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1.  Regulation of exocytosis by the exocyst subunit Sec6 and the SM protein Sec1 
Molecular Biology of the Cell  2012;23(2):337-346.
The Sec6 subunit of the multisubunit exocyst tethering complex interacts with the Sec1/Munc18 protein Sec1 and with the t-SNARE Sec9. Assembly of the exocyst upon vesicle arrival at sites of secretion is proposed to release Sec9 for SNARE complex assembly and to recruit Sec1 for interaction with SNARE complexes to facilitate fusion.
Trafficking of protein and lipid cargo through the secretory pathway in eukaryotic cells is mediated by membrane-bound vesicles. Secretory vesicle targeting and fusion require a conserved multisubunit protein complex termed the exocyst, which has been implicated in specific tethering of vesicles to sites of polarized exocytosis. The exocyst is directly involved in regulating soluble N-ethylmaleimide–sensitive factor (NSF) attachment protein receptor (SNARE) complexes and membrane fusion through interactions between the Sec6 subunit and the plasma membrane SNARE protein Sec9. Here we show another facet of Sec6 function—it directly binds Sec1, another SNARE regulator, but of the Sec1/Munc18 family. The Sec6–Sec1 interaction is exclusive of Sec6–Sec9 but compatible with Sec6–exocyst assembly. In contrast, the Sec6–exocyst interaction is incompatible with Sec6–Sec9. Therefore, upon vesicle arrival, Sec6 is proposed to release Sec9 in favor of Sec6–exocyst assembly and to simultaneously recruit Sec1 to sites of secretion for coordinated SNARE complex formation and membrane fusion.
doi:10.1091/mbc.E11-08-0670
PMCID: PMC3258177  PMID: 22114349
2.  The rab Exchange Factor Sec2p Reversibly Associates with the Exocyst 
Molecular Biology of the Cell  2006;17(6):2757-2769.
Activation of the rab GTPase, Sec4p, by its exchange factor, Sec2p, is needed for polarized transport of secretory vesicles to exocytic sites and for exocytosis. A small region in the C-terminal half of Sec2p regulates its localization. Loss of this region results in temperature-sensitive growth and the depolarized accumulation of secretory vesicles. Here, we show that Sec2p associates with the exocyst, an octameric effector of Sec4p involved in tethering secretory vesicles to the plasma membrane. Specifically, the exocyst subunit Sec15p directly interacts with Sec2p. This interaction normally occurs on secretory vesicles and serves to couple nucleotide exchange on Sec4p to the recruitment of the Sec4p effector. The mislocalization of Sec2p mutants correlates with dramatically enhanced binding to the exocyst complex. We propose that Sec2p is normally released from the exocyst after vesicle tethering so that it can recycle onto a new round of vesicles. The mislocalization of Sec2p mutants results from a failure to be released from Sec15p, blocking this recycling pathway.
doi:10.1091/mbc.E05-10-0917
PMCID: PMC1474791  PMID: 16611746
3.  The synaptobrevin homologue Snc2p recruits the exocyst to secretory vesicles by binding to Sec6p 
The Journal of Cell Biology  2013;202(3):509-526.
The exocyst is recruited to secretory vesicles by the combinatorial signals of Sec4-GTP and the Snc proteins to confer both specificity and directionality to vesicular traffic.
A screen for mutations that affect the recruitment of the exocyst to secretory vesicles identified genes encoding clathrin and proteins that associate or colocalize with clathrin at sites of endocytosis. However, no significant colocalization of the exocyst with clathrin was seen, arguing against a direct role in exocyst recruitment. Rather, these components are needed to recycle the exocytic vesicle SNAREs Snc1p and Snc2p from the plasma membrane into new secretory vesicles where they act to recruit the exocyst. We observe a direct interaction between the exocyst subunit Sec6p and the latter half of the SNARE motif of Snc2p. An snc2 mutation that specifically disrupts this interaction led to exocyst mislocalization and a block in exocytosis in vivo without affecting liposome fusion in vitro. Overexpression of Sec4p partially suppressed the exocyst localization defects of mutations in clathrin and clathrin-associated components. We propose that the exocyst is recruited to secretory vesicles by the combinatorial signals of Sec4-GTP and the Snc proteins. This could help to confer both specificity and directionality to vesicular traffic.
doi:10.1083/jcb.201211148
PMCID: PMC3734085  PMID: 23897890
4.  Fission Yeast Sec3 Bridges the Exocyst Complex to the Actin Cytoskeleton 
Traffic (Copenhagen, Denmark)  2012;13(11):1481-1495.
