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

The Arabidopsis exocyst subunit SEC3A is essential for embryo development and accumulates in transient puncta at the plasma membrane

The exocyst is a protein complex that is essential for polarized secretion in mammals and fungi. Although the exocyst is essential for plant development, its precise function has not been elucidated. We studied the role of exocyst subunit SEC3A in plant development and its subcellular localization. T-DNA insertional mutants were identified and complemented with a SEC3A-green fluorescent protein (GFP) fusion construct. SEC3A-GFP localization was determined using confocal microscopy. sec3a mutants are defective in the globular to heart stage transition in embryogenesis. SEC3A-GFP has similar cell plate localization to the other plant exocyst subunits. In interphase cells, SEC3A-GFP localizes to the cytoplasm and to the plasma membrane, where it forms immobile, punctate structures with discrete lifetimes of 2-40 s. These puncta are equally distributed over the cell surface of root epidermal cells and tip growing root hairs. The density of puncta does not decrease after growth termination of these cells, but decreases strongly when exocytosis is inhibited by treatment with brefeldin A. SEC3A does not appear to be involved in polarized secretion for cell expansion in tip growing root hairs. The landmark function performed by SEC3 in mammals and yeast is likely to be conserved in plants.

GEF-H1: orchestrating the interplay between cytoskeleton and vesicle trafficking

Vesicle trafficking is crucial for delivery of membrane compartments as well as signaling molecules to specific sites on the plasma membrane for regulation of diverse processes such as cell division, migration, polarity establishment and secretion. Rho GTPases are well-studied signaling molecules that regulate actin cytoskeleton in response to variety of extracellular stimuli. Increasing amounts of evidence suggest that Rho proteins play a critical role in vesicle trafficking in both the exocytic and endocytic pathways; however, the molecular mechanism underlying the process remains largely unclear. We recently defined a mechanism of action for RhoA in membrane trafficking pathways through regulation of the octameric complex exocyst in a manuscript published in Developmental Cell. We have shown that microtubule-associated RhoA-activating factor GEF-H1 is involved in endocytic and excocytic vesicle trafficking. GEF-H1 activates RhoA in response to RalA GTPase, which in turn regulates the localization and the assembly of exocyst components and exocytosis. Our work defines a mechanism for RhoA activation in response to RalA signaling and during vesicle trafficking. These results provide a framework for understanding how RhoA/GEF-H1 regulates the coordination of actin and microtubule cytoskeleton modulation and vesicle trafficking during migration and cell division.

Cdc42 interacts with the exocyst complex to promote phagocytosis

The process of phagocytosis in multicellular organisms is required for homeostasis, clearance of foreign particles, and establishment of long-term immunity, yet the molecular determinants of uptake are not well characterized. Cdc42, a Rho guanosine triphosphatase, is thought to orchestrate critical actin remodeling events needed for internalization. In this paper, we show that Cdc42 controls exocytic events during phagosome formation. Cdc42 inactivation led to a selective defect in large particle phagocytosis as well as a general decrease in the rate of membrane flow to the cell surface. Supporting the connection between Cdc42 and exocytic function, we found that the overproduction of a regulator of exocytosis, Rab11, rescued the large particle uptake defect in the absence of Cdc42. Additionally, we demonstrated a temporal interaction between Cdc42 and the exocyst complex during large particle uptake. Furthermore, disruption of exocyst function through Exo70 depletion led to a defect in large particle internalization, thereby establishing a functional role for the exocyst complex during phagocytosis.

Polarity in mammalian epithelial morphogenesis

Cell polarity is fundamental for the architecture and function of epithelial tissues. Epithelial polarization requires the intervention of several fundamental cell processes, whose integration in space and time is only starting to be elucidated. To understand what governs the building of epithelial tissues during development, it is essential to consider the polarization process in the context of the whole tissue. To this end, the development of three-dimensional organotypic cell culture models has brought new insights into the mechanisms underlying the establishment and maintenance of higher-order epithelial tissue architecture, and in the dynamic remodeling of cell polarity that often occurs during development of epithelial organs. Here we discuss some important aspects of mammalian epithelial morphogenesis, from the establishment of cell polarity to epithelial tissue generation.

Polarization and myelination in myelinating glia

Myelinating glia, oligodendrocytes in central nervous system and Schwann cells in peripheral nervous system, form myelin sheath, a multilayered membrane system around axons enabling salutatory nerve impulse conduction and maintaining axonal integrity. Myelin sheath is a polarized structure localized in the axonal side and therefore is supposed to be formed based on the preceding polarization of myelinating glia. Thus, myelination process is closely associated with polarization of myelinating glia. However, cell polarization has been less extensively studied in myelinating glia than other cell types such as epithelial cells. The ultimate goal of this paper is to provide insights for the field of myelination research by applying the information obtained in polarity study in other cell types, especially epithelial cells, to cell polarization of myelinating glia. Thus, in this paper, the main aspects of cell polarization study in general are summarized. Then, they will be compared with polarization in oligodendrocytes. Finally, the achievements obtained in polarization study for epithelial cells, oligodendrocytes, and other types of cells will be translated into polarization/myelination process by Schwann cells. Then, based on this model, the perspectives in the study of Schwann cell polarization/myelination will be discussed.

