The exocyst complex in polarized exocytosis

Exo70 interacts with phospholipids and mediates the targeting of the exocyst to the plasma membrane

The exocyst is an octameric protein complex implicated in the tethering of post-Golgi secretory vesicles to the plasma membrane before fusion. The function of individual exocyst components and the mechanism by which this tethering complex is targeted to sites of secretion are not clear. In this study, we report that the exocyst subunit Exo70 functions in concert with Sec3 to anchor the exocyst to the plasma membrane. We found that the C-terminal Domain D of Exo70 directly interacts with phosphatidylinositol 4,5-bisphosphate. In addition, we have identified key residues on Exo70 that are critical for its interaction with phospholipids and the small GTPase Rho3. Further genetic and cell biological analyses suggest that the interaction of Exo70 with phospholipids, but not Rho3, is essential for the membrane association of the exocyst complex. We propose that Exo70 mediates the assembly of the exocyst complex at the plasma membrane, which is a crucial step in the tethering of post-Golgi secretory vesicles for exocytosis.

Phosphatidylinositol 4,5-bisphosphate mediates the targeting of the exocyst to the plasma membrane for exocytosis in mammalian cells

The exocyst is an evolutionarily conserved octameric protein complex that tethers post-Golgi secretory vesicles at the plasma membrane for exocytosis. To elucidate the mechanism of vesicle tethering, it is important to understand how the exocyst physically associates with the plasma membrane (PM). In this study, we report that the mammalian exocyst subunit Exo70 associates with the PM through its direct interaction with phosphatidylinositol 4,5-bisphosphate (PI(4,5)P(2)). Furthermore, we have identified key conserved residues at the C-terminus of Exo70 that are crucial for the interaction of Exo70 with PI(4,5)P(2). Disrupting Exo70-PI(4,5)P(2) interaction abolished the membrane association of Exo70. We have also found that wild-type Exo70 but not the PI(4,5)P(2)-binding-deficient Exo70 mutant is capable of recruiting other exocyst components to the PM. Using the ts045 vesicular stomatitis virus glycoprotein trafficking assay, we demonstrate that Exo70-PI(4,5)P(2) interaction is critical for the docking and fusion of post-Golgi secretory vesicles, but not for their transport to the PM.

An NGF-induced Exo70-TC10 complex locally antagonises Cdc42-mediated activation of N-WASP to modulate neurite outgrowth

NGF-induced differentiation of PC12 cells is mediated by actin-polymerisation-driven membrane protrusion, involving GTPase signalling pathways that activate actin nucleation promoting factors such as the neural Wiskott-Aldrich syndrome protein (N-WASP). Expression of the exocyst subunit Exo70 in PC12 cells and neurons leads to the generation of numerous membrane protrusions, an effect that is strongly potentiated upon NGF-induced differentiation. Förster resonance energy transfer (FRET) imaging by fluorescence lifetime microscopy (FLIM) reveals an NGF-induced interaction of activated TC10 with Exo70. Expression of dominant-negative mutants and siRNA-mediated knockdown implicates N-WASP in NGF-induced Exo70-TC10-mediated membrane protrusion. However, FRET imaging of N-WASP activation levels of cells expressing Exo70 and/or constitutively active TC10 reveals that this complex locally antagonises the NGF-induced activation of N-WASP in membrane protrusions. Experiments involving siRNA-mediated knockdown of Cdc42 and overexpression of constitutively active Cdc42 confirm that the Exo70-TC10 complex mainly targets the NGF-induced Cdc42-dependent activation of N-WASP. Our results show that Exo70 is responsible for the correct targeting of the Exo70-TC10 complex to sites of membrane protrusion. The functional uncoupling between both pathways represents a novel regulatory mechanism that enables switching between morphologically distinct – Cdc42- or TC10-dominated – forms of cellular membrane outgrowth.

Functions of endosomal trafficking in Drosophila epithelial cells

The mechanisms underlying endosomal trafficking have been mostly dissected in yeast and mammalian tissue culture cells. Here, we review recent advances in the understanding of the role of endosomal trafficking in Drosophila epithelial cells. We focus on endosomal pathways that control cell polarization, cell growth, cell fate and epithelial cell rearrangement. We expect that mechanistic studies in mammalian cells and functional studies in invertebrates will continue to synergize to provide a comprehensive view of the role of endosomal trafficking in epithelial tissue organization and functions.

Splitting of the fission yeast septum

In cell-walled organisms, a cross wall (septum) is produced during cytokinesis, which then splits in certain organisms to allow the daughter cells to separate. The formation and the subsequent cleavage of the septum require wall synthesis and wall degradation, which need to be strictly coordinated in order to prevent cell lysis. The dividing fission yeast (Schizosaccharomyces) cell produces a three-layered septum in which the middle layer and a narrow band of the adjacent cell wall can be degraded without threatening the integrity of the separating daughter cells. This spatially very precise process requires the activity of the Agn1p 1,3-alpha-glucanase and the Eng1p 1,3-beta-glucanase, which are localized to the septum by a complex mechanism involving the formation of a septin ring and the directed activity of the exocyst system. The Sep1p-Ace2p transcription-factor cascade regulates the expression of many genes producing proteins for this complex process. Recent advances in research into the molecular mechanisms of separation and its regulation are discussed in this review.

Candida albicans hyphal morphogenesis occurs in Sec3p-independent and Sec3p-dependent phases separated by septin ring formation

The growing tips of Candida albicans hyphae are sites of polarized exocytosis. Mammalian septins have been implicated in regulating exocytosis and C. albicans septins are known to localize at hyphal tips, although their function here is unknown. Here, we report that C. albicans cells deleted of the exocyst subunit gene SEC3 can grow normal germ tubes, but are unable to maintain tip growth after assembly of the first septin ring, resulting in isotropic expansion of the tip. Deleting either of the septin genes CDC10 or CDC11 caused Sec3p mislocalization and surprisingly, also restored hyphal development in the sec3Δ mutant without rescuing the temperature sensitivity. Co-immunoprecipitation experiments detected association of the septin Cdc3p with the exocyst subunits Sec3p and Sec5p. Our results reveal that C. albicans hyphal development occurs through Sec3p-independent and dependent phases, and provide strong genetic and biochemical evidence for a role of septins in polarized exocytosis.

