Interaction of a Golgi-associated kinesin-like protein with Rab6

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.

The small GTPase Rab13 regulates assembly of functional tight junctions in epithelial cells

Junctional complexes such as tight junctions (TJ) and adherens junctions are required for maintaining cell surface asymmetry and polarized transport in epithelial cells. We have shown that Rab13 is recruited to junctional complexes from a cytosolic pool after cell-cell contact formation. In this study, we investigate the role of Rab13 in modulating TJ structure and functions in epithelial MDCK cells. We generate stable MDCK cell lines expressing inactive (T22N mutant) and constitutively active (Q67L mutant) Rab13 as GFP-Rab13 chimeras. Expression of GFP-Rab13Q67L delayed the formation of electrically tight epithelial monolayers as monitored by transepithelial electrical resistance (TER) and induced the leakage of small nonionic tracers from the apical domain. It also disrupted the TJ fence diffusion barrier. Freeze-fracture EM analysis revealed that tight junctional structures did not form a continuous belt but rather a discontinuous series of stranded clusters. Immunofluorescence studies showed that the expression of Rab13Q67L delayed the localization of the TJ transmembrane protein, claudin1, at the cell surface. In contrast, the inactive Rab13T22N mutant did not disrupt TJ functions, TJ strand architecture nor claudin1 localization. Our data revealed that Rab13 plays an important role in regulating both the structure and function of tight junctions.

Rab27a is an essential component of melanosome receptor for myosin Va

Melanocytes that lack the GTPase Rab27a (ashen) are disabled in myosin Va-dependent melanosome capture because the association of the myosin with the melanosome surface depends on the presence of this resident melanosomal membrane protein. One interpretation of these observations is that Rab27a functions wholly or in part as the melanosome receptor for myosin Va (Myo5a). Herein, we show that the ability of the myosin Va tail domain to localize to the melanosome and generate a myosin Va null (dilute) phenotype in wild-type melanocytes is absolutely dependent on the presence of exon F, one of two alternatively spliced exons present in the tail of the melanocyte-spliced isoform of myosin Va but not the brain-spliced isoform. Exon D, the other melanocyte-specific tail exon, is not required. Similarly, the ability of full-length myosin Va to colocalize with melanosomes and to rescue their distribution in dilute melanocytes requires exon F but not exon D. These results predict that an interaction between myosin Va and Rab27a should be exon F dependent. Consistent with this, Rab27a present in detergent lysates of melanocytes binds to beads coated with purified, full-length melanocyte myosin Va and melanocyte myosin Va lacking exon D, but not to beads coated with melanocyte myosin Va lacking exon F or brain myosin Va. Moreover, the preparation of melanocyte lysates in the presence of GDP rather than guanosine-5'-O-(3-thio)triphosphate reduces the amount of Rab27a bound to melanocyte myosin Va-coated beads by approximately fourfold. Finally, pure Rab27a does not bind to myosin Va-coated beads, suggesting that these two proteins interact indirectly. Together, these results argue that Rab27a is an essential component of a protein complex that serves as the melanosome receptor for myosin Va, suggest that this complex contains at least one additional protein capable of bridging the indirect interaction between Rab27a and myosin Va, and imply that the recruitment of myosin Va to the melanosome surface in vivo should be regulated by factors controlling the nucleotide state of Rab27a.

Rab coupling protein (RCP), a novel Rab4 and Rab11 effector protein

Rab4 and Rab11 are small GTPases belonging to the Ras superfamily. They both function as regulators along the receptor recycling pathway. We have identified a novel 80-kDa protein that interacts specifically with the GTP-bound conformation of Rab4, and subsequent work has shown that it also interacts strongly with Rab11. We name this protein Rab coupling protein (RCP). RCP is predominantly membrane-bound and is expressed in all cell lines and tissues tested. It colocalizes with early endosomal markers including Rab4 and Rab11 as well as with the transferrin receptor. Overexpression of the carboxyl-terminal region of RCP, which contains the Rab4- and Rab11-interacting domain, results in a dramatic tubulation of the transferrin compartment. Furthermore, expression of this mutant causes a significant reduction in endosomal recycling without affecting ligand uptake or degradation in quantitative assays. RCP is a homologue of Rip11 and therefore belongs to the recently described Rab11-FIP family.

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'.

Identification of an organelle receptor for myosin-Va

Little is known about how molecular motors bind to their vesicular cargo. Here we show that myosin-Va, an actin-based vesicle motor, binds to one of its cargoes, the melanosome, by interacting with a receptor-protein complex containing Rab27a and melanophilin, a postulated Rab27a effector. Rab27a binds to the melanosome first and then recruits melanophilin, which in turn recruits myosin-Va. Melanophilin creates this link by binding to Rab27a in a GTP-dependent fashion through its amino terminus, and to myosin-Va through its carboxy terminus. Moreover, this latter interaction, similar to the ability of myosin-Va to colocalize with melanosomes and influence their distribution in vivo, is absolutely dependent on the presence of exon-F, an alternatively spliced exon in the myosin-Va tail. These results provide the first molecular description of an organelle receptor for an actin-based motor, illustrate how alternate exon usage can be used to specify cargo, and further expand the functional repertoire of Rab GTPases and their effectors.

subito encodes a kinesin-like protein required for meiotic spindle pole formation in Drosophila melanogaster