The exocyst complex tethers post-Golgi secretory vesicles to the plasma membrane prior to docking and fusion. In this study, we identify Sec3, the missing component of the Schizosaccharomyces pombe exocyst complex (SpSec3). SpSec3 shares many properties with its orthologs, and its mutants are rescued by human Sec3/EXOC1. Although involved in exocytosis, SpSec3 does not appear to mark the site of exocyst complex assembly at the plasma membrane. It does, however, mark the sites of actin cytoskeleton recruitment and controls the organization of all three yeast actin structures: the actin cables, endocytic actin patches and actomyosin ring. Specifically, SpSec3 physically interacts with For3 and sec3 mutants have no actin cables as a result of a failure to polarize this nucleating formin. SpSec3 also interacts with actin patch components and sec3 mutants have depolarized actin patches of reduced endocytic capacity. Finally, the constriction and disassembly of the cytokinetic actomyosin ring is compromised in these sec3 mutant cells. We propose that a role of SpSec3 is to spatially couple actin machineries and their independently polarized regulators. As a consequence of its dual role in secretion and actin organization, Sec3 appears as a major co-ordinator of cell morphology in fission yeast.
doi:10.1111/j.1600-0854.2012.01408.x
PMCID: PMC3531892  PMID: 22891673
actin; endocytosis; exocyst; morphology; Schizosaccharomyces pombe
5.  The exocyst component Sec5 is present on endocytic vesicles in the oocyte of Drosophila melanogaster 
The Journal of Cell Biology  2005;169(6):953-963.
The exocyst is an octameric complex required for polarized secretion. Some components of the exocyst are found on the plasma membrane, whereas others are recruited to Golgi membranes, suggesting that exocyst assembly tethers vesicles to their site of fusion. We have found that in Drosophila melanogaster oocytes the majority of the exocyst component Sec5 is unexpectedly present in clathrin-coated pits and vesicles at the plasma membrane. In oocytes, the major substrate for clathrin-dependent endocytosis is the vitellogenin receptor Yolkless. A truncation mutant of Sec5 (sec5E13) allows the formation of normally sized oocytes but with greatly reduced yolk uptake. We find that in sec5E13 oocytes Yolkless accumulates aberrantly in late endocytic compartments, indicating a defect in the endocytic cycling of the receptor. An analogous truncation of the yeast SEC5 gene results in normal secretion but a temperature-sensitive defect in endocytic recycling. Thus, the exocyst may act in both Golgi to plasma membrane traffic and endocytic cycling, and hence in oocytes is recruited to clathrin-coated pits to facilitate the rapid recycling of Yolkless.
doi:10.1083/jcb.200411053
PMCID: PMC2171629  PMID: 15955846
6.  Visualization of the exocyst complex dynamics at the plasma membrane of Arabidopsis thaliana 
Molecular Biology of the Cell  2013;24(4):510-520.
The exocyst complex localizes to distinct foci at the plasma membrane of Arabidopsis thaliana cells. Their localization at the plasma membrane is insensitive to BFA treatment but is decreased in an exocyst-subunit mutant. In turn, exocyst-subunit mutants show decreased exocytosis.
The exocyst complex, an effector of Rho and Rab GTPases, is believed to function as an exocytotic vesicle tether at the plasma membrane before soluble N-ethylmaleimide–sensitive factor attachment protein receptor (SNARE) complex formation. Exocyst subunits localize to secretory-active regions of the plasma membrane, exemplified by the outer domain of Arabidopsis root epidermal cells. Using variable-angle epifluorescence microscopy, we visualized the dynamics of exocyst subunits at this domain. The subunits colocalized in defined foci at the plasma membrane, distinct from endocytic sites. Exocyst foci were independent of cytoskeleton, although prolonged actin disruption led to changes in exocyst localization. Exocyst foci partially overlapped with vesicles visualized by VAMP721 v-SNARE, but the majority of the foci represent sites without vesicles, as indicated by electron microscopy and drug treatments, supporting the concept of the exocyst functioning as a dynamic particle. We observed a decrease of SEC6–green fluorescent protein foci in an exo70A1 exocyst mutant. Finally, we documented decreased VAMP721 trafficking to the plasma membrane in exo70A1 and exo84b mutants. Our data support the concept that the exocyst-complex subunits dynamically dock and undock at the plasma membrane to create sites primed for vesicle tethering.
doi:10.1091/mbc.E12-06-0492
PMCID: PMC3571873  PMID: 23283982
7.  Rho GTPase regulation of exocytosis in yeast is independent of GTP hydrolysis and polarization of the exocyst complex 
The Journal of Cell Biology  2005;170(4):583-594.
