Rab GTPases, directors of vesicle docking

Expression in a RabGAP yeast mutant of two human homologues, one of which is an oncogene

The yeast proteins Msb3p and Msb4p are two Ypt/Rab-specific GTPase-activating proteins (GAPs) involved in cell growth polarization. Both proteins share with a wide variety of other proteins the highly conserved TBC domain forming the catalytically active RabGAP domain. In particular, Msb3p and Msb4p are similar to the human proteins oncTre210p (the 786-amino-acid product of the human Tre2 oncogene, implicated in Ewing's sarcoma) and RN-tre (a Rab5-GAP controlling endocytosis of the EGFR). To further understand the biochemical function of Tre2 oncogene, we expressed its cDNA and, as a control, the RN-tre cDNA, in an msb3 msb4 double mutant yeast strain. Complementation data show that RN-tre can, unlike Tre2, replace the function of the MSB3 and MSB4 genes. As two highly conserved amino acids, including the catalytic arginine, are mutated in the oncTre210p TBC domain, we restored these two amino acids and expressed the modified Tre2 cDNA in the yeast mutant.

Intracellular delivery of drugs to macrophages

Toxic side effects which often complicate successful therapy in a number of diseases possibly arise due to the fact that at therapeutically effective concentrations the non-target cells in the body are also exposed to the cytotoxic effects of the drugs. Minimization of such adverse reactions might be feasible through drug delivery modalities that would reduce the uptake of the drugs by non-target cells and selectively deliver the drug only to the target cells (and/or intracellular sites) at relatively low extracellular concentrations. The current generic approach to site-specific drug delivery consists of attaching the therapeutic agent to a carrier recognized only by the cells where the pharmacological action is desired. Two types of recognition elements on the surface of target cells are being exploited for this purpose, viz., (i) antigens capable of generating specific, non-cross reactive antibodies, and (ii) receptors on the cell surface capable of efficient transport of the ligands. In general, incomplete specificity for the target cells and poor internalization of antibody-drug conjugates still limit the usefulness of antibodies for site-specific drug delivery applications necessitating exploration of alternatives. The alternate possibility is to exploit the exquisite cell type specificity and high efficiency of endocytosis of macromolecules mediated by specific receptors present on the surface of target cells for delivering drugs. A large number of infectious, metabolic, and neoplastic diseases are associated with macrophages leading to morbidities and mortalities to millions of people worldwide, thus an appropriate design of a drug delivery system to macrophages will be of tremendous help.

Transcytosis: crossing cellular barriers

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

The chlamydial inclusion: escape from the endocytic pathway

Chlamydiae, bacterial obligate intracellular pathogens, are the etiologic agents of several human diseases. A large part of the chlamydial intracellular survival strategy involves the formation of a unique organelle called the inclusion that provides a protected site within which they replicate. The chlamydial inclusion is effectively isolated from endocytic pathways but is fusogenic with a subset of exocytic vesicles that deliver sphingomyelin from the Golgi apparatus to the plasma membrane. A combination of host and parasite functions contribute to the biogenesis of this compartment. Establishment of the mature inclusion is accompanied by the insertion of multiple chlamydial proteins, suggesting that chlamydiae actively modify the inclusion to define its interactions with the eukaryotic host cell. Despite being sequestered within a membrane-bound vacuole, chlamydiae clearly communicate with and manipulate the host cell from within this privileged intracellular niche.

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.

Regulation of host cell transcriptional physiology by the avian pneumovirus provides key insights into host-pathogen interactions

Infection with a viral pathogen triggers several pathways in the host cell that are crucial to eliminating infection, as well as those that are used by the virus to enhance its replication and virulence. We have here used suppression subtractive hybridization and cDNA microarray analyses to characterize the host transcriptional response in an avian pneumovirus model of infection. The results of our investigations reveal a dynamic host response that includes the regulation of genes with roles in a vast array of cellular functions as well as those that have not been described previously. The results show a considerable upregulation in transcripts representing the interferon-activated family of genes, predicted to play a role in virus replication arrest. The analysis also identified transcripts for proinflammatory leukocyte chemoattractants, adhesion molecules, and complement that were upregulated and may account for the inflammatory pathology that is the hallmark of viral respiratory infection. Interestingly, alterations in the transcription of several genes in the ubiquitin and endosomal protein trafficking pathways were observed, suggesting a role for these pathways in virus maturation and budding. Taken together, the results of our investigations provide key insights into individual genes and pathways that constitute the host cell's response to avian pneumovirus infection, and they have enabled the development of resources and a model of host-pathogen interaction for an important avian respiratory tract pathogen.