Exo70 subunit of the exocyst complex is involved in adhesion-dependent trafficking of caveolin-1

Caveolae are specialized domains of the plasma membrane, which play key roles in signaling, endocytosis and mechanosensing. Using total internal reflection fluorescent microscopy (TIRF-M), we observe that the exocyst subunit Exo70 forms punctuate structures at the plasma membrane and partially localizes with caveolin-1, the main component of caveolae. Upon cell detachment, we found that Exo70 accumulates with caveolin-1-positive vesicular structures. Upon cell re-adhesion, caveolin-1 traffics back to the plasma membrane in a multistep process involving microtubules and actin cytoskeleton. In addition, silencing of Exo70 redirects caveolin-1 to focal adhesions identified by markers such as α5 integrin or vinculin. Based on these findings, we conclude that Exo70 is involved in caveolin-1 recycling to the plasma membrane during re-adhesion of the cells to the substratum.

The microtubule-associated Rho activating factor GEF-H1 interacts with exocyst complex to regulate vesicle traffic

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.

Fission yeast sec3 bridges the exocyst complex to the actin cytoskeleton

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.

PIP2 signaling, an integrator of cell polarity and vesicle trafficking in directionally migrating cells

Cell migration is a fundamental cellular process required for embryonic development to wound healing and also plays a key role in tumor metastasis and atherosclerosis. Migration is regulated at multiple strata, from cytoskeletal reorganization to vesicle trafficking. In migrating cells, signaling pathways are integrated with vesicle trafficking machineries in a highly coordinated fashion to accomplish the recruitment and trafficking of the trans-membrane proteins toward the leading edge. Different signaling molecules regulate cell migration in different physio-pathological contexts, among them, phosphatidylinositol-4,5-biphosphate (PIP2) is an integral component of the plasma membrane and pleiotropic lipid signaling molecule modulating diverse biological processes, including actin cytoskeletal dynamics and vesicle trafficking required for cell migration. In this commentary, we provide a brief overview of our current understandings on the phosphoinositide signaling and its implication in regulation of cell polarity and vesicle trafficking in migrating cells. In addition, we highlight the coordinated role of PIPKIγi2, a focal adhesion-targeted enzyme that synthesizes PIP2, and the exocyst complex, a PIP2-effector, in the trafficking of E-cadherin in epithelial cells and integrins in migrating cancer cells.

Structures and mechanisms of vesicle coat components and multisubunit tethering complexes

Eukaryotic cells face a logistical challenge in ensuring prompt and precise delivery of vesicular cargo to specific organelles within the cell. Coat protein complexes select cargo and initiate vesicle formation, while multisubunit tethering complexes participate in the delivery of vesicles to target membranes. Understanding these macromolecular assemblies has greatly benefited from their structural characterization. Recent structural data highlight principles in coat recruitment and uncoating in both the endocytic and retrograde pathways, and studies on the architecture of tethering complexes provide a framework for how they might link vesicles to the respective acceptor compartments and the fusion machinery.

ERK1/2 regulate exocytosis through direct phosphorylation of the exocyst component Exo70

The exocyst is a multiprotein 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 of the exocyst complex in response to epidermal growth factor (EGF) signaling. We further demonstrate that ERK1/2 regulates exocytosis, because 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.

Inositol polyphosphate 5-phosphatases; new players in the regulation of cilia and ciliopathies

Phosphoinositides regulate numerous cellular events via the recruitment and activation of multiple lipid-binding effector proteins. The precise temporal and spatial regulation of phosphoinositide signals by the co-ordinated activities of phosphoinositide kinases and phosphatases is essential for homeostasis and development. Mutations in two inositol polyphosphate 5-phosphatases, INPP5E and OCRL, cause the cerebrorenal syndromes of Joubert and Lowe's, respectively. INPP5E and OCRL exhibit overlapping phosphoinositide substrate specificity and subcellular localisation, including an association with the primary cilia. Here, we review recent studies that identify a new role for these enzymes in the regulation of primary cilia function. Joubert syndrome has been extensively linked to primary cilia defects, and Lowe's may represent a new class of 'ciliopathy associated' syndromes.

OCRL1 modulates cilia length in renal epithelial cells

Lowe syndrome is an X-linked disorder characterized by cataracts at birth, mental retardation and progressive renal malfunction that results from loss of function of the OCRL1 (oculocerebrorenal syndrome of Lowe) protein. OCRL1 is a lipid phosphatase that converts phosphatidylinositol 4,5-bisphosphate to phosphatidylinositol 4-phosphate. The renal pathogenesis of Lowe syndrome patients has been suggested to result from alterations in membrane trafficking, but this cannot fully explain the disease progression. We found that knockdown of OCRL1 in zebrafish caused developmental defects consistent with disruption of ciliary function, including body axis curvature, pericardial edema, hydrocephaly and impaired renal clearance. In addition, cilia in the proximal tubule of the zebrafish pronephric kidney were longer in ocrl morphant embryos. We also found that knockdown of OCRL1 in polarized renal epithelial cells caused elongation of the primary cilium and disrupted formation of cysts in three-dimensional cultures. Calcium release in response to ATP was blunted in OCRL1 knockdown cells, suggesting changes in signaling that could lead to altered cell function. Our results suggest a new role for OCRL1 in renal epithelial cell function that could contribute to the pathogenesis of Lowe syndrome.

The exocyst complex in exocytosis and cell migration

Exocytosis is a fundamental membrane trafficking event in eukaryotic cells in which membrane proteins or lipids are incorporated into the plasma membrane and vesicle contents are secreted to the exterior of the cell. The exocyst, an evolutionarily conserved octameric protein complex, plays a crucial role in the targeting of secretory vesicles to the plasma membrane during exocytosis. The exocyst has been shown to be involved in diverse cellular processes requiring polarized exocytosis such as yeast budding, epithelial polarity establishment, and neurite outgrowth. Recently, the exocyst has also been implicated in cell migration through mechanisms independent of its role in exocytosis. In this review, we will first summarize our knowledge on the exocyst complex at a molecular and structural level. Then, we will discuss the specific functions of the exocyst in exocytosis in various cell types. Finally, we will review the emerging roles of the exocyst during cell migration and tumor cell invasion.

