Sec1p directly stimulates SNARE-mediated membrane fusion in vitro

SEC1A and SEC6 synergistically regulate pollen tube polar growth

Pollen tube polar growth is a key physiological activity for angiosperms to complete double fertilization, which is highly dependent on the transport of polar substances mediated by secretory vesicles. The exocyst and Sec1/Munc18 (SM) proteins are involved in the regulation of the tethering and fusion of vesicles and plasma membranes, but the molecular mechanism by which they regulate pollen tube polar growth is still unclear. In this study, we found that loss of function of SEC1A, a member of the SM protein family in Arabidopsis thaliana, resulted in reducing pollen tube growth and a significant increase in pollen tube width. SEC1A was diffusely distributed in the pollen tube cytoplasm, and was more concentrated at the tip of the pollen tube. Through co-immunoprecipitation-mass spectrometry screening, protein interaction analysis and in vivo microscopy, we found that SEC1A interacted with the exocyst subunit SEC6, and they mutually affected the distribution and secretion rate at the tip of the pollen tube. Meanwhile, the functional loss of SEC1A and SEC6 significantly affected the distribution of the SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptor) complex member SYP125 at the tip of the pollen tube, and led to the disorder of pollen tube cell wall components. Genetic analysis revealed that the pollen tube-related phenotype of the sec1a sec6 double mutant was significantly enhanced compared with their respective single mutants. Therefore, we speculated that SEC1A and SEC6 cooperatively regulate the fusion of secretory vesicles and plasma membranes in pollen tubes, thereby affecting the length and the width of pollen tubes.

Functional Roles of UNC-13/Munc13 and UNC-18/Munc18 in Neurotransmission

Neurotransmitters are released from synaptic and secretory vesicles following calcium-triggered fusion with the plasma membrane. These exocytotic events are driven by assembly of a ternary SNARE complex between the vesicle SNARE synaptobrevin and the plasma membrane-associated SNAREs syntaxin and SNAP-25. Proteins that affect SNARE complex assembly are therefore important regulators of synaptic strength. In this chapter, we review our current understanding of the roles played by two SNARE interacting proteins: UNC-13/Munc13 and UNC-18/Munc18. We discuss results from both invertebrate and vertebrate model systems, highlighting recent advances, focusing on the current consensus on molecular mechanisms of action and nanoscale organization, and pointing out some unresolved aspects of their functions.

Roles of Mso1 and the SM protein Sec1 in efficient vesicle fusion during fission yeast cytokinesis

Membrane trafficking during cytokinesis is essential for the delivery of membrane lipids and cargoes to the division site. However, the molecular mechanisms are still incompletely understood. In this study, we demonstrate the importance of uncharacterized fission yeast proteins Mso1 and Sec1 in membrane trafficking during cytokinesis. Fission yeast Mso1 shares homology with budding yeast Mso1 and human Mint1, proteins that interact with Sec1/Munc18 family proteins during vesicle fusion. Sec1/Munc18 proteins and their interactors are important regulators of SNARE complex formation during vesicle fusion. The roles of these proteins in vesicle trafficking during cytokinesis have been barely studied. Here, we show that fission yeast Mso1 is also a Sec1-binding protein and Mso1 and Sec1 localize to the division site interdependently during cytokinesis. The loss of Sec1 localization in mso1Δ cells results in a decrease in vesicle fusion and cytokinesis defects such as slow ring constriction, defective ring disassembly, and delayed plasma membrane closure. We also find that Mso1 and Sec1 may have functions independent of the exocyst tethering complex on the plasma membrane at the division site. Together, Mso1 and Sec1 play essential roles in regulating vesicle fusion and cargo delivery at the division site during cytokinesis.

Characteristics of SNARE proteins are defined by distinctive properties of SNARE motifs

BACKGROUND

Syntaxin-1A and Sso1 are syntaxin family SNARE proteins engaged in synaptic vesicle fusion and yeast exocytosis. The syntaxin-1A SNARE motif can form a fusogenic SNARE complex with Sso1 partners. However, a chimera in which the SNARE motif in syntaxin-1A is introduced into Sso1 was not functional in yeast because the chimera is retained in the ER. Through the analysis of the transport defect of Sso1/syntaxin-1A chimeric SNAREs, we found that their SNARE motifs have distinctive properties.

METHODS

Sso1, syntaxin-1A, and Sso1/syntaxin-1A chimeric SNAREs were expressed in yeast cells and their localization and interaction with other SNAREs are analyzed.

RESULTS

SNARE proteins containing the syntaxin-1A SNARE motif exhibit a transport defect because they form a cis-SNARE complex in the ER. Ectopic SNARE complex formation can be prevented in syntaxin-1A by binding to a Sec1/Munc-18-like (SM) protein. In contrast, the SNARE motif of Sso1 does not form an ectopic SNARE complex. Additionally, we found that the SNARE motif in syntaxin-1A, but not that in Sso1, self-interacts, even when it is in the inactive form and bound to the SM protein.

CONCLUSIONS

The SNARE motif in syntaxin-1A, but not in Sso1, likely forms ectopic SNARE complex. Because of this property, the SM protein is necessary for syntaxin-1A to prevent its promiscuous assembly and to promote its export from the ER.

GENERAL SIGNIFICANCE

Properties of SNARE motifs affect characteristics of SNARE proteins. The regulatory mechanisms of SNARE proteins are, in part, designed to handle such properties.

The Sec1/Munc18 (SM) protein Vps45 is involved in iron uptake, mitochondrial function and virulence in the pathogenic fungus Cryptococcus neoformans

The battle for iron between invading microorganisms and mammalian hosts is a pivotal determinant of the outcome of infection. The pathogenic fungus, Cryptococcus neoformans, employs multiple mechanisms to compete for iron during cryptococcosis, a disease primarily of immunocompromised hosts. In this study, we examined the role of endocytic trafficking in iron uptake by characterizing a mutant defective in the Sec1/Munc18 (SM) protein Vps45. This protein is known to regulate the machinery for vesicle trafficking and fusion via interactions with SNARE proteins. As expected, a vps45 deletion mutant was impaired in endocytosis and showed sensitivity to trafficking inhibitors. The mutant also showed poor growth on iron-limited media and a defect in transporting the Cfo1 ferroxidase of the high-affinity iron uptake system from the plasma membrane to the vacuole. Remarkably, we made the novel observation that Vps45 also contributes to mitochondrial function in that a Vps45-Gfp fusion protein associated with mitotracker, and a vps45 mutant showed enhanced sensitivity to inhibitors of electron transport complexes as well as changes in mitochondrial membrane potential. Consistent with mitochondrial function, the vps45 mutant was impaired in calcium homeostasis. To assess the relevance of these defects for virulence, we examined cell surface properties of the vps45 mutant and found increased sensitivity to agents that challenge cell wall integrity and to antifungal drugs. A change in cell wall properties was consistent with our observation of altered capsule polysaccharide attachment, and with attenuated virulence in a mouse model of cryptococcosis. Overall, our studies reveal a novel role for Vps45-mediated trafficking for iron uptake, mitochondrial function and virulence.

Cryptococcus neoformans is a causative agent of cryptococcal meningitis, a disease that is estimated to cause ~ 15% of AIDS-related deaths. In this context, cryptococosis is one of the most common causes of mortality in people with HIV/AIDS, closely behind tuberculosis. Unfortunately, very few antifungal drugs are available to treat this disease. However, understanding mechanisms involved in the pathogenesis of C. neoformans can lead to new therapeutic avenues. In this study, we discovered a new role for a regulatory protein involved in vesicle transport. Specifically, we found that the Vps45 protein, which regulates vesicle fusion, participates in the trafficking of iron into fungal cells, supports mitochondria function, mediates antifungal resistance and is required for virulence. These discoveries shed light on the molecular mechanisms underlying the uptake and use of iron as an essential nutrient for the virulence of C. neoformans. Further investigations could lead to the development of drugs that target Vps45-mediated processes.