Rho GTPases are important regulators of polarity in eukaryotic cells. In yeast they are involved in regulating the docking and fusion of secretory vesicles with the cell surface. Our analysis of a Rho3 mutant that is unable to interact with the Exo70 subunit of the exocyst reveals a normal polarization of the exocyst complex as well as other polarity markers. We also find that there is no redundancy between the Rho3–Exo70 and Rho1–Sec3 pathways in the localization of the exocyst. This suggests that Rho3 and Cdc42 act to polarize exocytosis by activating the exocytic machinery at the membrane without the need to first recruit it to sites of polarized growth. Consistent with this model, we find that the ability of Rho3 and Cdc42 to hydrolyze GTP is not required for their role in secretion. Moreover, our analysis of the Sec3 subunit of the exocyst suggests that polarization of the exocyst may be a consequence rather than a cause of polarized exocytosis.
doi:10.1083/jcb.200504108
PMCID: PMC2171504  PMID: 16103227
8.  Cyclical Regulation of the Exocyst and Cell Polarity Determinants for Polarized Cell Growth 
Molecular Biology of the Cell  2005;16(3):1500-1512.
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.
doi:10.1091/mbc.E04-10-0896
PMCID: PMC551511  PMID: 15647373
9.  Fission Yeast Sec3 and Exo70 Are Transported on Actin Cables and Localize the Exocyst Complex to Cell Poles 
PLoS ONE  2012;7(6):e40248.
The exocyst complex is essential for many exocytic events, by tethering vesicles at the plasma membrane for fusion. In fission yeast, polarized exocytosis for growth relies on the combined action of the exocyst at cell poles and myosin-driven transport along actin cables. We report here the identification of fission yeast Schizosaccharomyces pombe Sec3 protein, which we identified through sequence homology of its PH-like domain. Like other exocyst subunits, sec3 is required for secretion and cell division. Cells deleted for sec3 are only conditionally lethal and can proliferate when osmotically stabilized. Sec3 is redundant with Exo70 for viability and for the localization of other exocyst subunits, suggesting these components act as exocyst tethers at the plasma membrane. Consistently, Sec3 localizes to zones of growth independently of other exocyst subunits but depends on PIP2 and functional Cdc42. FRAP analysis shows that Sec3, like all other exocyst subunits, localizes to cell poles largely independently of the actin cytoskeleton. However, we show that Sec3, Exo70 and Sec5 are transported by the myosin V Myo52 along actin cables. These data suggest that the exocyst holocomplex, including Sec3 and Exo70, is present on exocytic vesicles, which can reach cell poles by either myosin-driven transport or random walk.
doi:10.1371/journal.pone.0040248
PMCID: PMC3386988  PMID: 22768263
10.  Exocyst Sec5 Regulates Exocytosis of Newcomer Insulin Granules Underlying Biphasic Insulin Secretion 
PLoS ONE  2013;8(7):e67561.
The exocyst complex subunit Sec5 is a downstream effector of RalA-GTPase which promotes RalA-exocyst interactions and exocyst assembly, serving to tether secretory granules to docking sites on the plasma membrane. We recently reported that RalA regulates biphasic insulin secretion in pancreatic islet β cells in part by tethering insulin secretory granules to Ca2+ channels to assist excitosome assembly. Here, we assessed β cell exocytosis by patch clamp membrane capacitance measurement and total internal reflection fluorescence microscopy to investigate the role of Sec5 in regulating insulin secretion. Sec5 is present in human and rodent islet β cells, localized to insulin granules. Sec5 protein depletion in rat INS-1 cells inhibited depolarization-induced release of primed insulin granules from both readily-releasable pool and mobilization from the reserve pool. This reduction in insulin exocytosis was attributed mainly to reduction in recruitment and exocytosis of newcomer insulin granules that undergo minimal docking time at the plasma membrane, but which encompassed a larger portion of biphasic glucose stimulated insulin secretion. Sec5 protein knockdown had little effect on predocked granules, unless vigorously stimulated by KCl depolarization. Taken together, newcomer insulin granules in β cells are more sensitive than predocked granules to Sec5 regulation.
doi:10.1371/journal.pone.0067561
PMCID: PMC3699660  PMID: 23844030
11.  The Microtubule-associated Rho Activating Factor GEF-H1 interacts with Exocyst complex to regulate Vesicle Traffic 
Developmental cell  2012;23(2):397-411.