Receptor-mediated regulation of tomosyn-syntaxin 1A interactions in bovine adrenal chromaffin cells

Tomosyn, a soluble R-SNARE protein identified as a binding partner of the Q-SNARE syntaxin 1A, is thought to be critical in setting the level of fusion-competent SNARE complexes for neurosecretion. To date, there has been no direct evaluation of the dynamics in which tomosyn transits through tomosyn-SNARE complexes or of the extent to which tomosyn-SNARE complexes are regulated by secretory demand. Here, we employed biochemical and optical approaches to characterize the dynamic properties of tomosyn-syntaxin 1A complexes in live adrenal chromaffin cells. We demonstrate that secretagogue stimulation results in the rapid translocation of tomosyn from the cytosol to plasma membrane regions and that this translocation is associated with an increase in the tomosyn-syntaxin 1A interaction, including increased cycling of tomosyn into tomosyn-SNARE complexes. The secretagogue-induced interaction was strongly reduced by pharmacological inhibition of the Rho-associated coiled-coil forming kinase, a result consistent with findings demonstrating secretagogue-induced activation of RhoA. Stimulation of chromaffin cells with lysophosphatidic acid, a nonsecretory stimulus that strongly activates RhoA, resulted in effects on tomosyn similar to that of application of the secretagogue. In PC-12 cells overexpressing tomosyn, secretagogue stimulation in the presence of lysophosphatidic acid resulted in reduced evoked secretory responses, an effect that was eliminated upon inhibition of Rho-associated coiled-coil forming kinase. Moreover, this effect required an intact interaction between tomosyn and syntaxin 1A. Thus, modulation of the tomosyn-syntaxin 1A interaction in response to secretagogue activation is an important mechanism allowing for dynamic regulation of the secretory response.

Exo70p mediates the secretion of specific exocytic vesicles at early stages of the cell cycle for polarized cell growth

In budding yeast, two classes of post-Golgi secretory vesicles carrying different sets of cargoes typified by Bgl2p and invertase are delivered to the plasma membrane for secretion. The exocyst is implicated in tethering these vesicles to the daughter cell membrane for exocytosis. In this study, we report that mutations in the exocyst component Exo70p predominantly block secretion of the Bgl2p vesicles. Furthermore, a defect in invertase vesicle trafficking caused by vps1Δ or pep12Δ in the exo70 mutant background is detrimental to the cell. The secretion defect in exo70 mutants was most pronounced during the early budding stage, which affected daughter cell growth. The selective secretion block does not occur at the vesicle formation or sorting stage because the exocytic vesicles are properly generated and protein processing is normal in the exo70 mutants. Our study suggests that Exo70p functions primarily at early stages of the cell cycle in Bgl2p vesicle secretion, which is critical for polarized cell growth.

Rab6 mediates membrane organization and determinant localization during Drosophila oogenesis

The Drosophila melanogaster body axes are defined by the precise localization and the restriction of molecular determinants in the oocyte. Polarization of the oocyte during oogenesis is vital for this process. The directed traffic of membranes and proteins is a crucial component of polarity establishment in various cell types and organisms. Here, we investigate the role of the small GTPase Rab6 in the organization of the egg chamber and in asymmetric determinant localization during oogenesis. We show that exocytosis is affected in rab6-null egg chambers, which display a loss of nurse cell plasma membranes. Rab6 is also required for the polarization of the oocyte microtubule cytoskeleton and for the posterior localization of oskar mRNA. We show that, in vivo, Rab6 is found in a complex with Bicaudal-D, and that Rab6 and Bicaudal-D cooperate in oskar mRNA localization. Thus, during Drosophila oogenesis, Rab6-dependent membrane trafficking is doubly required; first, for the general organization and growth of the egg chamber, and second, more specifically, for the polarization of the microtubule cytoskeleton and localization of oskar mRNA. These findings highlight the central role of vesicular trafficking in the establishment of polarity and in determinant localization in Drosophila.

The molecular mechanisms of the mammalian exocyst complex in exocytosis

Exocytosis is a highly ordered vesicle trafficking pathway that targets proteins to the plasma membrane for membrane addition or secretion. Research over the years has discovered many proteins that participate at various stages in the mammalian exocytotic pathway. At the early stage of exocytosis, co-atomer proteins and their respective adaptors and GTPases have been shown to play a role in the sorting and incorporation of proteins into secretory vesicles. At the final stage of exocytosis, SNAREs (soluble N-ethylmaleimide-sensitive fusion protein attachment protein receptor) and SNARE-associated proteins are believed to mediate the fusion of secretory vesicles at the plasma membrane. There are multiple events that may occur between the budding of secretory vesicles from the Golgi and the fusion of these vesicles at the plasma membrane. The most obvious and best-known event is the transport of secretory vesicles from Golgi to the vicinity of the plasma membrane via microtubules and their associated motors. At the vicinity of the plasma membrane, however, it is not clear how vesicles finally dock and fuse with the plasma membrane. Identification of proteins involved in these events should provide important insights into the mechanisms of this little known stage of the exocytotic pathway. Currently, a protein complex, known as the sec6/8 or the exocyst complex, has been implicated to play a role at this late stage of exocytosis.