SM protein Munc18-2 facilitates transition of Syntaxin 11-mediated lipid mixing to complete fusion for T-lymphocyte cytotoxicity

The atypical lipid-anchored Syntaxin 11 (STX11) and its binding partner, the Sec/Munc (SM) protein Munc18-2, facilitate cytolytic granule release by cytotoxic T lymphocytes (CTLs) and natural killer (NK) cells. Patients carrying mutations in these genes develop familial hemophagocytic lymphohistiocytosis, a primary immunodeficiency characterized by impaired lytic granule exocytosis. However, whether a SNARE such as STX11, which lacks a transmembrane domain, can support membrane fusion in vivo is uncertain, as is the precise role of Munc18-2 during lytic granule exocytosis. Here, using a reconstituted "flipped" cell-cell fusion assay, we show that lipid-anchored STX11 and its cognate SNARE proteins mainly support exchange of lipids but not cytoplasmic content between cells, resembling hemifusion. Strikingly, complete fusion is stimulated by addition of wild-type Munc18-2 to the assay, but not of Munc18-2 mutants with abnormal STX11 binding. Our data reveal that Munc18-2 is not just a chaperone of STX11 but also directly contributes to complete membrane merging by promoting SNARE complex assembly. These results further support the concept that SM proteins in general are part of the core fusion machinery. This fusion mechanism likely contributes to other cell-type-specific exocytic processes such as platelet secretion.

The Sec1/Munc18 Protein Groove Plays a Conserved Role in Interaction with Sec9p/SNAP-25

The Sec1/Munc18 (SM) proteins constitute a conserved family with essential functions in SNARE-mediated membrane fusion. Recently, a new protein-protein interaction site in Sec1p, designated the groove, was proposed. Here, we show that a sec1 groove mutant yeast strain, sec1(w24), displays temperature-sensitive growth and secretion defects. The yeast Sec1p and mammalian Munc18-1 grooves were shown to play an important role in the interaction with the SNAREs Sec9p and SNAP-25b, respectively. Incubation of SNAP-25b with the Munc18-1 groove mutant resulted in a lag in the kinetics of SNARE complex assembly in vitro when compared with wild-type Munc18-1. The SNARE regulator SRO7 was identified as a multicopy suppressor of sec1(w24) groove mutant and an intact Sec1p groove was required for the plasma membrane targeting of Sro7p-SNARE complexes. Simultaneous inactivation of Sec1p groove and SRO7 resulted in reduced levels of exocytic SNARE complexes. Our results identify the groove as a conserved interaction surface in SM proteins. The results indicate that this structural element is important for interactions with Sec9p/SNAP-25 and participates, in concert with Sro7p, in the initial steps of SNARE complex assembly.

The trans-SNARE-regulating function of Munc18-1 is essential to synaptic exocytosis

The fusion of neurotransmitter-filled synaptic vesicles with the plasma membrane requires two classes of molecules-SNAP receptor (SNARE) and Sec1/Munc18 (SM) protein. Reconstitution studies suggest that the SM protein Munc18-1 promotes the zippering of trans-SNARE complexes and accelerates the kinetics of SNARE-dependent membrane fusion. However, the physiological role of this trans-SNARE-regulating function in synaptic exocytosis remains to be established. Here we first demonstrate that two mutations in the vesicle-anchored v-SNARE selectively impair the ability of Munc18-1 to promote trans-SNARE zippering, whereas other known Munc18-1/SNARE-binding modes are unaffected. In cultured neurons, these v-SNARE mutations strongly inhibit spontaneous as well as evoked neurotransmitter release, providing genetic evidence for the trans-SNARE-regulating function of Munc18-1 in synaptic exocytosis. Finally, we show that the trans-SNARE-regulating function of Munc18-1 is compromised by a mutation associated with Ohtahara Syndrome, a severe form of epilepsy.

The origin and evolution of synaptic proteins - choanoflagellates lead the way

The origin of neurons was a key event in evolution, allowing metazoans to evolve rapid behavioral responses to environmental cues. Reconstructing the origin of synaptic proteins promises to reveal their ancestral functions and might shed light on the evolution of the first neuron-like cells in metazoans. By analyzing the genomes of diverse metazoans and their closest relatives, the evolutionary history of diverse presynaptic and postsynaptic proteins has been reconstructed. These analyses revealed that choanoflagellates, the closest relatives of metazoans, possess diverse synaptic protein homologs. Recent studies have now begun to investigate their ancestral functions. A primordial neurosecretory apparatus in choanoflagellates was identified and it was found that the mechanism, by which presynaptic proteins required for secretion of neurotransmitters interact, is conserved in choanoflagellates and metazoans. Moreover, studies on the postsynaptic protein homolog Homer revealed unexpected localization patterns in choanoflagellates and new binding partners, both which are conserved in metazoans. These findings demonstrate that the study of choanoflagellates can uncover ancient and previously undescribed functions of synaptic proteins.

Reconstituting Intracellular Vesicle Fusion Reactions: The Essential Role of Macromolecular Crowding

Intracellular vesicle fusion is mediated by SNAREs and Sec1/Munc18 (SM) proteins. Despite intensive efforts, the SNARE-SM mediated vesicle fusion reaction has not been faithfully reconstituted in biochemical assays. Here, we present an unexpected discovery that macromolecular crowding is required for reconstituting the vesicle fusion reaction in vitro. Macromolecular crowding is known to profoundly influence the kinetic and thermodynamic behaviors of macromolecules, but its role in membrane transport processes such as vesicle fusion remains unexplored. We introduced macromolecular crowding agents into reconstituted fusion reactions to mimic the crowded cellular environment. In this crowded assay, SNAREs and SM proteins acted in concert to drive efficient membrane fusion. In uncrowded assays, by contrast, SM proteins failed to associate with the SNAREs and the fusion rate decreased more than 30-fold, close to undetectable levels. The activities of SM proteins were strictly specific to their cognate SNARE isoforms and sensitive to biologically relevant mutations, further supporting that the crowded fusion assay accurately recapitulates the vesicle fusion reaction. Using this crowded fusion assay, we also showed that the SNARE-SM mediated fusion reaction can be modulated by two additional factors: NSF and α-SNAP. These findings suggest that the vesicle fusion machinery likely has been evolutionarily selected to function optimally in the crowded milieu of the cell. Accordingly, macromolecular crowding should constitute an integral element of any reconstituted fusion assay.

Tracking individual secretory vesicles during exocytosis reveals an ordered and regulated process

Post-Golgi secretory vesicle trafficking is a coordinated process, with transport and regulatory mechanisms to ensure appropriate exocytosis. While the contributions of many individual regulatory proteins to this process are well studied, the timing and dependencies of events have not been defined. Here we track individual secretory vesicles and associated proteins in vivo during tethering and fusion in budding yeast. Secretory vesicles tether to the plasma membrane very reproducibly for ∼18 s, which is extended in cells defective for membrane fusion and significantly lengthened and more variable when GTP hydrolysis of the exocytic Rab is delayed. Further, the myosin-V Myo2p regulates the tethering time in a mechanism unrelated to its interaction with exocyst component Sec15p. Two-color imaging of tethered vesicles with Myo2p, the GEF Sec2p, and several exocyst components allowed us to document a timeline for yeast exocytosis in which Myo2p leaves 4 s before fusion, whereas Sec2p and all the components of the exocyst disperse coincident with fusion.

