used to identify evolutionary conserved proteins that are likely to serve basic biological functions, and which can be updated when definitive information on the coding potential of both genomes becomes available.

The Plasmodium falciparum family of Rab GTPases

Rab GTPases are key regulators of vesicular traffic in eukaryotic cells. Here we sought a global characterization and description of the Plasmodium falciparum family of Rab GTPases. We used a combination of bioinformatic analyses, experimental testing of predictions, structure modelling and phylogenetics. These analyses led to the identification of seven new parasite Rabs. Accordingly we estimate that the P. falciparum family is made up of 11 genes. We show that ten members of this family are transcribed in infected erythrocytes. Concerning the various members of the family, a series of specific as well as global conclusions can be drawn. Rabs predicted to be compartment-specific show different subcellular distributions. This is demonstrated for PfRab1A and PfRab11A, with the generation of specific antisera. The sequence analyses reveal several peculiarities, with possible functional implications. One of the transcribed genes, Pfrab5b, does not encode a classical C-terminus, suggestive of a novel regulatory role for this GTPase. Another, Pfrab5a, previously identified as a rab gene located on chromosome 2, possesses a 30-amino-acid insertion in its GTP-binding domain. Structural considerations suggest that this insertion could represent a novel interaction interface. We used conserved RabF and RabSF motifs to discriminate between specific parasite Rabs, and followed their predicted change in position on the structure of PfRab6, as GTP is hydrolysed to GDP. This allowed us to propose their involvement in potential interaction surfaces, that we extended to human Rab6 and the motifs known to mediate Rabkinesine-6 binding. Finally, we compared the P. falciparum Rab family to those of Saccharomyces cerevisiae and Schizosaccharomyces pombe and found that parasite Rabs segregate into possible functional clads. Such grouping into clads may give clues to parasite Rab function, and may shed light on P. falciparum secretory/endocytic pathways.

Analysis of the small GTPase gene superfamily of Arabidopsis

Small GTP-binding proteins regulate diverse processes in eukaryotic cells such as signal transduction, cell proliferation, cytoskeletal organization, and intracellular membrane trafficking. These proteins function as molecular switches that cycle between "active" and "inactive" states, and this cycle is linked to the binding and hydrolysis of GTP. The Arabidopsis genome contains 93 genes that encode small GTP-binding protein homologs. Phylogenetic analysis of these genes shows that plants contain Rab, Rho, Arf, and Ran GTPases, but no Ras GTPases. We have assembled complete lists of these small GTPases families, as well as accessory proteins that control their activity, and review what is known of the functions of individual members of these families in Arabidopsis. We also discuss the possible roles of these GTPases in relation to their similarity to orthologs with known functions and localizations in yeast and/or animal systems.

Ras-effector interactions: after one decade

Ras effectors have convergently developed a common subdomain in their otherwise unrelated protein body for their interaction with Ras. Structural analysis revealed that the mode of interaction is highly similar for all Ras effectors, but is completely different from that of effectors of other subfamilies of small GTPases. Whereas the molecular mechanism of effector activation is still elusive, detailed knowledge about the thermodynamics and dynamics of the interaction with Ras has accumulated.

Structural determinants of Rab and Rab Escort Protein interaction: Rab family motifs define a conserved binding surface

Rab proteins are a large family of monomeric GTPases with 60 members identified in the human genome. Rab GTPases require an isoprenyl modification to their C-terminus for membrane association and function in the regulation of vesicular trafficking pathways. This reaction is catalysed by Rab geranylgeranyl transferase, which recognises as protein substrate any given Rab in a 1:1 complex with Rab Escort Protein (REP). REP is therefore able to bind many distinct Rab proteins but the molecular basis for this activity is still unclear. We recently identified conserved motifs in Rabs termed RabF motifs, which we proposed to mediate a conserved mode of interaction between Rabs and REPs. Here, we tested this hypothesis. We first used REP1 as a bait in the yeast two-hybrid system and isolated strictly full-length Rabs, suggesting that REP recognises multiple regions within and properly folded Rabs. We introduced point mutations in Rab3a as a model Rab and assessed the ability of the mutants to interact with REP using the yeast two-hybrid system and an in vitro prenylation assay. We identified several residues that affect REP:Rab binding in the RabF1, RabF3, and RabF4 regions (which include parts of the switch I and II regions), but not other RabF regions. These results support the hypothesis that Rabs bind REP via conserved RabF motifs and provide a molecular explanation for the preferential recognition of the GDP-bound conformation of Rab by REP.