SUMMARY
The exocyst complex plays a critical role in targeting and tethering vesicles to specific sites of the plasma membrane. These events are crucial for polarized delivery of membrane components to the cell surface, which is critical for cell motility and division. Though Rho GTPases are involved in regulating actin dynamics and membrane trafficking, their role in exocyst-mediated vesicle targeting is not very clear. Herein, we present evidence that depletion of GEF-H1, a guanine nucleotide exchange factor for Rho proteins, affects vesicle trafficking. Interestingly, we found that GEF-H1 directly binds to exocyst component Sec5 in a Ral GTPase-dependent manner. This interaction promotes RhoA activation, which then regulates exocyst assembly/localization and exocytosis. Taken together, our work defines a mechanism for RhoA activation in response to RalA-Sec5 signaling and involvement of GEF-H1/RhoA pathway in the regulation of vesicle trafficking.
doi:10.1016/j.devcel.2012.06.014
PMCID: PMC3422510  PMID: 22898781
12.  Dominant Negative Alleles of SEC10 Reveal Distinct Domains Involved in Secretion and Morphogenesis in Yeast 
Molecular Biology of the Cell  1998;9(7):1725-1739.
The accurate targeting of secretory vesicles to distinct sites on the plasma membrane is necessary to achieve polarized growth and to establish specialized domains at the surface of eukaryotic cells. Members of a protein complex required for exocytosis, the exocyst, have been localized to regions of active secretion in the budding yeast Saccharomyces cerevisiae where they may function to specify sites on the plasma membrane for vesicle docking and fusion. In this study we have addressed the function of one member of the exocyst complex, Sec10p. We have identified two functional domains of Sec10p that act in a dominant-negative manner to inhibit cell growth upon overexpression. Phenotypic and biochemical analysis of the dominant-negative mutants points to a bifunctional role for Sec10p. One domain, consisting of the amino-terminal two-thirds of Sec10p directly interacts with Sec15p, another exocyst component. Overexpression of this domain displaces the full-length Sec10 from the exocyst complex, resulting in a block in exocytosis and an accumulation of secretory vesicles. The carboxy-terminal domain of Sec10p does not interact with other members of the exocyst complex and expression of this domain does not cause a secretory defect. Rather, this mutant results in the formation of elongated cells, suggesting that the second domain of Sec10p is required for morphogenesis, perhaps regulating the reorientation of the secretory pathway from the tip of the emerging daughter cell toward the mother–daughter connection during cell cycle progression.
PMCID: PMC25411  PMID: 9658167
13.  The Exocyst Complex in Polarized Exocytosis 
Current opinion in cell biology  2009;21(4):537-542.
The exocyst is an octameric protein complex, which mediates the tethering of post-Golgi secretory vesicles to the plasma membrane prior to exocytic fusion. The exocyst assembles by side-by-side packing of rod-shaped subunits composed of helical bundles. The targeting of secretory vesicles to the plasma membrane involves direct interactions of the exocyst with PI(4,5)P2. In addition, a number of small GTP-binding proteins interact with components of the exocyst and regulate the assembly, localization, and function of this complex. Here we review the recent advances in the field, focusing on the function of the exocyst in polarized exocytosis.
doi:10.1016/j.ceb.2009.04.007
PMCID: PMC2725219  PMID: 19473826
14.  Live-cell imaging of exocyst links its spatiotemporal dynamics to various stages of vesicle fusion 
The Journal of Cell Biology  2013;201(5):673-680.
Live-cell imaging of the exocyst subunit Sec8 reveals how the protein’s spatiotemporal dynamics correlate with its roles in vesicle fusion.
