Defining the functions of trans-SNARE pairs

Rab GTPases as coordinators of vesicle traffic

Membrane trafficking between organelles by vesiculotubular carriers is fundamental to the existence of eukaryotic cells. Central in ensuring that cargoes are delivered to their correct destinations are the Rab GTPases, a large family of small GTPases that control membrane identity and vesicle budding, uncoating, motility and fusion through the recruitment of effector proteins, such as sorting adaptors, tethering factors, kinases, phosphatases and motors. Crosstalk between multiple Rab GTPases through shared effectors, or through effectors that recruit selective Rab activators, ensures the spatiotemporal regulation of vesicle traffic. Functional impairments of Rab pathways are associated with diseases, such as immunodeficiencies, cancer and neurological disorders.

Capture and release of partially zipped trans-SNARE complexes on intact organelles

Soluble N-ethyl-maleimide sensitive fusion protein attachment protein receptors (SNAREs) are hypothesized to trigger membrane fusion by complexing in trans through their membrane-distal N termini and zippering toward their membrane-embedded C termini, which in turn drives the two membranes together. In this study, we use a set of truncated SNAREs to trap kinetically stable, partially zipped trans-SNARE complexes on intact organelles in the absence of hemifusion and content mixing. We show that the C-terminal zippering of SNARE cytoplasmic domains controls the onset of lipid mixing but not the subsequent transition from hemifusion to full fusion. Moreover, we find that a partially zipped nonfusogenic trans-complex is rescued by Sec17, a universal SNARE cochaperone. Rescue occurs independently of the Sec17-binding partner Sec18, and it exhibits steep cooperativity, indicating that Sec17 engages multiple stalled trans-complexes to drive fusion. These experiments delineate distinct functions within the trans-complex, provide a straightforward method to trap and study prefusion complexes on native membranes, and reveal that Sec17 can rescue a stalled, partially zipped trans-complex.

Identifying critical components of native Ca2+-triggered membrane fusion. Integrating studies of proteins and lipids

Ca(2+)-triggered membrane fusion is the defining step of exocytosis. Despite realization that the fusion machinery must include lipids and proteins working in concert, only of late has work in the field focused more equally on both these components. Here we use isolated sea urchin egg cortical vesicles (CV), a stage-specific preparation of Ca(2+)-sensitive release-ready vesicles that enables the tight coupling of molecular and functional analyses necessary to dissect molecular mechanisms. The stalk-pore hypothesis proposes that bilayer merger proceeds rapidly via transient, high-negative curvature, intermediate membrane structures. Consistent with this, cholesterol, a major component of the CV membrane, contributes to a critical local negative curvature that supports formation of lipidic fusion intermediates. Following cholesterol depletion, structurally dissimilar lipids having intrinsic negative curvature greater than or equal to cholesterol recover the ability of CV to fuse but do not recover fusion efficiency (Ca(2+) sensitivity and kinetics). Conversely, cholesterol- and sphingomyelin-enriched microdomains regulate the efficiency of the fusion mechanism, presumably by contributing spatial and functional organization of other critical lipids and proteins at the fusion site. Critical proteins are thought to participate in Ca(2+) sensing, initiating membrane deformations, and facilitating fusion pore expansion. Capitalizing on a novel effect of the thiol-reactive reagent iodoacetamide (IA), potentiation of the Ca(2+) sensitivity and kinetics, a fluorescently tagged IA has been used to enhance fusion efficiency and simultaneously label the proteins involved. Isolation of cholesterol-enriched CV membrane fractions, using density gradient centrifugation, is being used to narrow the list of protein candidates potentially critical to the mechanism of fast Ca(2+)-triggered membrane fusion.

SNARE function is not involved in early endosome docking

Docking and fusion of transport vesicles constitute elementary steps in intracellular membrane traffic. While docking is thought to be initiated by Rab-effector complexes, fusion is mediated by SNARE (N-ethylmaleimide-sensitive factor [NSF] attachment receptor) proteins. However, it has been recently debated whether SNAREs also play a role in the establishment or maintenance of a stably docked state. To address this question, we have investigated the SNARE dependence of docking and fusion of early endosomes, one of the central sorting compartments in the endocytic pathway. A new, fluorescence-based in vitro assay was developed, which allowed us to investigate fusion and docking in parallel. Similar to homotypic fusion, docking of early endosomes is dependent on the presence of ATP and requires physiological temperatures. Unlike fusion, docking is insensitive to the perturbation of SNARE function by means of soluble SNARE motifs, SNARE-specific F(ab) fragments, or by a block of NSF activity. In contrast, as expected, docking is strongly reduced by interfering with the synthesis of phosphatidyl inositol (PI)-3 phosphate, with the function of Rab-GTPases, as well as with early endosomal autoantigen 1 (EEA1), an essential tethering factor. We conclude that docking of early endosomes is independent of SNARE function.