High resolution crystal structures of human Rab5a and five mutants with substitutions in the catalytically important phosphate-binding loop

GTPase domain crystal structures of Rab5a wild type and five variants with mutations in the phosphate-binding loop are reported here at resolutions up to 1.5 A. Of particular interest, the A30P mutant was crystallized in complexes with GDP, GDP+AlF(3), and authentic GTP, respectively. The other variant crystals were obtained in complexes with a non-hydrolyzable GTP analog, GppNHp. All structures were solved in the same crystal form, providing an unusual opportunity to compare structures of small GTPases with different catalytic rates. The A30P mutant exhibits dramatically reduced GTPase activity and forms a GTP-bound complex stable enough for crystallographic analysis. Importantly, the A30P structure with bound GDP plus AlF(3) has been solved in the absence of a GTPase-activating protein, and it may resemble that of a transition state intermediate. Conformational changes are observed between the GTP-bound form and the transition state intermediate, mainly in the switch II region containing the catalytic Gln(79) residue and independent of A30P mutation-induced local alterations in the P-loop. The structures suggest an important catalytic role for a P-loop backbone amide group, which is eliminated in the A30P mutant, and support the notion that the transition state of GTPase-mediated GTP hydrolysis is of considerable dissociative character.

The Arabidopsis Rab GTPase family: another enigma variation

The Arabidopsis genome sequence reveals that gene families such as the Rab GTPase family, which encodes key determinants of vesicle-targeting specificity, are considerably more diverse in plants and mammals than in yeast. In mammals, this diversity appears to reflect the complexity of membrane trafficking. Phylogenetic analyses indicate that, despite its large size, the Arabidopsis Rab family lacks most of the Rab subclasses found in mammals. The Arabidopsis Rab family has, however, undergone a distinct 'adaptive radiation' that has given rise to proteins that may perform plant-specific functions.

Slac2-c (synaptotagmin-like protein homologue lacking C2 domains-c), a novel linker protein that interacts with Rab27, myosin Va/VIIa, and actin

Slac2-a (synaptotagmin-like protein (Slp) homologue lacking C2 domains-a)/melanophilin is a melanosome-associated protein that links Rab27A on melanosomes with myosin Va, an actin-based motor protein, and formation of the tripartite protein complex (Rab27A.Slac2-a.myosin Va) has been suggested to regulate melanosome transport (Fukuda, M., Kuroda, T. S., and Mikoshiba, K. (2002) J. Biol. Chem. 277, 12432-12436). Here we report the structure of a novel form of Slac2, named Slac2-c, that is homologous to Slac2-a. Slac2-a and Slac2-c exhibit the same overall structure, consisting of a highly conserved N-terminal Slp homology domain (about 50% identity) and a less conserved C-terminal myosin Va-binding domain (about 20% identity). As with other Slac2 members and the Slp family, the Slp homology domain of Slac2-c was found to interact specifically with the GTP-bound form of Rab27A/B both in vitro and in intact cells, and the C-terminal domain of Slac2-c interacted with myosin Va and myosin VIIa. In addition, we discovered that the most C-terminal conserved region of Slac2-a (amino acids 400-590) and Slac2-c (amino acids 670-856), which is not essential for myosin Va binding, directly binds actin and that expression of these regions in PC12 cells and melanoma cells colocalized with actin filaments at the cell periphery, suggesting a novel role of Slac2-a/c in capture of Rab27-containing organelles in the actin-enriched cell periphery.

Synaptotagmin-like protein (Slp) homology domain 1 of Slac2-a/melanophilin is a critical determinant of GTP-dependent specific binding to Rab27A

The N-terminal synaptotagmin-like protein (Slp) homology domain (SHD) of the Slp and Slac2 families has recently been identified as a specific Rab27A-binding domain (Kuroda, T. S., Fukuda, M., Ariga, H., and Mikoshiba, K. (2002) J. Biol. Chem. 277, 9212-9218; Fukuda, M., Kuroda, T. S., and Mikoshiba, K. (2002) J. Biol. Chem. 277, 12432-12436). The SHD consists of two conserved alpha-helical regions (SHD1 and SHD2) that are often separated by two zinc finger motifs. However, the structural basis of Rab27A recognition by the SHD (i.e. involvement of each region (SHD1, zinc finger motifs, and SHD2) in Rab27A recognition and critical residue(s) for Rab27A/SHD interaction) had never been elucidated. In this study, systematic deletion analysis and Ala-based site-directed mutagenesis showed that SHD1 of Slac2-a/melanophilin alone is both necessary and sufficient for high affinity specific recognition of the GTP-bound form of Rab27A. By contrast, the zinc finger motifs and SHD2 are not an autonomous Rab27A-binding site and seem to be important for stabilization of the structure of the SHD or higher affinity Rab27A binding. In addition, chimeric analysis of Rab3A and Rab27A showed that the specific sequence of the switch II region of Rab27 isoforms (especially Leu-84, Phe-88, and Asp-91 of Rab27A), which is not conserved in the Rab3 or Rab8 isoforms, is essential for recognition by the Slac2-a SHD. Based on these findings, I propose that SHD1 of the Slp and Slac2 families be referred to as RBD27 (Rab-binding domain specific for Rab27 isoforms).

