Sec1p binds to SNARE complexes and concentrates at sites of secretion

SNARE Proteins in Synaptic Vesicle Fusion

Neurotransmitters are stored in small membrane-bound vesicles at synapses; a subset of synaptic vesicles is docked at release sites. Fusion of docked vesicles with the plasma membrane releases neurotransmitters. Membrane fusion at synapses, as well as all trafficking steps of the secretory pathway, is mediated by SNARE proteins. The SNAREs are the minimal fusion machinery. They zipper from N-termini to membrane-anchored C-termini to form a 4-helix bundle that forces the apposed membranes to fuse. At synapses, the SNAREs comprise a single helix from syntaxin and synaptobrevin; SNAP-25 contributes the other two helices to complete the bundle. Unc13 mediates synaptic vesicle docking and converts syntaxin into the permissive "open" configuration. The SM protein, Unc18, is required to initiate and proofread SNARE assembly. The SNAREs are then held in a half-zippered state by synaptotagmin and complexin. Calcium removes the synaptotagmin and complexin block, and the SNAREs drive vesicle fusion. After fusion, NSF and alpha-SNAP unwind the SNAREs and thereby recharge the system for further rounds of fusion. In this chapter, we will describe the discovery of the SNAREs, their relevant structural features, models for their function, and the central role of Unc18. In addition, we will touch upon the regulation of SNARE complex formation by Unc13, complexin, and synaptotagmin.

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.

High-resolution secretory timeline from vesicle formation at the Golgi to fusion at the plasma membrane in S. cerevisiae

Most of the components in the yeast secretory pathway have been studied, yet a high-resolution temporal timeline of their participation is lacking. Here, we define the order of acquisition, lifetime, and release of critical components involved in late secretion from the Golgi to the plasma membrane. Of particular interest is the timing of the many reported effectors of the secretory vesicle Rab protein Sec4, including the myosin-V Myo2, the exocyst complex, the lgl homolog Sro7, and the small yeast-specific protein Mso1. At the trans-Golgi network (TGN) Sec4's GEF, Sec2, is recruited to Ypt31-positive compartments, quickly followed by Sec4 and Myo2 and vesicle formation. While transported to the bud tip, the entire exocyst complex, including Sec3, is assembled on to the vesicle. Before fusion, vesicles tether for 5 s, during which the vesicle retains the exocyst complex and stimulates lateral recruitment of Rho3 on the plasma membrane. Sec2 and Myo2 are rapidly lost, followed by recruitment of cytosolic Sro7, and finally the SM protein Sec1, which appears for just 2 s prior to fusion. Perturbation experiments reveal an ordered and robust series of events during tethering that provide insights into the function of Sec4 and effector exchange.

On the difficulties of characterizing weak protein interactions that are critical for neurotransmitter release

The mechanism of neurotransmitter release has been extensively characterized, showing that vesicle fusion is mediated by the SNARE complex formed by syntaxin-1, SNAP-25 and synaptobrevin. This complex is disassembled by N-ethylmaleimide sensitive factor (NSF) and SNAPs to recycle the SNAREs, whereas Munc18-1 and Munc13s organize SNARE complex assembly in an NSF-SNAP-resistant manner. Synaptotagmin-1 acts as the Ca²⁺ sensor that triggers exocytosis in a tight interplay with the SNAREs and complexins. Here, we review technical aspects associated with investigation of protein interactions underlying these steps, which is hindered because the release machinery is assembled between two membranes and is highly dynamic. Moreover, weak interactions, which are difficult to characterize, play key roles in neurotransmitter release, for instance by lowering energy barriers that need to be overcome in this highly regulated process. We illustrate the crucial role that structural biology has played in uncovering mechanisms underlying neurotransmitter release, but also discuss the importance of considering the limitations of the techniques used, including lessons learned from research in our lab and others. In particular, we emphasize: (a) the promiscuity of some protein sequences, including membrane-binding regions that can mediate irrelevant interactions with proteins in the absence of their native targets; (b) the need to ensure that weak interactions observed in crystal structures are biologically relevant; and (c) the limitations of isothermal titration calorimetry to analyze weak interactions. Finally, we stress that even studies that required re-interpretation often helped to move the field forward by improving our understanding of the system and providing testable hypotheses.

Synaptic Secretion and Beyond: Targeting Synapse and Neurotransmitters to Treat Neurodegenerative Diseases

The nervous system is important, because it regulates the physiological function of the body. Neurons are the most basic structural and functional unit of the nervous system. The synapse is an asymmetric structure that is important for neuronal function. The chemical transmission mode of the synapse is realized through neurotransmitters and electrical processes. Based on vesicle transport, the abnormal information transmission process in the synapse can lead to a series of neurorelated diseases. Numerous proteins and complexes that regulate the process of vesicle transport, such as SNARE proteins, Munc18-1, and Synaptotagmin-1, have been identified. Their regulation of synaptic vesicle secretion is complicated and delicate, and their defects can lead to a series of neurodegenerative diseases. This review will discuss the structure and functions of vesicle-based synapses and their roles in neurons. Furthermore, we will analyze neurotransmitter and synaptic functions in neurodegenerative diseases and discuss the potential of using related drugs in their treatment.