The female meiotic spindle lacks a centrosome or microtubule-organizing center in many organisms. During cell division, these spindles are organized by the chromosomes and microtubule-associated proteins. Previous studies in Drosophila melanogaster implicated at least one kinesin motor protein, NCD, in tapering the microtubules into a bipolar spindle. We have identified a second Drosophila kinesin-like protein, SUB, that is required for meiotic spindle function. At meiosis I in males and females, sub mutations affect only the segregation of homologous chromosomes. In female meiosis, sub mutations have a similar phenotype to ncd; even though chromosomes are joined by chiasmata they fail to segregate at meiosis I. Cytological analyses have revealed that sub is required for bipolar spindle formation. In sub mutations, we observed spindles that were unipolar, multipolar, or frayed with no defined poles. On the basis of these phenotypes and the observation that sub mutations genetically interact with ncd, we propose that SUB is one member of a group of microtubule-associated proteins required for bipolar spindle assembly in the absence of the centrosomes. sub is also required for the early embryonic divisions but is otherwise dispensable for most mitotic divisions.

CHO1, a mammalian kinesin-like protein, interacts with F-actin and is involved in the terminal phase of cytokinesis

CHO1 is a kinesin-like protein of the mitotic kinesin-like protein (MKLP)1 subfamily present in central spindles and midbodies in mammalian cells. It is different from other subfamily members in that it contains an extra approximately 300 bp in the COOH-terminal tail. Analysis of the chicken genomic sequence showed that heterogeneity is derived from alternative splicing, and exon 18 is expressed in only the CHO1 isoform. CHO1 and its truncated isoform MKLP1 are coexpressed in a single cell. Surprisingly, the sequence encoded by exon 18 possesses a capability to interact with F-actin, suggesting that CHO1 can associate with both microtubule and actin cytoskeletons. Microinjection of exon 18-specific antibodies did not result in any inhibitory effects on karyokinesis and early stages of cytokinesis. However, almost completely separated daughter cells became reunited to form a binulceate cell, suggesting that the exon 18 protein may not have a role in the formation and ingression of the contractile ring in the cortex. Rather, it might be involved directly or indirectly in the membrane events necessary for completion of the terminal phase of cytokinesis.

The small GTPase Rab22 interacts with EEA1 and controls endosomal membrane trafficking

Rab22a is a small GTPase that is expressed ubiquitously in mammalian tissues and displays the highest sequence homology to Rab5. In BHK-21 cells,overexpression of the wild-type Rab22a caused formation of abnormally large vacuole-like structures containing the early-endosomal antigen EEA1 but not Rab11, a marker of recycling endosomes or the late-endosomal/lysosomal markers LAMP-1 and lyso-bis-phosphatidic acid. In HeLa cells, overexpressed Rab22a was found on smaller EEA1-positive endosomes, but a portion of the protein was also found in the Golgi complex. Using the yeast two-hybrid system and a biochemical pull-down assay, the GTP-bound form of Rab22a was found to interact with the N-terminus of EEA1. In HeLa cells overexpressing Rab22a or its mutants affected in the GTPase cycle, no significant changes were observed in the uptake of Alexa-transferrin. However, the GTPase-deficient Rab22a Q64L mutant caused a redistribution of transferrin-positive endosomes to the leading edges of cells and a fragmentation of the Golgi complex. In BHK cells,the Q64L mutant caused the accumulation of a fluid phase marker,TRITC-dextran, and a lysosomal hydrolase, aspartylglucosaminidase, in abnormal vacuole-like structures that contained both early and late endosome markers. Both the wild-type Rab22a and the Q64L mutant were found to interfere with the degradation of EGF. These results suggest that Rab22a may regulate the dynamic interactions of endosomal compartments and it may be involved in the communication between the biosynthetic and early endocytic pathways.

Early/recycling endosomes-to-TGN transport involves two SNARE complexes and a Rab6 isoform

The molecular mechanisms underlying early/recycling endosomes-to-TGN transport are still not understood. We identified interactions between the TGN-localized putative t-SNAREs syntaxin 6, syntaxin 16, and Vti1a, and two early/recycling endosomal v-SNAREs, VAMP3/cellubrevin, and VAMP4. Using a novel permeabilized cell system, these proteins were functionally implicated in the post-Golgi retrograde transport step. The function of Rab6a' was also required, whereas its closely related isoform, Rab6a, has previously been implicated in Golgi-to-endoplasmic reticulum transport. Thus, our study shows that membrane exchange between the early endocytic and the biosynthetic/secretory pathways involves specific components of the Rab and SNARE machinery, and suggests that retrograde transport between early/recycling endosomes and the endoplasmic reticulum is critically dependent on the sequential action of two members of the Rab6 subfamily.

Rabs grab motors: defining the connections between Rab GTPases and motor proteins

Rab GTPases and their effectors regulate membrane traffic by determining, along with cognate SNAREs, the specificity of transport vesicle docking and fusion steps. Recent studies have also implicated Rabs in the movement of these transport vesicles from their site of formation to their site of fusion, and several Rabs have been linked to specific microtubule- or actin-based motor proteins. Analyses of Rab and motor protein mutants, coupled with advanced imaging techniques, have led to the suggestion that certain Rabs function as essential components of the vesicle receptor for specific motor proteins.