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.

Genetic interactions link ARF1, YPT31/32 and TRS130

A genetic screen for synthetic lethal interactions with arf1(-) identified a recessive mutation in TRS130, one of 10 components in the trafficking protein particle (TRAPP) complex (Sacher et al., 2000). As TRS130 is an essential gene, the synthetic lethal allele (trs130-101) is a novel one that requires ARF1 for viability. This allele was found to exhibit no defects in secretory function, i.e. processing of carboxypeptidase Y or invertase. YPT31 and YPT32 were identified in a subsequent screen as high-copy suppressors of arf1(-)trs130-101. Increasing the gene dosage of YPT31/32 also suppressed lethality resulting from deletion of TRS130 or TRS120 but not three other essential TRAPP subunit-encoding genes. Although unable to suppress defects in several alleles of ARF1, increasing the gene dosage of YPT31/32 suppressed the cold sensitivity of gcs1(-), an Arf GTPase-activating protein (GAP). Thus, these genetic interactions provide initial evidence for linkage of Arf and TRAPP signalling and for Ypt31/32 proteins functioning downstream of both components in the TRAPP complex and of Arf signalling via the Gcs1 Arf GAP.

Rab4 function in membrane recycling from early endosomes depends on a membrane to cytoplasm cycle

The monomeric GTPase rab4 is associated with early endosomes and regulates recycling vesicle formation. Because the function of rab proteins in the biosynthetic pathway does not appear to depend on cycling between membranes and cytosol, we were interested to investigate whether or not this holds true for rab function in the endocytic pathway. We created a chimeric rab4 protein (NHrab4cbvn) in which the carboxyl-terminal prenylation motif was replaced by the transmembrane domain of cellubrevin. The chimeric protein was permanently attached to membranes, properly targeted to early endosomes, and bound guanine nucleotide to the same extent as wild type rab4. However, in transport assays we found that basolaterally endocytosed transferrin was less efficiently transported to the apical cell surface in Madin-Darby canine kidney cells transfected with NHrab4cbvn than in cells expressing wild type rab4. Hence, rab4 function requires ongoing cycles of association and dissociation from early endosomes. This cycle is altered during mitosis when rab4 accumulates in the cytoplasm through phosphorylation by a mitotic kinase. We show here, using a rab4 construct that is permanently hooked onto membranes, that the membrane-bound pool of rab4 is targeted by a mitotic kinase.

Functional redundancy of Rab27 proteins and the pathogenesis of Griscelli syndrome

Griscelli syndrome (GS) patients and the corresponding mouse model ashen exhibit defects mainly in two types of lysosome-related organelles, melanosomes in melanocytes and lytic granules in CTLs. This disease is caused by loss-of-function mutations in RAB27A, which encodes 1 of the 60 known Rab GTPases, critical regulators of vesicular transport. Here we present evidence that Rab27a function can be compensated by a closely related protein, Rab27b. Rab27b is expressed in platelets and other tissues but not in melanocytes or CTLs. Morphological and functional tests in platelets derived from ashen mice are all within normal limits. Both Rab27a and Rab27b are found associated with the limiting membrane of platelet-dense granules and to a lesser degree with alpha-granules. Ubiquitous transgenic expression of Rab27a or Rab27b rescues ashen coat color, and melanocytes derived from transgenic mice exhibit widespread peripheral distribution of melanosomes instead of the perinuclear clumping observed in ashen melanocytes. Finally, transient expression in ashen melanocytes of Rab27a or Rab27b, but not other Rab's, restores peripheral distribution of melanosomes. Our data suggest that Rab27b is functionally redundant with Rab27a and that the pathogenesis of GS is determined by the relative expression of Rab27a and Rab27b in specialized cell types.

Rab27b association with melanosomes: dominant negative mutants disrupt melanosomal movement

The movement of melanosomes from post-Golgi compartments to the periphery of melanocytes is known to be regulated by factors including myosin Va and at least one Rab protein, Rab27a. Mutations in the genes encoding either protein in the mouse result in a hypopigmented phenotype mimicking the human disease Griscelli syndrome. Rab27b and Rab27a share 72% identity and they belong to the same melanocyte/platelet subfamily of Rab proteins. Rab27a orchestrates the transport of melanosomes by recruitment of the actin motor, myosin Va, onto melanosomes. By contrast, the function of Rab27b has remained elusive. In this study, we found that Rab27b mRNA is present in melanocytes and demonstrated the intrinsic GTPase activity of Rab27b protein. We explored the function of Rab27b by overexpression of two dominant negative mutants as well as the wild-type Rab27b in melan-a melanocytes. Green-fluorescent-protein-tagged Rab27b colocalizes with the melanosome marker tyrosinase-related protein 1 and with myosin Va at the cell periphery, whereas Rab27b mutants do not decorate melanosomes, and melanosomes in these mutant transfected cells redistribute from cell periphery to the perinuclear region. Furthermore, transient overexpression of the dominant negative forms of Rab27b caused diminution in both numbers and length of dendrites of melan-a cells. Our results suggest that Rab27b may regulate the outward movement of melanosomes and the formation or maintenance of dendritic extensions in melanocytes.