From protein-protein complexes to interactomics

Protein-protein interactions (or PPIs) are key elements for the normal functioning of a living cell. A large description of the protein interactomics field is given in this review where different aspects will be discussed. We first give an introduction of the different large scale experimental approaches from yeast two-hybrid to mass spectrometry used to discover PPIs and build protein interaction maps. Single PPI validation techniques such as co-immunoprecipitation or fluorescence methods are then presented as they are more and more integrated in global PPI discovery strategy. Data from different experimental sets are compared and an assessment of the different large scale technologies is presented. Bioinformatics tools can also predict with a good accuracy PPIs in silico, PPIs databases are now numerous and topological analysis has led to interesting insights into the nature of network connection. Finally, PPI, as an association of two proteins, has been structurally characterized for many protein complexes and is largely discussed throughout existing examples. The results obtained so far already provide the biologist with a large set of structured data from which knowledge on pathways and associated protein function can be extracted.

Exo70 interacts with the Arp2/3 complex and regulates cell migration

The exocyst is a multiprotein complex essential for tethering secretory vesicles to specific domains of the plasma membrane for exocytosis. Here, we report that the exocyst component Exo70 interacts with the Arp2/3 complex, a key regulator of actin polymerization. We further show that the exocyst-Arp2/3 interaction is regulated by epidermal growth factor (EGF) signalling. Inhibition of Exo70 by RNA interference (RNAi) or antibody microinjection blocks the formation of actin-based membrane protrusions and affects various aspects of cell motility. We propose that Exo70, in addition to functioning in exocytosis, also regulates actin at the leading edges of migrating cells, therefore coordinating cytoskeleton and membrane traffic during cell migration.

The yeast actin cytoskeleton: from cellular function to biochemical mechanism

All cells undergo rapid remodeling of their actin networks to regulate such critical processes as endocytosis, cytokinesis, cell polarity, and cell morphogenesis. These events are driven by the coordinated activities of a set of 20 to 30 highly conserved actin-associated proteins, in addition to many cell-specific actin-associated proteins and numerous upstream signaling molecules. The combined activities of these factors control with exquisite precision the spatial and temporal assembly of actin structures and ensure dynamic turnover of actin structures such that cells can rapidly alter their cytoskeletons in response to internal and external cues. One of the most exciting principles to emerge from the last decade of research on actin is that the assembly of architecturally diverse actin structures is governed by highly conserved machinery and mechanisms. With this realization, it has become apparent that pioneering efforts in budding yeast have contributed substantially to defining the universal mechanisms regulating actin dynamics in eukaryotes. In this review, we first describe the filamentous actin structures found in Saccharomyces cerevisiae (patches, cables, and rings) and their physiological functions, and then we discuss in detail the specific roles of actin-associated proteins and their biochemical mechanisms of action.

AtEXO70A1, a member of a family of putative exocyst subunits specifically expanded in land plants, is important for polar growth and plant development

The exocyst is a hetero-oligomeric protein complex involved in exocytosis and has been extensively studied in yeast and animal cells. Evidence is now accumulating that the exocyst is also present in plants. Bioinformatic analysis of genes encoding plant homologs of the exocyst subunit, Exo70, revealed that three Exo70 subgroups are evolutionarily conserved among angiosperms, lycophytes and mosses. Arabidopsis and rice contain 22 and approximately 39 EXO70 genes, respectively, which can be classified into nine clusters considered to be ancient in angiosperms (one has been lost in Arabidopsis). We characterized two independent T-DNA insertional mutants of the AtEXO70A1 gene (exo70A1-1 and exo70A1-2). Heterozygous EXO70A1/exo70A1 plants appear to be normal and segregate in a 1:2:1 ratio, suggesting that neither male nor female gametophytes are affected by the EXO70A1 disruption. However, both exo70A1-1 and exo70A1-2 homozygotes exhibit an array of phenotypic defects. The polar growth of root hairs and stigmatic papillae is disturbed. Organs are generally smaller, plants show a loss of apical dominance and indeterminate growth where instead of floral meristems new lateral inflorescences are initiated in a reiterative manner. Both exo70A1 mutants have dramatically reduced fertility. These results suggest that the putative exocyst subunit EXO70A1 is involved in cell and organ morphogenesis.

Organelles and trafficking machinery for postsynaptic plasticity

Neurons are among the largest and most complex cells in the body. Their immense size and intricate geometry pose many unique cell-biological problems. How is dendritic architecture established and maintained? How do neurons traffic newly synthesized integral membrane proteins over such long distances to synapses? Functionally, protein trafficking to and from the postsynaptic membrane has emerged as a key mechanism underlying various forms of synaptic plasticity. Which organelles are involved in postsynaptic trafficking, and how do they integrate and respond to activity at individual synapses? Here we review what is currently known about long-range trafficking of newly synthesized postsynaptic proteins as well as the local rules that govern postsynaptic trafficking at individual synapses.

Ral GTPases: corrupting the exocyst in cancer cells

The Ras-like small G-proteins RalA and RalB have achieved some notoriety as components of one of a growing variety of candidate Ras effector pathways. Recent work has demonstrated that Ral GTPase activation is required to support both the initiation and maintenance of tumorigenic transformation of human cells. The mechanistic basis for this support remains to be defined. However, the discovery that the exocyst is a direct effector complex for activated Ral proteins suggests that mobilization of polarized exocytosis might be a basic component of the biological framework supporting tumorigenic progression.

Rho GTPase signaling in Dictyostelium discoideum: Insights from the genome

Rho GTPases are ubiquitously expressed across the eukaryotes where they act as molecular switches participating in the regulation of many cellular processes. We present an inventory of proteins involved in Rho-regulated signaling pathways in Dictyostelium discoideum that have been identified in the completed genome sequence. In Dictyostelium the Rho family is encoded by 18 genes and one pseudogene. Some of the Rho GTPases (Rac1a/b/c, RacF1/F2 and RacB) are members of the Rac subfamily, and one, RacA, belongs to the RhoBTB subfamily. The Cdc42 and Rho subfamilies, characteristic of metazoa and fungi, are absent. The activities of these GTPases are regulated by two members of the RhoGDI family, by eight members of the Dock180/zizimin family and by a surprisingly large number of proteins carrying RhoGEF (42 genes) or RhoGAP (43 genes) domains or both (three genes). Most of these show domain compositions not found in other organisms, although some have clear homologs in metazoa and/or fungi. Among the (in many cases putative) effectors found in Dictyostelium are the CRIB domain proteins (WASP and two related proteins, eight PAK kinases and a novel gelsolin-related protein), components of the Scar/WAVE complex, 10 formins, four IQGAPs, two members of the PCH family, numerous lipid kinases and phospholipases, and components of the NADPH oxidase and the exocyst complexes. In general, the repertoire of Rho signaling components of Dictyostelium is similar to that of metazoa and fungi.