Direct interaction between Rab3b and the polymeric immunoglobulin receptor controls ligand-stimulated transcytosis in epithelial cells

We have examined the role of rab3b in epithelial cells. In MDCK cells, rab3b localizes to vesicular structures containing the polymeric immunoglobulin receptor (pIgR) and located subjacent to the apical surface. We found that GTP-bound rab3b directly interacts with the cytoplasmic domain of pIgR. Binding of dIgA to pIgR causes a dissociation of the interaction with rab3b, a process that requires dIgA-mediated signaling, Arg657 in the cytoplasmic domain of pIgR, and possibly GTP hydrolysis by rab3b. Binding of dIgA to pIgR at the basolateral surface stimulates subsequent transcytosis to the apical surface. Overexpression of GTP-locked rab3b inhibits dIgA-stimulated transcytosis. Together, our data demonstrate that a rab protein can bind directly to a specific cargo protein and thereby control its trafficking.

Molecular motor KIF1C is not essential for mouse survival and motor-dependent retrograde Golgi apparatus-to-endoplasmic reticulum transport

KIF1C is a new member of the kinesin superfamily of proteins (KIFs), which act as microtubule-based molecular motors involved in intracellular transport. We cloned full-length mouse kif1C cDNA, which turned out to have a high homology to a mitochondrial motor KIF1Balpha and to be expressed ubiquitously. To investigate the in vivo significance of KIF1C, we generated kif1C(-/-) mice by knocking in the beta-galactosidase gene into the motor domain of kif1C gene. On staining of LacZ, we detected its expression in the heart, liver, hippocampus, and cerebellum. Unexpectedly, kif1C(-/-) mice were viable and showed no obvious abnormalities. Because immunocytochemistry showed partial colocalization of KIF1C with the Golgi marker protein, we compared the organelle distribution in primary lung fibroblasts from kif1C(+/+) and kif1C(-/-) mice. We found that there was no significant difference in the distribution of the Golgi apparatus or in the transport from the Golgi apparatus to the endoplasmic reticulum (ER) facilitated by brefeldin A between the two cells. This retrograde membrane transport was further confirmed to be normal by time-lapse analysis. Consequently, KIF1C is dispensable for the motor-dependent retrograde transport from the Golgi apparatus to the ER.

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.

Animal cell cytokinesis

Cytokinesis creates two daughter cells endowed with a complete set of chromosomes and cytoplasmic organelles. This conceptually simple event is mediated by a complex and dynamic interplay between the microtubules of the mitotic spindle, the actomyosin cytoskeleton, and membrane fusion events. For many decades the study of cytokinesis was driven by morphological studies on specimens amenable to physical manipulation. The studies led to great insights into the cellular structures that orchestrate cell division, but the underlying molecular machinery was largely unknown. Molecular and genetic approaches have now allowed the initial steps in the development of a molecular understanding of this fundamental event in the life of a cell. This review provides an overview of the literature on cytokinesis with a particular emphasis on the molecular pathways involved in the division of animal cells.

Motor-cargo interactions: the key to transport specificity

Eukaryotic cells organize their cytoplasm by moving different organelles and macromolecular complexes along microtubules and actin filaments. These movements are powered by numerous motor proteins that must recognize their respective cargoes in order to function. Recently, several proteins that interact with motors have been identified by yeast two-hybrid and biochemical analyses, and their roles in transport are now being elucidated. In several cases, analysis of the binding partners helped to identify new transport pathways, new types of cargo, and transport regulated at the level of motor-cargo binding. We discuss here how different motors of the kinesin, dynein and myosin families recognize their cargo and how motor-cargo interactions are regulated.

Central spindle assembly and cytokinesis require a kinesin-like protein/RhoGAP complex with microtubule bundling activity

A late step in cytokinesis requires the central spindle, which forms during anaphase by the bundling of antiparallel nonkinetochore microtubules. Microtubule bundling and completion of cytokinesis require ZEN-4/CeMKLP-1, a kinesin-like protein, and CYK-4, which contains a RhoGAP domain. We show that CYK-4 and ZEN-4 exist in a complex in vivo that can be reconstituted in vitro. The N terminus of CYK-4 binds the central region of ZEN-4, including the neck linker. Genetic suppression data prove the functional significance of this interaction. An analogous complex, containing equimolar amounts of a CYK-4 ortholog and MKLP-1, was purified from mammalian cells. Biochemical studies indicate that this complex, named centralspindlin, is a heterotetramer. Centralspindlin, but not its individual components, strongly promotes microtubule bundling in vitro.

Identification and characterization of nine novel human small GTPases showing variable expressions in liver cancer tissues

Digestion and detoxification are the two most important functions of the liver, and liver cells always keep a high metabolism level and active vesicular traffic. The malfunction of the vesicular traffic system might be a cause of the abnormal biological behavior of cancerous liver cells. The Ras superfamily is known to regulate various steps of vesicular traffic in eukaryotic cells. It would be significant to determine the change of vesicular transport molecules such as the members of Ras superfamily in carcinogenesis of liver cells. In the present study, we have cloned nine novel genes encoding human small GTPases: RAB1B, RAB4B, RAB10, RAB22A, RAB24, RAB25 ARL5, SARA1, and SARA2, among which the former six belong to the RAB family and the latter three belong to the ARF/SAR1 family. The identification of these new genes has greatly enlarged the pool of the Ras superfamily. It is interesting to find that they are upregulated in most of the 11 hepatocellular carcinoma and 1 cholangiohepatoma cases. Furthermore, the expression in 16 normal human adult tissues, the chromosome loci, and the gene structures of the nine genes are also described. The above findings could be valuable for understanding the vesicular transport system and elucidating the molecular basis of liver cancer carcinogenesis.