Munc18a does not alter fusion rates mediated by neuronal SNAREs, synaptotagmin, and complexin

Sec1/Munc18 (SM) proteins are essential for membrane trafficking, but their molecular mechanism remains unclear. Using a single vesicle-vesicle content-mixing assay with reconstituted neuronal SNAREs, synaptotagmin-1, and complexin-1, we show that the neuronal SM protein Munc18a/nSec1 has no effect on the intrinsic kinetics of both spontaneous fusion and Ca²⁺-triggered fusion between vesicles that mimic synaptic vesicles and the plasma membrane. However, wild type Munc18a reduced vesicle association ∼50% when the vesicles bearing the t-SNAREs syntaxin-1A and SNAP-25 were preincubated with Munc18 for 30 min. Single molecule experiments with labeled SNAP-25 indicate that the reduction of vesicle association is a consequence of sequestration of syntaxin-1A by Munc18a and subsequent release of SNAP-25 (i.e. Munc18a captures syntaxin-1A via its high affinity interaction). Moreover, a phosphorylation mimic mutant of Munc18a with reduced affinity to syntaxin-1A results in less reduction of vesicle association. In summary, Munc18a does not directly affect fusion, although it has an effect on the t-SNARE complex, depending on the presence of other factors and experimental conditions. Our results suggest that Munc18a primarily acts at the prefusion stage.

Hemophagocytic lymphohistiocytosis caused by dominant-negative mutations in STXBP2 that inhibit SNARE-mediated membrane fusion

Familial hemophagocytic lymphohistiocytosis (F-HLH) and Griscelli syndrome type 2 (GS) are life-threatening immunodeficiencies characterized by impaired cytotoxic T lymphocyte (CTL) and natural killer (NK) cell lytic activity. In the majority of cases, these disorders are caused by biallelic inactivating germline mutations in genes such as RAB27A (GS) and PRF1, UNC13D, STX11, and STXBP2 (F-HLH). Although monoallelic (ie, heterozygous) mutations have been identified in certain patients, the clinical significance and molecular mechanisms by which these mutations influence CTL and NK cell function remain poorly understood. Here, we characterize 2 novel monoallelic hemophagocytic lymphohistiocytosis (HLH)-associated mutations affecting codon 65 of STXPB2, the gene encoding Munc18-2, a member of the SEC/MUNC18 family. Unlike previously described Munc18-2 mutants, Munc18-2(R65Q) and Munc18-2(R65W) retain the ability to interact with and stabilize syntaxin 11. However, presence of Munc18-2(R65Q/W) in patient-derived lymphocytes and forced expression in control CTLs and NK cells diminishes degranulation and cytotoxic activity. Mechanistic studies reveal that mutations affecting R65 hinder membrane fusion in vitro by arresting the late steps of soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE)-complex assembly. Collectively, these results reveal a direct role for SEC/MUNC18 proteins in promoting SNARE-complex assembly in vivo and suggest that STXBP2 R65 mutations operate in a novel dominant-negative fashion to impair lytic granule fusion and contribute to HLH.

SM proteins Sly1 and Vps33 co-assemble with Sec17 and SNARE complexes to oppose SNARE disassembly by Sec18

Secretory and endolysosomal fusion events are driven by SNAREs and cofactors, including Sec17/α-SNAP, Sec18/NSF, and Sec1/Munc18 (SM) proteins. SMs are essential for fusion in vivo, but the basis of this requirement is enigmatic. We now report that, in addition to their established roles as fusion accelerators, SM proteins Sly1 and Vps33 directly shield SNARE complexes from Sec17- and Sec18-mediated disassembly. In vivo, wild-type Sly1 and Vps33 function are required to withstand overproduction of Sec17. In vitro, Sly1 and Vps33 impede SNARE complex disassembly by Sec18 and ATP. Unexpectedly, Sec17 directly promotes selective loading of Sly1 and Vps33 onto cognate SNARE complexes. A large thermodynamic barrier limits SM binding, implying that significant conformational rearrangements are involved. In a working model, Sec17 and SMs accelerate fusion mediated by cognate SNARE complexes and protect them from NSF-mediated disassembly, while mis-assembled or non-cognate SNARE complexes are eliminated through kinetic proofreading by Sec18.

DOI:http://dx.doi.org/10.7554/eLife.02272.001

Eukaryotic organisms, from single-celled yeast to humans, divide their cells into membrane-bound compartments (organelles) of distinct function. To move from one compartment to another, or to enter or exit a cell, large molecules like proteins are packaged into small membrane sacs called vesicles.

To release its cargo, the membrane of a vesicle must fuse with the membrane of the correct destination compartment. The SNARE family of proteins plays a key role in this fusion process. As the membranes of a vesicle and target compartment come close, SNARE proteins located on each membrane form a SNARE complex that tethers the vesicle in place and causes the two membranes fuse. SNARE proteins do not act alone in this process: the SM family of proteins also plays an essential role in SNARE-mediated membrane fusion. However, it is still not clear exactly why the SM proteins are needed.

Lobingier et al. used the yeast model organism and biochemical studies with purified proteins to show that SM proteins help SNARE complexes form at the right time by regulating the delicate balance between SNARE complex formation and disassembly. This is achieved through the interplay of SM proteins and two other proteins (Sec17 and Sec18). Sec17 is known to load Sec18 onto SNARE complexes to break them apart. Lobingier et al. showed that Sec17 can also load SM proteins on SNARE complexes. This hinders Sec18 action, and so helps to keep the SNARE complexes intact. Because each SM protein tested only binds to the SNARE complex that should function at the membrane where the SM protein resides, these findings suggest SM proteins perform quality control at potential sites of membrane fusion.

DOI:http://dx.doi.org/10.7554/eLife.02272.002

Insulin stimulates syntaxin4 SNARE complex assembly via a novel regulatory mechanism

Insulin stimulates glucose transport into fat and muscle cells by increasing the exocytic trafficking rate of the GLUT4 facilitative glucose transporter from intracellular stores to the plasma membrane. Delivery of GLUT4 to the plasma membrane is mediated by formation of functional SNARE complexes containing syntaxin4, SNAP23, and VAMP2. Here we have used an in situ proximity ligation assay to integrate these two observations by demonstrating for the first time that insulin stimulation causes an increase in syntaxin4-containing SNARE complex formation in adipocytes. Furthermore, we demonstrate that insulin brings about this increase in SNARE complex formation by mobilizing a pool of syntaxin4 held in an inactive state under basal conditions. Finally, we have identified phosphorylation of the regulatory protein Munc18c, a direct target of the insulin receptor, as a molecular switch to coordinate this process. Hence, this report provides molecular detail of how the cell alters membrane traffic in response to an external stimulus, in this case, insulin.

Milligram quantities of homogeneous recombinant full-length mouse Munc18c from Escherichia coli cultures

Vesicle fusion is an indispensable cellular process required for eukaryotic cargo delivery. The Sec/Munc18 protein Munc18c is essential for insulin-regulated trafficking of glucose transporter4 (GLUT4) vesicles to the cell surface in muscle and adipose tissue. Previously, our biophysical and structural studies have used Munc18c expressed in SF9 insect cells. However to maximize efficiency, minimize cost and negate any possible effects of post-translational modifications of Munc18c, we investigated the use of Escherichia coli as an expression host for Munc18c. We were encouraged by previous reports describing Munc18c production in E. coli cultures for use in in vitro fusion assay, pulldown assays and immunoprecipitations. Our approach differs from the previously reported method in that it uses a codon-optimized gene, lower temperature expression and autoinduction media. Three N-terminal His-tagged constructs were engineered, two with a tobacco etch virus (TEV) or thrombin protease cleavage site to enable removal of the fusion tag. The optimized protocol generated 1–2 mg of purified Munc18c per L of culture at much reduced cost compared to Munc18c generated using insect cell culture. The purified recombinant Munc18c protein expressed in bacteria was monodisperse, monomeric, and functional. In summary, we developed methods that decrease the cost and time required to generate functional Munc18c compared with previous insect cell protocols, and which generates sufficient purified protein for structural and biophysical studies.