HOPS proofreads the trans-SNARE complex for yeast vacuole fusion

The fusion of yeast vacuoles, like other organelles, requires a Rab-family guanosine triphosphatase (Ypt7p), a Rab effector and Sec1/Munc18 (SM) complex termed HOPS (homotypic fusion and vacuole protein sorting), and soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs). The central 0-layer of the four bundled vacuolar SNAREs requires the wild-type three glutaminyl (Q) and one arginyl (R) residues for optimal fusion. Alterations of this layer dramatically increase the K(m) value for SNAREs to assemble trans-SNARE complexes and to fuse. We now find that added purified HOPS complex strongly suppresses the fusion of vacuoles bearing 0-layer alterations, but it has little effect on the fusion of vacuoles with wild-type SNAREs. HOPS proofreads at two levels, inhibiting the formation of trans-SNARE complexes with altered 0-layers and suppressing the ability of these mismatched 0-layer trans-SNARE complexes to support membrane fusion. HOPS proofreading also extends to other parts of the SNARE complex, because it suppresses the fusion of trans-SNARE complexes formed without the N-terminal Phox homology domain of Vam7p (Q(c)). Unlike some other SM proteins, HOPS proofreading does not require the Vam3p (Q(a)) N-terminal domain. HOPS thus proofreads SNARE domain and N-terminal domain structures and regulates the fusion capacity of trans-SNARE complexes, only allowing full function for wild-type SNARE configurations. This is the most direct evidence to date that HOPS is directly involved in the fusion event.

Photolysis of a caged peptide reveals rapid action of N-ethylmaleimide sensitive factor before neurotransmitter release

The time at which the N-ethylmaleimide-sensitive factor (NSF) acts during synaptic vesicle (SV) trafficking was identified by time-controlled perturbation of NSF function with a photoactivatable inhibitory peptide. Photolysis of this caged peptide in the squid giant presynaptic terminal caused an abrupt (0.2 s) slowing of the kinetics of the postsynaptic current (PSC) and a more gradual (2-3 s) reduction in PSC amplitude. Based on the rapid rate of these inhibitory effects relative to the speed of SV recycling, we conclude that NSF functions in reactions that immediately precede neurotransmitter release. Our results indicate the locus of SNARE protein recycling in presynaptic terminals and reveal NSF as a potential target for rapid regulation of transmitter release.

Sec18p and Vam7p remodel trans-SNARE complexes to permit a lipid-anchored R-SNARE to support yeast vacuole fusion

Intracellular membrane fusion requires SNARE proteins in a trans-complex, anchored to apposed membranes. Proteoliposome studies have suggested that SNAREs drive fusion by stressing the lipid bilayer via their transmembrane domains (TMDs), and that SNARE complexes require a TMD in each docked membrane to promote fusion. Yeast vacuole fusion is believed to require three Q-SNAREs from one vacuole and the R-SNARE Nyv1p from its fusion partner. In accord with this model, we find that fusion is abolished when the TMD of Nyv1p is replaced by lipid anchors, even though lipid-anchored Nyv1p assembles into trans-SNARE complexes. However, normal fusion is restored by the addition of both Sec18p and the soluble SNARE Vam7p. In restoring fusion, Sec18p promotes the disassembly of trans-SNARE complexes, and Vam7p enhances their assembly. Thus, either the TMD of this R-SNARE is not essential for fusion, and TMD-mediated membrane stress is not the only mode of trans-SNARE complex action, or these SNAREs have more flexibility than heretofore appreciated to form alternate functional complexes that violate the 3Q:1R rule.

Characterization of cocaine-elicited cell vacuolation: the involvement of calcium/calmodulin in organelle deregulation

The sizes of organelles are tightly regulated in the cells. However, little is known on how cells maintain the homeostasis of these intracellular compartments. Using cocaine as a model compound, we have characterized the mechanism of deregulated vacuolation in cultured rat liver epithelial Clone 9 cells. The vacuoles were observed as early as 10 min following cocaine treatment. Removal of cocaine led to vacuole degeneration, indicating vacuolation is a reversible process. The vacuoles could devour intracellular materials and the vacuoles originated from late endosome/lysosome as indicated by immunofluorescence studies. Instant calcium influx and calmodulin were required for the initiation of vacuole formation. The unique properties of these late endosome/lysosome-derived vacuoles were further discussed. In summary, cocaine elicited a new type of deregulated vacuole and the involvement of calcium/calmodulin in vacuolation could shed light on prevention or treatment of cocaine-induced cytotoxicity.