The molecular mechanism of targeted vesicle transport in cytokinesis

Recent studies have demonstrated that vesicle transport to cleavage furrow is indispensable for cytokinesis. Some animal and plant cells form distinct structures during cell division known as central spindle and phragmoplast, respectively. Several essential factors involved in the vesicle transport have been isolated so far. SNARE proteins and molecular motors play a central role in this process. For future research of cytokinesis, it is important to investigate these factors as well as cytoskeletal components of the contractile ring in detail. This review focuses on the molecular mechanism of targeted vesicle transport in cytokinesis.

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.

IgE receptor type I-dependent regulation of a Rab3D-associated kinase: a possible link in the calcium-dependent assembly of SNARE complexes

Following activation through high affinity IgE receptors (FcepsilonRI), mast cells release, within a few minutes, their granule content of inflammatory and allergic mediators. FcepsilonRI-induced degranulation is a SNARE (soluble N-ethylmaleimide attachment protein receptors)-dependent fusion process. It is regulated by Rab3D, a subfamily member of Rab GTPases. Evidence exists showing that Rab3 action is calcium-regulated although the molecular mechanisms remain unclear. To obtain an understanding of Rab3D function we have searched for Rab3D-associated effectors that respond to allergic triggering through FcepsilonRI. Using the RBL-2H3 mast cell line we detected a Ser/Thr kinase activity, termed here Rak3D (from Rab3D-associated kinase), because it was specifically co-immunoprecipitated with anti-Rab3D antibody. Rak3D activity, as measured by its auto- or transphosphorylation, was maximal in resting cells and decreased upon stimulation. The down-regulation of the observed activity was blocked with EGTA, but not with other degranulation inhibitors, suggesting that its activity functions downstream of calcium influx. We found that Rak3D phosphorylates the NH(2)-terminal regulatory domain of the t-SNARE syntaxin 4, but not syntaxin 2 or 3. The phosphorylation of syntaxin 4 decreased its binding to its partner SNAP23. Thus, we propose a novel phosphorylation-dependent mechanism by which Rab3D controls SNARE assembly in a calcium-dependent manner.

Insulin can regulate GLUT4 internalization by signaling to Rab5 and the motor protein dynein

Insulin stimulates glucose transport by promoting translocation of the insulin-sensitive glucose transporter isoform 4 (GLUT4) from an intracellular compartment to the cell surface. This movement is accomplished by stimulation of GLUT4 exocytosis as well as inhibition of endocytosis. However, the molecular mechanisms for these effects remain unclear. In this study, we found that the GTP-binding protein Rab5 physically associated with the motor protein dynein in immunoprecipitants from both untransfected cells and cells transfected with GFP-Rab5 constructs. Microinjection of anti-Rab5 or anti-dynein antibody into 3T3-L1 adipocytes increased the basal level of surface GLUT4, did not change the insulin-stimulated surface GLUT4 level, and inhibited GLUT4 internalization after the removal of insulin. Photoaffinity labeling of Rab5 with [gamma-(32)P]GTP-azidoanilide showed that insulin inhibited Rab5-GTP loading. By using microtubule-capture assays, we found that insulin also caused a significant decrease in the binding of dynein to microtubules. Furthermore, pretreatment of cells with the PI3-kinase inhibitor LY294002 inhibited the effects of insulin on both Rab5-GTP loading and dynein binding to microtubules. In conclusion, these data indicate that insulin signaling inhibits Rab5 activity and the interaction of dynein with microtubules in a PI3-kinase-dependent manner, and that these effects may inhibit the rate of GLUT4 internalization. As such, our results present a previously uncharacterized insulin-signaling pathway involving Rab5, the motor protein dynein, and the cytoskeleton to regulate directional GLUT4 movement, facilitating GLUT4 distribution to the cell surface.

Rab4 regulates formation of synaptic-like microvesicles from early endosomes in PC12 cells

Early endosomes in PC12 cells are an important site for the formation of synaptic-like microvesicles and constitutive recycling vesicles. By immunogold electron microscopy, the small GTPase rab4 was localized to early endosomes and numerous small vesicles in the cell periphery and Golgi area of PC12 cells. Overexpression of GTPase-deficient Q67Lrab4 increased the number of early endosome-associated and cytoplasmic vesicles, whereas expression of GDP-bound S22Nrab4 significantly increased the length of early endosomal tubules. In parallel, Q67Lrab4 induced a shift in rab4, VAMP2, and TfR label from early endosomes to peripheral vesicles, whereas S22Nrab4 increased early endosome labeling of all three proteins. These observations were corroborated by early endosome budding assays. Together, our data document a thus far unrecognized role for rab4 in the formation of synaptic-like microvesicles and add to our understanding of the formation of constitutive recycling vesicles from early endosomes.