Homologues of Oxysterol-Binding Proteins Affect Cdc42p- and Rho1p-Mediated Cell Polarization in Saccharomyces cerevisiae

Polarized cell growth requires the establishment of an axis of growth along which secretion can be targeted to a specific site on the cell cortex. How polarity establishment and secretion are choreographed is not fully understood, though Rho GTPase- and Rab GTPase-mediated signaling is required. Superimposed on this regulation are the functions of specific lipids and their cognate binding proteins. In a screen for Saccharomyces cerevisiae genes that interact with Rho family CDC42 to promote polarity establishment, we identified KES1/OSH4, which encodes a homologue of mammalian oxysterol-binding protein (OSBP). Other yeast OSH genes (OSBP homologues) had comparable genetic interactions with CDC42, implicating OSH genes in the regulation of CDC42-dependent polarity establishment. We found that the OSH gene family (OSH1-OSH7) promotes cell polarization by maintaining the proper localization of septins, the Rho GTPases Cdc42p and Rho1p, and the Rab GTPase Sec4p. Disruption of all OSH gene function caused specific defects in polarized exocytosis, indicating that the Osh proteins are collectively required for a secretory pathway implicated in the maintenance of polarized growth.

Endosome dynamics during development

Endocytosis has traditionally been studied in isolated cells. More recently, however, the analysis of protein trafficking in whole organisms has revealed that it plays exciting roles during development. Endocytic trafficking of cell adhesion molecules regulates epithelial polarity and cell migration. Developmental signaling pathways are regulated by the trafficking of receptors and their ligands through the endocytic pathway. Finally, impairment of the endocytic machinery can affect proliferation control and contribute to tumor development.

Lethal giant puzzle of Lgl

Cell polarity is one of the most basic properties of all normal cells and loss of polarity is a hallmark of cancer. While multiple proteins have been implicated in the maintenance of cell polarity, the functionally related neoplastic tumor suppressors Lethal giant larvae (Lgl), Scribble and Disks large comprise a unique group of molecules that are not only involved in the maintenance of cell polarity, but also in the regulation of cell proliferation and cancer. Lgl is the first identified member of this group. Loss of Lgl leads to massive tissue disorganization, tumor-like growth and lethal phenotypes in both Drosophila and mice. Lgl mutant cells display disruption of cell polarity, failure of asymmetric cell division, deregulation of Notch signaling and loss of proper cell fate determination. Lgl is a critical downstream target of the Par6/aPKC cell polarity complex; however, the functional role of Lgl itself and, specifically, the mechanisms of Lgl function in cell polarity and regulation of cell proliferation remain enigmatic. This minireview summarizes available information and discusses potential mechanisms of Lgl function.

Light-induced exocytosis in cell development and differentiation

Calcium-dependent exocytosis of fluorescently labeled single secretory vesicles in PC12 cells and primary embryonic telencephalon cells can be triggered by illumination with visible light and imaged by TIRF or epifluorescence microscopy. Opsin 3 was identified by quantitative PCR expression analysis as the putative light receptor molecule for light-induced exocytosis. In primary chicken telencephalon cells, light-induced exocytosis is restricted to a specific period during embryonic development, and involves fusion of rather large vesicles. Strictly calcium-dependent exocytosis starts after a delay of a few seconds of illumination and lasts for up to 2 min. We analyzed the frequency, time course and spatial distribution of exocytotic events. Exocytosis in PC12 cells and telencephalon cells occurs at the periphery or the interface between dividing cells, and the duration of single secretion events varies considerably. Our observation strongly supports the idea that light induced exocytosis is most likely a mechanism for building plasma membrane during differentiation, development and proliferation rather than for calcium-dependent neurotransmitter release.

Purification of active HOPS complex reveals its affinities for phosphoinositides and the SNARE Vam7p

Coupling of Rab GTPase activation and SNARE complex assembly during membrane fusion is poorly understood. The homotypic fusion and vacuole protein sorting (HOPS) complex links these two processes: it is an effector for the vacuolar Rab GTPase Ypt7p and is required for vacuolar SNARE complex assembly. We now report that pure, active HOPS complex binds phosphoinositides and the PX domain of the vacuolar SNARE protein Vam7p. These binding interactions support HOPS complex association with the vacuole and explain its enrichment at the same microdomains on docked vacuoles as phosphoinositides, Ypt7p, Vam7p, and the other SNARE proteins. Concentration of the HOPS complex at these microdomains may be a key factor for coupling Rab GTPase activation to SNARE complex assembly.

Systemic RNAi of the cockroach vitellogenin receptor results in a phenotype similar to that of the Drosophila yolkless mutant

During vitellogenesis, one of the most tightly regulated processes in oviparous reproduction, vitellogenins are incorporated into the oocyte through vitellogenin receptor (VgR)-mediated endocytosis. In this paper, we report the cloning of the VgR cDNA from Blattella germanica, as well as the first functional analysis of VgR following an RNA interference (RNAi) approach. We characterized the VgR, VgR mRNA and protein expression patterns in pre-adult and adult stages of this cockroach, as well as VgR immunolocalization in ovarioles, belonging to the panoistic type. We then specifically disrupted VgR gene function using RNAi techniques. Knockdown of VgR expression led to a phenotype characterized by low yolk content in the ovary and high vitellogenin concentration in the haemolymph. This phenotype is equivalent to that of the yolkless mutant of Drosophila melanogaster, which have the yl (VgR) gene disrupted. The results additionally open the perspective that development genes can be functionally analyzed via systemic RNAi in this basal species.