Fission yeast Sec3 and Exo70 are transported on actin cables and localize the exocyst complex to cell poles

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.

Exo70 stimulates the Arp2/3 complex for lamellipodia formation and directional cell migration

Directional cell migration requires the coordination of actin assembly and membrane remodeling. The exocyst is an octameric protein complex essential for exocytosis and plasma membrane remodeling. A component of the exocyst, Exo70, directly interacts with the Arp2/3 complex, a core nucleating factor for the generation of branched actin networks for cell morphogenesis and migration. Using in vitro actin polymerization assay and time-lapse total internal reflection fluorescence microscopy, we found that Exo70 functions as a kinetic activator of the Arp2/3 complex that promotes actin filament nucleation and branching. We further found that the effect of Exo70 on actin is mediated by promoting the interaction of the Arp2/3 complex with WAVE2, a member of the N-WASP/WAVE family of nucleation promoting factors. At the cellular level, the stimulatory effect of Exo70 on the Arp2/3 complex is required for lamellipodia formation and maintaining directional persistence of cell migration. Our findings provide a novel mechanism for regulating actin polymerization and branching for effective membrane protrusion during cell morphogenesis and migration.

Rab11 regulates exocytosis of recycling vesicles at the plasma membrane

Rab11 is known to associate primarily with perinuclear recycling endosomes and regulate recycling of endocytosed proteins. However, the recycling step in which Rab11 participates remains unknown. We show here that, in addition to causing tubulation of recycling endosomes, Rab11 depletion gives rise to accumulation of recycling carriers containing endocytosed transferrin and transferrin receptor beneath the plasma membrane. We also show that the carriers are transported from perinuclear recycling endosomes to the cell periphery along microtubules. Total internal reflection fluorescence microscopy of cells expressing EGFP-tagged transferrin receptor revealed that Rab11 depletion inhibits tethering and fusion of recycling carriers to the plasma membrane. Depletion of Sec15, which interacts with Rab11, or Exo70, both components of the exocyst tethering complex, leads to essentially the same phenotypes as those of Rab11 depletion. Thus, in addition to its role in recycling processes at perinuclear recycling endosomes, Rab11 is transported along microtubules to the cell periphery through association with recycling carriers, and directly regulates vesicle exocytosis at the plasma membrane in concert with the exocyst.

Regulation of glucose transport by insulin: traffic control of GLUT4

Despite daily fasting and feeding, plasma glucose levels are normally maintained within a narrow range owing to the hormones insulin and glucagon. Insulin increases glucose uptake into fat and muscle cells through the regulated trafficking of vesicles that contain glucose transporter type 4 (GLUT4). New insights into insulin signalling reveal that phosphorylation events initiated by the insulin receptor regulate key GLUT4 trafficking proteins, including small GTPases, tethering complexes and the vesicle fusion machinery. These proteins, in turn, control GLUT4 movement through the endosomal system, formation and retention of specialized GLUT4 storage vesicles and targeted exocytosis of these vesicles. Understanding these processes may help to explain the development of insulin resistance in type 2 diabetes and provide new potential therapeutic targets.

The dual PH domain protein Opy1 functions as a sensor and modulator of PtdIns(4,5)P₂ synthesis

Phosphatidylinositol-4,5-bisphosphate, PtdIns(4,5)P(2), is an essential signalling lipid that regulates key processes such as endocytosis, exocytosis, actin cytoskeletal organization and calcium signalling. Maintaining proper levels of PtdIns(4,5)P(2) at the plasma membrane (PM) is crucial for cell survival and growth. We show that the conserved PtdIns(4)P 5-kinase, Mss4, forms dynamic, oligomeric structures at the PM that we term PIK patches. The dynamic assembly and disassembly of Mss4 PIK patches may provide a mechanism to precisely modulate Mss4 kinase activity, as needed, for localized regulation of PtdIns(4,5)P(2) synthesis. Furthermore, we identify a tandem PH domain-containing protein, Opy1, as a novel Mss4-interacting protein that partially colocalizes with PIK patches. Based upon genetic, cell biological, and biochemical data, we propose that Opy1 functions as a coincidence detector of the Mss4 PtdIns(4)P 5-kinase and PtdIns(4,5)P(2) and serves as a negative regulator of PtdIns(4,5)P(2) synthesis at the PM. Our results also suggest that additional conserved tandem PH domain-containing proteins may play important roles in regulating phosphoinositide signalling.

Exorcising the exocyst complex

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.

Phosphoinositide signaling regulates the exocyst complex and polarized integrin trafficking in directionally migrating cells

Polarized delivery of signaling and adhesion molecules to the leading edge is required for directional migration of cells. Here, we describe a role for the PIP(2)-synthesizing enzyme, PIPKIγi2, in regulation of exocyst complex control of cell polarity and polarized integrin trafficking during migration. Loss of PIPKIγi2 impaired directional migration, formation of cell polarity, and integrin trafficking to the leading edge. Upon initiation of directional migration, PIPKIγi2 via PIP(2) generation controls the integration of the exocyst complex into an integrin-containing trafficking compartment that requires the talin-binding ability of PIPKIγi2, and talin for integrin recruitment to the leading edge. A PIP(2) requirement is further emphasized by inhibition of PIPKIγi2-regulated directional migration by an Exo70 mutant deficient in PIP(2) binding. These results reveal how phosphoinositide generation orchestrates polarized trafficking of integrin in coordination with talin that links integrins to the actin cytoskeleton, processes that are required for directional migration.