The Rab7 effector protein RILP controls lysosomal transport by inducing the recruitment of dynein-dynactin motors

Many intracellular compartments, including MHC class II-containing lysosomes, melanosomes, and phagosomes, move along microtubules in a bidirectional manner and in a stop-and-go fashion due to the alternating activities of a plus-end directed kinesin motor and a minus-end directed dynein-dynactin motor. It is largely unclear how motor proteins are targeted specifically to different compartments. Rab GTPases recruit and/or activate several proteins involved in membrane fusion and vesicular transport. They associate with specific compartments after activation, which makes Rab GTPases ideal candidates for controlling motor protein binding to specific membranes. We and others [7] have identified a protein, called RILP (for Rab7-interacting lysosomal protein), that interacts with active Rab7 on late endosomes and lysosomes. Here we show that RILP prevents further cycling of Rab7. RILP expression induces the recruitment of functional dynein-dynactin motor complexes to Rab7-containing late endosomes and lysosomes. Consequently, these compartments are transported by these motors toward the minus end of microtubules, effectively inhibiting their transport toward the cell periphery. This signaling cascade may be responsible for timed and selective dynein motor recruitment onto late endosomes and lysosomes.

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.

Identification and characterization of a family of Rab11-interacting proteins

Rab11a is a small GTP-binding protein enriched in the pericentriolar plasma membrane recycling systems. We hypothesized that Rab11a-binding proteins exist as downstream effectors of its action. Here we define a family of four Rab11-interacting proteins: Rab11-Family Interacting Protein 1 (Rab11-FIP1), Rab11-Family Interacting Protein 2 (Rab11-FIP2), Rab11-Family Interacting Protein 3 (Rab11-FIP3), and pp75/Rip11. All four interacting proteins associated with wild type Rab11a and dominant active Rab11a (Rab11aS20V) as well as Rab11b and Rab25. Rab11-FIP2 also interacted with dominant negative Rab11a (Rab11aS25N) and the tail of myosin Vb. The binding of Rab11-FIP1, Rab11-FIP2, and Rab11-FIP3 to Rab11a was dependent upon a conserved carboxyl-terminal amphipathic alpha-helix. Rab11-FIP1, Rab11-FIP2, and pp75/Rip11 colocalized with Rab11a in plasma membrane recycling systems in both non-polarized HeLa cells and polarized Madin-Darby canine kidney cells. GFP-Rab11-FIP3 also colocalized with Rab11a in HeLa cells. Rab11-FIP1, Rab11-FIP2, and pp75/Rip11 also coenriched with Rab11a and H(+)K(+)-ATPase on parietal cell tubulovesicles, and Rab11-FIP1 and Rab11-FIP2 translocated with Rab11a and the H(+)K(+)-ATPase upon stimulating parietal cells with histamine. The results suggest that the function of Rab11a in plasma membrane recycling systems is dependent upon a compendium of protein effectors.

Identification of a novel kinesin-related protein, KRMP1, as a target for mitotic peptidyl-prolyl isomerase Pin1

Mitosis utilizes a number of kinesin-related proteins (KRPs). Here we report the identification of a novel KRP termed KRMP1, which has a deduced 1780-amino acid sequence composed of ternary domains. The amino-terminal head domain is most similar to the kinesin motor domain of the MKLP-1 subfamily and has an intrinsic ATPase activity that is diminished by substituting the consensus Lys-168 with Arg. The central stalk domain is predicted to form a long alpha-helical coiled-coil, and can interact with each other in vivo. An in vivo labeling experiment revealed that KRMP1 is phosphorylated, and we also found that the region within the tail domain containing Thr-1604 as the cdc2 kinase phosphorylation site differs from the bimC box conserved in the bimC subfamily of KRPs. Immunofluorescence analysis showed that endogenous KRMP1 was localized predominantly to the cytoplasm during interphase and dispersed throughout the cell during mitosis. Consistent with this finding, overexpressed KRMP1 was detected in a complicated nuclear or cytoplasmic pattern reflecting multiple nuclear localization/export signals. Furthermore, KRMP1 interacted with the mitotic peptidyl-prolyl isomerase Pin1 in vivo, and an in vitro interaction was detected between the tail domain of KRMP1 and the WW domain of Pin1. Overexpression of KRMP1 caused COS-7 cells to arrest at G(2)-M, and co-expression of Pin1 reversed this effect, indicating their physiological interaction. Together, our results suggest that KRMP1 is a mitotic target regulated by Pin1 and vice versa.

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.

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.

Interactions between adaptor protein-1 of the clathrin coat and microtubules via type 1a microtubule-associated proteins

The classical view suggests that adaptor proteins of the clathrin coat mediate the sorting of cargo protein passengers into clathrin-coated pits and the recruitment of clathrin into budding areas in the donor membrane. In the present study, we provide biochemical and morphological evidence that the adaptor protein 1 (AP-1) adaptor of the trans-Golgi network clathrin interacts with microtubules. AP-1 in cytosolic extracts interacted with in vitro assembled microtubules, and these interactions were inhibited by ATP depletion of the extracts or in the presence of 5'-adenylylimidodiphosphate. An overexpressed gamma-subunit of the AP-1 complex associated with microtubules, suggesting that this subunit may mediate the interaction of AP-1 with the cytoskeleton. Purified AP-1 did not interact with purified microtubules, but interaction occurred when an isolated microtubule-associated protein fraction was added to the reaction mix. The gamma-adaptin subunit of AP-1 specifically co-immunoprecipitated with a microtubule-associated protein of type 1a from rat brain cytosol. This suggests that type 1a microtubule-associated protein may mediate the association of AP-1 with microtubules in the cytoplasm. The microtubule binding activity of AP-1 was markedly inhibited in cytosol of mitotic cells. By means of its interaction with microtubule-associated proteins, we propose novel roles for AP-1 adaptors in modulating the dynamics of the cytoskeleton, the stability and shape of coated organelles, and the loading of nascent AP-1-coated vesicles onto appropriate microtubular tracks.