Envelopment of human cytomegalovirus occurs by budding into Golgi-derived vacuole compartments positive for gB, Rab 3, trans-golgi network 46, and mannosidase II

Although considerable progress has been made towards characterizing virus assembly processes, assignment of the site of tegumentation and envelopment for human cytomegalovirus (HCMV) is still not clear. In this study, we examined the envelopment of HCMV particles in human lung fibroblasts (HF) HL 411 and HL 19, human umbilical vein endothelial cells, human pulmonary arterial endothelial cells, and arterial smooth muscle cells at different time points after infection by electron microscopy (EM), immunohistochemistry, and confocal microscopy analysis. Double-immunofluorescence labeling experiments demonstrated colocalization of the HCMV glycoprotein B (gB) with the Golgi resident enzyme mannosidase II, the Golgi marker TGN (trans-Golgi network) 46, and the secretory vacuole marker Rab 3 in all cell types investigated. Final envelopment of tegumented capsids was observed at 5 days postinfection by EM, when tegumented capsids budded into subcellular compartments located in the cytoplasm, in close proximity to the Golgi apparatus. Immunogold labeling and EM analysis confirmed staining of the budding compartment with HCMV gB, Rab 3, and mannosidase II in HL 411 cells. However, the markers Rab 1, Rab 2, Rab 7, Lamp 1 (late endosomes and lysosomes), and Lamp 2 (lysosomes) neither showed specific staining of the budding compartment in the immunogold labeling experiments nor colocalized with gB in the immunofluorescent colocalization experiments in any cell type studied. Together, these results suggest that the final envelopment of HCMV particles takes place mainly into a Golgi-derived secretory vacuole destined for the plasma membrane, which may release new infectious virus particles by fusion with the plasma membrane.

Hepatic gene expression profiling of streptozotocin-induced diabetes

Diabetes induces biochemical, morphological, and functional alterations in the liver. The liver is a major target of insulin action, and plays a critical role in maintaining blood glucose homeostasis. We investigated the effects of streptozotocin-induced diabetes (SID) on global hepatic gene expression in mice. We induced SID in mice by intraperitoneal injection of streptozotocin. Affymetrix (Santa Clara, CA) microarrays containing probe sets for approximately 11,000 murine genes and expressed sequence tags were used to assess the effects of SID on hepatic gene expression in mice. SID significantly altered the expression of 87 known genes in the liver. SID increased the expression of genes associated with cytoprotective stress responses, oxidative and reductive xenobiotic metabolism, cell cycle inhibition, growth arrest, apoptosis induction, and protein degradation. SID decreased the expression of genes associated with cell proliferation, growth factor signaling, protein synthesis, and xenobiotic metabolism. The novel results reported here should open new areas of investigation in diabetes research and facilitate the development of novel strategies for gene therapy and drug discovery.

Hermansky-Pudlak syndrome: vesicle formation from yeast to man

The disorders known as Hermansky-Pudlak syndrome (HPS) are a group of genetic diseases resulting from abnormal formation of intracellular vesicles. In HPS, dysfunction of melanosomes results in oculocutaneous albinism, and absence of platelet dense bodies causes a bleeding diathesis. In addition, some HPS patients suffer granulomatous colitis or fatal pulmonary fibrosis, perhaps due to mistrafficking of a subset of lysosomes. The impaired function of specific organelles indicates that the causative genes encode proteins operative in the formation of certain vesicles. Four such genes, HPS1, ADTB3A, HPS3, and HPS4, are associated with the four known subtypes of HPS, i.e. HPS-1, HPS-2, HPS-3, and HPS-4. ADTB3A codes for the beta 3 A subunit of adaptor complex-3, known to assist in vesicle formation from the trans-Golgi network or late endosome. However, the functions of the HPS1, HPS3, and HPS4 gene products remain unknown. These three genes arose with the evolution of mammals and have no homologs in yeast, reflecting their specialized function. In contrast, all four known HPS-causing genes have homologs in mice, a species with 14 different models of HPS, i.e. hypopigmentation and a platelet storage pool deficiency. Pursuit of the mechanism of mammalian vesicle formation and trafficking, impaired in HPS, relies upon investigation of these mouse models as well as studies of protein complexes involved in yeast vacuole formation.

Expression of mammalian Rab Escort protein-1 and -2 in yeast Saccharomyces cerevisiae

Rab GTPases are post-translationally geranylgeranylated on their C-terminal cysteines by Rab geranylgeranyl transferase (RabGGTase) and this modification is essential for their biological activity. Rab Escort Protein (REP) is a molecular chaperone that assists in the prenylation reaction carried out by RabGGTase. Mutations in the REP-1 gene lead to progressive retinal degradation and blindness in humans. Despite the significant interest in REP proteins, their preparative production remains challenging. We report here an inducible expression system for the production of REP-1 and REP-2 in yeast Saccharomyces cerevisiae at levels 3.5 and 2mg/L, respectively. The REP-1 was found to be toxic for yeast cells and its toxicity is proposed to be associated with the formation of unproductive Ypt:REP-1 complexes. To minimize the toxic effect of REP-1, the recombinant protein expression was induced at the end of the exponential phase. Under these conditions, the GAL1 promoter is no longer repressed due to exhaustion of glucose and utilization of ethanol as a carbon source. This expression procedure was successfully scaled up to 30L for both proteins. The REP-1 and REP-2 were purified using a combination of affinity and anion-exchange chromatography. Purified proteins were functionally active, as determined by a fluorescent Rab binding assay and in vitro prenylation. The reported procedure provides a reliable source of REP proteins for biochemical and structural studies.