Fusion proteins and select lipids cooperate as membrane receptors for the soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) Vam7p

Vam7p, the vacuolar soluble Qc-SNARE, is essential for yeast vacuole fusion. The large tethering complex, homotypic fusion and vacuole protein sorting complex (HOPS), and phosphoinositides, which interact with the Vam7p PX domain, have each been proposed to serve as its membrane receptors. Studies with the isolated organelle cannot determine whether these receptor elements suffice and whether ligands or mutations act directly or indirectly on Vam7p binding to the membrane. Using pure components that are active in reconstituted vacuolar fusion, we now find that Vam7p binds to membranes through its combined affinities for several vacuolar membrane constituents: HOPS, phosphatidylinositol 3-phosphate, SNAREs, and acidic phospholipids. Acidic lipids allow low concentrations of Vam7p to suffice for fusion; without acidic lipids, the block to fusion is partially bypassed by high concentrations of Vam7p.

Comparative studies of Munc18c and Munc18-1 reveal conserved and divergent mechanisms of Sec1/Munc18 proteins

Sec1/Munc18 (SM) family proteins are essential for every vesicle fusion pathway. The best-characterized SM protein is the synaptic factor Munc18-1, but it remains unclear whether its functions represent conserved mechanisms of SM proteins or specialized activities in neurotransmitter release. To address this question, we dissected Munc18c, a functionally distinct SM protein involved in nonsynaptic exocytic pathways. We discovered that Munc18c binds to the trans-SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptor) complex and strongly accelerates the fusion rate. Further analysis suggests that Munc18c recognizes both vesicle-rooted SNARE and target membrane-associated SNAREs, and promotes trans-SNARE zippering at the postdocking stage of the fusion reaction. The stimulation of fusion by Munc18c is specific to its cognate SNARE isoforms. Because Munc18-1 regulates fusion in a similar manner, we conclude that one conserved function of SM proteins is to bind their cognate trans-SNARE complexes and accelerate fusion kinetics. Munc18c also binds syntaxin-4 monomer but does not block target membrane-associated SNARE assembly, in agreement with our observation that six- to eightfold increases in Munc18c expression do not inhibit insulin-stimulated glucose uptake in adipocytes. Thus, the inhibitory "closed" syntaxin binding mode demonstrated for Munc18-1 is not conserved in Munc18c. Unexpectedly, we found that Munc18c recognizes the N-terminal region of the vesicle-rooted SNARE, whereas Munc18-1 requires the C-terminal sequences, suggesting that the architecture of the SNARE/SM complex likely differs across fusion pathways. Together, these comparative studies of two distinct SM proteins reveal conserved as well as divergent mechanisms of SM family proteins in intracellular vesicle fusion.

LegC3, an effector protein from Legionella pneumophila, inhibits homotypic yeast vacuole fusion in vivo and in vitro

During infection, the intracellular pathogenic bacterium Legionella pneumophila causes an extensive remodeling of host membrane trafficking pathways, both in the construction of a replication-competent vacuole comprised of ER-derived vesicles and plasma membrane components, and in the inhibition of normal phagosome:endosome/lysosome fusion pathways. Here, we identify the LegC3 secreted effector protein from L. pneumophila as able to inhibit a SNARE- and Rab GTPase-dependent membrane fusion pathway in vitro, the homotypic fusion of yeast vacuoles (lysosomes). This vacuole fusion inhibition appeared to be specific, as similar secreted coiled-coiled domain containing proteins from L. pneumophila, LegC7/YlfA and LegC2/YlfB, did not inhibit vacuole fusion. The LegC3-mediated fusion inhibition was reversible by a yeast cytosolic extract, as well as by a purified soluble SNARE, Vam7p. LegC3 blocked the formation of trans-SNARE complexes during vacuole fusion, although we did not detect a direct interaction of LegC3 with the vacuolar SNARE protein complexes required for fusion. Additionally, LegC3 was incapable of inhibiting a defined synthetic model of vacuolar SNARE-driven membrane fusion, further suggesting that LegC3 does not directly inhibit the activity of vacuolar SNAREs, HOPS complex, or Sec17p/18p during membrane fusion. LegC3 is likely utilized by Legionella to modulate eukaryotic membrane fusion events during pathogenesis.

Munc18b is an essential gene in mice whose expression is limiting for secretion by airway epithelial and mast cells

Airway mucin secretion and MC (mast cell) degranulation must be tightly controlled for homoeostasis of the lungs and immune system respectively. We found the exocytic protein Munc18b to be highly expressed in mouse airway epithelial cells and MCs, and localized to the apical pole of airway secretory cells. To address its functions, we created a mouse with a severely hypomorphic Munc18b allele such that protein expression in heterozygotes was reduced by ~50%. Homozygous mutant mice were not viable, but heterozygotes showed a ~50% reduction in stimulated release of mucin from epithelial cells and granule contents from MCs. The defect in MCs affected only regulated secretion and not constitutive or transporter-mediated secretion. The severity of passive cutaneous anaphylaxis was also reduced by ~50%, showing that reduction of Munc18b expression results in an attenuation of physiological responses dependent on MC degranulation. The Munc18b promoter is controlled by INR (initiator), Sp1 (specificity protein 1), Ets, CRE (cAMP-response element), GRE (glucocorticoid-response element), GATA and E-box elements in airway epithelial cells; however, protein levels did not change during mucous metaplasia induced by allergic inflammation. Taken together, the results of the present study identify Munc18b as an essential gene that is a limiting component of the exocytic machinery of epithelial cells and MCs.

Sec1/Munc18 protein Vps33 binds to SNARE domains and the quaternary SNARE complex

Vps33, a member of the Sec1/Munc18 family of soluble N-ethylmaleimide–sensitive factor attachment protein receptor (SNARE) chaperones, is a subunit of the homotypic fusion and protein sorting and class C core vacuole/endosome tethering complexes and essential for endolysosomal transport. In this study, Vps33 interactions with SNARE proteins are investigated using genetic and biochemical approaches.

Soluble N-ethylmaleimide–sensitive factor attachment protein receptor (SNARE) proteins catalyze membrane fusion events in the secretory and endolysosomal systems, and all SNARE-mediated fusion processes require cofactors of the Sec1/Munc18 (SM) family. Vps33 is an SM protein and subunit of the Vps-C complexes HOPS (homotypic fusion and protein sorting) and CORVET (class C core vacuole/endosome tethering), which are central regulators of endocytic traffic. Here we present biochemical studies of interactions between Saccharomyces cerevisiae vacuolar SNAREs and the HOPS holocomplex or Vps33 alone. HOPS binds the N-terminal H_abc domain of the Qa-family SNARE Vam3, but Vps33 is not required for this interaction. Instead, Vps33 binds the SNARE domains of Vam3, Vam7, and Nyv1. Vps33 directly binds vacuolar quaternary SNARE complexes, and the affinity of Vps33 for SNARE complexes is greater than for individual SNAREs. Through targeted mutational analyses, we identify missense mutations of Vps33 that produce a novel set of defects, including cargo missorting and the loss of Vps33-HOPS association. Together these data suggest a working model for membrane docking: HOPS associates with N-terminal domains of Vam3 and Vam7 through Vps33-independent interactions, which are followed by binding of Vps33, the HOPS SM protein, to SNARE domains and finally to the quaternary SNARE complex. Our results also strengthen the hypothesis that SNARE complex binding is a core attribute of SM protein function.