Predicting the distribution, conservation, and functions of SNAREs and related proteins in fungi

Hyphal tip growth, the hallmark of the fungi, requires highly polarized and localized exocytosis, but how this requirement is met is unknown. Members of conserved protein families called SNAREs and Rabs mediate vesicle trafficking and fusion at virtually every step of the intracellular pathway in all examined eukaryotes. We have searched the available nearly complete fungal genomes, established the presence or absence of members of the SNARE and Rab families in these genomes, and predicted their evolutionary relationships to one another. Comparisons with the extensively studied Saccharomyces cerevisiae indicate that, in general, most of the members of these families (including those involved in mediating exocytosis) are conserved. The presence of exceptional SNAREs and Rabs in some fungi that are not conserved in S. cerevisiae may be indicative of specialized steps that occur in these fungi. The implications of these findings for current tip growth models are discussed.

Intracellular membrane trafficking pathways in bone-resorbing osteoclasts revealed by cloning and subcellular localization studies of small GTP-binding rab proteins

A variety of intracellular membrane trafficking pathways are involved in establishing the polarization of resorbing osteoclasts and regulating bone resorption activities. Small GTP-binding proteins of rab family have been implicated as key regulators of membrane trafficking in mammalian cells. Here we used a RT-PCR-based cloning method and confocal laser scanning microscopy to explore the expression array and subcellular localization of rab proteins in osteoclasts. Rab1B, rab4B, rab5C, rab7, rab9, rab11B, and rab35 were identified from rat osteoclasts in this study. Rab5C may be associated with early endosomes, while rab11B is localized at perinuclear recycling compartments and may function in the ruffled border membrane turnover and osteoclast motility. Interestingly, late endosomal rabs, rab7, and rab9, were found to localize at the ruffled border membrane indicating a late endosomal nature of this specialized plasma membrane domain in resorbing osteoclasts. This also suggests that late endocytotic pathways may play an important role in the secretion of lysosomal enzymes, such as cathepsin K, during bone resorption.

TbRAB18, a developmentally regulated Golgi GTPase from Trypanosoma brucei

The trypanosomal secretory system is broadly similar to that of higher eukaryotes as proteins enter the system via the endoplasmic reticulum and are transported to the Golgi complex for elaboration of glycan chains. Importantly N-glycan processing is stage specific with only the bloodstream form (BSF) processing beyond the oligomannose form. Increased complexity of the BSF Golgi apparatus, as evidenced by morphological studies, may underpin this higher activity, but few trypanosome-specific Golgi proteins have been described that may play a role in this developmental alteration. Here we describe a novel member of the T. brucei Rab family, TbRAB18, which is stage-regulated and highly expressed in the BSF whilst barely detectable in the insect stage. This stage-specific expression suggests the presence of a TbRAB18-dependent transport pathway required for survival in the mammalian host. Furthermore, data indicate that TbRAB18 localises to membranes in close juxtaposition to structures stained with BODIPY-ceramide, a Golgi marker. Wild type TbRAB18, ectopically expressed in insect stage cells colocalises with TbRAB31, and hence is targeted to the Golgi complex, consistent with the location of the endogenous protein in the bloodstream form, whilst GTP and GDP-locked mutant isoforms demonstrate distinct localisations, suggesting that Golgi-targetting of TbRAB18 is nucleotide-state dependent. We also find that ectopic expression of TbRAB18 wild type and mutant isoforms has no detectable effect on the synthetic anteriograde trafficking probe, TbBiPN. Finally, the location, and hence function, of TbRAB18 are distinct from the closest metazoan homologue, murine Rab18; the latter protein is involved in endocytic transport pathways whilst clearly TbRAB18 is not. Our data indicate further complexity in the evolution of small GTPases, and highlight the need for robust functional data prior to assignment of members of complex gene families.

Rab-subfamily-specific regions of Ypt7p are structurally different from other RabGTPases

The GTPase Ypt7p from S. cerevisiae is involved in late endosome-to-vacuole transport and homotypic vacuole fusion. We present crystal structures of the GDP- and GppNHp-bound conformation of Ypt7p solved at 1.35 and 1.6 A resolution, respectively. Despite the similarity of the overall structure to other Ypt/Rab proteins, Ypt7p displays small but significant differences. The Ypt7p-specific residues Tyr33 and Tyr37 cause a difference in the main chain trace of the RabSF2 region and form a characteristic surface epitope. Ypt7p*GppNHp does not display the helix alpha2, characteristic of the Ras-superfamily, but instead possess an extended loop L4/L5. Due to insertions in loops L3 and L7, the neighboring RabSF1 and RabSF4 regions are different in their conformations to those of other Ypt/Rab proteins.