The Polarized Redistribution of the Contractile Vacuole to the Rear of the Cell is Critical for Streaming and is Regulated by PI(4,5)P2-Mediated Exocytosis

Dictyostelium discoideum amoebae align in a head to tail manner during the process of streaming during fruiting body formation. The chemoattractant cAMP is the chemoattractant regulating cell migration during this process and is released from the rear of cells. The process by which this cAMP release occurs has eluded investigators for many decades, but new findings suggest that this release can occur through expulsion during contractile vacuole (CV) ejection. The CV is an organelle that performs several functions inside the cell including the regulation of osmolarity, and discharges its content via exocytosis. The CV localizes to the rear of the cell and appears to be part of the polarity network, with the localization under the influence of the plasma membrane (PM) lipids, including the phosphoinositides (PIs), among those is PI(4,5)P2, the most abundant PI on the PM. Research on D. discoideum and neutrophils have shown that PI(4,5)P2 is enriched at the rear of migrating cells. In several systems, it has been shown that the essential regulator of exocytosis is through the exocyst complex, mediated in part by PI(4,5)P2-binding. This review features the role of the CV complex in D. discoideum signaling with a focus on the role of PI(4,5)P2 in regulating CV exocytosis and localization. Many of the regulators of these processes are conserved during evolution, so the mechanisms controlling exocytosis and membrane trafficking in D. discoideum and mammalian cells will be discussed, highlighting their important functions in membrane trafficking and signaling in health and disease.

Interspecies complementation identifies a pathway to assemble SNAREs

Unc18 and SNARE proteins form the core of the membrane fusion complex at synapses. To understand the functional interactions within the core machinery, we adopted an "interspecies complementation" approach in Caenorhabditis elegans. Substitutions of individual SNAREs and Unc18 proteins with those from yeast fail to rescue fusion. However, synaptic transmission could be restored in worm-yeast chimeras when two key interfaces were present: an Habc-Unc18 contact site and an Unc18-SNARE motif contact site. A constitutively open form of Unc18 bypasses the requirement for the Habc-Unc18 interface. These data suggest that the Habc domain of syntaxin is required for Unc18 to adopt an open conformation; open Unc18 then templates SNARE complex formation. Finally, we demonstrate that the SNARE and Unc18 machinery in the nematode C. elegans can be replaced by yeast proteins and still carry out synaptic transmission, pointing to the deep evolutionary conservation of these two interfaces.

SEC1A is a major Arabidopsis Sec1/Munc18 gene in vesicle trafficking during pollen tube tip growth

Pollen tubes (PTs) grow by the targeted secretion of new cell wall material to their expanding tip region. Sec1/Munc18 (SM) proteins promote membrane fusion through regulation of the SNARE complex. We have previously shown that disruption of protein glycosylation in the Arabidopsis thaliana hpat1 hpat3 double mutant leads to PT growth defects that can be suppressed by reducing secretion. Here, we identified five point mutant alleles of the SM protein SEC1A as hpat1/3 suppressors. The suppressors increased seed set, reduced PT growth defects and reduced the rate of glycoprotein secretion. In the absence of the hpat mutations, sec1a reduced pollen germination and PT elongation producing shorter and wider PTs. Consistent with a defect in membrane fusion, sec1a PTs accumulated secretory vesicles. Though sec1a had significantly reduced male transmission, homozygous sec1a plants maintained full seed set, demonstrating that SEC1A was ultimately dispensable for pollen fertility. However, when combined with a mutation in another SEC1-like SM gene, keule, pollen fertility was totally abolished. Mutation in sec1b, the final member of the Arabidopsis SEC1 clade, did not enhance the sec1a phenotype. Thus, SEC1A is the major SM protein promoting pollen germination and tube elongation, but in its absence KEULE can partially supply this activity. When we examined the expression of the SM protein family in other species for which pollen expression data were available, we found that at least one Sec1-like protein was highly expressed in pollen samples, suggesting a conserved role in pollen fertility in other species.

Room for Two: The Synaptophysin/Synaptobrevin Complex

Synaptic vesicle release is regulated by upwards of 30 proteins at the fusion complex alone, but disruptions in any one of these components can have devastating consequences for neuronal communication. Aberrant molecular responses to calcium signaling at the pre-synaptic terminal dramatically affect vesicle trafficking, docking, fusion, and release. At the organismal level, this is reflected in disorders such as epilepsy, depression, and neurodegeneration. Among the myriad pre-synaptic proteins, perhaps the most functionally mysterious is synaptophysin (SYP). On its own, this vesicular transmembrane protein has been proposed to function as a calcium sensor, a cholesterol-binding protein, and to form ion channels across the phospholipid bilayer. The downstream effects of these functions are largely unknown. The physiological relevance of SYP is readily apparent in its interaction with synaptobrevin (VAMP2), an integral element of the neuronal SNARE complex. SNAREs, soluble NSF attachment protein receptors, comprise a family of proteins essential for vesicle fusion. The complex formed by SYP and VAMP2 is thought to be involved in both trafficking to the pre-synaptic membrane as well as regulation of SNARE complex formation. Recent structural observations specifically implicate the SYP/VAMP2 complex in anchoring the SNARE assembly at the pre-synaptic membrane prior to vesicle fusion. Thus, the SYP/VAMP2 complex appears vital to the form and function of neuronal exocytotic machinery.