Tethers play ubiquitous roles in membrane trafficking and influence the specificity of vesicle attachment. Unlike soluble N-ethyl-maleimide–sensitive fusion attachment protein receptors (SNAREs), the spatiotemporal dynamics of tethers relative to vesicle fusion are poorly characterized. The most extensively studied tethering complex is the exocyst, which spatially targets vesicles to sites on the plasma membrane. By using a mammalian genetic replacement strategy, we were able to assemble fluorescently tagged Sec8 into the exocyst complex, which was shown to be functional by biochemical, trafficking, and morphological criteria. Ultrasensitive live-cell imaging revealed that Sec8-TagRFP moved to the cell cortex on vesicles, which preferentially originated from the endocytic recycling compartment. Surprisingly, Sec8 remained with vesicles until full dilation of the fusion pore, supporting potential coupling with SNARE fusion machinery. Fluorescence recovery after photobleaching analysis of Sec8 at cell protrusions revealed that a significant fraction was immobile. Additionally, Sec8 dynamically repositioned to the site of membrane expansion, suggesting that it may respond to local cues during early cell polarization.
doi:10.1083/jcb.201212103
PMCID: PMC3664709  PMID: 23690179
15.  Exocyst Is Involved in Cystogenesis and Tubulogenesis and Acts by Modulating Synthesis and Delivery of Basolateral Plasma Membrane and Secretory Proteins 
Molecular Biology of the Cell  2000;11(12):4259-4275.
Epithelial cyst and tubule formation are critical processes that involve transient, highly choreographed changes in cell polarity. Factors controlling these changes in polarity are largely unknown. One candidate factor is the highly conserved eight-member protein complex called the exocyst. We show that during tubulogenesis in an in vitro model system the exocyst relocalized along growing tubules consistent with changes in cell polarity. In yeast, the exocyst subunit Sec10p is a crucial component linking polarized exocytic vesicles with the rest of the exocyst complex and, ultimately, the plasma membrane. When the exocyst subunit human Sec10 was exogenously expressed in epithelial Madin-Darby canine kidney cells, there was a selective increase in the synthesis and delivery of apical and basolateral secretory proteins and a basolateral plasma membrane protein, but not an apical plasma membrane protein. Overexpression of human Sec10 resulted in more efficient and rapid cyst formation and increased tubule formation upon stimulation with hepatocyte growth factor. We conclude that the exocyst plays a central role in the development of epithelial cysts and tubules.
PMCID: PMC15071  PMID: 11102522
16.  Exorcising the Exocyst Complex 
Traffic (Copenhagen, Denmark)  2012;13(7):898-907.
The exocyst complex is an evolutionarily conserved multisubunit protein complex implicated in tethering secretory vesicles to the plasma membrane. Originally identified two decades ago in budding yeast, investigations using several different eukaryotic systems have since made great progress toward determination of the overall structure and organization of the eight exocyst subunits. Studies point to a critical role for the complex as a spatiotemporal regulator through the numerous protein and lipid interactions of its subunits, although a molecular understanding of exocyst function has been challenging to elucidate. Recent progress demonstrates that the exocyst is also important for additional trafficking steps and cellular processes beyond exocytosis, with links to development and disease. In this review, we discuss current knowledge of exocyst architecture, assembly, regulation and its roles in a variety of cellular trafficking pathways.
doi:10.1111/j.1600-0854.2012.01353.x
PMCID: PMC3374049  PMID: 22420621
17.  Sec3p Is Needed for the Spatial Regulation of Secretion and for the Inheritance of the Cortical Endoplasmic ReticulumV⃞ 
Molecular Biology of the Cell  2003;14(12):4770-4782.
Sec3p is a component of the exocyst complex that tethers secretory vesicles to the plasma membrane at exocytic sites in preparation for fusion. Unlike all other exocyst structural genes, SEC3 is not essential for growth. Cells lacking Sec3p grow and secrete surprisingly well at 25°C; however, late markers of secretion, such as the vesicle marker Sec4p and the exocyst subunit Sec8p, localize more diffusely within the bud. Furthermore, sec3Δ cells are strikingly round relative to wild-type cells and are unable to form pointed mating projections in response to α factor. These phenotypes support the proposed role of Sec3p as a spatial landmark for secretion. We also find that cells lacking Sec3p exhibit a dramatic defect in the inheritance of cortical ER into the bud, whereas the inheritance of mitochondria and Golgi is unaffected. Overexpression of Sec3p results in a prominent patch of the endoplasmic reticulum (ER) marker Sec61p-GFP at the bud tip. Cortical ER inheritance in yeast has been suggested to involve the capture of ER tubules at the bud tip. Sec3p may act in this process as a spatial landmark for cortical ER inheritance.
doi:10.1091/mbc.E03-04-0229
PMCID: PMC284782  PMID: 12960429
18.  ERK1/2 Regulate Exocytosis through Direct Phosphorylation of the Exocyst Component Exo70 
Developmental Cell  2012;22(5):967-978.