Rab8a regulates the exocyst-mediated kiss-and-run discharge of the Dictyostelium contractile vacuole

A molecular dissection of contractile vacuole (CV) discharge shows that Rab8a is recruited to the CV a few seconds before the exocyst. Together they tether it to the plasma membrane and commit it to fusion. GTP hydrolysis is necessary for vacuole detethering, a process in which LvsA, a protein of the Chédiak–Higashi family, plays a crucial role.

Water expulsion by the contractile vacuole (CV) in Dictyostelium is carried out by a giant kiss-and-run focal exocytic event during which the two membranes are only transiently connected but do not completely merge. We present a molecular dissection of the GTPase Rab8a and the exocyst complex in tethering of the contractile vacuole to the plasma membrane, fusion, and final detachment. Right before discharge, the contractile vacuole bladder sequentially recruits Drainin, a Rab11a effector, Rab8a, the exocyst complex, and LvsA, a protein of the Chédiak–Higashi family. Rab8a recruitment precedes the nucleotide-dependent arrival of the exocyst to the bladder by a few seconds. A dominant-negative mutant of Rab8a strongly binds to the exocyst and prevents recruitment to the bladder, suggesting that a Rab8a guanine nucleotide exchange factor activity is associated with the complex. Absence of Drainin leads to overtethering and blocks fusion, whereas expression of constitutively active Rab8a allows fusion but blocks vacuole detachment from the plasma membrane, inducing complete fragmentation of tethered vacuoles. An indistinguishable phenotype is generated in cells lacking LvsA, implicating this protein in postfusion detethering. Of interest, overexpression of a constitutively active Rab8a mutant reverses the lvsA-null CV phenotype.

PIP kinases from the cell membrane to the nucleus

Phosphatidylinositol 4,5-bisphosphate (PIP(2)) is a membrane bound lipid molecule with capabilities to affect a wide array of signaling pathways to regulate very different cellular processes. PIP(2) is used as a precursor to generate the second messengers PIP(3), DAG and IP(3), indispensable molecules for signaling events generated by membrane receptors. However, PIP(2) can also directly regulate a vast array of proteins and is emerging as a crucial messenger with the potential to distinctly modulate biological processes critical for both normal and pathogenic cell physiology. PIP(2) directly associates with effector proteins via unique phosphoinositide binding domains, altering their localization and/or enzymatic activity. The spatial and temporal generation of PIP(2) synthesized by the phosphatidylinositol phosphate kinases (PIPKs) tightly regulates the activation of receptor signaling pathways, endocytosis and vesicle trafficking, cell polarity, focal adhesion dynamics, actin assembly and 3' mRNA processing. Here we discuss our current understanding of PIPKs in the regulation of cellular processes from the plasma membrane to the nucleus.

Role of PI(4,5)P(2) in vesicle exocytosis and membrane fusion

A role for phosphatidylinositol 4,5-bisphosphate (PI(4,5)P(2)) in membrane fusion was originally identified for regulated dense-core vesicle exocytosis in neuroendocrine cells. Subsequent studies demonstrated essential roles for PI(4,5)P(2) in regulated synaptic vesicle and constitutive vesicle exocytosis. For regulated dense-core vesicle exocytosis, PI(4,5)P(2) appears to be primarily required for priming, a stage in vesicle exocytosis that follows vesicle docking and precedes Ca(2) (+)-triggered fusion. The priming step involves the organization of SNARE protein complexes for fusion. A central issue concerns the mechanisms by which PI(4,5)P(2) exerts an essential role in membrane fusion events at the plasma membrane. The observed microdomains of PI(4,5)P(2) in the plasma membrane of neuroendocrine cells at fusion sites has suggested possible direct effects of the phosphoinositide on membrane curvature and tension. More likely, PI(4,5)P(2) functions in vesicle exocytosis as in other cellular processes to recruit and activate PI(4,5)P(2)-binding proteins. CAPS and Munc13 proteins, which bind PI(4,5)P(2) and function in vesicle priming to organize SNARE proteins, are key candidates as effectors for the role of PI(4,5)P(2) in vesicle priming. Consistent with roles prior to fusion that affect SNARE function, subunits of the exocyst tethering complex involved in constitutive vesicle exocytosis also bind PI(4,5)P(2). Additional roles for PI(4,5)P(2) in fusion pore dilation have been described, which may involve other PI(4,5)P(2)-binding proteins such as synaptotagmin. Lastly, the SNARE proteins that mediate exocytic vesicle fusion contain highly basic membrane-proximal domains that interact with acidic phospholipids that likely affect their function.