The Close Relationship between the Golgi Trafficking Machinery and Protein Glycosylation

Glycosylation is the most common post-translational modification of proteins; it mediates their correct folding and stability, as well as their transport through the secretory transport. Changes in N- and O-linked glycans have been associated with multiple pathological conditions including congenital disorders of glycosylation, inflammatory diseases and cancer. Glycoprotein glycosylation at the Golgi involves the coordinated action of hundreds of glycosyltransferases and glycosidases, which are maintained at the correct location through retrograde vesicle trafficking between Golgi cisternae. In this review, we describe the molecular machinery involved in vesicle trafficking and tethering at the Golgi apparatus and the effects of mutations in the context of glycan biosynthesis and human diseases.

AtNSF regulates leaf serration by modulating intracellular trafficking of PIN1 in Arabidopsis thaliana

In eukaryotes, N-ethylmaleimide-sensitive factor (NSF) is a conserved AAA+ ATPase and a key component of the membrane trafficking machinery that promotes the fusion of secretory vesicles with target membranes. Here, we demonstrate that the Arabidopsis thaliana genome contains a single copy of NSF, AtNSF, which plays an essential role in the regulation of leaf serration. The AtNSF knock-down mutant, atnsf-1, exhibited more serrations in the leaf margin. Moreover, polar localization of the PIN-FORMED1 (PIN1) auxin efflux transporter was diffuse around the margins of atnsf-1 leaves and root growth was inhibited in the atnsf-1 mutant. More PIN1-GFP accumulated in the intracellular compartments of atnsf-1 plants, suggesting that AtNSF is required for intracellular trafficking of PIN between the endosome and plasma membrane. Furthermore, the serration phenotype was suppressed in the atnsf-1 pin1-8 double mutant, suggesting that AtNSF is required for PIN1-mediated polar auxin transport to regulate leaf serration. The CUP-SHAPED COTYLEDON2 (CUC2) transcription factor gene is up-regulated in atnsf-1 plants and the cuc2-3 single mutant exhibits smooth leaf margins, demonstrating that AtNSF also functions in the CUC2 pathway. Our results reveal that AtNSF regulates the PIN1-generated auxin maxima with a CUC2-mediated feedback loop to control leaf serration. This article is protected by copyright. All rights reserved.

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-like proteins TgSec1 and TgVps45 play pivotal roles in assembly of the pellicle and sub-pellicle network in Toxoplasma gondii

Post-Golgi vesicle trafficking is indispensable for precise movement of proteins to the pellicle, the sub-pellicle network and apical secretory organelles in Apicomplexa. However, only a small number of molecular complexes involved in trafficking, tethering and fusion of vesicles have been identified in Toxoplasma gondii. Consequently, it is unclear how complicated vesicle trafficking is accomplished in this parasite. Sec1/Munc18-like (SM) proteins are essential components of protein complexes involved in vesicle fusion. Here, we found that depletion of the SM protein TgSec1 using an auxin-inducible degron-based conditional knockout strategy led to mislocalization of plasma membrane proteins. By contrast, conditional depletion of the SM protein TgVps45 led to morphological changes, asymmetrical loss of the inner membrane complex and defects in nucleation of sub-pellicular microtubules, polarization and symmetrical assembly of daughter parasites during repeated endodyogeny. TgVps45 interacts with the SNARE protein TgStx16 and TgVAMP4-1. Conditional ablation of TgStx16 causes the similar growth defect like TgVps45 deficiency suggested they work together for the vesicle fusion at TGN. These findings indicate that these two SM proteins are crucial for assembly of pellicle and sub-pellicle network in T. gondii respectively.

A VPS33A-binding motif on syntaxin 17 controls autophagy completion in mammalian cells

Autophagy is an intracellular degradation pathway that transports cytoplasmic material to the lysosome for hydrolysis. It is completed by SNARE-mediated fusion of the autophagosome and endolysosome membranes. This process must be carefully regulated to maintain the organization of the membrane system and prevent mistargeted degradation. As yet, models of autophagosomal fusion have not been verified within a cellular context because of difficulties with assessing protein interactions in situ Here, we used high-resolution fluorescence lifetime imaging (FLIM)-FRET of HeLa cells to identify protein interactions within the spatiotemporal framework of the cell. We show that autophagosomal syntaxin 17 (Stx17) heterotrimerizes with synaptosome-associated protein 29 (SNAP29) and vesicle-associated membrane protein 7 (VAMP7) in situ, highlighting a functional role for VAMP7 in autophagosome clearance that has previously been sidelined in favor of a role for VAMP8. Additionally, we identified multimodal regulation of SNARE assembly by the Sec1/Munc18 (SM) protein VPS33A, mirroring other syntaxin-SM interactions and therefore suggesting a unified model of SM regulation. Contrary to current theoretical models, we found that the Stx17 N-peptide appears to interact in a positionally conserved, but mechanistically divergent manner with VPS33A, providing a late "go, no-go" step for autophagic fusion via a phosphoserine master-switch. Our findings suggest that Stx17 fusion competency is regulated by a phosphosite in its N-peptide, representing a previously unknown regulatory step in mammalian autophagy.