The exocyst is a multi-protein complex essential for exocytosis and plasma membrane remodeling. The assembly of the exocyst complex mediates the tethering of post-Golgi secretory vesicles to the plasma membrane prior to fusion. Elucidating the mechanisms regulating exocyst assembly is important for the understanding of exocytosis. Here we show that the exocyst component Exo70 is a direct substrate of the Extracellular signal-Regulated Kinases 1/2 (ERK1/2). ERK1/2 phosphorylation enhances the binding of Exo70 to other exocyst components and promotes the assembly the exocyst complex in response to epidermal growth factor (EGF) signaling. We further demonstrate that ERK1/2 regulates exocytosis as blocking ERK1/2 signaling by a chemical inhibitor or the expression of an Exo70 mutant defective in ERK1/2 phosphorylation inhibited exocytosis. In tumor cells, blocking Exo70 phosphorylation inhibits matrix metalloproteinase secretion and invadopodia formation. ERK1/2 phosphorylation of Exo70 may thus coordinate exocytosis with other cellular events in response to growth factor signaling.
doi:10.1016/j.devcel.2012.03.005
PMCID: PMC3356571  PMID: 22595671
ERK1/2; exocyst; Exo70; vesicle tethering; exocytosis; EGF; invadopodia
19.  Vesicles carry most exocyst subunits to exocytic sites marked by the remaining two subunits, Sec3p and Exo70p 
The Journal of Cell Biology  2004;167(5):889-901.
Exocytosis in the budding yeast Saccharomyces cerevisiae occurs at discrete domains of the plasma membrane. The protein complex that tethers incoming vesicles to sites of secretion is known as the exocyst. We have used photobleaching recovery experiments to characterize the dynamic behavior of the eight subunits that make up the exocyst. One subset (Sec5p, Sec6p, Sec8p, Sec10p, Sec15p, and Exo84p) exhibits mobility similar to that of the vesicle-bound Rab family protein Sec4p, whereas Sec3p and Exo70p exhibit substantially more stability. Disruption of actin assembly abolishes the ability of the first subset of subunits to recover after photobleaching, whereas Sec3p and Exo70p are resistant. Immunogold electron microscopy and epifluorescence video microscopy indicate that all exocyst subunits, except for Sec3p, are associated with secretory vesicles as they arrive at exocytic sites. Assembly of the exocyst occurs when the first subset of subunits, delivered on vesicles, joins Sec3p and Exo70p on the plasma membrane. Exocyst assembly serves to both target and tether vesicles to sites of exocytosis.
doi:10.1083/jcb.200408124
PMCID: PMC2172445  PMID: 15583031
20.  An Internal Domain of Exo70p Is Required for Actin-independent Localization and Mediates Assembly of Specific Exocyst Components 
Molecular Biology of the Cell  2009;20(1):153-163.
The exocyst consists of eight rod-shaped subunits that align in a side-by-side manner to tether secretory vesicles to the plasma membrane in preparation for fusion. Two subunits, Sec3p and Exo70p, localize to exocytic sites by an actin-independent pathway, whereas the other six ride on vesicles along actin cables. Here, we demonstrate that three of the four domains of Exo70p are essential for growth. The remaining domain, domain C, is not essential but when deleted, it leads to synthetic lethality with many secretory mutations, defects in exocyst assembly of exocyst components Sec5p and Sec6p, and loss of actin-independent localization. This is analogous to a deletion of the amino-terminal domain of Sec3p, which prevents an interaction with Cdc42p or Rho1p and blocks its actin-independent localization. The two mutations are synthetically lethal, even in the presence of high copy number suppressors that can bypass complete deletions of either single gene. Although domain C binds Rho3p, loss of the Exo70p-Rho3p interaction does not account for the synthetic lethal interactions or the exocyst assembly defects. The results suggest that either Exo70p or Sec3p must associate with the plasma membrane for the exocyst to function as a vesicle tether.
doi:10.1091/mbc.E08-02-0157
PMCID: PMC2613103  PMID: 18946089
21.  Membrane association and functional regulation of Sec3 by phospholipids and Cdc42 
The Journal of Cell Biology  2008;180(1):145-158.