An association between type Iγ PI4P 5-kinase and Exo70 directs E-cadherin clustering and epithelial polarization

E-Cadherin-mediated formation of adherens junctions (AJs) is essential for the morphogenesis of epithelial cells. However, the mechanisms underlying E-cadherin clustering and AJ maturation are not fully understood. Here we report that type Iγ phosphatidylinositol-4-phosphate 5-kinase (PIPKIγ) associates with the exocyst via a direct interaction with Exo70, the exocyst subunit that guides the polarized targeting of exocyst to the plasma membrane. By means of this interaction, PIPKIγ mediates the association between E-cadherin and Exo70 and determines the targeting of Exo70 to AJs. Further investigation revealed that Exo70 is necessary for clustering of E-cadherin on the plasma membrane and extension of nascent E-cadherin adhesions, which are critical for the maturation of cohesive AJs. In addition, we observed phosphatidylinositol-4,5-bisphosphate (PI4,5P(2)) accumulation at E-cadherin clusters during the assembly of E-cadherin adhesions. PIPKIγ-generated PI4,5P(2) is required for recruiting Exo70 to newly formed E-cadherin junctions and facilitates the assembly and maturation of AJs. These results support a model in which PIPKIγ and PIPKIγ-generated PI4,5P(2) pools at nascent E-cadherin contacts cue Exo70 targeting and orient the tethering of exocyst-associated E-cadherin. This could be an important mechanism that regulates E-cadherin clustering and AJ maturation, which is essential for the establishment of solid, polarized epithelial structures.

Lowe Syndrome protein OCRL1 supports maturation of polarized epithelial cells

Mutations in the inositol polyphosphate 5-phosphatase OCRL1 cause Lowe Syndrome, leading to cataracts, mental retardation and renal failure. We noted that cell types affected in Lowe Syndrome are highly polarized, and therefore we studied OCRL1 in epithelial cells as they mature from isolated individual cells into polarized sheets and cysts with extensive communication between neighbouring cells. We show that a proportion of OCRL1 targets intercellular junctions at the early stages of their formation, co-localizing both with adherens junctional components and with tight junctional components. Correlating with this distribution, OCRL1 forms complexes with junctional components α-catenin and zonula occludens (ZO)-1/2/3. Depletion of OCRL1 in epithelial cells growing as a sheet inhibits maturation; cells remain flat, fail to polarize apical markers and also show reduced proliferation. The effect on shape is reverted by re-expressed OCRL1 and requires the 5'-phosphatase domain, indicating that down-regulation of 5-phosphorylated inositides is necessary for epithelial development. The effect of OCRL1 in epithelial maturation is seen more strongly in 3-dimensional cultures, where epithelial cells lacking OCRL1 not only fail to form a central lumen, but also do not have the correct intracellular distribution of ZO-1, suggesting that OCRL1 functions early in the maturation of intercellular junctions when cells grow as cysts. A role of OCRL1 in junctions of polarized cells may explain the pattern of organs affected in Lowe Syndrome.

Divide and polarize: recent advances in the molecular mechanism regulating epithelial tubulogenesis

Epithelial organs are generated from groups of non-polarized cells by a combination of processes that induce the acquisition of cell polarity, lumen formation, and the subsequent steps required for tubulogenesis. The subcellular mechanisms associated to these processes are still poorly understood. The extracellular environment provides a cue for the initial polarization, while cytoskeletal rearrangements build up the three-dimensional architecture that supports the central lumen. The proper orientation of cell division in the epithelium has been found to be required for the normal formation of the central lumen in epithelial morphogenesis. Moreover, recent data in cellular models and in vivo have shed light into the underlying mechanisms that connect the spindle orientation machinery with cell polarity. In addition, recent work has clarified the core molecular components of the vesicle trafficking machinery in epithelial morphogenesis, including Rab-GTPases and the Exocyst, as well as an increasing list of microtubule-binding and actin-binding proteins and motors, most of which are conserved from yeast to humans. In this review we will focus on the discussion of novel findings that have unveiled important clues for the mechanisms that regulate epithelial tubulogenesis.

Molecular regulation of lumen morphogenesis

The asymmetric polarization of cells allows specialized functions to be performed at discrete subcellular locales. Spatiotemporal coordination of polarization between groups of cells allowed the evolution of metazoa. For instance, coordinated apical-basal polarization of epithelial and endothelial cells allows transport of nutrients and metabolites across cell barriers and tissue microenvironments. The defining feature of such tissues is the presence of a central, interconnected luminal network. Although tubular networks are present in seemingly different organ systems, such as the kidney, lung, and blood vessels, common underlying principles govern their formation. Recent studies using in vivo and in vitro models of lumen formation have shed new light on the molecular networks regulating this fundamental process. We here discuss progress in understanding common design principles underpinning de novo lumen formation and expansion.

Endocytosis, recycling, and regulated exocytosis of glucose transporter 4

Glucose transporter 4 (GLUT4) is responsible for the uptake of glucose into muscle and adipose tissues. Under resting conditions, GLUT4 is dynamically retained through idle cycling among selective intracellular compartments, from whence it undergoes slow recycling to the plasma membrane (PM). This dynamic retention can be released by command from intracellular signals elicited by insulin and other stimuli, which result in 2-10-fold increases in the surface level of GLUT4. Insulin-derived signals promote translocation of GLUT4 to the PM from a specialized compartment termed GLUT4 storage vesicles (GSV). Much effort has been devoted to the characterization of the intracellular compartments and dynamics of GLUT4 cycling and to the signals by which GLUT4 is sorted into, and recruited from, GSV. This review summarizes our understanding of intracellular GLUT4 traffic during its internalization from the membrane, its slow, constitutive recycling, and its regulated exocytosis in response to insulin. In spite of specific differences in GLUT4 dynamic behavior in adipose and muscle cells, the generalities of its endocytic and exocytic itineraries are consistent and an array of regulatory proteins that regulate each vesicular traffic event emerges from these cell systems.