Munc18-1 mutations that strongly impair SNARE-complex binding support normal synaptic transmission

Synaptic transmission depends critically on the Sec1p/Munc18 protein Munc18-1, but it is unclear whether Munc18-1 primarily operates as a integral part of the fusion machinery or has a more upstream role in fusion complex assembly. Here, we show that point mutations in Munc18-1 that interfere with binding to the free Syntaxin1a N-terminus and strongly impair binding to assembled SNARE complexes all support normal docking, priming and fusion of synaptic vesicles, and normal synaptic plasticity in munc18-1 null mutant neurons. These data support a prevailing role of Munc18-1 before/during SNARE-complex assembly, while its continued association to assembled SNARE complexes is dispensable for synaptic transmission.

Network motif comparison rationalizes Sec1/Munc18-SNARE regulation mechanism in exocytosis

BACKGROUND

Network motifs, recurring subnetwork patterns, provide significant insight into the biological networks which are believed to govern cellular processes.

METHODS

We present a comparative network motif experimental approach, which helps to explain complex biological phenomena and increases the understanding of biological functions at the molecular level by exploring evolutionary design principles of network motifs.

RESULTS

Using this framework to analyze the SM (Sec1/Munc18)-SNARE (N-ethylmaleimide-sensitive factor activating protein receptor) system in exocytic membrane fusion in yeast and neurons, we find that the SM-SNARE network motifs of yeast and neurons show distinct dynamical behaviors. We identify the closed binding mode of neuronal SM (Munc18-1) and SNARE (syntaxin-1) as the key factor leading to mechanistic divergence of membrane fusion systems in yeast and neurons. We also predict that it underlies the conflicting observations in SM overexpression experiments. Furthermore, hypothesis-driven lipid mixing assays validated the prediction.

CONCLUSION

Therefore this study provides a new method to solve the discrepancies and to generalize the functional role of SM proteins.

Regulation of exocytosis by the exocyst subunit Sec6 and the SM protein Sec1

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.

SNARE proteins are required for macroautophagy

Macroautophagy mediates the degradation of long-lived proteins and organelles via the de novo formation of double-membrane autophagosomes that sequester cytoplasm and deliver it to the vacuole/lysosome; however, relatively little is known about autophagosome biogenesis. Atg8, a phosphatidylethanolamine-conjugated protein, was previously proposed to function in autophagosome membrane expansion, based on the observation that it mediates liposome tethering and hemifusion in vitro. We show here that with physiological concentrations of phosphatidylethanolamine, Atg8 does not act as a fusogen. Rather, we provide evidence for the involvement of exocytic Q/t-SNAREs in autophagosome formation, acting in the recruitment of key autophagy components to the site of autophagosome formation, and in regulating the organization of Atg9 into tubulovesicular clusters. Additionally, we found that the endosomal Q/t-SNARE Tlg2 and the R/v-SNAREs Sec22 and Ykt6 interact with Sso1-Sec9, and are required for normal Atg9 transport. Thus, multiple SNARE-mediated fusion events are likely to be involved in autophagosome biogenesis.

Dual roles of Munc18-1 rely on distinct binding modes of the central cavity with Stx1A and SNARE complex

Sec1/Munc18 proteins play a fundamental role in multiple steps of intracellular membrane trafficking. Dual functions have been attributed to Munc18-1: it can act as a chaperone when it interacts with monomeric syntaxin 1A, and it can activate soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs) for membrane fusion when it binds to SNARE complexes. Although both modes of binding involve the central cavity of Munc18-1, their precise molecular mechanisms of action are not fully understood. In this paper, we describe a novel Munc18-1 mutant in the central cavity that showed a reduced interaction with syntaxin 1A and impaired chaperone function, but still bound to assembled SNARE complexes and promoted liposome fusion and secretion in neuroendocrine cells. Soluble syntaxin 1A H3 domain partially blocks Munc18-1 activation of liposome fusion by occupying the Munc18-1 central cavity. Our findings lead us to propose a transition model between the two distinct binding modes by which Munc18 can control and assist in SNARE-complex assembly during neurotransmitter release.

Characterization of Mug33 reveals complementary roles for actin cable-dependent transport and exocyst regulators in fission yeast exocytosis

Although endocytosis and exocytosis have been extensively studied in budding yeast, there have been relatively few investigations of these complex processes in the fission yeast Schizosaccharomyces pombe. Here we identify and characterize fission yeast Mug33, a novel Tea1-interacting protein, and show that Mug33 is involved in exocytosis. Mug33 is a Sur7/PalI-family transmembrane protein that localizes to the plasma membrane at the cell tips and to cytoplasmic tubulovesicular elements (TVEs). A subset of Mug33 TVEs make long-range movements along actin cables, co-translocating with subunits of the exocyst complex. TVE movement depends on the type V myosin Myo52. Although mug33Δ mutants are viable, with only a mild cell-polarity phenotype, mug33Δ myo52Δ double mutants are synthetically lethal. Combining mug33 Δ with deletion of the formin For3 (for3Δ) leads to synthetic temperature-sensitive growth and strongly reduced levels of exocytosis. Interestingly, mutants in non-essential genes involved in exocyst function behave in a manner similar to mug33Δ when combined with myo52Δ and for3Δ. By contrast, combining mug33Δ with mutants in non-essential exocyst genes has only minor effects on growth. We propose that Mug33 contributes to exocyst function and that actin cable-dependent vesicle transport and exocyst function have complementary roles in promoting efficient exocytosis in fission yeast.

Topological arrangement of the intracellular membrane fusion machinery

The topology of the SNARE complex is strictly restricted: of all the possible topological combinations, only one is fusogenic—the topology compatible with both the basal fusion and the SM activation. A fusogenic SNARE complex must contain a complete set of the QabcR SNARE helices.

Soluble N-ethylmaleimide–sensitive factor attachment protein receptors (SNAREs) form a four-helix coiled-coil bundle that juxtaposes two bilayers and drives a basal level of membrane fusion. The Sec1/Munc18 (SM) protein binds to its cognate SNARE bundle and accelerates the basal fusion reaction. The question of how the topological arrangement of the SNARE helices affects the reactivity of the fusion proteins remains unanswered. Here we address the problem for the first time in a reconstituted system containing both SNAREs and SM proteins. We find that to be fusogenic a SNARE topology must support both basal fusion and SM stimulation. Certain topological combinations of exocytic SNAREs result in basal fusion but cannot support SM stimulation, whereas other topologies support SM stimulation without inducing basal fusion. It is striking that of all the possible topological combinations of exocytic SNARE helices, only one induces efficient fusion. Our results suggest that the intracellular membrane fusion complex is designed to fuse bilayers according to one genetically programmed topology.

CytoModeler: a tool for bridging large-scale network analysis and dynamic quantitative modeling

Summary: CytoModeler is an open-source Java application based on the Cytoscape platform. It integrates large-scale network analysis and quantitative modeling by combining omics analysis on the Cytoscape platform, access to deterministic and stochastic simulators, and static and dynamic network context visualizations of simulation results.

Availability: Implemented in Java, CytoModeler runs with Cytoscape 2.6 and 2.7. Binaries, documentation and video walkthroughs are freely available at http://vrac.iastate.edu/~jlv/cytomodeler/.

Contact:julied@iastate.edu; netscape@iastate.edu

Supplementary Information:Supplementary data are available at Bioinformatics online.

Syntaxin N-terminal peptide motif is an initiation factor for the assembly of the SNARE-Sec1/Munc18 membrane fusion complex

Intracellular membrane fusion is mediated by the concerted action of N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs) and Sec1/Munc18 (SM) proteins. During fusion, SM proteins bind the N-terminal peptide (N-peptide) motif of the SNARE subunit syntaxin, but the function of this interaction is unknown. Here, using FRET-based biochemical reconstitution and Caenorhabditis elegans genetics, we show that the N-peptide of syntaxin-1 recruits the SM protein Munc18-1/nSec1 to the SNARE bundle, facilitating their assembly into a fusion-competent complex. The recruitment is achieved through physical tethering rather than allosteric activation of Munc18-1. Consistent with the recruitment role, the N-peptide is not spatially constrained along syntaxin-1, and it is functional when translocated to another SNARE subunit SNAP-25 or even when simply anchored in the target membrane. The N-peptide function is restricted to an early initiation stage of the fusion reaction. After association, Munc18-1 and the SNARE bundle together drive membrane merging without further involving the N-peptide. Thus, the syntaxin N-peptide is an initiation factor for the assembly of the SNARE-SM membrane fusion complex.