The exocyst is an octameric protein complex implicated in tethering post-Golgi secretory vesicles at the plasma membrane in preparation for fusion. However, it is not clear how the exocyst is targeted to and physically associates with specific domains of the plasma membrane and how its functions are regulated at those regions. We demonstrate that the N terminus of the exocyst component Sec3 directly interacts with phosphatidylinositol 4,5-bisphosphate. In addition, we have identified key residues in Sec3 that are critical for its binding to the guanosine triphosphate–bound form of Cdc42. Genetic analyses indicate that the dual interactions of Sec3 with phospholipids and Cdc42 control its function in yeast cells. Disrupting these interactions not only blocks exocytosis and affects exocyst polarization but also leads to defects in cell morphogenesis. We propose that the interactions of Sec3 with phospholipids and Cdc42 play important roles in exocytosis and polarized cell growth.
doi:10.1083/jcb.200704128
PMCID: PMC2213614  PMID: 18195105
22.  The Function of the Exocyst is Regulated by Effector Phosphorylation 
Nature cell biology  2011;13(5):580-588.
The exocyst complex tethers vesicles at sites of fusion through interactions with small GTPases. The G-protein RalA resides on Glut4 vesicles, and binds to the exocyst after activation by insulin, but must then disengage to ensure continuous exocytosis. Here we report that after recognition of the exocyst by activated RalA, disengagement occurs through phosphorylation of its effector Sec5, rather than RalA inactivation. Sec5 undergoes phosphorylation in the G-protein binding domain, allosterically reducing RalA interaction. The phosphorylation event is catalyzed by protein kinase C and is reversed by an exocyst-associated phosphatase. Introduction of Sec5 bearing mutations of the phosphorylation site to either alanine or aspartate disrupts insulin-stimulated Glut4 exocytosis, as well as other trafficking processes in polarized epithelial cells and during development of zebrafish embryos. The exocyst thus serves as a “gatekeeper” for exocytic vesicles through a circuit of engagement, disengagement, and re-engagement with G proteins.
doi:10.1038/ncb2226
PMCID: PMC3904505  PMID: 21516108
23.  Lethal giant larvae proteins interact with the exocyst complex and are involved in polarized exocytosis 
The Journal of Cell Biology  2005;170(2):273-283.
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.
doi:10.1083/jcb.200502055
PMCID: PMC2171422  PMID: 16027223
24.  Essential function of Drosophila Sec6 in apical exocytosis of epithelial photoreceptor cells 
The Journal of Cell Biology  2005;169(4):635-646.
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.
doi:10.1083/jcb.200410081
PMCID: PMC2171699  PMID: 15897260
25.  The Role of the Exocyst in Matrix Metalloproteinase Secretion and Actin Dynamics during Tumor Cell Invadopodia Formation 
Molecular Biology of the Cell  2009;20(16):3763-3771.
Invadopodia are actin-rich membrane protrusions formed by tumor cells that degrade the extracellular matrix for invasion. Invadopodia formation involves membrane protrusions driven by Arp2/3-mediated actin polymerization and secretion of matrix metalloproteinases (MMPs) at the focal degrading sites. The exocyst mediates the tethering of post-Golgi secretory vesicles at the plasma membrane for exocytosis and has recently been implicated in regulating actin dynamics during cell migration. Here, we report that the exocyst plays a pivotal role in invadopodial activity. With RNAi knockdown of the exocyst component Exo70 or Sec8, MDA-MB-231 cells expressing constitutively active c-Src failed to form invadopodia. On the other hand, overexpression of Exo70 promoted invadopodia formation. Disrupting the exocyst function by siEXO70 or siSEC8 treatment or by expression of a dominant negative fragment of Exo70 inhibited the secretion of MMPs. We have also found that the exocyst interacts with the Arp2/3 complex in cells with high invasion potential; blocking the exocyst-Arp2/3 interaction inhibited Arp2/3-mediated actin polymerization and invadopodia formation. Together, our results suggest that the exocyst plays important roles in cell invasion by mediating the secretion of MMPs at focal degrading sites and regulating Arp2/3-mediated actin dynamics.
doi:10.1091/mbc.E08-09-0967
PMCID: PMC2777935  PMID: 19535457

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