Multisubunit tethering complexes and their role in membrane fusion

Protein trafficking within eukaryotic cells depends on vesicular carriers that fuse with organelles to deliver their lipid and protein content. Cells have developed an elaborate system to capture vesicles at organelles that involves the action of Rab GTPases and tethers. Vesicle fusion then takes place with the help of SNARE proteins. In this review we focus on the role of multisubunit tethering complexes of eukaryotic cells. In particular, we discuss the tethering complexes of the secretory pathway and the endolysosomal system and highlight recent evidence for the role of these complexes in interaction with Rabs, coat recognition and cooperation with SNAREs during the fusion cascade.

Cell polarity during motile processes: keeping on track with the exocyst complex

Motile processes are critical for several physiological and pathological situations such as embryonic development, tumour dissemination and metastasis. Migrating cells, or developing neurons, need to establish front–rear polarity consisting of actin-driven extension of the leading edge and traffic of components that are essential for membrane extension and cell adhesion at the front. Previously, several studies have suggested that the exocyst complex is critical for the establishment and maintenance of cell polarity. This octameric complex controls the docking and insertion of exocytic vesicles to growing areas of the plasma membrane. The aim of the present review is to detail recent advances concerning the molecular and structural organization of the exocyst complex that help to elucidate its role in cell polarity. We will also review the function of the exocyst complex and some of its key interacting partners [including the small GTP-binding protein Ral, aPKCs (atypical protein kinase Cs) and proteins involved in actin assembly] in the formation of plasma extensions at the leading edge, growth cone formation during axonal extension and generation of cell movement.

Faulty epithelial polarity genes and cancer

Epithelial architecture is formed in tissues and organs when groups of epithelial cells are organized into polarized structures. The epithelial function and integrity as well as signaling across the epithelial layer is orchestrated by apical junctional complexes (AJCs), which are landmarks for PAR/CRUMBS and lateral SCRIB polarity modules and by dynamic interactions of the cells with underlying basement membrane (BM). These highly organized epithelial architectures are demolished in cancer. In all advanced epithelial cancers, malignant cells have lost polarity and connections to the basement membrane and they have become proliferative, motile, and invasive. Clearly, loss of epithelial integrity associates with tumor progression but does it contribute to tumor development? Evidence from studies in Drosophila and recently also in vertebrate models have suggested that even the oncogene-driven enforced cell proliferation can be conditional, dependant on the influence of cell-cell or cell-microenvironment contacts. Therefore, loss of epithelial integrity may not only be an obligate consequence of unscheduled proliferation of malignant cells but instead, malignant epithelial cells may need to acquire capacity to break free from the constraints of integrity to freely and autonomously proliferate. We discuss how epithelial polarity complexes form and regulate epithelial integrity, highlighting the roles of enzymes Rho GTPases, aPKCs, PI3K, and type II transmembrane serine proteases (TTSPs). We also discuss relevance of these pathways to cancer in light of genetic alterations found in human cancers and review molecular pathways and potential pharmacological strategies to revert or selectively eradicate disorganized tumor epithelium.

Bacterial invasion: entry through the exocyst door

Salmonella entry into host cells involves rearrangements of actin and mobilization of membranes. Here we discuss new findings showing that Salmonella recruits the exocyst complex, which plays a role in vesicle secretion, to the site of invasion to promote its entry.

Actin cables and the exocyst form two independent morphogenesis pathways in the fission yeast

In fission yeast, long-range transport and vesicle tethering by the exocyst are individually dispensable but together essential for cell morphogenesis. Both pathways function downstream of Cdc42. The exocyst localizes to growing cell tips independently of the cytoskeleton and instead depends on PIP₂.

Cell morphogenesis depends on polarized exocytosis. One widely held model posits that long-range transport and exocyst-dependent tethering of exocytic vesicles at the plasma membrane sequentially drive this process. Here, we describe that disruption of either actin-based long-range transport and microtubules or the exocyst did not abolish polarized growth in rod-shaped fission yeast cells. However, disruption of both actin cables and exocyst led to isotropic growth. Exocytic vesicles localized to cell tips in single mutants but were dispersed in double mutants. In contrast, a marker for active Cdc42, a major polarity landmark, localized to discreet cortical sites even in double mutants. Localization and photobleaching studies show that the exocyst subunits Sec6 and Sec8 localize to cell tips largely independently of the actin cytoskeleton, but in a cdc42 and phospholipid phosphatidylinositol 4,5-bisphosphate (PIP₂)–dependent manner. Thus in fission yeast long-range cytoskeletal transport and PIP₂-dependent exocyst represent parallel morphogenetic modules downstream of Cdc42, raising the possibility of similar mechanisms in other cell types.

ARF6-mediated endocytic recycling impacts cell movement, cell division and lipid homeostasis

A wide range of cellular activities depends upon endocytic recycling. ARF6, a small molecular weight GTPase, regulates the processes of endocytosis and endocytic recycling in concert with various effector molecules and other small GTPases. This review highlights three critical processes that involve ARF6-mediated endosomal membrane trafficking-cell motility, cytokinesis, and cholesterol homeostasis. In each case, the function of ARF6-mediated trafficking varies-including localization of specific protein and lipid cargo, regulation of bulk membrane movement, and modulation of intracellular signaling. As described in this review, mis-regulation of endocytic traffic can result in human disease when it compromises the cell's ability to regulate cell movement and invasion, cell division, and lipid homeostasis.