Characterization of novel Rab6-interacting proteins involved in endosome-to-TGN transport

Rab6 GTPase regulates intracellular transport at the level of the Golgi complex. Using the yeast two-hybrid screen, we have isolated two clones that specifically interact with the three isoforms of Rab6 present in mammalian cells (Rab6A, A' and B). The cDNAs encode two proteins of 976 and 1120 amino acids (calculated molecular mass of 112 and 128 kDa, respectively) that we named Rab6IP2A and Rab6IP2B (for Rab6 Interacting Protein 2). The two proteins likely correspond to spliced variants of the same gene. Rab6IP2s have no significant homology with other known proteins, including Rab effectors or partners. They are ubiquitously expressed, mostly cytosolic and found in high molecular mass complexes in brain cytosol. We show that Rab6IP2s can be recruited on Golgi membranes in a Rab6:GTP-dependent manner. The overexpression of any form of Rab6IP2 has no detectable effect on the secretory pathway. In contrast, the retrograde transport of the Shiga toxin B subunit between the plasma membrane and the Golgi complex is partly inhibited in cells overexpressing the Rab6-binding domain of Rab6IP2. Our data suggest that Rab6IP2s is involved in the pathway regulated by Rab6A'.

Toward understanding vesicle traffic and the guard cell model

Vesicular trafficking and membrane fusion are integral to cell growth and development with SNARE proteins, RabGTPases and their associates implicated in membrane fusion and secretion throughout the plant endomembrane system. Although the overall pattern of function is similar to that of animals and yeast, many aspects of endomembrane organization and vesicle trafficking appear unique to plants, for example, the control of cell and vacuolar expansion, asymmetric growth and cell division. However, the dominant membrane trafficking pathways have yet to be defined. Comparative genomics provide important information about vesicle trafficking elements but assigning biological roles based on sequence similarities is extremely difficult. Cellular and genetic approaches are reviewed here that have allowed visualization of vesicle trafficking in plants, including capacitance and dye methods, imaging and marker techniques, protein interactions and reverse genetics. Stomatal guard cells are discussed as cell models for identifying vesicle trafficking pathways and evidence points to a role for vesicle trafficking in stomatal function. For plants generally, kinetic analyses and biochemical studies suggest that several different pools of vesicles, and possibly different mechanisms for delivery, are available for vesicle traffic between endomembrane compartments and the plasma membrane. Characterizing these pathways, their functions and controls provides a major challenge for the future.

Molecular determinants of regulated exocytosis

Regulated exocytosis forms the basis for many intercellular signaling processes, for example, in hormone secretion or neurotransmitter release. During regulated exocytosis, the membrane of a secretory vesicle fuses with the plasma membrane in a tightly controlled reaction that is most often triggered by calcium. Recent advances have allowed major insights into the molecular mechanisms that mediate regulated exocytosis. In the present review, we will briefly discuss two key features of regulated exocytosis that have been particularly well studied recently. First, we will examine the current understanding of the membrane fusion reaction that underlies regulated exocytosis and that is effected by SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptor) and munc18-like proteins similar to other membrane fusion reactions. Second, we will describe the role of the major candidates for the calcium sensors that trigger exocytosis, a protein family called synaptotagmins. Although our understanding of regulated exocytosis is as yet incomplete, the results from the studies of SNAREs, munc18s, and synaptotagmins have provided a molecular anchor for a more complete future description.