Sec1p and Mso1p C-terminal tails cooperate with the SNAREs and Sec4p in polarized exocytosis

It is shown that Sec1p C-terminal tail is needed for proper Sec1p-SNARE complex interaction. Furthermore, evidence is provided that the Mso1p C terminus collaborates with the GTP-bound form of Sec4p in the bud. These results reveal a role for the Sec1p C-terminal tail in SNARE complex binding and suggest Mso1p as an effector for Sec4p.

The Sec1/Munc18 protein family members perform an essential, albeit poorly understood, function in association with soluble n-ethylmaleimide sensitive factor adaptor protein receptor (SNARE) complexes in membrane fusion. The Saccharomyces cerevisiae Sec1p has a C-terminal tail that is missing in its mammalian homologues. Here we show that deletion of the Sec1p tail (amino acids 658–724) renders cells temperature sensitive for growth, reduces sporulation efficiency, causes a secretion defect, and abolishes Sec1p-SNARE component coimmunoprecipitation. The results show that the Sec1p tail binds preferentially ternary Sso1p-Sec9p-Snc2p complexes and it enhances ternary SNARE complex formation in vitro. The bimolecular fluorescence complementation (BiFC) assay results suggest that, in the SNARE-deficient sso2–1 Δsso1 cells, Mso1p, a Sec1p binding protein, helps to target Sec1p(1–657) lacking the C-terminal tail to the sites of secretion. The results suggest that the Mso1p C terminus is important for Sec1p(1–657) targeting. We show that, in addition to Sec1p, Mso1p can bind the Rab-GTPase Sec4p in vitro. The BiFC results suggest that Mso1p acts in close association with Sec4p on intracellular membranes in the bud. This association depends on the Sec4p guanine nucleotide exchange factor Sec2p. Our results reveal a novel binding mode between the Sec1p C-terminal tail and the SNARE complex, and suggest a role for Mso1p as an effector of Sec4p.

Glutamate enhances the surface distribution and release of Munc18 in cerebral cortical neurons

OBJECTIVE

Munc18 is considered as an intracellular protein that plays an important role in exocytosis of neurotransmitters. Previous studies have demonstrated the presence of autoantibodies against Munc18 in a subgroup of Rasmussen's encephalitis patients. However, the machinery of Munc18 autoimmunity is still elusive. The present study was aimed to investigate Munc18 release from primary cultured neurons, Munc18 distribution on the outer plasma membrane of neurons, and the neurotoxicity of Munc18 antibody.

METHODS

The cerebral cortical neurons from embryonic day 17 Sprague-Dawley rats were prepared and cultured in neurobasal medium. The proteins in culture medium were precipitated with 10 % trichloroacetic acid, and analyzed by immunoblotting. The proteins on neuronal surface were biotinylated with EZ-Link-sulfo-NHS-LC-Biotin, and collected with avidin-conjugated agarose beads followed by immunoblotting analysis. For cell surface immunofluorescent staining, the living neurons were labeled with anti-Munc18 antibody at 4 degrees C. Neuronal injury was assessed by lactate dehydrogenase(LDH) release.

RESULTS

Munc18 was detected in culture medium by immunoblotting analysis. After treatment with 50 micromol/L glutamate for 1 h, Munc18 content in medium was increased. Meanwhile, beta-actin and syntaxin1 were not detected in culture medium, and LDH release was not significantly increased. Moreover, glutamate enhanced Munc18 distribution on outer plasma membrane. Living neuron staining also demonstrated the localization of Munc18 on neuronal surface after glutamate treatment, especially at contacting regions between neurons. Glutamate-induced increase of surface Munc18 distribution was suppressed by NMDA receptor antagonist MK801, but not by AMPA receptor antagonist NBQX. Moreover, compared with c-Fos antibody, Munc18 antibody could induce neuronal injury, when culture medium contained the components of serum.

CONCLUSION

A portion of Munc18 can be released from neurons or distributed on neuronal surface, which can be enhanced by glutamate treatment via activation of NMDA receptors. Besides, Munc18 antibody-induced neuronal injury depends on the serum components.

Protein determinants of SNARE-mediated lipid mixing

Soluble N-ethylmaleimide sensitive factor attachment protein receptor (SNARE)-mediated lipid mixing can be efficiently recapitulated in vitro by the incorporation of purified vesicle membrane (-v) SNARE and target membrane (t-) SNARE proteins into separate liposome populations. Despite the strong correlation between the observed activities in this system and the known SNARE physiology, some recent works have suggested that SNARE-mediated lipid mixing may be limited to circumstances where membrane defects arise from artifactual reconstitution conditions (such as nonphysiological high-protein concentrations or unrealistically small liposome populations). Here, we show that the previously published strategies used to reconstitute SNAREs into liposomes do not significantly affect either the physical parameters of the proteoliposomes or the ability of SNAREs to drive lipid mixing in vitro. The surface density of SNARE proteins turns out to be the most critical parameter, which controls both the rate and the extent of SNARE-mediated liposome fusion. In addition, the specific activity of the t-SNARE complex is significantly influenced by expression and reconstitution protocols, such that we only observe optimal lipid mixing when the t-SNARE proteins are coexpressed before purification.

Munc18b regulates core SNARE complex assembly and constitutive exocytosis by interacting with the N-peptide and the closed-conformation C-terminus of syntaxin 3

The interaction between SM (Sec1/Munc18) and SNARE (soluble N-ethylmaleimide-sensitive factor-attachment receptor) proteins constitutes the core eukaryotic membrane fusion machinery which manages exocytosis by mediating fusion of constitutively exocytic vesicles with the plasma membrane. However, mechanistic details on the nature and the physiological impact of SM-SNARE interactions remain largely elusive. Detailed characterization of the interaction profiles between Munc18b and its cognate SNAREs, Stx3 (syntaxin 3), SNAP-23 (soluble N-ethylmaleimide-attachment protein 23) and VAMP8 (vesicle-associated membrane protein 8), revealed that Munc18b binds Stx3, VAMP8 and the assembled core SNARE complex consisting of Stx3, SNAP-23 and VAMP8. Dissection of the Munc18b-Stx3 heterodimer suggested that Munc18b interacts with Stx3's conserved N-peptide as well as with its closed-conformation C-terminus encompassing the Habc domain, a linker and the SNARE (H3) motif. Deletion of the Habc domain or mutations interrupting the intramolecular binding of the Habc and H3 domains abrogated the Munc18b-Stx3 interaction. Although only the N-peptide deletion mutant, but not the soluble wild-type Stx3, is assembled into the core SNARE complex in the presence of Munc18b in vitro, ectopic expression of this SM protein increases constitutive exocytosis in mammalian cells. Our results suggest that Munc18b is functionally coupled to the assembly of exocytic SNARE complexes and increases exocytosis by interacting with the N-peptide and closed-conformation C-terminus of Stx3, thereby neutralizing the secretion-inhibitory effect of this SNARE.

SNARE bundle and syntaxin N-peptide constitute a minimal complement for Munc18-1 activation of membrane fusion

Whittling away SNARE complex components reveals essential domains for Munc18-1–mediated membrane fusion.