SNARE zippering requires activation by SNARE-like peptides in Sec1/Munc18 proteins

Soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs) catalyze membrane fusion by forming coiled-coil bundles between membrane bilayers. The SNARE bundle zippers progressively toward the membranes, pulling the lipid bilayers into close proximity to fuse. In this work, we found that the +1 and +2 layers in the C-terminal domains (CTDs) of SNAREs are dispensable for reconstituted SNARE-mediated fusion reactions. By contrast, all CTD layers are required for fusion reactions activated by the cognate Sec1/Munc18 (SM) protein or a synthetic Vc peptide derived from the vesicular (v-) SNARE, correlating with strong acceleration of fusion kinetics. These results suggest a similar mechanism underlying the stimulatory functions of SM proteins and Vc peptide in SNARE-dependent membrane fusion. Unexpectedly, we identified a conserved SNARE-like peptide (SLP) in SM proteins that structurally and functionally resembles Vc peptide. Like Vc peptide, SLP binds and activates target (t-) SNAREs, accelerating the fusion reaction. Disruption of the t-SNARE-SLP interaction inhibits exocytosis in vivo. Our findings demonstrated that a t-SNARE-SLP intermediate must form before SNAREs can drive efficient vesicle fusion.

Lipid-dependent regulation of exocytosis in S. cerevisiae by OSBP homolog (Osh) 4

Polarized exocytosis is an essential process in many organisms and cell types for correct cell division or functional specialization. Previous studies established that homologs of the oxysterol-binding protein (OSBP) in S. cerevisiae, which comprise the Osh protein family, are necessary for efficient polarized exocytosis by supporting a late post-Golgi step. We define this step as the docking of a specific sub-population of exocytic vesicles with the plasma membrane. In the absence of other Osh proteins, yeast Osh4p can support this process in a manner dependent upon two lipid ligands, PI4P and sterol. Osh6p, which binds PI4P and phosphatidylserine, is also sufficient to support polarized exocytosis, again in a lipid-dependent manner. These data suggest that Osh-mediated exocytosis depends upon lipid binding and exchange without a strict requirement for sterol. We propose a two-step mechanism for Osh protein-mediated regulation of polarized exocytosis by using Osh4p as a model. We describe a specific in vivo role for lipid binding by an OSBP-related protein (ORP) in the process of polarized exocytosis, guiding our understanding of where and how OSBP and ORPs may function in more complex organisms.

Munc18a clusters SNARE-bearing liposomes prior to trans-SNARE zippering

Sec1-Munc18 (SM) proteins co-operate with SNAREs {SNAP [soluble NSF (N-ethylmaleimide-sensitive factor) attachment protein] receptors} to mediate membrane fusion in eukaryotic cells. Studies of Munc18a/Munc18-1/Stxbp1 in neurotransmission suggest that SM proteins accelerate fusion kinetics primarily by activating the partially zippered trans-SNARE complex. However, accumulating evidence has argued for additional roles for SM proteins in earlier steps in the fusion cascade. Here, we investigate the function of Munc18a in reconstituted exocytic reactions mediated by neuronal and non-neuronal SNAREs. We show that Munc18a plays a direct role in promoting proteoliposome clustering, underlying vesicle docking during exocytosis. In the three different fusion reactions examined, Munc18a-dependent clustering requires an intact N-terminal peptide (N-peptide) motif in syntaxin that mediates the binary interaction between syntaxin and Munc18a. Importantly, clustering is preserved under inhibitory conditions that abolish both trans-SNARE complex formation and lipid mixing, indicating that Munc18a promotes membrane clustering in a step that is independent of trans-SNARE zippering and activation.

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.

Sec3 promotes the initial binary t-SNARE complex assembly and membrane fusion

The soluble N-ethylmaleimide-sensitive factor-attachment protein receptors (SNAREs) constitute the core machinery for membrane fusion during eukaryotic cell vesicular trafficking. However, how the assembly of the SNARE complex is initiated is unknown. Here we report that Sec3, a component of the exocyst complex that mediates vesicle tethering during exocytosis, directly interacts with the t-SNARE protein Sso2. This interaction promotes the formation of an Sso2-Sec9 'binary' t-SNARE complex, the early rate-limiting step in SNARE complex assembly, and stimulates membrane fusion. The crystal structure of the Sec3-Sso2 complex suggests that Sec3 binding induces conformational changes of Sso2 that are crucial for the relief of its auto-inhibition. Interestingly, specific disruption of the Sec3-Sso2 interaction in cells blocks exocytosis without affecting the function of Sec3 in vesicle tethering. Our study reveals an activation mechanism for SNARE complex assembly, and uncovers a role of the exocyst in promoting membrane fusion in addition to vesicle tethering.

The cell polarity proteins Boi1p and Boi2p stimulate vesicle fusion at the plasma membrane of yeast cells

Eukaryotic cells can direct secretion to defined regions of their plasma membrane. These regions are distinguished by an elaborate architecture of proteins and lipids that are specialized to capture and fuse post-Golgi vesicles. Here we show that the proteins Boi1p and Boi2p are important elements of this area of active exocytosis at the tip of growing yeast cells. Cells lacking Boi1p and Boi2p accumulate secretory vesicles in their bud. The essential PH domains of Boi1p and Boi2p interact with Sec1p, a protein required for SNARE complex formation and vesicle fusion. Sec1p loses its tip localization in cells depleted of Boi1p and Boi2p but can partially compensate for their loss upon overexpression. The capacity to simultaneously bind phospholipids, Sec1p, multiple subunits of the exocyst, Cdc42p, and the module for generating active Cdc42p identify Boi1p and Boi2p as essential mediators between exocytosis and polar growth.