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.

Diverting intracellular trafficking of Salmonella to the lysosome through activation of the late endocytic Rab7 by intracellular delivery of muramyl dipeptide

Previously, we showed that live Salmonella-containing phagosomes (LSP) recruit early acting Rab5 and promote fusion with early endosomes, thus avoiding transport to the lysosomes. Therefore, live Salmonella survive in a specialized compartment. Here we show that scavenger-receptor-mediated intracellular delivery of muramyl dipeptide (MDP) to macrophages leads to efficient killing of Salmonella both in vitro and in vivo. To understand the intracellular trafficking modulation of Salmonella by delivery of MDP, we investigated the levels of endocytic Rab proteins, which are the major regulators of vesicular transport. Western blot analysis reveals reduced Rab5 and enhanced Rab7 content in the maleylated bovine serum albumin-MDP (MBSA-MDP)-treated cells. The reduced content of Rab5 in the treated cells and on phagosomes inhibits the fusion of Salmonella-containing phagosomes with early endosomes, and the enhanced Rab7 content in these cells facilitated targeting of LSP to lysosomes, which contain cathepsin D and vacuolar ATPase, for killing. In vitro reconstitution of lysosomal transport demonstrated that a reduced content of Rab5 and an enhanced level of Rab7 in MBSA-MDP-treated cells is primarily responsible for targeting Salmonella to lysosomes. Intracellular delivery of MDP thus offers a general strategy against macrophage-associated infections caused by intracellular pathogens that survive in the host cell by resisting transport to lysosomes.

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

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

The exosome pathway in K562 cells is regulated by Rab11

During maturation, reticulocytes lose some membrane proteins that are not required on the mature red cell surface. The proteins are released into the extracellular medium associated with vesicles that are formed by budding of the endosomal membrane into the lumen of the compartment; this process results in the formation of multivesicular bodies (MVBs). Fusion of MVBs with the plasma membrane results in secretion of the small internal vesicles, termed exosomes. K562 cells release exosomes with similar characteristics to reticulocyte exosomes, in particular the transferrin receptor (TfR) is found associated with the vesicles. Interestingly, this cell line has been shown to possess high amounts of Rab11 compared with other Rab proteins. To assess the regulation of transferrin receptor release via exosome secretion by Rab11 in this cell type, K562 cells were stably transfected with GFP-Rab11wt or the GTP- and GDP-locked mutants. The distribution of the proteins was assessed by fluorescence microscopy. Transferrin recycling and the number of TfRs present on the surface of the transfected cells were reduced by overexpression of either Rab11wt or the mutants. The amount of released exosomes was analyzed by measuring different molecular markers present on these vesicles either biochemically or by western blot. Overexpression of the dominant-negative mutant Rab11S25N inhibited exosome release, whereas the secretion of exosomes was slightly stimulated in cells transfected with Rab11wt. Taken together, the results demonstrate that in K562 cells Rab11 modulates the exosome pathway although the exact step involved is still not known.

Reduction of nucleoside diphosphate kinase B, Rab GDP-dissociation inhibitor beta and histidine triad nucleotide-binding protein in fetal Down syndrome brain

Information on the various factors leading to impairments in the developing brain of fetal Down Syndrome patients is limited to few histological reports. We therefore attempted to describe expression levels of proteins in brain using the proteomic technique of two-dimensional electrophoresis with subsequent mass spectroscopical identification of protein spots and quantification with specific software. Cortical tissue was obtained from autopsy of human fetal abortus. Protein levels of GTP-binding nuclear protein ran, guanine nucleotide-binding protein g(o), alpha subunit 2, guanine nucleotide-binding protein g(i)/g(s)/g(t) beta subunit 1, -beta subunit 2, guanine nucleotide-binding protein beta subunit 5, nucleoside diphosphate kinase A, nucleoside diphosphate kinase B, Rab GDP-dissociation inhibitor beta, Rho GDP-dissociation inhibitor 1, biphosphate 3'-nucleotidase, small glutamine-rich tetra-tricopeptide repeat-containing protein and histidine triad nucleotide-binding protein were studied. Quantification revealed statistically significant reduced levels of nucleoside diphosphate kinase B, Rab GDP-dissociation inhibitor beta and histidine triad nucleotide-binding protein in fetal DS brain as compared to controls. We conclude that in early prenatal life proteins involved in neural differentiation, migration and synaptic transmission are impaired in DS cortex. These results may help to understand the abundant mechanisms leading to abnormalities in the wiring, structure and function of DS brain.

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.