Compartmentalization of the exocyst complex in lipid rafts controls Glut4 vesicle tethering

Lipid raft microdomains act as organizing centers for signal transduction. We report here that the exocyst complex, consisting of Exo70, Sec6, and Sec8, regulates the compartmentalization of Glut4-containing vesicles at lipid raft domains in adipocytes. Exo70 is recruited by the G protein TC10 after activation by insulin and brings with it Sec6 and Sec8. Knockdowns of these proteins block insulin-stimulated glucose uptake. Moreover, their targeting to lipid rafts is required for glucose uptake and Glut4 docking at the plasma membrane. The assembly of this complex also requires the PDZ domain protein SAP97, a member of the MAGUKs family, which binds to Sec8 upon its translocation to the lipid raft. Exocyst assembly at lipid rafts sets up targeting sites for Glut4 vesicles, which transiently associate with these microdomains upon stimulation of cells with insulin. These results suggest that the TC10/exocyst complex/SAP97 axis plays an important role in the tethering of Glut4 vesicles to the plasma membrane in adipocytes.

AS160, the Akt substrate regulating GLUT4 translocation, has a functional Rab GTPase-activating protein domain

Recently, we described a 160 kDa protein (designated AS160, for Akt substrate of 160 kDa) with a predicted Rab GAP (GTPase-activating protein) domain that is phosphorylated on multiple sites by the protein kinase Akt. Phosphorylation of AS160 in adipocytes is required for insulin-stimulated translocation of the glucose transporter GLUT4 to the plasma membrane. The aim of the present study was to determine whether AS160 is in fact a GAP for Rabs, and, if so, what its specificity is. We first identified a group of 16 Rabs in a preparation of intracellular vesicles containing GLUT4 by MS. We then prepared the recombinant GAP domain of AS160 and examined its activity against many of these Rabs, as well as several others. The GAP domain was active against Rabs 2A, 8A, 10 and 14. There was no significant activity against 14 other Rabs. GAP activity was further validated by the finding that the recombinant GAP domain with the predicted catalytic arginine residue replaced by lysine was inactive. Finally, it was found by immunoblotting that Rabs 2A, 8A and 14 are present in GLUT4 vesicles. These results indicate that AS160 is a Rab GAP, and suggest novel Rabs that may participate in GLUT4 translocation.

Cell polarity in filamentous fungi: shaping the mold

The formation of highly polarized hyphae that grow by apical extension is a defining feature of the filamentous fungi. High-resolution microscopy and mathematical modeling have revealed the importance of the cytoskeleton and the Spitzenkorper (an apical vesicle cluster) in hyphal morphogenesis. However, the underlying molecular mechanisms remain poorly characterized. In this review, the pathways and functions known to be involved in polarized hyphal growth are summarized. A central theme is the notion that the polarized growth of hyphae is more complex than in yeast, though similar sets of core pathways are likely utilized. In addition, a model for the establishment and maintenance of hyphal polarity is presented. Key features of the model include the idea that polarity establishment is a stochastic process that occurs independent of internal landmarks. Moreover, the stabilization of nascent polarity axes may be the critical step that permits the emergence of a new hypha.

Increased synaptic microtubules and altered synapse development in Drosophila sec8 mutants

Background

Sec8 is highly expressed in mammalian nervous systems and has been proposed to play a role in several aspects of neural development and function, including neurite outgrowth, calcium-dependent neurotransmitter secretion, trafficking of ionotropic glutamate receptors and regulation of neuronal microtubule assembly. However, these models have never been tested in vivo. Nervous system development and function have not been described after mutation of sec8 in any organism.

Results

We identified lethal sec8 mutants in an unbiased forward genetic screen for mutations causing defects in development of glutamatergic Drosophila neuromuscular junctions (NMJs). The Drosophila NMJ is genetically malleable and accessible throughout development to electrophysiology and immunocytochemistry, making it ideal for examination of the sec8 mutant synaptic phenotype. We developed antibodies to Drosophila Sec8 and showed that Sec8 is abundant at the NMJ. In our sec8 null mutants, in which the sec8 gene is specifically deleted, Sec8 immunoreactivity at the NMJ is eliminated but immunoblots reveal substantial maternal contribution in the rest of the animal. Contrary to the hypothesis that Sec8 is required for neurite outgrowth or synaptic terminal growth, immunocytochemical examination revealed that sec8 mutant NMJs developed more branches and presynaptic terminals during larval development, compared to controls. Synaptic electrophysiology showed no evidence that Sec8 is required for basal neurotransmission, though glutamate receptor trafficking was mildly disrupted in sec8 mutants. The most dramatic NMJ phenotype in sec8 mutants was an increase in synaptic microtubule density, which was approximately doubled compared to controls.

Conclusion

Sec8 is abundant in the Drosophila NMJ. Sec8 is required in vivo for regulation of synaptic microtubule formation, and (probably secondarily) regulation of synaptic growth and glutamate receptor trafficking. We did not find any evidence that Sec8 is required for basal neurotransmission.

Sec15, a component of the exocyst, promotes notch signaling during the asymmetric division of Drosophila sensory organ precursors

Asymmetric division of sensory organ precursors (SOPs) in Drosophila generates different cell types of the mature sensory organ. In a genetic screen designed to identify novel players in this process, we have isolated a mutation in Drosophila sec15, which encodes a component of the exocyst, an evolutionarily conserved complex implicated in intracellular vesicle transport. sec15(-) sensory organs contain extra neurons at the expense of support cells, a phenotype consistent with loss of Notch signaling. A vesicular compartment containing Notch, Sanpodo, and endocytosed Delta accumulates in basal areas of mutant SOPs. Based on the dynamic traffic of Sec15, its colocalization with the recycling endosomal marker Rab11, and the aberrant distribution of Rab11 in sec15 clones, we propose that a defect in Delta recycling causes cell fate transformation in sec15(-) sensory lineages. Our data indicate that Sec15 mediates a specific vesicle trafficking event to ensure proper neuronal fate specification in Drosophila.