Distinct roles of the two VPS33 proteins in the endolysosomal system in Caenorhabditis elegans

Sec1/Munc-18 (SM) family proteins are essential regulators in intracellular transport in eukaryotic cells. The SM protein Vps33 functions as a core subunit of two tethering complexes, class C core vacuole/endosome tethering (CORVET) and homotypic fusion and vacuole protein sorting (HOPS) in the endocytic pathway in yeast. Metazoan cells possess two Vps33 proteins, VPS33A and VPS33B, but their precise roles remain unknown. Here, we present a comparative analysis of Caenorhabditis elegans null mutants for these proteins. We found that the vps-33.1 (VPS33A) mutants exhibited severe defects in both endocytic function and endolysosomal biogenesis in scavenger cells. Furthermore, vps-33.1 mutations caused endocytosis defects in other tissues, and the loss of maternal and zygotic VPS-33.1 resulted in embryonic lethality. By contrast, vps-33.2 mutants were viable but sterile, with terminally arrested spermatocytes. The spermatogenesis phenotype suggests that VPS33.2 is involved in the formation of a sperm-specific organelle. The endocytosis defect in the vps-33.1 mutant was not restored by the expression of VPS-33.2, which indicates that these proteins have nonredundant functions. Together, our data suggest that VPS-33.1 shares most of the general functions of yeast Vps33 in terms of tethering complexes in the endolysosomal system, whereas VPS-33.2 has tissue/organelle specific functions in C. elegans.

The Secret Life of Tethers: The Role of Tethering Factors in SNARE Complex Regulation

Trafficking in eukaryotic cells is a tightly regulated process to ensure correct cargo delivery to the proper destination organelle or plasma membrane. In this review, we focus on how the vesicle fusion machinery, the SNARE complex, is regulated by the interplay of the multisubunit tethering complexes (MTC) with the SNAREs and Sec1/Munc18 (SM) proteins. Although these factors are used in different stages of membrane trafficking, e.g., Golgi to plasma membrane transport vs. vacuolar fusion, and in a variety of diverse eukaryotic cell types, many commonalities between their functions are being revealed. We explore the various protein-protein interactions and findings from functional reconstitution studies in order to highlight both their common features and the differences in their modes of regulation. These studies serve as a starting point for mechanistic explorations in other systems.

Bridging the Gap between Glycosylation and Vesicle Traffic

Glycosylation is recognized as a vitally important posttranslational modification. The structure of glycans that decorate proteins and lipids is largely dictated by biosynthetic reactions occurring in the Golgi apparatus. This biosynthesis relies on the relative distribution of glycosyltransferases and glycosidases, which is maintained by retrograde vesicle traffic between Golgi cisternae. Tethering of vesicles at the Golgi apparatus prior to fusion is regulated by Rab GTPases, coiled-coil tethers termed golgins and the multisubunit tethering complex known as the conserved oligomeric Golgi (COG) complex. In this review we discuss the mechanisms involved in vesicle tethering at the Golgi apparatus and highlight the importance of tethering in the context of glycan biosynthesis and a set of diseases known as congenital disorders of glycosylation.

The Synaptic Vesicle Release Machinery

Extensive research has yielded crucial insights into the mechanism of neurotransmitter release, and working models for the functions of key proteins involved in release. The SNAREs Syntaxin-1, Synaptobrevin, and SNAP-25 play a central role in membrane fusion, forming SNARE complexes that bridge the vesicle and plasma membranes and that are disassembled by NSF-SNAPs. Exocytosis likely starts with Syntaxin-1 folded into a self-inhibited closed conformation that binds to Munc18-1. Munc13s open Syntaxin-1, orchestrating SNARE complex assembly in an NSF-SNAP-resistant manner together with Munc18-1. In the resulting primed state, with partially assembled SNARE complexes, fusion is inhibited by Synaptotagmin-1 and Complexins, which also perform active functions in release. Upon influx of Ca(2+), Synaptotagmin-1 activates fast release, likely by relieving the inhibition caused by Complexins and cooperating with the SNAREs in bringing the membranes together. Although alternative models exist and fundamental questions remain unanswered, a definitive description of the basic release mechanism may be available soon.

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.

Regulation of dendrite growth and maintenance by exocytosis

Dendrites lengthen by several orders of magnitude during neuronal development, but how membrane is allocated in dendrites to facilitate this growth remains unclear. Here, we report that Ras opposite (Rop), the Drosophila ortholog of the key exocytosis regulator Munc18-1 (also known as STXBP1), is an essential factor mediating dendrite growth. Neurons with depleted Rop function exhibit reduced terminal dendrite outgrowth followed by primary dendrite degeneration, suggestive of differential requirements for exocytosis in the growth and maintenance of different dendritic compartments. Rop promotes dendrite growth together with the exocyst, an octameric protein complex involved in tethering vesicles to the plasma membrane, with Rop-exocyst complexes and exocytosis predominating in primary dendrites over terminal dendrites. By contrast, membrane-associated proteins readily diffuse from primary dendrites into terminals, but not in the reverse direction, suggesting that diffusion, rather than targeted exocytosis, supplies membranous material for terminal dendritic growth, revealing key differences in the distribution of materials to these expanding dendritic compartments.

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.