Sly1 protein bound to Golgi syntaxin Sed5p allows assembly and contributes to specificity of SNARE fusion complexes

Fusion of transport vesicles with their target organelles involves specific membrane proteins, SNAREs, which form tight complexes bridging the membranes to be fused. Evidence from yeast and mammals indicates that Sec1 family proteins act as regulators of membrane fusion by binding to the target membrane SNAREs. In experiments with purified proteins, we now made the observation that the ER to Golgi core SNARE fusion complex could be assembled on syntaxin Sed5p tightly bound to the Sec1-related Sly1p. Sly1p also bound to preassembled SNARE complexes in vitro and was found to be part of a vesicular/target membrane SNARE complex immunoprecipitated from yeast cell lysates. This is in marked contrast to the exocytic SNARE assembly in neuronal cells where high affinity binding of N-Sec1/Munc-18 to syntaxin 1A precluded core SNARE fusion complex formation. We also found that the kinetics of SNARE complex formation in vitro with either Sly1p-bound or free Sed5p was not significantly different. Importantly, several presumably nonphysiological SNARE complexes easily generated with Sed5p did not form when the syntaxin was first bound to Sly1p. This indicates for the first time that a Sec1 family member contributes to the specificity of SNARE complex assembly.

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

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

Mutation of melanosome protein RAB38 in chocolate mice

Mutations of genes needed for melanocyte function can result in oculocutaneous albinism. Examination of similarities in human gene expression patterns by using microarray analysis reveals that RAB38, a small GTP binding protein, demonstrates a similar expression profile to melanocytic genes. Comparative genomic analysis localizes human RAB38 to the mouse chocolate (cht) locus. A G146T mutation occurs in the conserved GTP binding domain of RAB38 in cht mice. Rab38(cht)/Rab38(cht) mice exhibit a brown coat similar in color to mice with a mutation in tyrosinase-related protein 1 (Tyrp1), a mouse model for oculocutaneous albinism. The targeting of TYRP1 protein to the melanosome is impaired in Rab38(cht)/Rab38(cht) melanocytes. These observations, and the fact that green fluorescent protein-tagged RAB38 colocalizes with end-stage melanosomes in wild-type melanocytes, suggest that RAB38 plays a role in the sorting of TYRP1. This study demonstrates the utility of expression profile analysis to identify mammalian disease genes.

Rab2 GTPase regulates vesicle trafficking between the endoplasmic reticulum and the Golgi bodies and is important to pollen tube growth

Pollen tube elongation depends on the secretion of large amounts of membrane and cell wall materials at the pollen tube tip to sustain rapid growth. A large family of RAS-related small GTPases, Rabs or Ypts, is known to regulate both anterograde and retrograde trafficking of transport vesicles between different endomembrane compartments and the plasma membrane in mammalian and yeast cells. Studies on the functional roles of analogous plant proteins are emerging. We report here that a tobacco pollen-predominant Rab2, NtRab2, functions in the secretory pathway between the endoplasmic reticulum and the Golgi in elongating pollen tubes. Green fluorescent protein-NtRab2 fusion protein localized to the Golgi bodies in elongating pollen tubes. Dominant-negative mutations in NtRab2 proteins inhibited their Golgi localization, blocked the delivery of Golgi-resident as well as plasmalemma and secreted proteins to their normal locations, and inhibited pollen tube growth. On the other hand, when green fluorescent protein-NtRab2 was over-expressed in transiently transformed leaf protoplasts and epidermal cells, in which NtRab2 mRNA have not been observed to accumulate to detectable levels, these proteins did not target efficiently to Golgi bodies. Together, these observations indicate that NtRab2 is important for trafficking between the endoplasmic reticulum and the Golgi bodies in pollen tubes and may be specialized to optimally support the high secretory demands in these tip growth cells.

The biology of cortical granules

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

Rabphilin potentiates soluble N-ethylmaleimide sensitive factor attachment protein receptor function independently of rab3

Rabphilin, a putative rab effector, interacts specifically with the GTP-bound form of the synaptic vesicle-associated protein rab3a. In this study, we define in vivo functions for rabphilin through the characterization of mutants that disrupt the Caenorhabditis elegans rabphilin homolog. The mutants do not display the general synaptic defects associated with rab3 lesions, as assayed at the pharmacological, physiological, and ultrastructural level. However, rabphilin mutants exhibit severe lethargy in the absence of mechanical stimulation. Furthermore, rabphilin mutations display strong synergistic interactions with hypomorphic lesions in the syntaxin, synaptosomal-associated protein of 25 kDa, and synaptobrevin soluble N-ethylmaleimide sensitive factor attachment protein receptor (SNARE) genes; double mutants were nonresponsive to mechanical stimulation. These synergistic interactions were independent of rab3 function and were not observed in rab3-SNARE double mutants. Our data reveal rab3-independent functions for rabphilin in the potentiation of SNARE function.