Molecular cloning, bacterial expression and properties of Rab31 and Rab32

GTP-binding proteins of the Rab family were cloned from human platelets using RT-PCR. Clones corresponding to two novel Rab proteins, Rab31 and Rab32, and to Rab11A, which had not been detected in platelets previously, were isolated. The coding sequence of Rab31 (GenBank accession no. U59877) corresponded to a 194 amino-acid protein of 21.6 kDa. The Rab32 sequence was extended to 1000 nucleotides including 630 nucleotides of coding sequence (GenBank accession no. U59878) but the 5' coding sequence was only completed later by others (GenBank accession no. U71127). Human Rab32 cDNA encodes a 225 amino-acid protein of 25.0 kDa with the unusual GTP-binding sequence DIAGQE in place of DTAGQE. Northern blots for Rab31 and Rab32 identified 4.4 kb and 1.35 kb mRNA species, respectively, in some human tissues and in human erythroleukemia (HEL) cells. Rabbit polyclonal anti-peptide antibodies to Rab31, Rab32 and Rab11A detected platelet proteins of 22 kDa, 28 kDa and 26 kDa, respectively. Human platelets were highly enriched in Rab11A (0.85 microg x mg of platelet protein(-1)) and contained substantial amounts of Rab32 (0.11 microg x mg protein(-1)). Little Rab31 was present (0.005 microg x mg protein(-1)). All three Rab proteins were found in both granule and membrane fractions from platelets. In rat platelets, the 28-kDa Rab32 was replaced by a 52-kDa immunoreactive protein. Rab31 and Rab32, expressed as glutathione S-transferase (GST)-fusion proteins, did not bind [alpha-(32)P]GTP on nitrocellulose blots but did bind [(35)S]GTP[S] in a Mg(2+)-dependent manner. Binding of [(35)S]GTP[S] was optimal with 5 microm Mg(2+)(free) and was markedly inhibited by higher Mg(2+) concentrations in the case of GST-Rab31 but not GST-Rab32. Both proteins displayed low steady-state GTPase activities, which were not inhibited by mutations (Rab31(Q64L) and Rab32(Q85L)) that abolish the GTPase activities of most low-M(r) GTP-binding proteins.

Rab GTPases, intracellular traffic and disease

Membrane and protein traffic in the secretory and endocytic pathways is mediated by vesicular transport. Recent studies of certain key regulators of vesicular transport, the Rab GTPases, have linked Rab dysfunction to human disease. Mutations in Rab27a result in Griscelli syndrome, caused by defects in melanosome transport in melanocytes and loss of cytotoxic killing activity in Tcells. Other genetic diseases are caused by partial dysfunction of multiple Rab proteins resulting from mutations in general regulators of Rab activity; Rab escort protein-1 (choroideremia), Rab geranylgeranyl transferase (Hermansky-Pudlak syndrome) and Rab GDP dissociation inhibitor-alpha (X-linked mental retardation). In infectious diseases caused by intracellular microorganisms, the function of endocytic Rabs is altered either as part of host defences or as part of survival strategy of the pathogen. The human genome is predicted to contain 60 RAB genes, suggesting that future work could reveal further links between Rab dysfunction and disease.

Evolution of the Rab family of small GTP-binding proteins

Rab proteins are small GTP-binding proteins that form the largest family within the Ras superfamily. Rab proteins regulate vesicular trafficking pathways, behaving as membrane-associated molecular switches. Here, we have identified the complete Rab families in the Caenorhabditis elegans (29 members), Drosophila melanogaster (29), Homo sapiens (60) and Arabidopsis thaliana (57), and we defined criteria for annotation of this protein family in each organism. We studied sequence conservation patterns and observed that the RabF motifs and the RabSF regions previously described in mammalian Rabs are conserved across species. This is consistent with conserved recognition mechanisms by general regulators and specific effectors. We used phylogenetic analysis and other approaches to reconstruct the multiplication of the Rab family and observed that this family shows a strict phylogeny of function as opposed to a phylogeny of species. Furthermore, we observed that Rabs co-segregating in phylogenetic trees show a pattern of similar cellular localisation and/or function. Therefore, animal and fungi Rab proteins can be grouped in "Rab functional groups" according to their segregating patterns in phylogenetic trees. These functional groups reflect similarity of sequence, localisation and/or function, and may also represent shared ancestry. Rab functional groups can help the understanding of the functional evolution of the Rab family in particular and vesicular transport in general, and may be used to predict general functions for novel Rab sequences.

Downregulation of small GTPase Rab7 impairs osteoclast polarization and bone resorption

During skeletal growth and remodeling the mineralized bone matrix is resorbed by osteoclasts through the constant secretion of protons and proteases to the bone surface. This relies on the formation of specialized plasma membrane domains, the sealing zone and the ruffled border, and vectorial transportation of intracellular vesicles in bone-resorbing osteoclasts. Here we show that Rab7, a small GTPase that is associated with late endosomes, is highly expressed and is predominantly localized at the ruffled border in bone-resorbing osteoclasts. The decreased expression of Rab7 in cultured osteoclasts by antisense oligodeoxynucleotides disrupted the polarization of the osteoclasts and the targeting of vesicles to the ruffled border. These impairments caused a significant inhibition of bone resorption in vitro. The results indicate that the late endocytotic pathway is involved in the osteoclast polarization and bone resorption and underscore the importance of Rab7 in osteoclast function.

The melanosome: membrane dynamics in black and white

Melanosomes are morphologically and functionally unique organelles within which melanin pigments are synthesized and stored. Melanosomes share some characteristics with lysosomes, but can be distinguished from them in many ways. The biogenesis and intracellular movement of melanosomes and related organelles are disrupted in several genetic disorders in mice and humans. The recent characterization of genes defective in these diseases has reinvigorated interest in the melanosome as a model system for understanding the molecular mechanisms that underlie intracellular membrane dynamics.