Cyclin-dependent kinase 5 regulates degranulation in human eosinophils

Degranulation from eosinophils in response to secretagogue stimulation is a regulated process that involves exocytosis of granule proteins through specific signalling pathways. One potential pathway is dependent on cyclin-dependent kinase 5 (Cdk5) and its effector molecules, p35 and p39, which play a central role in neuronal cell exocytosis by phosphorylating Munc18, a regulator of SNARE binding. Emerging evidence suggests a role for Cdk5 in exocytosis in immune cells, although its role in eosinophils is not known. We sought to examine the expression of Cdk5 and its activators in human eosinophils, and to assess the role of Cdk5 in eosinophil degranulation. We used freshly isolated human eosinophils and analysed the expression of Cdk5, p35, p39 and Munc18c by Western blot, RT-PCR, flow cytometry and immunoprecipitation. Cdk5 kinase activity was determined following eosinophil activation. Cdk5 inhibitors were used (roscovitine, AT7519 and small interfering RNA) to determine its role in eosinophil peroxidase (EPX) secretion. Cdk5 was expressed in association with Munc18c, p35 and p39, and phosphorylated following human eosinophil activation with eotaxin/CCL11, platelet-activating factor, and secretory IgA-Sepharose. Cdk5 inhibitors (roscovitine, AT7519) reduced EPX release when cells were stimulated by PMA or secretory IgA. In assays using small interfering RNA knock-down of Cdk5 expression in human eosinophils, we observed inhibition of EPX release. Our findings suggest that in activated eosinophils, Cdk5 is phosphorylated and binds to Munc18c, resulting in Munc18c release from syntaxin-4, allowing SNARE binding and vesicle fusion, with subsequent eosinophil degranulation. Our work identifies a novel role for Cdk5 in eosinophil mediator release by agonist-induced degranulation.

The GTPase-activating protein Rga1 interacts with Rho3 GTPase and may regulate its function in polarized growth in budding yeast

In budding yeast, Rga1 negatively regulates the Rho GTPase Cdc42 by acting as a GTPase-activating protein (GAP) for Cdc42. To gain insight into the function and regulation of Rga1, we overexpressed Rga1 and an N-terminally truncated Rga1-C538 (a.a. 538-1007) segment. Overexpression of Rga1-C538 but not full-length Rga1 severely impaired growth and cell morphology in wild-type cells. We show that Rga1 is phosphorylated during the cell cycle. The lack of phenotype for full-length Rga1 upon overexpression may result from a negative regulation by G1-specific Pho85, a cyclin-dependent kinase (CDK). From a high-copy suppressor screen, we isolated RHO3, SEC9, SEC1, SSO1, SSO2, and SRO7, genes involved in exocytosis, as suppressors of the growth defect caused by Rga1-C538 overexpression. Moreover, we detected that Rga1 interacts with Rho3 in two-hybrid and bimolecular fluorescence complementation (BiFC) assays. Rga1 preferentially interacts with the GTP-bound form of Rho3 and the interaction requires the GAP domain and additional sequence upstream of the GAP domain. Our data suggest that the interaction of Rga1 with Rho3 may regulate Rho3’s function in polarized bud growth.

The molecular machinery of neurotransmitter release (Nobel lecture)

The most important property of synaptic transmission is its speed, which is crucial for the overall workings of the brain. In his Nobel Lecture, T. C. Südhof explains how the synaptic vesicle and the plasma membrane undergo rapid fusion during neurotransmitter release and how this process is spatially organized, such that opening of Ca(2+) -channels allows rapid translation of the entering Ca(2+) signal into a fusion event.

The SM protein Sly1 accelerates assembly of the ER-Golgi SNARE complex

Soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) and Sec1/Munc18 (SM) proteins constitute the core of an ancient vesicle fusion machine that diversified into distinct sets that now function in different trafficking steps in eukaryotic cells. Deciphering their precise mode of action has proved challenging. SM proteins are thought to act primarily through one type of SNARE protein, the syntaxins. Despite high structural similarity, however, contrasting binding modes have been found for different SM proteins and syntaxins. Whereas the secretory SM protein Munc18 binds to the ‟closed conformation" of syntaxin 1, the ER-Golgi SM protein Sly1 interacts only with the N-peptide of Sed5. Recent findings, however, indicate that SM proteins might interact simultaneously with both syntaxin regions. In search for a common mechanism, we now reinvestigated the Sly1/Sed5 interaction. We found that individual Sed5 adopts a tight closed conformation. Sly1 binds to both the closed conformation and the N-peptide of Sed5, suggesting that this is the original binding mode of SM proteins and syntaxins. In contrast to Munc18, however, Sly1 facilitates SNARE complex formation by loosening the closed conformation of Sed5.