Salmonella-directed recruitment of new membrane to invasion foci via the host exocyst complex

Salmonella attachment to the intestinal epithelium triggers delivery of bacterial effector proteins into the host cytosol through a type III secretion system (T3SS), leading to pronounced membrane ruffling and macropinocytic uptake of attached bacteria. The tip of the T3SS is made up of two proteins, SipB and SipC, which insert into the host plasma membrane, forming a translocation pore. Both the N and C termini of SipC are exposed in the host cytosol and have been shown to directly modulate actin cytoskeleton assembly. We have identified a direct interaction between SipC and Exo70, a component of the exocyst complex, which mediates docking and fusion of exocytic vesicles with the plasma membrane. Here, we show that exocyst components coprecipitate with SipC and accumulate at sites of invasion by Salmonella typhimurium. Exocyst assembly requires activation of the small GTPase RalA, which we show is triggered during Salmonella infection by the translocated effector, SopE. Knockdown of RalA or Sec5 results in reduced membrane ruffling at sites of attachment and impairs bacterial entry into host cells. These findings suggest that S. typhimurium enhances invasion efficiency by promoting localized membrane expansion, directly through SipC-dependent recruitment of the exocyst and indirectly via SopE-dependent activation of RalA.

HOPS initiates vacuole docking by tethering membranes before trans-SNARE complex assembly

Vacuole homotypic fusion has been reconstituted with all purified components: vacuolar lipids, four soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) proteins, Sec17p, Sec18p, the Rab Ypt7p, and the hexameric homotypic fusion and vacuole protein sorting complex (HOPS). HOPS is a Rab-effector with direct affinity for SNAREs (presumably via its Sec1-Munc18 homologous subunit Vps33p) and for certain vacuolar lipids. Each of these pure vacuolar proteins was required for optimal proteoliposome clustering, raising the question of which was most directly involved. We now present model subreactions of clustering and fusion that reveal that HOPS is the direct agent of tethering. The Rab and vacuole lipids contribute to tethering by supporting the membrane association of HOPS. HOPS indirectly facilitates trans-SNARE complex formation by tethering membranes, because the synthetic liposome tethering factor polyethylene glycol can also stimulate trans-SNARE complex formation and fusion. SNAREs further stabilize the associations of HOPS-tethered membranes. HOPS then protects newly formed trans-SNARE complexes from disassembly by Sec17p/Sec18p.

Molecular mechanisms of protein and lipid targeting to ciliary membranes

Cilia are specialized surface regions of eukaryotic cells that serve a variety of functions, ranging from motility to sensation and to regulation of cell growth and differentiation. The discovery that a number of human diseases, collectively known as ciliopathies, result from defective cilium function has expanded interest in these structures. Among the many properties of cilia, motility and intraflagellar transport have been most extensively studied. The latter is the process by which multiprotein complexes associate with microtubule motors to transport structural subunits along the axoneme to and from the ciliary tip. By contrast, the mechanisms by which membrane proteins and lipids are specifically targeted to the cilium are still largely unknown. In this Commentary, we review the current knowledge of protein and lipid targeting to ciliary membranes and outline important issues for future study. We also integrate this information into a proposed model of how the cell specifically targets proteins and lipids to the specialized membrane of this unique organelle.

Phosphatidylinositol 4,5-bisphosphate directs spermatid cell polarity and exocyst localization in Drosophila

During spermiogenesis, Drosophila melanogaster spermatids coordinate their elongation in interconnected cysts that become highly polarized, with nuclei localizing to one end and sperm tail growth occurring at the other. Remarkably little is known about the signals that drive spermatid polarity and elongation. Here we identify phosphoinositides as critical regulators of these processes. Reduction of plasma membrane phosphatidylinositol 4,5-bisphosphate (PIP(2)) by low-level expression of the PIP(2) phosphatase SigD or mutation of the PIP(2) biosynthetic enzyme Skittles (Sktl) results in dramatic defects in spermatid cysts, which become bipolar and fail to fully elongate. Defects in polarity are evident from the earliest stages of elongation, indicating that phosphoinositides are required for establishment of polarity. Sktl and PIP(2) localize to the growing end of the cysts together with the exocyst complex. Strikingly, the exocyst becomes completely delocalized when PIP(2) levels are reduced, and overexpression of Sktl restores exocyst localization and spermatid cyst polarity. Moreover, the exocyst is required for polarity, as partial loss of function of the exocyst subunit Sec8 results in bipolar cysts. Our data are consistent with a mechanism in which localized synthesis of PIP(2) recruits the exocyst to promote targeted membrane delivery and polarization of the elongating cysts.

At the poles across kingdoms: phosphoinositides and polar tip growth

Phosphoinositides (PIs) are minor, but essential phospholipid constituents of eukaryotic membranes, and are involved in the regulation of various physiological processes. Recent genetic and cell biological advances indicate that PIs play important roles in the control of polar tip growth in plant cells. In root hairs and pollen tubes, PIs control directional membrane trafficking required for the delivery of cell wall material and membrane area to the growing tip. So far, the exact mechanisms by which PIs control polarity and tip growth are unresolved. However, data gained from the analysis of plant, fungal and animal systems implicate PIs in the control of cytoskeletal dynamics, ion channel activity as well as vesicle trafficking. The present review aims at giving an overview of PI roles in eukaryotic cells with a special focus on functions pertaining to the control of cell polarity. Comparative screening of plant and fungal genomes suggests diversification of the PI system with increasing organismic complexity. The evolutionary conservation of the PI system among eukaryotic cells suggests a role for PIs in tip growing cells in models where PIs so far have not been a focus of attention, such as fungal hyphae.