Sec1/Munc18 (SM) proteins activate intracellular membrane fusion through binding to cognate SNAP receptor (SNARE) complexes. The synaptic target membrane SNARE syntaxin 1 contains a highly conserved H_abc domain, which connects an N-peptide motif to the SNARE core domain and is thought to participate in the binding of Munc18-1 (the neuronal SM protein) to the SNARE complex. Unexpectedly, we found that mutation or complete removal of the H_abc domain had no effect on Munc18-1 stimulation of fusion. The central cavity region of Munc18-1 is required to stimulate fusion but not through its binding to the syntaxin H_abc domain. SNAP-25, another synaptic SNARE subunit, contains a flexible linker and exhibits an atypical conjoined Q_bc configuration. We found that neither the linker nor the Q_bc configuration is necessary for Munc18-1 promotion of fusion. As a result, Munc18-1 activates a SNARE complex with the typical configuration, in which each of the SNARE core domains is individually rooted in the membrane bilayer. Thus, the SNARE four-helix bundle and syntaxin N-peptide constitute a minimal complement for Munc18-1 activation of fusion.

At the junction of SNARE and SM protein function

Sec1/Munc18 (SM) proteins bind to and function with soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs) at each vesicle fusion site in the cell. The purpose for these interactions is becoming clearer, as what had been interpreted as functional divergence between SM proteins acting at different vesicle trafficking steps, or in specialized cells, is giving way to more recent evidence for common functions among all SM proteins. What is emerging is a picture of SM proteins acting not merely as SNARE regulators, but also as central components of the membrane fusion apparatus. The available data suggest sequential models that describe how the soluble SM protein might first regulate SNARE complex assembly and then cooperate with SNAREs to stimulate membrane fusion.

Mso1p regulates membrane fusion through interactions with the putative N-peptide-binding area in Sec1p domain 1

We show that the putative N-peptide binding area in Sec1p domain 1 is important for Mso1p binding and that Mso1p can interact with Sso1p and Sso2p. Our results suggest that Mso1p mimics N-peptide binding to facilitate membrane fusion.

Sec1p/Munc18 (SM) family proteins regulate SNARE complex function in membrane fusion through their interactions with syntaxins. In addition to syntaxins, only a few SM protein interacting proteins are known and typically, their binding modes with SM proteins are poorly characterized. We previously identified Mso1p as a Sec1p-binding protein and showed that it is involved in membrane fusion regulation. Here we demonstrate that Mso1p and Sec1p interact at sites of exocytosis and that the Mso1p–Sec1p interaction site depends on a functional Rab GTPase Sec4p and its GEF Sec2p. Random and targeted mutagenesis of Sec1p, followed by analysis of protein interactions, indicates that Mso1p interacts with Sec1p domain 1 and that this interaction is important for membrane fusion. In many SM family proteins, domain 1 binds to a N-terminal peptide of a syntaxin family protein. The Sec1p-interacting syntaxins Sso1p and Sso2p lack the N-terminal peptide. We show that the putative N-peptide binding area in Sec1p domain 1 is important for Mso1p binding, and that Mso1p can interact with Sso1p and Sso2p. Our results suggest that Mso1p mimics N-peptide binding to facilitate membrane fusion.

Minimal membrane docking requirements revealed by reconstitution of Rab GTPase-dependent membrane fusion from purified components

Rab GTPases and their effectors mediate docking, the initial contact of intracellular membranes preceding bilayer fusion. However, it has been unclear whether Rab proteins and effectors are sufficient for intermembrane interactions. We have recently reported reconstituted membrane fusion that requires yeast vacuolar SNAREs, lipids, and the homotypic fusion and vacuole protein sorting (HOPS)/class C Vps complex, an effector and guanine nucleotide exchange factor for the yeast vacuolar Rab GTPase Ypt7p. We now report reconstitution of lysis-free membrane fusion that requires purified GTP-bound Ypt7p, HOPS complex, vacuolar SNAREs, ATP hydrolysis, and the SNARE disassembly catalysts Sec17p and Sec18p. We use this reconstituted system to show that SNAREs and Sec17p/Sec18p, and Ypt7p and the HOPS complex, are required for stable intermembrane interactions and that the three vacuolar Q-SNAREs are sufficient for these interactions.

Yeast Sec1p functions before and after vesicle docking

Sec1/Munc18 (SM) proteins bind cognate soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) complexes and stimulate vesicle membrane fusion. Before fusion, vesicles are docked to specific target membranes. Regulation of vesicle docking is attributed to some but not all SM proteins, suggesting specialization of this earlier function. Yeast Sec1p seems to function only after vesicles are docked and SNARE complexes are assembled. Here, we show that yeast Sec1p is required before and after SNARE complex assembly, in support of general requirements for SM proteins in both vesicle docking and fusion. Two classes of sec1 mutants were isolated. Class A mutants are tightly blocked in cell growth and secretion at a step before SNARE complex assembly. Class B mutants have a SNARE complex binding defect, with a range in severity of cell growth and secretion defects. Mapping the mutations onto an SM protein structure implicates a peripheral bundle of helices for the early, docking function and a deep groove, opposite the syntaxin-binding cleft on nSec1/Munc-18, for the interaction between Sec1p and the exocytic SNARE complex.

Direct interaction between the COG complex and the SM protein, Sly1, is required for Golgi SNARE pairing

The crucial roles of Sec1/Munc18 (SM)-like proteins in membrane fusion have been evidenced in genetic and biochemical studies. SM proteins interact directly with SNAREs and contribute to SNARE pairing by a yet unclear mechanism. Here, we show that the SM protein, Sly1, interacts directly with the conserved oligomeric Golgi (COG) tethering complex. The Sly1-COG interaction is mediated by the Cog4 subunit, which also interacts with Syntaxin 5 through a different binding site. We provide evidence that disruption of Cog4-Sly1 interaction impairs pairing of SNAREs involved in intra-Golgi transport thereby markedly attenuating Golgi-to-ER retrograde transport. These results highlight the mechanism by which SM proteins link tethering to SNAREpin assembly.

Molecular engineering of exocytic vesicle traffic enhances the productivity of Chinese hamster ovary cells

A complex vesicle trafficking system manages the precise and regulated distribution of proteins, membranes and other molecular cargo between cellular compartments as well as the secretion of (heterologous) proteins in mammalian cells. Sec1/Munc18 (SM) proteins are key components of the system by regulating membrane fusion. However, it is not clear how SM proteins contribute to the overall exocytosis. Here, functional analysis of the SM protein Sly1 and Munc18c suggested a united, positive impact upon SNARE-based fusion of ER-to-Golgi- and Golgi-to-plasma membrane-addressed exocytic vesicles and increased the secretory capacity of different therapeutic proteins in Chinese hamster ovary cells up to 40 pg/cell/day. Sly1- and Munc18c-based vesicle traffic engineering cooperated with Xbp-1-mediated ER/Golgi organelle engineering. Our study supports a model for united function of SM proteins in stimulating vesicle trafficking machinery and provides a generic secretion engineering strategy to improve biopharmaceutical manufacturing of important protein therapeutics.