The complexity of vesicle transport factors in plants examined by orthology search

Vesicle transport is a central process to ensure protein and lipid distribution in eukaryotic cells. The current knowledge on the molecular components and mechanisms of this process is majorly based on studies in Saccharomyces cerevisiae and Arabidopsis thaliana, which revealed 240 different proteinaceous factors either experimentally proven or predicted to be involved in vesicle transport. In here, we performed an orthologue search using two different algorithms to identify the components of the secretory pathway in yeast and 14 plant genomes by using the 'core-set' of 240 factors as bait. We identified 4021 orthologues and (co-)orthologues in the discussed plant species accounting for components of COP-II, COP-I, Clathrin Coated Vesicles, Retromers and ESCRTs, Rab GTPases, Tethering factors and SNAREs. In plants, we observed a significantly higher number of (co-)orthologues than yeast, while only 8 tethering factors from yeast seem to be absent in the analyzed plant genomes. To link the identified (co-)orthologues to vesicle transport, the domain architecture of the proteins from yeast, genetic model plant A. thaliana and agriculturally relevant crop Solanum lycopersicum has been inspected. For the orthologous groups containing (co-)orthologues from yeast, A. thaliana and S. lycopersicum, we observed the same domain architecture for 79% (416/527) of the (co-)orthologues, which documents a very high conservation of this process. Further, publically available tissue-specific expression profiles for a subset of (co-)orthologues found in A. thaliana and S. lycopersicum suggest that some (co-)orthologues are involved in tissue-specific functions. Inspection of localization of the (co-)orthologues based on available proteome data or localization predictions lead to the assignment of plastid- as well as mitochondrial localized (co-)orthologues of vesicle transport factors and the relevance of this is discussed.

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

Exocyst complex protein expression in the human placenta

INTRODUCTION

Protein production and secretion are essential to syncytiotrophoblast function and are associated with cytotrophoblast cell fusion and differentiation. Syncytiotrophoblast hormone secretion is a crucial determinant of maternal-fetal health, and can be misregulated in pathological pregnancies. Although, polarized secretion is a key component of placental function, the mechanisms underlying this process are poorly understood.

OBJECTIVE

While the octameric exocyst complex is classically regarded as a master regulator of secretion in various mammalian systems, its expression in the placenta remained unexplored. We hypothesized that the syncytiotrophoblast would express all exocyst complex components and effector proteins requisite for vesicle-mediated secretion more abundantly than cytotrophoblasts in tissue specimens.

METHODS

A two-tiered immunobiological approach was utilized to characterize exocyst and ancillary proteins in normal, term human placentas. Exocyst protein expression and localization was documented in tissue homogenates via immunoblotting and immunofluorescence labeling of placental sections.

RESULTS

The eight exocyst proteins, EXOC1, 2, 3, 4, 5, 6, 7, and 8, were found in the human placenta. In addition, RAB11, an important exocyst complex modulator, was also expressed. Exocyst and Rab protein expression appeared to be regulated during trophoblast differentiation, as the syncytiotrophoblast expressed these proteins with little, if any, expression in cytotrophoblast cells. Additionally, exocyst proteins were localized at or near the syncytiotrophoblast apical membrane, the major site of placental secretion.

DISCUSSION/CONCLUSION

Our findings highlight exocyst protein expression as novel indicators of trophoblast differentiation. The exocyst's regulated localization within the syncytiotrophoblast in conjunction with its well known functions suggests a possible role in placental polarized secretion.

Neurotransmitter release: the last millisecond in the life of a synaptic vesicle

During an action potential, Ca(2+) entering a presynaptic terminal triggers synaptic vesicle exocytosis and neurotransmitter release in less than a millisecond. How does Ca(2+) stimulate release so rapidly and precisely? Work over the last decades revealed that Ca(2+) binding to synaptotagmin triggers release by stimulating synaptotagmin binding to a core fusion machinery composed of SNARE and SM proteins that mediates membrane fusion during exocytosis. Complexin adaptor proteins assist synaptotagmin by activating and clamping this core fusion machinery. Synaptic vesicles containing synaptotagmin are positioned at the active zone, the site of vesicle fusion, by a protein complex containing RIM proteins. RIM proteins activate docking and priming of synaptic vesicles and simultaneously recruit Ca(2+) channels to active zones, thereby connecting in a single complex primed synaptic vesicles to Ca(2+) channels. This architecture allows direct flow of Ca(2+) ions from Ca(2+) channels to synaptotagmin, which then triggers fusion, thus mediating tight millisecond coupling of an action potential to neurotransmitter release.

Syntaxin binding mechanism and disease-causing mutations in Munc18-2

Mutations in either syntaxin 11 (Stx11) or Munc18-2 abolish cytotoxic T lymphocytes (CTL) and natural killer cell (NK) cytotoxicity, and give rise to familial hemophagocytic lymphohistiocytosis (FHL4 or FHL5, respectively). Although Munc18-2 is known to interact with Stx11, little is known about the molecular mechanisms governing the specificity of this interaction or how in vitro IL-2 activation leads to compensation of CTL and NK cytotoxicity. To understand how mutations in Munc18-2 give rise to disease, we have solved the structure of human Munc18-2 at 2.6 Å resolution and mapped 18 point mutations. The four surface mutations identified (R39P, L130S, E132A, P334L) map exclusively to the predicted syntaxin and soluble N-ethylmaleimide-sensitive factor accessory protein receptor binding sites of Munc18-2. We find that Munc18-2 binds the N-terminal peptide of Stx11 with a ~20-fold higher affinity than Stx3, suggesting a potential role in selective binding. Upon IL-2 activation, levels of Stx3 are increased, favoring Munc18-2 binding when Stx11 is absent. Similarly, Munc18-1, expressed in IL-2-activated CTL, is capable of binding Stx11. These findings provide potential explanations for restoration of Munc18-Stx function and cytotoxicity in IL-2-activated cells.