Ypt/rab gtpases: regulators of protein trafficking

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

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.

Ypt and Rab GTPases: insight into functions through novel interactions

Ypt/Rab GTPases are key regulators of vesicular transport in eukaryotic cells. During the past two years, a number of new Ypt/Rab-interacting proteins have been identified and shown to serve as either upstream regulators or downstream effectors. Proteins that interact with these regulators and effectors of Ypt/Rabs have also been identified, and together they provide new insights into Ypt/Rab mechanisms of action. The picture that emerges from these studies suggests that Ypt/Rabs function in multiple and diverse aspects of vesicular transport. In addition, not only are Ypt/Rabs highly conserved, but their functions and interactions are as well. Interestingly, crosstalk among Ypt/Rabs and between Ypt/Rabs and other signaling factors, suggest the possibility of coordination of the individual vesicular transport steps and of the protein transport machinery with other cellular processes.

Rab23 is an essential negative regulator of the mouse Sonic hedgehog signalling pathway

The mouse open brain (opb) and Sonic hedgehog (Shh) genes have opposing roles in neural patterning: opb is required for dorsal cell types and Shh is required for ventral cell types in the spinal cord. Here we show that opb acts downstream of Shh. Ventral cell types that are absent in Shh mutants, including the floor plate, are present in Shh opb double mutants. The organization of ventral cell types in Shh opb double mutants reveals that Shh-independent mechanisms can pattern the neural tube along its dorsal-ventral axis. We cloned opb by a map-based approach and found that it encodes Rab23, a member of the Rab family of vesicle transport proteins. The data indicate that dorsalizing signals activate transcription of Rab23 in order to silence the Shh pathway in dorsal neural cells.

Prenylation of Rab GTPases: molecular mechanisms and involvement in genetic disease

Small GTPases of the Rab family regulate membrane transport pathways. More than 50 mammalian Rab proteins are known, many with transport step-specific localisation. Rabs must associate with cellular membranes for activity and membrane attachment is mediated by prenyl (geranylgeranyl) post-translational modification. Mutations in genes encoding proteins essential for the geranylgeranylation reaction, Rab escort protein and Rab geranylgeranyl transferase, underlie genetic diseases. Choroideremia patients have loss of function mutations in REP1 and the murine Hermansky-Pudlak syndrome model gunmetal possesses a splice-site mutation in the alpha-subunit of RGGT. Here we discuss recent insights into Rab prenylation and advances towards our understanding of both diseases.

Expression of mammalian geranylgeranyltransferase type-II in Escherichia coli and its application for in vitro prenylation of Rab proteins

Mammalian geranylgeranyltransferase type II (GGTase-II) is a 100-kDa heterodimer that catalyzes the transfer of two 20-carbon geranylgeranyl groups from geranylgeranyl pyrophosphate onto C-terminal cysteine residues of Rab GTPases. This modification is essential for the biological activity of Rab proteins. Geranylgeranylation can be performed in vitro using recombinant GGTase-II but so far large-scale production of the enzyme was challenging. We report here the design of a two plasmid expression system that will produce GGTase-II at levels as high as 15 mg/L in Escherichia coli. The protein was produced as a heterodimer with the alpha subunit bearing a cleavable tandem 6His-glutathione S-transferase (GST) tag that was used for two-step purification of the enzyme. Purified enzyme was functionally active as determined by in vitro prenylation and phosphoisoprenoid binding assay. Furthermore, the GST-tagged GGTase-II was used for preparative in vitro prenylation of the Rab7:REP-1 complex. Using this procedure, 10 mg of doubly prenylated Rab7:REP-1 complex were obtained.

Normal and abnormal secretion by haemopoietic cells

The secretory lysosomes found in haemopoietic cells provide a very efficient mechanism for delivering the effector proteins of many immune cells in response to antigen recognition. Although secretion shows some similarities to the secretion of specialized granules in other secretory cell types, some aspects of secretory lysosome release appear to be unique to melanocytes and cells of the haemopoietic lineage. Mast cells and platelets have provided excellent models for studying secretion, but recent advances in characterizing the immunological synapse allow a very fine dissection of the secretory process in T lymphocytes. These studies show that secretory lysosomes are secreted from the centre of the talin ring at the synapse. Proper secretion requires a series of Rab and cytoskeletal elements which play critical roles in the specialized secretion of lysosomes in haemopoietic cells.