The glycine transporter GLYT1 interacts with Sec3, a component of the exocyst complex

Evidence is accumulating that the glycine transporter GLYT1 regulates NMDA receptor function by modulating the glycine concentration in glutamatergic synapses. In this article, we describe a physical and functional interaction between GLYT1 and the exocyst complex. Through a yeast two-hybrid screen to search for proteins capable of interacting with the intracellular C-terminal tail of GLYT1, we identified a protein that is highly homologous to the human and mouse Sec3 protein, a component of the exocyst complex. Pull-down and immunoprecipitation assays confirmed the physical interaction between the C-terminus of GLYT1 and Sec3. Subsequently, immunofluorescence experiments indicated that Sec3-GFP was partially recruited to the plasma membrane upon coexpression with GLYT1. The interaction of GLYT1 with exocyst components was also observed in the native rat brain since complexes immunoprecipitated from brain extracts with anti-GLYT1 antibodies contained both Sec6 and Sec8. Functional assays revealed that Sec3 increased the transporter capacity of GLYT1, suggesting that the exocyst favors insertion of GLYT1 into the plasma membrane.

Lethal giant larvae proteins interact with the exocyst complex and are involved in polarized exocytosis

The tumor suppressor lethal giant larvae (Lgl) plays a critical role in epithelial cell polarization. However, the molecular mechanism by which Lgl carries out its functions is unclear. In this study, we report that the yeast Lgl proteins Sro7p and Sro77p directly interact with Exo84p, which is a component of the exocyst complex that is essential for targeting vesicles to specific sites of the plasma membrane for exocytosis, and that this interaction is important for post-Golgi secretion. Genetic analyses demonstrate a molecular pathway from Rab and Rho GTPases through the exocyst and Lgl to SNAREs, which mediate membrane fusion. We also found that overexpression of Lgl and t-SNARE proteins not only improves exocytosis but also rescues polarity defects in exocyst mutants. We propose that, although Lgl is broadly distributed in the cells, its localized interaction with the exocyst and kinetic activation are important for the establishment and reenforcement of cell polarity.

Role of septins and the exocyst complex in the function of hydrolytic enzymes responsible for fission yeast cell separation

Cell separation in Schizosaccharomyces pombe is achieved by the concerted action of the Eng1 endo-beta-1,3-glucanase and the Agn1 endo-alpha-1,3-glucanase, which are transported to the septum and localize to a ringlike structure that surrounds the septum. The requirements for the correct localization of both hydrolases as a ring were analyzed using green fluorescent protein fusion proteins. Targeting to the septum required a functional exocyst, because both proteins failed to localize correctly in sec8-1 or exo70delta mutants, suggesting that Agn1 and Eng1 might be two of the cargo proteins present in the vesicles that accumulate in exocyst mutants. Septins and Mid2 were also required for correct formation of a ring. In their absence, Eng1 and Agn1 were found in a disk-like structure that spanned the septum, rather than in a ring. Even though septin and mid2delta mutants have a cell separation defect, the septum and the distribution of linear beta-1,3-glucans were normal in these cells, suggesting that mislocalization of Eng1 and Agn1 might be the reason underlying the failure to separate efficiently. Thus, one of the functions of the septin ring would be to act as a positional marker for the localization of hydrolytic proteins to the medial region.

Septins: traffic control at the cytokinesis intersection

The physical division of one cell into two requires the highly orchestrated separation of genetic and cytoplasmic contents during M phase of the cell cycle. Mitosis, the physical segregation of the genetic material of a cell into two daughter cells, has traditionally received more attention than cytokinesis, the partitioning of the cytoplasmic contents, yet clearly the two processes must be intimately co-ordinated and tightly regulated. While plant cells divide by the formation of a membranous cell barrier called the phragmoplast, animal cell division is largely driven by contraction of an actomyosin ring. However, recent evidence has suggested that membranes derived from one or more intracellular compartments are also required to break the cytoplasmic bridge connecting two dividing cells during late telophase. In this review, we focus on studies of animal cell cytokinesis that support a requirement for specific endomembrane fusion during fission, define molecular components of the membrane fusion apparatus that may be involved and point to possible roles for an emerging family of cytoskeletal proteins, the septins, in this process.

SEC8, a subunit of the putative Arabidopsis exocyst complex, facilitates pollen germination and competitive pollen tube growth

The exocyst, a complex of eight proteins, contributes to the morphogenesis of polarized cells in a broad range of eukaryotes. In these organisms, the exocyst appears to facilitate vesicle docking at the plasma membrane during exocytosis. Although we had identified orthologs for each of the eight exocyst components in Arabidopsis (Arabidopsis thaliana), no function has been demonstrated for any of them in plants. The gene encoding one exocyst component ortholog, AtSEC8, is expressed in pollen and vegetative tissues of Arabidopsis. Genetic studies utilizing an allelic series of six independent T-DNA mutations reveal a role for SEC8 in male gametophyte function. Three T-DNA insertions in SEC8 cause an absolute, male-specific transmission defect that can be complemented by expression of SEC8 from the LAT52 pollen promoter. Microscopic analysis shows no obvious abnormalities in the microgametogenesis of the SEC8 mutants, and the mutant pollen grains appear to respond to the signals that initiate germination. However, in vivo assays indicate that these mutant pollen grains are unable to germinate a pollen tube. The other three T-DNA insertions are associated with a partial transmission defect, such that the mutant allele is transmitted through the pollen at a reduced frequency. The partial transmission defect is only evident when mutant gametophytes must compete with wild-type gametophytes, and arises in part from a reduced pollen tube growth rate. These data support the hypothesis that one function of the putative plant exocyst is to facilitate the initiation and maintenance of the polarized growth of pollen tubes.