The GRIP domain is a specific targeting sequence for a population of trans-Golgi network derived tubulo-vesicular carriers

Vesicular carriers for intracellular transport associate with unique sets of accessory molecules that dictate budding and docking on specific membrane domains. Although many of these accessory molecules are peripheral membrane proteins, in most cases the targeting sequences responsible for their membrane recruitment have yet to be identified. We have previously defined a novel Golgi targeting domain (GRIP) shared by a family of coiled-coil peripheral membrane Golgi proteins implicated in membrane trafficking. We show here that the docking site for the GRIP motif of p230 is a specific domain of Golgi membranes. By immuno-electron microscopy of HeLa cells stably expressing a green fluorescent protein (GFP)-p230GRIP fusion protein, we show binding specifically to a subset of membranes of the trans-Golgi network (TGN). Real-time imaging of live HeLa cells revealed that the GFP-p230GRIP was associated with highly dynamic tubular extensions of the TGN, which have the appearance and behaviour of transport carriers. To further define the nature of the GRIP membrane binding site, in vitro budding assays were performed using purified rat liver Golgi membranes and cytosol from GFP-p230GRIP-transfected cells. Analysis of Golgi-derived vesicles by sucrose gradient fractionation demonstrated that GFP-p230GRIP binds to a specific population of vesicles distinct from those labelled for beta-COP or gamma-adaptin. The GFP-p230GRIP fusion protein is recruited to the same vesicle population as full-length p230, demonstrating that the GRIP domain is solely proficient as a targeting signal for membrane binding of the native molecule. Therefore, p230 GRIP is a targeting signal for recruitment to a highly selective membrane attachment site on a specific population of trans-Golgi network tubulo-vesicular carriers.

The Ras-like GTPase Gem is involved in cell shape remodelling and interacts with the novel kinesin-like protein KIF9

Gem belongs to the Rad/Gem/Kir (RGK) subfamily of Ras-related GTPases, which also comprises Rem, Rem2 and Ges. The RGK family members Ges and Rem have been shown to produce endothelial cell sprouting and reorganization of the actin cytoskeleton upon overexpression. Here we show that high intracellular Gem levels promote profound changes in cell morphology and we investigate how this phenotype arises dynamically. We also show that this effect requires intact microtubules and microfilaments, and that Gem is associated with both cytoskeletal components. In order to investigate the mechanisms of Gem recruitment to the cytoskeleton, we performed a yeast two-hybrid screen and identified a novel kinesin-like protein, termed KIF9, as a new Gem interacting partner. We further show that Gem and KIF9 interact by co-immunoprecipitation. Furthermore, Gem and KIF9 display identical patterns of gene expression in different tissues and developmental stages. The Gem- KIF9 interaction reported here is the first molecular link between RGK family members and the microtubule cytoskeleton.

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.

A developmentally regulated Rab11 homologue in Trypanosoma brucei is involved in recycling processes

Endocytosis in the parasitic protozoan Trypanosoma brucei, a deeply divergent eukaryote, is implicated as important in both general cellular function and virulence, and is strongly developmentally regulated. We report the characterisation of a previously undefined endosomal compartment in T. brucei based on identification of a new trypanosome gene (TbRAB11) homologous to Rab11/Ypt31. Northern and western analyses indicated that TbRAB11 expression was significantly upregulated in the bloodstream stage of the parasite, the first trypanosome Rab to be identified with a developmentally regulated expression profile. In procyclic form parasites TbRAB11 localised to a compartment positioned close to the basal body, similar to mammalian Rab11. By contrast, in bloodstream form parasites, TbRAB11-containing structures were more extensive and the TbRAB11 compartment extended towards the posterior face of the nucleus, was more elaborate and was not always adjacent to the basal body. Colocalisation studies by light and confocal microscopy demonstrated that TbRAB11 was located on a compartment that did not correspond to other established trypanosomal organelles or markers. Using concanavalin A internalisation and temperature block procedures, TbRAB11 was observed on endomembranes anterior to the flagellar pocket that are juxtaposed to the collecting tubules. TbRAB11 colocalised with the trypanosomal transferrin receptor and internalised antivariant surface glycoprotein. Further, we show that the collecting tubules contain TbRAB5A, suggesting that they are the trypanosomatid early endosome. Hence, TbRAB11 is present on endosomal structures that contain recycling cargo molecules and is under developmental regulation, suggesting a role in stage-dependent endocytic processes.

The Golgi complex is a microtubule-organizing organelle

We show that the Golgi complex can directly stimulate microtubule nucleation in vivo and in vitro and thus behaves as a potent microtubule-organizing organelle in interphase cells. With the use of nocodazole wash-out experiments in hepatic cells, we found that the occurrence of noncentrosomal, early stabilized microtubules is highly correlated with the subcellular localization of Golgi membranes. With the use of in vitro reconstituted microtubule assembly systems with or without cytosol, we also found that, in contrast to centrosomally attached microtubules, the distal ends of Golgi-attached microtubules are remotely stabilized in a way that requires additional cytosolic component(s). Finally, we demonstrate that Golgi-based microtubule nucleation is direct and involves a subset of gamma-tubulin bound to the cytoplasmic face of the organelle.