Rab1 interaction with a GM130 effector complex regulates COPII vesicle cis--Golgi tethering

Members of the Rab family of small molecular weight GTPases regulate the fusion of transport intermediates to target membranes along the biosynthetic and endocytic pathways. We recently demonstrated that Rab1 recruitment of the tethering factor p115 into a cis-SNARE complex programs coat protein II vesicles budding from the endoplasmic reticulum (donor compartment) for fusion with the Golgi apparatus (acceptor compartment) (Allan BB, Moyer BD, Balch WE. Science 2000; 289: 444-448). However, the molecular mechanism(s) of Rab regulation of Golgi acceptor compartment function in endoplasmic reticulum to Golgi transport are unknown. Here, we demonstrate that the cis-Golgi tethering protein GM130, complexed with GRASP65 and other proteins, forms a novel Rab1 effector complex that interacts with activated Rab1-GTP in a p115-independent manner and is required for coat protein II vesicle targeting/fusion with the cis-Golgi. We propose a 'homing hypothesis' in which the same Rab interacts with distinct tethering factors at donor and acceptor membranes to program heterotypic membrane fusion events between transport intermediates and their target compartments.

Traffic control: Rab GTPases and the regulation of interorganellar transport

Membrane proteins, membrane lipids, and luminal contents are exchanged among the intracellular compartments of eukaryotic cells by vesicular transport. This process must be highly ordered to maintain cellular architecture in the face of rapid membrane turnover. The Ras-related Rab GTPases play multiple roles in regulating this traffic.

Rab27a regulates the peripheral distribution of melanosomes in melanocytes

Rab GTPases are regulators of intracellular membrane traffic. We report a possible function of Rab27a, a protein implicated in several diseases, including Griscelli syndrome, choroideremia, and the Hermansky-Pudlak syndrome mouse model, gunmetal. We studied endogenous Rab27a and overexpressed enhanced GFP-Rab27a fusion protein in several cultured melanocyte and melanoma-derived cell lines. In pigmented cells, we observed that Rab27a decorates melanosomes, whereas in nonpigmented cells Rab27a colocalizes with melanosome-resident proteins. When dominant interfering Rab27a mutants were expressed in pigmented cells, we observed a redistribution of pigment granules with perinuclear clustering. This phenotype is similar to that observed by others in melanocytes derived from the ashen and dilute mutant mice, which bear mutations in the Rab27a and MyoVa loci, respectively. We also found that myosinVa coimmunoprecipitates with Rab27a in extracts from melanocytes and that both Rab27a and myosinVa colocalize on the cytoplasmic face of peripheral melanosomes in wild-type melanocytes. However, the amount of myosinVa in melanosomes from Rab27a-deficient ashen melanocytes is greatly reduced. These results, together with recent data implicating myosinVa in the peripheral capture of melanosomes, suggest that Rab27a is necessary for the recruitment of myosinVa, so allowing the peripheral retention of melanosomes in melanocytes.

Chromosomal mapping, gene structure and characterization of the human and murine RAB27B gene

BACKGROUND

Rab GTPases are regulators of intracellular membrane traffic. The Rab27 subfamily consists of Rab27a and Rab27b. Rab27a has been recently implicated in Griscelli Disease, a disease combining partial albinism with severe immunodeficiency. Rab27a plays a key role in the function of lysosomal-like organelles such as melanosomes in melanocytes and lytic granules in cytotoxic T lymphocytes. Little is known about Rab27b.

RESULTS

The human RAB27B gene is organised in six exons, spanning about 69 kb in the chromosome 18q21.1 region. Exon 1 is non-coding and is separated from the others by 49 kb of DNA and exon 6 contains a long 3' untranslated sequence (6.4 kb). The mouse Rab27b cDNA shows 95% identity with the human cDNA at the protein level and maps to mouse chromosome 18. The mouse mRNA was detected in stomach, large intestine, spleen and eye by RT-PCR, and in heart, brain, spleen and kidney by Northern blot. Transient over-expression of EGF-Rab27b fusion protein in cultured melanocytes revealed that Rab27b is associated with melanosomes, as observed for EGF-Rab27a.

CONCLUSIONS

Our results indicate that the Rab27 subfamily of Ras-like GTPases is highly conserved in mammals. There is high degree of conservation in sequence and gene structure between RAB27A and RAB27B genes. Exogenous expression of Rab27b in melanocytes results in melanosomal association as observed for Rab27a, suggesting the two Rab27 proteins are functional homologues. As with RAB27A in Griscelli Disease, RAB27B may be also associated with human disease mapping to chromosome 18.

The Rab GTPase family

SUMMARY

The Rab family is part of the Ras superfamily of small GTPases. There are at least 60 Rab genes in the human genome, and a number of Rab GTPases are conserved from yeast to humans. The different Rab GTPases are localized to the cytosolic face of specific intracellular membranes, where they function as regulators of distinct steps in membrane traffic pathways. In the GTP-bound form, the Rab GTPases recruit specific sets of effector proteins onto membranes. Through their effectors, Rab GTPases regulate vesicle formation, actin- and tubulin-dependent vesicle movement, and membrane fusion.