Golgi tethering factors

Transport of cargo to, through and from the Golgi complex is mediated by vesicular carriers and transient tubular connections. In this review, we describe vesicle tethering events with the understanding that similar events occur during transport via larger structures. Tethering factors can be generally divided into a group of coiled-coil proteins and a group of multi-subunit complexes. Current evidence suggests that these factors function in a variety of membrane-membrane tethering events at the Golgi complex, interact with SNARE molecules, and are regulated by small GTPases of the Rab and Arl families.

Mutations in Drosophila sec15 reveal a function in neuronal targeting for a subset of exocyst components

The exocyst is a complex of proteins originally identified in yeast that has been implicated in polarized secretion. Components of the exocyst have been implicated in neurite outgrowth, cell polarity, and cell viability. We have isolated an exocyst component, sec15, in a screen for genes required for synaptic specificity. Loss of sec15 causes a targeting defect of photoreceptors that coincides with mislocalization of specific cell adhesion and signaling molecules. Additionally, sec15 mutant neurons fail to localize other exocyst members like Sec5 and Sec8, but not Sec6, to neuronal terminals. However, loss of sec15 does not cause cell lethality in contrast to loss of sec5 or sec6. Our data suggest a role of Sec15 in an exocyst-like subcomplex for the targeting and subcellular distribution of specific proteins. The data also show that functions of other exocyst components persist in the absence of sec15, suggesting that different exocyst components have separable functions.

Essential function of Drosophila Sec6 in apical exocytosis of epithelial photoreceptor cells

Polarized exocytosis plays a major role in development and cell differentiation but the mechanisms that target exocytosis to specific membrane domains in animal cells are still poorly understood. We characterized Drosophila Sec6, a component of the exocyst complex that is believed to tether secretory vesicles to specific plasma membrane sites. sec6 mutations cause cell lethality and disrupt plasma membrane growth. In developing photoreceptor cells (PRCs), Sec6 but not Sec5 or Sec8 shows accumulation at adherens junctions. In late PRCs, Sec6, Sec5, and Sec8 colocalize at the rhabdomere, the light sensing subdomain of the apical membrane. PRCs with reduced Sec6 function accumulate secretory vesicles and fail to transport proteins to the rhabdomere, but show normal localization of proteins to the apical stalk membrane and the basolateral membrane. Furthermore, we show that Rab11 forms a complex with Sec5 and that Sec5 interacts with Sec6 suggesting that the exocyst is a Rab11 effector that facilitates protein transport to the apical rhabdomere in Drosophila PRCs.

At the Heart of Cell Polarity and the Cytoskeleton

Researchers working on cell polarity and cytoskeletal processes met at a Keystone meeting in Coeur d'Alene, Idaho in March to present and discuss the newest findings in these rapidly moving fields. The unexpectedly warm weather and the lack of snow favored discussions at this very interactive meeting. To fill the 6 hr break in the afternoon, walks in the beautiful surroundings and shopping trips were organized, during which microtubules, PAR proteins, and small G proteins were the guests of honor.

Modulation of neurotransmitter release by the second messenger-activated protein kinases: implications for presynaptic plasticity

Activity-dependent modulation of synaptic function and structure is emerging as one of the key mechanisms underlying synaptic plasticity. Whereas over the past decade considerable progress has been made in identifying postsynaptic mechanisms for synaptic plasticity, the presynaptic mechanisms involved have remained largely elusive. Recent evidence implicates that second messenger regulation of the protein interactions in synaptic vesicle release machinery is one mechanism by which cellular events modulate synaptic transmission. Thus, identifying protein kinases and their targets in nerve terminals, particularly those functionally regulated by synaptic activity or intracellular [Ca2+], is critical to the elucidation of the molecular mechanisms underlying modulation of neurotransmitter release and presynaptic plasticity. The phosphorylation and dephosphorylation states of synaptic proteins that mediate vesicle exocytosis could regulate the biochemical pathways leading from synaptic vesicle docking to fusion. However, functional evaluation of the activity-dependent phosphorylation events for modulating presynaptic functions still represents a considerable challenge. Here, we present a brief overview of the data on the newly identified candidate targets of the second messenger-activated protein kinases in the presynaptic release machinery and discuss the potential impact of these phosphorylation events in synaptic strength and presynaptic plasticity.

The critical role of Exo84p in the organization and polarized localization of the exocyst complex

The exocyst complex plays an essential role in tethering secretory vesicles to specific domains of the plasma membrane for exocytosis. However, how the exocyst complex is assembled and targeted to sites of secretion is unclear. Here, we have investigated the role of the exocyst component Exo84p in these processes. We have generated an array of temperature-sensitive yeast exo84 mutants. Electron microscopy and cargo protein traffic analyses of these mutants indicated that Exo84p is specifically involved in the post-Golgi stage of secretion. Using various yeast mutants, we systematically studied the localization of Exo84p and other exocyst proteins by fluorescence microscopy. We found that pre-Golgi traffic and polarized actin organization are required for Exo84p localization. However, none of the exocyst proteins controls Exo84p polarization. In addition, Sec3p is not responsible for the polarization of Exo84p or any other exocyst component to the daughter cell. On the other hand, several exocyst members, including Sec10p, Sec15p, and Exo70p, clearly require Exo84p for their polarization. Biochemical analyses of the exocyst composition indicated that the assembly of Sec10p, Sec15p, and Exo70p with the rest of the complex requires Exo84p. We propose that there are at least two distinct regulatory mechanisms for exocyst polarization, one for Sec3p and one for the other members, including Exo84p. Exo84p plays a critical role in both the assembly of the exocyst and its targeting to sites of secretion.