All kinesin superfamily protein, KIF, genes in mouse and human

Intracellular transport is essential for morphogenesis and functioning of the cell. The kinesin superfamily proteins (KIFs) have been shown to transport membranous organelles and protein complexes in a microtubule- and ATP-dependent manner. More than 30 KIFs have been reported in mice. However, the nomenclature of KIFs has not been clearly established, resulting in various designations and redundant names for a single KIF. Here, we report the identification and classification of all KIFs in mouse and human genome transcripts. Previously unidentified murine KIFs were found by a PCR-based search. The identification of all KIFs was confirmed by a database search of the total human genome. As a result, there are a total of 45 KIFs. The nomenclature of all KIFs is presented. To understand the function of KIFs in intracellular transport in a single tissue, we focused on the brain. The expression of 38 KIFs was detected in brain tissue by Northern blotting or PCR using cDNA. The brain, mainly composed of highly differentiated and polarized cells such as neurons and glia, requires a highly complex intracellular transport system as indicated by the increased number of KIFs for their sophisticated functions. It is becoming increasingly clear that the cell uses a number of KIFs and tightly controls the direction, destination, and velocity of transportation of various important functional molecules, including mRNA. This report will set the foundation of KIF and intracellular transport research.

Myosin vb is associated with plasma membrane recycling systems

Myosin Va is associated with discrete vesicle populations in a number of cell types, but little is known of the function of myosin Vb. Yeast two-hybrid screening of a rabbit parietal cell cDNA library with dominant active Rab11a (Rab11aS20V) identified myosin Vb as an interacting protein for Rab11a, a marker for plasma membrane recycling systems. The isolated clone, corresponding to the carboxyl terminal 60 kDa of the myosin Vb tail, interacted with all members of the Rab11 family (Rab11a, Rab11b, and Rab25). GFP-myosin Vb and endogenous myosin Vb immunoreactivity codistributed with Rab11a in HeLa and Madin-Darby canine kidney (MDCK) cells. As with Rab11a in MDCK cells, the myosin Vb immunoreactivity was dispersed with nocodazole treatment and relocated to the apical corners of cells with taxol treatment. A green fluorescent protein (GFP)-myosin Vb tail chimera overexpressed in HeLa cells retarded transferrin recycling and caused accumulation of transferrin and the transferrin receptor in pericentrosomal vesicles. Expression of the myosin Vb tail chimera in polarized MDCK cells stably expressing the polymeric IgA receptor caused accumulation of basolaterally endocytosed polymeric IgA and the polymeric IgA receptor in the pericentrosomal region. The myosin Vb tail had no effects on transferrin trafficking in polarized MDCK cells. The GFP-myosin Va tail did not colocalize with Rab11a and had no effects on recycling system vesicle distribution in either HeLa or MDCK cells. The results indicate myosin Vb is associated with the plasma membrane recycling system in nonpolarized cells and the apical recycling system in polarized cells. The dominant negative effects of the myosin Vb tail chimera indicate that this unconventional myosin is required for transit out of plasma membrane recycling systems.

Regulation of molecular motor proteins

Motor proteins in the kinesin, dynein, and myosin superfamilies are tightly regulated to perform multiple functions in the cell requiring force generation. Although motor proteins within families are diverse in sequence and structure, there are general mechanisms by which they are regulated. We first discuss the regulation of the subset of kinesin family members for which such information exists, and then address general mechanisms of kinesin family regulation. We review what is known about the regulation of axonemal and cytoplasmic dyneins. Recent work on cytoplasmic dynein has revealed the existence of multiple isoforms for each dynein chain, making the study of dynein regulation more complicated than previously realized. Finally, we discuss the regulation of myosins known to be involved in membrane trafficking. Myosins and kinesins may be evolutionarily related, and there are common themes of regulation between these two classes of motors.

Completion of cytokinesis in C. elegans requires a brefeldin A-sensitive membrane accumulation at the cleavage furrow apex

BACKGROUND

The terminal phase of cytokinesis in eukaryotic cells involves breakage of the intercellular canal containing the spindle midzone and resealing of the daughter cells. Recent observations suggest that the spindle midzone is required for this process. In this study, we investigated the possibility that targeted secretion in the vicinity of the spindle midzone is required for the execution of the terminal phase of cytokinesis.

RESULTS

We inhibited secretion in early C. elegans embryos by treatment with brefeldin A (BFA). Using 4D recordings of dividing cells, we showed that BFA induced stereotyped failures in the terminal phase of cytokinesis; although the furrow ingressed normally, after a few minutes the furrow completely regressed, even though spindle midzone and midbody microtubules appeared normal. In addition, using an FM1-43 membrane probe, we found that membrane accumulated locally at the apices of the late cleavage furrows that form the persisting intercellular canals between daughter cells. However, in BFA-treated embryos this membrane accumulation did not occur, which possibly accounts for the observed cleavage failures.

CONCLUSIONS

We have shown that BFA disrupts the terminal phase of cytokinesis in the embryonic blastomeres of C. elegans. We observed that membrane accumulates at the apices of the late cleavage furrow by means of a BFA-sensitive mechanism. We suggest that this local membrane accumulation is necessary for the completion of cytokinesis and speculate that the spindle midzone region of animal cells is functionally equivalent to the phragmoplast of plants and acts to target secretion to the equatorial plane of a cleaving cell.

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.