Rab5 association with the angiotensin II type 1A receptor promotes Rab5 GTP binding and vesicular fusion

Previous studies have demonstrated that the internalization of the angiotensin II type 1A receptor (AT(1A)R) may be mediated by both beta-arrestin-sensitive and -insensitive mechanisms. Therefore, we have used the AT(1A)R carboxyl-terminal tail to screen a rat brain yeast two-hybrid expression library for novel AT(1A)R-interacting proteins that might contribute to the regulation of AT(1A)R internalization. We have identified Rab5a as an AT(1A)R-binding protein that selectively associates with the AT(1A)R and not with the beta2-adrenergic receptor. A Rab5a-S34N mutant defective in GTP binding does not prevent the internalization of the AT(1A)R but does prevent the trafficking of the AT(1A)R into larger hollow cored vesicular structures. Agonist activation of the AT(1A)R promotes both the formation of Rab5a.AT(1A)R protein complexes and Rab5a GTP binding. Rab5a interactions with the AT(1A)R are mediated in part by the last 10 amino acid residues of the AT(1A)R carboxyl-terminal tail, and although a mutant receptor lacking these residues internalizes normally, it does not redistribute into larger hollow vesicles. Our data suggest that AT(1A)R activation modulates Rab5a activity leading to the homotypic fusion of endocytic vesicles. These observations suggest that vesicular cargo proteins, such as the AT(1A)R, may control their targeting between intracellular compartments by directly regulating the activity of components of the intracellular trafficking machinery such as Rab5a.

Regulation of Golgi structure and function by ARF-like protein 1 (Arl1)

Arl1 is a member of the ARF-like protein (Arl) subfamily of small GTPases. Nothing is known about the function of Arl1 except for the fact that it is essential for normal development in Drosophila and that it is associated with the Golgi apparatus. In this study, we first demonstrate that Arl1 is enriched at the trans side of the Golgi, marked by AP-1. Association of Arl1 with the Golgi is saturable in intact cells and depends on N-terminal myristoylation. Over-expression of Arl1(T31N), which is expected to be restricted to the GDP-bound form and thus function as a dominant-negative mutant, causes the disappearance of the Golgi apparatus (marked by Golgi SNARE GS28), suggesting that Arl1 is necessary for maintaining normal Golgi structure. Overexpression of Arl1(Q71L), a mutant restricted primarily to the activated GTP-bound form, causes an expansion of the Golgi apparatus with massive and stable Golgi association of COPI and AP-1 coats. Interestingly, Golgi ARFs also become stably associated with the expanded Golgi. Transport of the envelope protein of vesicular stomatitis virus (VSV-G) along the secretory pathway is arrested at the expanded Golgi upon expression of Arl1(Q71L). The structure of stacked cisternae of the Golgi is disrupted in cells expressing Arl1(Q71L), resulting in the transformation of the Golgi into an extensive vesicule-tubule network. In addition, the GTP form of Arl1 interacts with arfaptin-2/POR1 but not GGA1, both of which interact with GTP-restricted ARF1, suggesting that Arl1 and ARF1 share some common effectors in regulating cellular events. On the basis of these observations, we propose that one of the mechanisms for the cell to regulate the structure and function of the Golgi apparatus is through the action of Arl1.

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.

Rab22a affects the morphology and function of the endocytic pathway

Soon after endocytosis, internalized material is sorted along different pathways in a process that requires the coordinated activity of several Rab proteins. Although abundant information is available about the subcellular distribution and function of some of the endocytosis-specific Rabs (e.g. Rab5 and Rab4), very little is known about some other members of this family of proteins. To unveil some of the properties of Rab22a, one of the less studied endosome-associated small GTPases, we have expressed the protein tagged with the green fluorescent protein in CHO cells. The results indicate that Rab22a associates with early and late endosomes (labeled by a 5 minute rhodamine-transferrin uptake and the cation-independent mannose 6-phosphate receptor, respectively) but not with lysosomes (labeled by 1 hour rhodamine horseradish peroxidase uptake followed by 1 hour chase). Overexpression of the protein causes a prominent morphological enlargement of the early and late endosomes. Two mutants were generated by site-directed mutagenesis, a negative mutant (Rab22aS19N, with reduced affinity for GTP) and a constitutively active mutant (Rab22aQ64L, with reduced endogenous GTPase activity). The distribution of the negative mutant was mostly cytosolic, whereas the positive mutant associated with early and late endosomes and, interestingly also with lysosomes and autophagosomes (labeled with monodansylcadaverine). Cells expressing Rab22a wild type and Rab22aS19N displayed decreased endocytosis of a fluid phase marker. Conversely, overexpression of Rab22aQ64L, which strongly affects the morphology of endosomes, did not inhibit bulk endocytosis. Our results show that Rab22a has a unique distribution along the endocytic pathway that is not shared by any other Rab protein, and that it strongly affects the morphology and function of endosomes.

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

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

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.