Structure-function study of the N-terminal domain of exocyst subunit Sec3

The exocyst is an evolutionarily conserved octameric complex involved in polarized exocytosis from yeast to humans. The Sec3 subunit of the exocyst acts as a spatial landmark for exocytosis through its ability to bind phospholipids and small GTPases. The structure of the N-terminal domain of Sec3 (Sec3N) was determined ab initio and defines a new subclass of pleckstrin homology (PH) domains along with a new family of proteins carrying this domain. Respectively, N- and C-terminal to the PH domain Sec3N presents an additional alpha-helix and two beta-strands that mediate dimerization through domain swapping. The structure identifies residues responsible for phospholipid binding, which when mutated in cells impair the localization of exocyst components at the plasma membrane and lead to defects in exocytosis. Through its ability to bind the small GTPase Cdc42 and phospholipids, the PH domain of Sec3 functions as a coincidence detector at the plasma membrane.

The plant exocyst

The exocyst is an octameric vesicle tethering complex that functions upstream of SNARE mediated exocytotic vesicle fusion with the plasma membrane. All proteins in the complex have been conserved during evolution, and genes that encode the exocyst subunits are present in the genomes of all plants investigated to date. Although the plant exocyst has not been studied in great detail, it is likely that the basic function of the exocyst in vesicle tethering is conserved. Nevertheless, genomic and genetic studies suggest that the exocyst complex in plants may have more diversified roles than that in budding yeast. In this review, we compare the knowledge about the exocyst in plant cells to the well-studied exocyst in budding yeast, in order to explore similarities and differences in expression and function between these organisms, both of which have walled cells.

Structural basis for the Rho- and phosphoinositide-dependent localization of the exocyst subunit Sec3

The exocyst complex is a hetero-octameric protein complex that functions during cell polarization by tethering the secretory vesicle to the target membrane. The yeast exocyst subunit Sec3 binds to phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P(2)) and the small GTPases Rho1 and Cdc42 via its N-terminal domain (Sec3-N), and these interactions target Sec3 to the plasma membrane. Here we report the crystal structure of the Sec3-N in complex with Rho1 at 2.6-A resolution. Sec3-N adopts a pleckstrin homology (PH) fold, despite having no detectable sequence homology with other PH domains of known structure. Clusters of conserved basic residues constitute a positively charged cleft, which was identified as a binding site for PtdIns(4,5)P(2). Residues Phe77, Ile115 and Leu131 of Sec3 bind to an extended hydrophobic surface formed around switch regions I and II of Rho1. To our knowledge, these are the first structural insights into how an exocyst subunit might interact with both protein and phospholipid factors on the target membrane.

Sec3-containing exocyst complex is required for desmosome assembly in mammalian epithelial cells

In epithelial cells, Sec3 associates with Exocyst complexes enriched at desmosomes and centrosomes, distinct from Sec6/8 complexes at the apical junctional complex. RNAi-mediated suppression of Sec3 alters trafficking of desmosomal cadherins and impairs desmosome morphology and function, without noticeable effect on adherens junctions.

The Exocyst is a conserved multisubunit complex involved in the docking of post-Golgi transport vesicles to sites of membrane remodeling during cellular processes such as polarization, migration, and division. In mammalian epithelial cells, Exocyst complexes are recruited to nascent sites of cell–cell contact in response to E-cadherin–mediated adhesive interactions, and this event is an important early step in the assembly of intercellular junctions. Sec3 has been hypothesized to function as a spatial landmark for the development of polarity in budding yeast, but its role in epithelial cells has not been investigated. Here, we provide evidence in support of a function for a Sec3-containing Exocyst complex in the assembly or maintenance of desmosomes, adhesive junctions that link intermediate filament networks to sites of strong intercellular adhesion. We show that Sec3 associates with a subset of Exocyst complexes that are enriched at desmosomes. Moreover, we found that membrane recruitment of Sec3 is dependent on cadherin-mediated adhesion but occurs later than that of the known Exocyst components Sec6 and Sec8 that are recruited to adherens junctions. RNA interference-mediated suppression of Sec3 expression led to specific impairment of both the morphology and function of desmosomes, without noticeable effect on adherens junctions. These results suggest that two different exocyst complexes may function in basal–lateral membrane trafficking and will enable us to better understand how exocytosis is spatially organized during development of epithelial plasma membrane domains.

Membrane organization and dynamics in cell polarity

The establishment and maintenance of cell polarity is important to a wide range of biological processes ranging from chemotaxis to embryogenesis. An essential feature of cell polarity is the asymmetric organization of proteins and lipids in the plasma membrane. In this article, we discuss how polarity regulators such as small GTP-binding proteins and phospholipids spatially and kinetically control vesicular trafficking and membrane organization. Conversely, we discuss how membrane trafficking contributes to cell polarization through delivery of polarity determinants and regulators to the plasma membrane.

Phosphoinositides, exocytosis and polarity in yeast: all about actin?

Cell polarity is necessary for cell division, morphogenesis and motility in eukaryotes, and is determined by dynamic control of the cytoskeleton and secretory pathway to promote directional growth. In yeast, three essential and tightly-regulated processes orchestrate polarization and facilitate bud growth. These processes include phosphoinositide (PI) signaling, Rho GTPase regulation of the actin cytoskeleton, and exocytosis. As yet, the interplay between these different processes is unclear, and two main models (Spatial Landmark and Allosteric Local Activation) have been proposed for Rho GTPase control of polarization in yeast. Here, we summarize the inter-relationship between these growth processes and present a more unified model, the Exocytic Signal model, which proposes that exocytosis and actin regulation are fully integrated events mediated by PI signaling.