Rab8b and its interacting partner TRIP8b are involved in regulated secretion in AtT20 cells

Rab proteins are a family of small GTPases that regulate intracellular vesicle traffic. Rab8b, because of its homology with Rab8, has been suggested to function in vesicle transport to the plasma membrane. Using the yeast two-hybrid system, we identified a Rab8b interacting clone, termed TRIP8b, from a rat brain cDNA library. The gene encodes a 66-kDa protein with homology to the peroxisomal targeting signal 1 receptor. The interaction between Rab8b and TRIP8b was further verified by in vitro binding assays and co-immunoprecipitation studies. Additional experiments with Rab8b mutants demonstrated that Rab8b requires a guanine nucleotide but not prenylation for its interaction with TRIP8b. Western immunoblot analysis showed that TRIP8b was primarily expressed in brain. Subcellular fractionation of AtT20 cells revealed that TRIP8b was present in both cytosolic and membrane fractions. To investigate the function of Rab8b and TRIP8b in secretion, we examined the release of ACTH from AtT20 cells. Results from stable cell lines expressing Rab8b or TRIP8b indicated that both proteins had a stimulatory effect on cAMP-induced secretion of ACTH. In summary, these data suggest that Rab8b and TRIP8b interact with each other and are involved in the regulated secretory pathway in AtT20 cells.

The human kinesin-like protein RB6K is under tight cell cycle control and is essential for cytokinesis

Several members of the kinesin superfamily are known to play a prominent role in the motor-driven transport processes that occur in mitotic cells. Here we describe a new mitotic human kinesin-like protein, RB6K (Rabkinesin 6), distantly related to MKLP-1. Expression of RB6K is regulated during the cell cycle at both the mRNA and protein level and, similar to cyclin B, shows a maximum during M phase. Isolation of the RB6K promoter allowed identification of a CDE-CHR element and promoter activity was shown to be maximal during M phase. Immunofluorescence microscopy using antibodies raised against RB6K showed a weak signal in interphase Golgi but a 10-fold higher signal in prophase nuclei. During M phase, the newly synthesized RB6K does not colocalise with Rab6. In later stages of mitosis RB6K localized to the spindle midzone and appeared on the midbodies during cytokinesis. The functional significance of this localization during M phase was revealed by antibody microinjection studies which resulted exclusively in binucleate cells, showing a complete failure of cytokinesis. These results substantiate a crucial role for RB6K in late anaphase B and/or cytokinesis, clearly distinct from the role of MKLP-1.

Molecular motors and membrane traffic in Dictyostelium

Phagocytosis and membrane traffic in general are largely dependent on the cytoskeleton and their associated molecular motors. The myosin family of motors, especially the unconventional myosins, interact with the actin cortex to facilitate the internalization of external materials during the early steps of phagocytosis. Members of the kinesin and dynein motor families, which mediate transport along microtubules (MTs), facilitate the intracellular processing of the internalized materials and the movement of membrane. Recent studies indicate that some unconventional myosins are also involved in membrane transport, and that the MT- and actin-dependent transport systems might interact with each other. Studies in Dictyostelium have led to the discovery of many motors involved in critical steps of phagocytosis and membrane transport. With the ease of genetic and biochemical approaches, the established functional analysis to test phagocytosis and vesicle transport, and the effort of the Dictyostelium cDNA and Genome Projects, Dictyostelium will continue to be a superb model system to study phagocytosis in particular and cytoskeleton and motors in general.

Rab27a enables myosin Va-dependent melanosome capture by recruiting the myosin to the organelle

The peripheral accumulation of melanosomes characteristic of wild-type mouse melanocytes is driven by a cooperative process involving long-range, bidirectional, microtubule-dependent movements coupled to capture and local movement in the actin-rich periphery by myosin Va, the product of the dilute locus. Genetic evidence suggests that Rab27a, the product of the ashen locus, functions with myosin Va in this process. Here we show that ashen melanocytes, like dilute melanocytes, exhibit normal dendritic morphology and melanosome biogenesis, an abnormal accumulation of end-stage melanosomes in the cell center, and rapid, bidirectional, microtubule-dependent melanosome movements between the cell center and the periphery. This phenotype suggests that ashen melanocytes, like dilute melanocytes, are defective in peripheral melanosome capture. Consistent with this, introduction into ashen melanocytes of cDNAs encoding wild-type and GTP-bound versions of Rab27a restores the peripheral accumulation of melanosomes in a microtubule-dependent manner. Conversely, introduction into wild-type melanocytes of the GDP-bound version of Rab27a generates an ashen/dilute phenotype. Rab27a colocalizes with end-stage melanosomes in wild-type cells, and is most concentrated in melanosome-rich dendritic tips, where it also colocalizes with myosin Va. Finally, neither endogenous myosin Va nor an expressed, GFP-tagged, myosin Va tail domain fusion protein colocalize with melanosomes in ashen melanocytes, in contrast to that seen previously in wild-type cells. These results argue that Rab27a serves to enable the myosinVa-dependent capture of melanosomes delivered to the periphery by bidirectional, microtubule-dependent transport, and that it does so by recruiting the myosin to the melanosome surface. We suggest that Rab27a, in its GTP-bound and melanosome-associated form, predominates in the periphery, and that it is this form that recruits the myosin, enabling capture. These results argue that Rab27a serves as a myosin Va ‘receptor’, and add to the growing evidence that Rab GTPases regulate vesicle motors as well as SNARE pairing.