Vesicle-mediated trafficking of parasite proteins to the host cell cytosol and erythrocyte surface membrane in Plasmodium falciparum infected erythrocytes

During the development of the asexual stage of the malaria parasite, Plasmodium falciparum, the composition, structure and function of the host cell membrane is dramatically altered, including the ability to adhere to vascular endothelium. Crucial to these changes is the transport of parasite proteins, which become associated with or inserted into the erythrocyte membrane. Protein and membrane targeting beyond the parasite plasma membrane must require unique pathways, given the parasites intracellular location within a parasitophorous vacuolar membrane and the lack of organelles and biosynthetic machinery in the host cell necessary to support a secretory system. It is not clear how these proteins cross the parasitophorous vacuolar membrane or how they traverse the erythrocyte cytosol to reach their final destinations. The identification of: (1) a P. falciparum homologue of the protein Sar1p, which is an essential component of the COPII-based secretory system in mammalian cells and yeast and (2) electron-dense, possibly coated, secretory vesicles bearing P. falciparum erythrocyte membrane protein 1 and P. falciparum erythrocyte membrane protein 3 in the host cell cytosol of P. falciparum infected erythrocytes recently provided the first direct evidence of a vesicle-mediated pathway for the trafficking of some parasite proteins to the erythrocyte membrane. The major advance in uncovering the parasite-induced secretory pathway was made by incubating infected erythrocytes with aluminium tetrafluoride, an activator of guanidine triphosphate-binding proteins, which resulted in the accumulation of the vesicles into multiple vesicle strings. These vesicle complexes were often associated with and closely abutted the erythrocyte membrane, but were apparently prevented from fusing by the aluminium fluoride treatment, making their capture by electron microscopy possible. It appears that malaria parasites export proteins into the host cell cytosol to support a vesicle-mediated protein trafficking pathway.

Rab3a is involved in transport of synaptic vesicles to the active zone in mouse brain nerve terminals

The rab family of GTP-binding proteins regulates membrane transport between intracellular compartments. The major rab protein in brain, rab3A, associates with synaptic vesicles. However, rab3A was shown to regulate the fusion probability of synaptic vesicles, rather than their transport and docking. We tested whether rab3A has a transport function by analyzing synaptic vesicle distribution and exocytosis in rab3A null-mutant mice. Rab3A deletion did not affect the number of vesicles and their distribution in resting nerve terminals. The secretion response upon a single depolarization was also unaffected. In normal mice, a depolarization pulse in the presence of Ca(2+) induces an accumulation of vesicles close to and docked at the active zone (recruitment). Rab3A deletion completely abolished this activity-dependent recruitment, without affecting the total number of vesicles. Concomitantly, the secretion response in the rab3A-deficient terminals recovered slowly and incompletely after exhaustive stimulation, and the replenishment of docked vesicles after exhaustive stimulation was also impaired in the absence of rab3A. These data indicate that rab3A has a function upstream of vesicle fusion in the activity-dependent transport of synaptic vesicles to and their docking at the active zone.

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.

Stabilities and isomeric equilibria in solutions of monomeric metal-ion complexes of guanosine 5'-triphosphate (GTP4-) and inosine 5'-triphosphate (ITP4-) in comparison with those of adenosine 5'-triphosphate (ATP4-)

Under experimental conditions in which the self-association of the purine-nucleoside 5'-triphosphates (PuNTPs) GTP and ITP is negligible, potentiometric pH titrations were carried out to determine the stabilities of the M(H;PuNTP) and M(PuNTP)2-complexes where M2+ = Mg2+, Ca2+, Sr2+. Ba2+, Mn2+, Co2+, Ni2+, Cu2+, Zn2+, or Cd2+ (I = 0.1 M, 25 degrees C). The stabilities of all M(GTP)2- and M(ITP)2- complexes are significantly larger than those of the corresponding complexes formed with pyrimidine-nucleoside 5'-triphosphates (PyNTPs), which had been determined previously under the same conditions. This increased complex stability is attributed, in agreement with previous 1H MNR shift studies, to the formation of macrochelates of the phosphate-coordinated metal ions with N7 of the purine residues. A similar enhanced stability (despite relatively large error limits) was observed for the M(H;PuNTP) complexes, in which H+ is bound to the terminal y-phosphate group, relative to the stability of the M(H;PyNTP)- species. The percentage of the macrochelated isomers in the M(GTP)2- and M(ITP)2- systems was quantified by employing the difference log KMM(PuNTP)-log KMM(PyNTP); the lowest and highest formation degrees of the macrochelates were observed for Mg(ITP)2- and Cu(GTP)2- with 17 +/- 11% and 97 +/- 1%, respectively. From previous studies of M(ATP)2- complexes, it is known that innersphere and outersphere macrochelates may form; that is, in the latter case a water molecule is between N7 and the phosphate-coordinated M2+. Similar conclusions are reached now by comparisons with earlier 1H MNR shift measurements, that is, that Mg(GTP)2- (21 +/- 11%), for example, exists largely in the form of an outersphere macrochelate and Zn(GTP)2- (68 +/- 4%) as an innersphere one. Generally, the overall percentage of macrochelate falls off for a given metal ion in the order M(GTP)2- > M(ITP)2- > M(ATP)2-; this is in accord with the decreasing basicity of N7 and the steric inhibition of the (C6)NH2 group in the adenine residue. Furthermore, although the absolute stability constants of the previously studied M(GMP), M(IMP), and M(AMP) complexes differ by about two to three log units from the present M(PuNTP)2- results, the formation degrees of the macrochelates are astonishingly similar for the two series of nucleotides for a given metal ion and purine-nucleobase residue. The conclusion that N7 of the guanine residue is an especially favored binding site for metal ions is also in accord with observations made for nucleic acids.