Cyclical regulation of the exocyst and cell polarity determinants for polarized cell growth

Polarized exocytosis is important for morphogenesis and cell growth. The exocyst is a multiprotein complex implicated in tethering secretory vesicles at specific sites of the plasma membrane for exocytosis. In the budding yeast, the exocyst is localized to sites of bud emergence or the tips of small daughter cells, where it mediates secretion and cell surface expansion. To understand how exocytosis is spatially controlled, we systematically analyzed the localization of Sec15p, a member of the exocyst complex and downstream effector of the rab protein Sec4p, in various mutants. We found that the polarized localization of Sec15p relies on functional upstream membrane traffic, activated rab protein Sec4p, and its guanine exchange factor Sec2p. The initial targeting of both Sec4p and Sec15p to the bud tip depends on polarized actin cable. However, different recycling mechanisms for rab and Sec15p may account for the different kinetics of polarization for these two proteins. We also found that Sec3p and Sec15p, though both members of the exocyst complex, rely on distinctive targeting mechanisms for their localization. The assembly of the exocyst may integrate various cellular signals to ensure that exocytosis is tightly controlled. Key regulators of cell polarity such as Cdc42p are important for the recruitment of the exocyst to the budding site. Conversely, we found that the proper localization of these cell polarity regulators themselves also requires a functional exocytosis pathway. We further report that Bem1p, a protein essential for the recruitment of signaling molecules for the establishment of cell polarity, interacts with the exocyst complex. We propose that a cyclical regulatory network contributes to the establishment and maintenance of polarized cell growth in yeast.

Mammalian Bet3 functions as a cytosolic factor participating in transport from the ER to the Golgi apparatus

The TRAPP complex identified in yeast regulates vesicular transport in the early secretory pathway. Although some components of the TRAPP complex are structurally conserved in mammalian cells, the function of the mammalian components has not been examined. We describe our biochemical and functional analysis of mammalian Bet3, the most conserved component of the TRAPP complex. Bet3 mRNA is ubiquitously expressed in all tissues. Antibodies raised against recombinant Bet3 specifically recognize a protein of 22 kDa. In contrast to yeast Bet3p, the majority of Bet3 is present in the cytosol. To investigate the possible involvement of Bet3 in transport events in mammalian cells, we utilized a semi-intact cell system that reconstitutes the transport of the envelope glycoprotein of vesicular stomatitis virus (VSV-G) from the ER to the Golgi apparatus. In this system, antibodies against Bet3 inhibit transport in a dose-dependent manner, and cytosol that is immunodepleted of Bet3 is also defective in this transport. This defect can be rescued by supplementing the Bet3-depleted cytosol with recombinant GST-Bet3. We also show that Bet3 acts after COPII but before Rab1, alpha-SNAP and the EGTA-sensitive stage during ER-Golgi transport. Gel filtration analysis demonstrates that Bet3 exists in two distinct pools in the cytosol, the high-molecular-weight pool may represent the TRAPP complex, whereas the other probably represents the monomeric Bet3.

Interaction of BIG2, a brefeldin A-inhibited guanine nucleotide-exchange protein, with exocyst protein Exo70

Guanine nucleotide-exchange proteins activate ADP-ribosylation factors by accelerating the replacement of bound GDP with GTP. Mammalian brefeldin A-inhibited guanine nucleotide-exchange proteins, BIG1 and BIG2, are important activators of ADP-ribosylation factors for vesicular trafficking. To identify proteins that interact with BIG2, we used cDNA constructs encoding BIG2 sequences in a yeast two-hybrid screen of a human heart library. Clone p2-5-3, encoding a form of human exocyst protein Exo70, interacted with BIG2 amino acids 1-643 and 1-832, but not 644-832, which was confirmed by coimmunoprecipitation of in vitro-translated BIG2 N-terminal segments and 2-5-3. By immunofluorescence microscopy, endogenous BIG2 and Exo70 in HepG2 cells were visualized at Golgi membranes and apparently at the microtubule-organizing center (MTOC). Both were identified in purified centrosomes. Immunoreactive Exo70 and BIG2 partially or completely overlapped with gamma-tubulin at the MTOC in cells inspected by confocal microscopy. In cells incubated with brefeldin A, most of the BIG2, Exo70, and trans-Golgi protein p230 were widely dispersed from their perinuclear concentrations, but small amounts always remained, apparently at the MTOC. After disruption of microtubules with nocodazole, BIG2 and Exo70 were widely distributed in cells and remained only partially colocalized with p230, BIG2 more so than Exo70. We conclude that in HepG2 cells BIG2 and Exo70 interact in trans-Golgi network and centrosomes, as well as in exocyst structures or complexes that move along microtubules to the plasma membrane, consistent with a functional association in both early and late stages of vesicular trafficking.

Sec15 Is an Effector for the Rab11 GTPase in Mammalian Cells

Rab/Ypt GTPases play key roles in the regulation of vesicular trafficking. They perform most of their functions in a GTP-bound form by interacting with specific downstream effectors. The exocyst is a complex of eight polypeptides involved in constitutive secretion and functions as an effector for multiple Ras-related small GTPases, including the Rab protein Sec4p in yeast. In this study, we have examined the localization and function of the Sec15 exocyst subunit in mammalian cells. Overexpressed Sec15 associated with clusters of tubular/vesicular elements that were concentrated in the perinuclear region. The tubular/vesicular clusters were dispersed throughout the cytoplasm upon treatment with the microtubule-depolymerizing agent nocodazole and were accessible to endocytosed transferrin, but not exocytic cargo (vesicular stomatitis virus glycoprotein). Consistent with these observations, Sec15 colocalized selectively with the recycling endosome marker Rab11 and exhibited a GTP-dependent interaction with the Rab11 GTPase, but not with Rab4, Rab6, or Rab7. These findings provide the first evidence that the exocyst functions as a Rab effector complex in mammalian cells.

Mediterranean views on epithelial polarity

Developmental and cell biologists viewed polarity through each other's eyes at the EMBO workshop on Epithelial Polarity in Development and Disease, March 27-31 2004, in Carry-le-Rouet, France, a small village west of Marseille on the rocky Mediterranean coast. The presentations highlighted our growing understanding, not only of the molecular mechanisms underlying polarity and the conservation of polarity complexes from worms to mammals, but also the diverse roles that epithelial polarity has during development.