Regulation of cytokinesis by exocyst subunit SEC6 and KEULE in Arabidopsis thaliana

Proper vesicle tethering and membrane fusion at the cell plate are essential for cytokinesis. Both the vesicle tethering complex exocyst and membrane fusion regulator KEULE were shown to function in cell plate formation, but the exact mechanisms still remain to be explored. In this study, using yeast two-hybrid (Y-2-H) assay, we found that SEC6 interacted with KEULE, and that a small portion of C-terminal region of KEULE was required for the interaction. The direct SEC6-KEULE interaction was supported by further studies using in vitro pull-down assay, immunoprecipitation, and in vivo bimolecular fluorescence complementation (BIFC) microscopy. sec6 mutants were male gametophytic lethal as reported; however, pollen-rescued sec6 mutants (PRsec6) displayed cytokinesis defects in the embryonic cells and later in the leaf pavement cells and the guard cells. SEC6 and KEULE proteins were co-localized to the cell plate during cytokinesis in transgenic Arabidopsis. Furthermore, only SEC6 but not other exocyst subunits located in the cell plate interacted with KEULE in vitro. These results demonstrated that, like KEULE, SEC6 plays a physiological role in cytokinesis, and the SEC6-KEULE interaction may serve as a novel molecular linkage between arriving vesicles and membrane fusion machinery or directly regulate membrane fusion during cell plate formation in plants.

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.

The Golgi puppet master: COG complex at center stage of membrane trafficking interactions

The central organelle within the secretory pathway is the Golgi apparatus, a collection of flattened membranes organized into stacks. The cisternal maturation model of intra-Golgi transport depicts Golgi cisternae that mature from cis to medial to trans by receiving resident proteins, such as glycosylation enzymes via retrograde vesicle-mediated recycling. The conserved oligomeric Golgi (COG) complex, a multi-subunit tethering complex of the complexes associated with tethering containing helical rods family, organizes vesicle targeting during intra-Golgi retrograde transport. The COG complex, both physically and functionally, interacts with all classes of molecules maintaining intra-Golgi trafficking, namely SNAREs, SNARE-interacting proteins, Rabs, coiled-coil tethers, vesicular coats, and molecular motors. In this report, we will review the current state of the COG interactome and analyze possible scenarios for the molecular mechanism of the COG orchestrated vesicle targeting, which plays a central role in maintaining glycosylation homeostasis in all eukaryotic cells.

Ultrahigh-resolution imaging reveals formation of neuronal SNARE/Munc18 complexes in situ

Membrane fusion is mediated by complexes formed by SNAP-receptor (SNARE) and Secretory 1 (Sec1)/mammalian uncoordinated-18 (Munc18)-like (SM) proteins, but it is unclear when and how these complexes assemble. Here we describe an improved two-color fluorescence nanoscopy technique that can achieve effective resolutions of up to 7.5-nm full width at half maximum (3.2-nm localization precision), limited only by stochastic photon emission from single molecules. We use this technique to dissect the spatial relationships between the neuronal SM protein Munc18-1 and SNARE proteins syntaxin-1 and SNAP-25 (25 kDa synaptosome-associated protein). Strikingly, we observed nanoscale clusters consisting of syntaxin-1 and SNAP-25 that contained associated Munc18-1. Rescue experiments with syntaxin-1 mutants revealed that Munc18-1 recruitment to the plasma membrane depends on the Munc18-1 binding to the N-terminal peptide of syntaxin-1. Our results suggest that in a primary neuron, SNARE/SM protein complexes containing syntaxin-1, SNAP-25, and Munc18-1 are preassembled in microdomains on the presynaptic plasma membrane. Our superresolution imaging method provides a framework for investigating interactions between the synaptic vesicle fusion machinery and other subcellular systems in situ.

Crystal Structures of the Sec1/Munc18 (SM) Protein Vps33, Alone and Bound to the Homotypic Fusion and Vacuolar Protein Sorting (HOPS) Subunit Vps16*

Intracellular membrane fusion requires the regulated assembly of SNARE (soluble N-ethylmaleimide-sensitive factor (NSF) attachment protein receptor) proteins anchored in the apposed membranes. To exert the force required to drive fusion between lipid bilayers, juxtamembrane SNARE motifs zipper into four-helix bundles. Importantly, SNARE function is regulated by additional factors, none more extensively studied than the SM (Sec1/Munc18-like) proteins. SM proteins interact with both individual SNAREs and SNARE complexes, likely chaperoning SNARE complex formation and protecting assembly intermediates from premature disassembly by NSF. Four families of SM proteins have been identified, and representative members of two of these families (Sec1/Munc18 and Sly1) have been structurally characterized. We report here the 2.6 Å resolution crystal structure of an SM protein from the third family, Vps33. Although Vps33 shares with the first two families the same basic three-domain architecture, domain 1 is displaced by 15 Å, accompanied by a 40° rotation. A unique feature of the Vps33 family of SM proteins is that its members function as stable subunits within a multi-subunit tethering complex called HOPS (homotypic fusion and vacuolar protein sorting). Integration into the HOPS complex depends on the interaction between Vps33 and a second HOPS subunit, Vps16. The crystal structure of Vps33 bound to a C-terminal portion of Vps16, also at 2.6 Å resolution, reveals the structural basis for this interaction. Despite the extensive interface between the two HOPS subunits, the conformation of Vps33 is only subtly affected by binding to Vps16.