Rim1 and rabphilin-3 bind Rab3-GTP by composite determinants partially related through N-terminal alpha -helix motifs

Rim1 is a protein of the presynaptic active zone, the area of the plasma membrane specialized for neurotransmitter exocytosis, and interacts with Rab3, a small GTPase implicated in neurotransmitter vesicle dynamics. Here, we have studied the molecular determinants of Rim1 that are responsible for Rab3 binding, employing surface plasmon resonance and recombinant, bacterially expressed Rab3 and Rim1 proteins. A site that binds GTP- but not GDP-saturated Rab3 was localized to a short alpha-helical sequence near the Rim1 N terminus (amino acids 19-55). Rab3 isoforms A, C, and D were bound with similar affinities (K(d) = 1-2 microm). Low affinity binding of Rab6A-GTP was also observed (K(d) = 16 microm), whereas Rab1B, -5, -7, -8, or -11A did not bind. Adjacent sequences up to amino acid 387, encompassing differentially spliced sequences, the zinc finger module, and the SGAWFF motif of Rim1, did not significantly contribute to the strength or the specificity of Rab3 binding, whereas a point mutation within the helix (R33G) abolished binding. This Rab3 binding site of Rim1 is reminiscent of the N-terminal alpha-helix that is part of the Rab3-binding region of rabphilin-3, and indeed we observed low affinity, specific binding of Rab3A (K(d) on the order of magnitude of 10-100 microm) to this region of rabphilin-3 alone (amino acids 40-88), whereas additional sequences up to amino acid 178 are needed for high affinity Rab3A binding to rabphilin-3 (K(d) = 10-20 nm). In contrast, an N-terminal alpha-helix motif in aczonin, with sequence similarity to the Rab3-binding site of Rim1, did not bind Rab3A, -C, or -D or several other Rab proteins. These results were qualitatively confirmed in pull-down experiments with native, prenylated Rab3 from brain lysate in Triton X-100. Munc13 bound to the zinc finger domain of Rim1 but not to the rabphilin-3 or aczonin zinc fingers. Pull-down experiments from brain lysate in the presence of cholate as detergent detected binding to downstream Rim1 sequences, between amino acids 56 and 387, of syntaxin and of Rab3. The latter, however, was inhibited rather than stimulated by GTP.

GRAB: a physiologic guanine nucleotide exchange factor for Rab3A, which interacts with inositol hexakisphosphate kinase

Diphosphoinositol-pentakisphosphate (InsP7) and bis-diphosphoinositol tetrakisphosphate (InsP8) possess pyrophosphate bonds. InsP7 is formed from inositol hexakisphosphate (InsP6) by recently identified InsP6 kinases designated InsP6K1 and InsP6K2. We now report the identification, cloning, and characterization of a novel protein, GRAB (guanine nucleotide exchange factor for Rab3A), which interacts with both InsP6K1 and Rab3A, a Ras-like GTPase that regulates synaptic vesicle exocytosis. GRAB is a physiologic GEF (guanine nucleotide exchange factor) for Rab3A. Consistent with a role of Rab3A in synaptic vesicle exocytosis, GRAB regulates depolarization-induced release of dopamine from PC12 cells and nicotinic agonist-induced hGH release from bovine adrenal chromaffin cells. The association of InsP6K1 with GRAB fits with a role for InsP7 in vesicle exocytosis.

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.

Vps45p stabilizes the syntaxin homologue Tlg2p and positively regulates SNARE complex formation

Sec1p-like/Munc-18 (SM) proteins bind to t-SNAREs and inhibit ternary complex formation. Paradoxically, the absence of SM proteins does not result in constitutive membrane fusion. Here, we show that in yeast cells lacking the SM protein Vps45p, the t-SNARE Tlg2p is down-regulated, to undetectable levels, by rapid proteasomal degradation. In the absence of Vps45p, Tlg2p can be stabilized through abolition of proteasome activity. Surprisingly, the stabilized Tlg2p was targeted to the correct intracellular location. However, the stabilized Tlg2p is non-functional and unable to bind its cognate SNARE binding partners, Tlg1p and Vti1p, in the absence of Vps45p. A truncation mutant lacking the first 230 residues of Tlg2p no longer bound Vps45p but was able to form complexes with Tlg1p and Vti1p in the absence of the SM protein. These data provide us with two valuable insights into the function of SM proteins. First, SM proteins act as chaperone-like molecules for their cognate t-SNAREs. Secondly, SM proteins play an essential role in the activation process allowing their cognate t-SNARE to participate in ternary complex formation.