Binding of UNC-18 to the N-terminus of syntaxin is essential for neurotransmission in Caenorhabditis elegans

SNAREs (soluble N-ethylmaleimide-sensitive fusion protein-attachment protein receptors) are widely accepted to drive all intracellular membrane fusion events. SM (Sec1/Munc18-like) proteins bind to SNAREs and this interaction may underlie their ubiquitous requirement for efficient membrane fusion. SM proteins bind to SNAREs in at least three modes: (i) to a closed conformation of syntaxin; (ii) to the syntaxin N-terminus; and (iii) to the assembled SNARE complex. Munc18-1 exhibits all three binding modes and recent in vitro reconstitution assays suggest that its interaction with the syntaxin N-terminus is essential for neuronal SNARE complex binding and efficient membrane fusion. To investigate the physiological relevance of these binding modes, we studied the UNC-18/UNC-64 SM/SNARE pair, which is essential for neuronal exocytosis in Caenorhabditis elegans. Mutations in the N-terminus of UNC-64 strongly inhibited binding to UNC-18, as did mutations targeting closed conformation binding. Complementary mutations in UNC-18 designed to selectively impair binding to either closed syntaxin or its N-terminus produced a similarly strong inhibition of UNC-64 binding. Therefore high-affinity UNC18/UNC-64 interaction in vitro involves both binding modes. To determine the physiological relevance of each mode, unc-18-null mutant worms were transformed with wild-type or mutant unc-18 constructs. The UNC-18(R39C) construct, that is defective in closed syntaxin binding, fully rescued the locomotion defects of the unc-18 mutant. In contrast, the UNC-18(F113R) construct, that is defective in binding to the N-terminus of UNC-64, provided no rescue. These results suggest that binding of UNC-18 to closed syntaxin is dispensable for membrane fusion, whereas interaction with the syntaxin N-terminus is essential for neuronal exocytosis in vivo.

The functions of Munc18-1 in regulated exocytosis

The activation of regulated exocytosis occurs by a rise in cytosolic Ca(2+) concentration. Synaptotagmins act as the Ca(2+) sensors, whereas the machinery that allows fusion of secretory vesicles with the plasma membrane consists of the soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) proteins, including syntaxin 1, SNAP-25, and VAMP. Within the pathway leading to exocytosis, there is an essential requirement for a member of the conserved Sec1/Munc18 (SM) protein family, which in neurotransmitter and neurohormone release in mammalian cells is Munc18-1. The exact role of Munc18-1 and the steps within exocytosis in which it acts have been intensively investigated. Current evidence suggests that Munc18-1 acts via distinct modes of interactions with syntaxin 1 and the other SNARE proteins and influences all of the steps leading to exocytosis, including vesicle recruitment, tethering, docking, priming, and membrane fusion.

Negative regulation of syntaxin4/SNAP-23/VAMP2-mediated membrane fusion by Munc18c in vitro

Background

Translocation of the facilitative glucose transporter GLUT4 from an intracellular store to the plasma membrane is responsible for the increased rate of glucose transport into fat and muscle cells in response to insulin. This represents a specialised form of regulated membrane trafficking. Intracellular membrane traffic is subject to multiple levels of regulation by conserved families of proteins in all eukaryotic cells. Notably, all intracellular fusion events require SNARE proteins and Sec1p/Munc18 family members. Fusion of GLUT4-containing vesicles with the plasma membrane of insulin-sensitive cells involves the SM protein Munc18c, and is regulated by the formation of syntaxin 4/SNAP23/VAMP2 SNARE complexes.

Methodology/Principal Findings

Here we have used biochemical approaches to characterise the interaction(s) of Munc18c with its cognate SNARE proteins and to examine the role of Munc18c in regulating liposome fusion catalysed by syntaxin 4/SNAP23/VAMP2 SNARE complex formation. We demonstrate that Munc18c makes contacts with both t- and v-SNARE proteins of this complex, and directly inhibits bilayer fusion mediated by the syntaxin 4/SNAP23/VAMP2 SNARE complex.

Conclusion/Significance

Our reductionist approach has enabled us to ascertain a direct inhibitory role for Munc18c in regulating membrane fusion mediated by syntaxin 4/SNAP23/VAMP2 SNARE complex formation. It is important to note that two different SM proteins have recently been shown to stimulate liposome fusion mediated by their cognate SNARE complexes. Given the structural similarities between SM proteins, it seems unlikely that different members of this family perform opposing regulatory functions. Hence, our findings indicate that Munc18c requires a further level of regulation in order to stimulate SNARE-mediated membrane fusion.

UNC-18 modulates ethanol sensitivity in Caenorhabditis elegans

Acute ethanol exposure affects the nervous system as a stimulant at low concentrations and as a depressant at higher concentrations, eventually resulting in motor dysfunction and uncoordination. A recent genetic study of two mouse strains with varying ethanol preference indicated a correlation with a polymorphism (D216N) in the synaptic protein Munc18-1. Munc18-1 functions in exocytosis via a number of discrete interactions with the soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) protein syntaxin-1. We report that the mutation affects binding to syntaxin but not through either a closed conformation mode of interaction or through binding to the syntaxin N terminus. The D216N mutant instead has a specific impairment in binding the assembled SNARE complex. Furthermore, the mutation broadens the duration of single exocytotic events. Expression of the orthologous mutation (D214N) in the Caenorhabditis elegans UNC-18 null background generated transgenic rescues with phenotypically similar locomotion to worms rescued with the wild-type protein. Strikingly, D214N worms were strongly resistant to both stimulatory and sedative effects of acute ethanol. Analysis of an alternative Munc18-1 mutation (I133V) supported the link between reduced SNARE complex binding and ethanol resistance. We conclude that ethanol acts, at least partially, at the level of vesicle fusion and that its acute effects are ameliorated by point mutations in UNC-18.

Munc18a scaffolds SNARE assembly to promote membrane fusion

Munc18a is an SM protein required for SNARE-mediated fusion. The molecular details of how Munc18a acts to enhance neurosecretion have remained elusive. Here, we use in vitro fusion assays to characterize how specific interactions between Munc18a and the neuronal SNAREs enhance the rate and extent of fusion. We show that Munc18a interacts directly and functionally with the preassembled t-SNARE complex. Analysis of Munc18a point mutations indicates that Munc18a interacts with helix C of the Syntaxin1a NRD in the t-SNARE complex. Replacement of the t-SNARE SNAP25b with yeast Sec9c had little effect, suggesting that Munc18a has minimal contact with SNAP25b within the t-SNARE complex. A chimeric Syntaxin built of the Syntaxin1a NRD and the H3 domain of yeast Sso1p and paired with Sec9c eliminated stimulation of fusion, suggesting that Munc18a/Syntaxin1a H3 domain contacts are important. Additionally, a Syntaxin1A mutant lacking a flexible linker region that allows NRD movement abolished stimulation of fusion. These experiments suggest that Munc18a binds to the Syntaxin1a NRD and H3 domain within the assembled t-SNARE complex, positioning them for productive VAMP2 binding. In this capacity, Munc18a serves as a platform for trans-SNARE complex assembly that facilitates efficient SNARE-mediated membrane fusion.

A random mutagenesis approach to isolate dominant-negative yeast sec1 mutants reveals a functional role for domain 3a in yeast and mammalian Sec1/Munc18 proteins

SNAP receptor (SNARE) and Sec1/Munc18 (SM) proteins are required for all intracellular membrane fusion events. SNAREs are widely believed to drive the fusion process, but the function of SM proteins remains unclear. To shed light on this, we screened for dominant-negative mutants of yeast Sec1 by random mutagenesis of a GAL1-regulated SEC1 plasmid. Mutants were identified on the basis of galactose-inducible growth arrest and inhibition of invertase secretion. This effect of dominant-negative sec1 was suppressed by overexpression of the vesicle (v)-SNAREs, Snc1 and Snc2, but not the target (t)-SNAREs, Sec9 and Sso2. The mutations isolated in Sec1 clustered in a hotspot within domain 3a, with F361 mutated in four different mutants. To test if this region was generally involved in SM protein function, the F361-equivalent residue in mammalian Munc18-1 (Y337) was mutated. Overexpression of the Munc18-1 Y337L mutant in bovine chromaffin cells inhibited the release kinetics of individual exocytosis events. The Y337L mutation impaired binding of Munc18-1 to the neuronal SNARE complex, but did not affect its binary interaction with syntaxin1a. Taken together, these data suggest that domain 3a of SM proteins has a functionally important role in membrane fusion. Furthermore, this approach of screening for dominant-negative mutants in yeast may be useful for other conserved proteins, to identify functionally important domains in their mammalian homologs.