The class C Vps complex functions at multiple stages of the vacuolar transport pathway

The Class C Vps complex, consisting of Vps11, Vps16, Vps18, and Vps33, is required for SNARE-mediated membrane fusion at the lysosome-like yeast vacuole. However, Class C vps mutants display more severe and pleiotropic phenotypes than mutants specifically defective in endosome-to-vacuole transport, suggesting that there are additional functions for the Class C Vps complex. A SNARE double mutant which is defective for both Golgi-to-endosome and endosome-to-vacuole trafficking replicates many of the phenotypes observed in Class C vps mutants. We show that genetic interactions exist between Class C vps alleles and alleles of the Class D vps group, which are defective in the docking and fusion of Golgi-derived vesicles at the endosome. Moreover, the Class D protein Vac1 was found to physically bind to the Class C Vps complex through a direct association with Vps11. Finally, using a random mutagenic screen, a temperature-conditional allele which shares many of the phenotypes of mutants which are selectively defective in Golgi-to-endosome trafficking was isolated (vps11-3ts). Collectively, these results indicate that the Class C Vps complex plays essential roles in the processes of membrane docking and fusion at both the Golgi-to-endosome and endosome-to-vacuole stages of transport.

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

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

SopE acts as an Rab5-specific nucleotide exchange factor and recruits non-prenylated Rab5 on Salmonella-containing phagosomes to promote fusion with early endosomes

Rab-GTPase regulates the fusion between two specific vesicles. It is well documented that, for their biological function, Rab proteins need to be prenylated for attachment to the vesicle membrane. In contrast, we showed in the present investigation that SopE, a type III secretory protein of Salmonella, translocates onto Salmonella-containing phagosomes (LSP) and mediates the recruitment of non-prenylated Rab5 (Rab5:DeltaC4) on LSP in GTP form. Simultaneously, SopE present in infected cell cytosol acts as an Rab5-specific exchange factor and converts the inactive Rab-GDP to the GTP form. The non-prenylated Rab5 subsequently promoted efficient fusion of LSP with early endosomes. This is the first demonstration that a prenylation-deficient Rab protein retains biological activity and can promote vesicle fusion, if it is recruited on the membrane by some other method.

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.

Role of Rab9 GTPase in facilitating receptor recruitment by TIP47

Mannose 6-phosphate receptors (MPRs) deliver lysosomal hydrolases from the Golgi to endosomes and then return to the Golgi complex. TIP47 recognizes the cytoplasmic domains of MPRs and is required for endosome-to-Golgi transport. Here we show that TIP47 also bound directly to the Rab9 guanosine triphosphatase (GTPase) in its active, GTP-bound conformation. Moreover, Rab9 increased the affinity of TIP47 for its cargo. A functional Rab9 binding site was required for TIP47 stimulation of MPR transport in vivo. Thus, a cytosolic cargo selection device may be selectively recruited onto a specific organelle, and vesicle budding might be coupled to the presence of an active Rab GTPase.

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.

Synexin and GTP increase surfactant secretion in permeabilized alveolar type II cells

We have previously suggested that synexin (annexin VII), a Ca(2+)-dependent phospholipid binding protein, may have a role in surfactant secretion, since it promotes membrane fusion between isolated lamellar bodies (the surfactant-containing organelles) and plasma membranes. In this study, we investigated whether exogenous synexin can augment surfactant phosphatidylcholine (PC) secretion in synexin-deficient lung epithelial type II cells. Isolated rat type II cells were cultured for 20-22 h with [(3)H]choline to label cellular PC. The cells were then treated with beta-escin, which forms pores in the cell membrane and releases cytoplasmic proteins including synexin. These cells, however, retained lamellar bodies. The permeabilized type II cells were evaluated for PC secretion during a 30-min incubation. Compared with PC secretion under basal conditions, the presence of Ca(2+) (up to 10 microM) did not increase PC secretion. In the presence of 1 microM Ca(2+), synexin increased PC secretion in a concentration-dependent manner, which reached a maximum at approximately 5 microg/ml synexin. The secretagogue effect of synexin was abolished when synexin was inactivated by heat treatment (30 min at 65 degrees C) or by treatment with synexin antibodies. GTP or its nonhydrolyzable analog beta:gamma-imidoguanosine-5'-triphosphate also increased PC secretion in permeabilized type II cells. The PC secretion was further increased in an additive manner when a maximally effective concentration of synexin was added in the presence of 1 mM GTP, suggesting that GTP acts by a synexin-independent mechanism to increase membrane fusion. Thus our results support a direct role for synexin in surfactant secretion. Our study also suggests that membrane fusion during surfactant secretion may be mediated by two independent mechanisms.

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.

Vesicular trafficking machinery, the actin cytoskeleton, and H+-K+-ATPase recycling in the gastric parietal cell

Gastric HCl secretion by the parietal cell involves the secretagogue-regulated re-cycling of the H+-K+-ATPase at the apical membrane. The trafficking of the H+-K+-ATPase and the remodelling of the apical membrane during this process are likely to involve the co-ordination of the function of vesicular trafficking machinery and the cytoskeleton. This review summarizes the progress made in the identification and characterization of components of the vesicular trafficking machinery that are associated with the H+-K+-ATPase and of components of the actin-based cytoskeleton that are associated with the apical membrane of the parietal cell. Since many of these proteins are also expressed at the apical pole of other epithelial cells, the parietal cell may represent a model system to characterize the protein- protein interactions that regulate apical membrane trafficking in many other epithelial 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.