Glutamate-induced exocytosis of glutamate from astrocytes

Recent studies indicate that astrocytes can play a much more active role in neuronal circuits than previously believed, by releasing neurotransmitters such as glutamate and ATP. Here we report that local application of glutamate or glutamine synthetase inhibitors induces astrocytic release of glutamate, which activates a slowly decaying transient inward current (SIC) in CA1 pyramidal neurons and a transient inward current in astrocytes in hippocampal slices. The occurrence of SICs was accompanied by an appearance of large vesicles around the puffing pipette. The frequency of SICs was positively correlated with [glutamate]o. EM imaging of anti-glial fibrillary acid protein-labeled astrocytes showed glutamate-induced large astrocytic vesicles. Imaging of FM 1-43 fluorescence using two-photon laser scanning microscopy detected glutamate-induced formation and fusion of large vesicles identified as FM 1-43-negative structures. Fusion of large vesicles, monitored by collapse of vesicles with a high intensity FM 1-43 stain in the vesicular membrane, coincided with SICs. Glutamate induced two types of large vesicles with high and low intravesicular [Ca2+]. The high [Ca2+] vesicle plays a major role in astrocytic release of glutamate. Vesicular fusion was blocked by infusing the Ca2+ chelator, 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid, or the SNARE blocker, tetanus toxin, suggesting Ca2+- and SNARE-dependent fusion. Infusion of the vesicular glutamate transport inhibitor, Rose Bengal, reduced astrocytic glutamate release, suggesting the involvement of vesicular glutamate transports in vesicular transport of glutamate. Our results demonstrate that local [glutamate]o increases induce formation and exocytotic fusion of glutamate-containing large astrocytic vesicles. These large vesicles could play important roles in the feedback control of neuronal circuits and epileptic seizures.

Coats, tethers, Rabs, and SNAREs work together to mediate the intracellular destination of a transport vesicle

Tethering factors have been shown to interact with Rabs and SNAREs and, more recently, with coat proteins. Coat proteins are required for cargo selection and membrane deformation to bud a transport vesicle from a donor compartment. It was once thought that a vesicle must uncoat before it recognizes its target membrane. However, recent findings have revealed a role for the coat in directing a vesicle to its correct intracellular destination. In this review we will discuss the literature that links coat proteins to vesicle targeting events.

Trans-SNARE complex assembly and yeast vacuole membrane fusion

cis-SNARE complexes (anchored in one membrane) are disassembled by Sec17p (alpha-SNAP) and Sec18p (NSF), permitting the unpaired SNAREs to assemble in trans. We now report a direct assay of trans-SNARE complex formation during yeast vacuole docking. SNARE complex assembly and fusion is promoted by high concentrations of the SNARE Vam7p or Nyv1p or by addition of HOPS (homotypic fusion and vacuole protein sorting), a Ypt7p (Rab)-effector complex with a Sec1/Munc18-family subunit. Inhibitors that target Ypt7p, HOPS, or key regulatory lipids prevent trans-SNARE complex assembly and ensuing fusion. Strikingly, the lipid ligand MED (myristoylated alanine-rich C kinase substrate effector domain) or elevated concentrations of Sec17p, which can displace HOPS from SNARE complexes, permit full trans-SNARE pairing but block fusion. These findings suggest that efficient fusion requires trans-SNARE complex associations with factors such as HOPS and subsequent regulated lipid rearrangements.

Interaction of SPIN90 with syndapin is implicated in clathrin-mediated endocytic pathway in fibroblasts

SPIN90, a 90-kDa Nck-interacting protein with a SH3 domain, plays a role in sarcomere formation and myofibril assembly, and its phosphorylation is modulated by cell adhesion and Erk activation. Here we demonstrate that SPIN90 participates in receptor-mediated endocytic pathway in fibroblasts. We identified syndapin (synaptic dynamin-binding protein) as a SPIN90 interacting protein using yeast two-hybrid screening. SPIN90 directly binds the SH3 domain of syndapin via its proline rich domain in vitro and in vivo and also associates with clathrin. Over-expression of SPIN90-full length in COS-7 cells inhibited transferrin uptake, a marker of endocytosis. Interestingly, SPIN90-PRD, a syndapin-binding domain, significantly inhibited endocytosis, and the inhibition was reversed by co-expression of syndapin. Depleting SPIN90 through antibody microinjection or Knocking it down using siRNAs also significantly inhibited transferrin internalization. Moreover, early endosomal marker proteins (EEA1 and Rab5) appeared to closely associate or partially co-localize with SPIN90 in endosomes and an internalized FITC-dextran and Texas Red-EGF were found on the endosomes in association with SPIN90. Time-lapse video showed that GFP-SPIN90 travels with moving vesicles within living cells. Taken together, these findings suggest that SPIN90 is implicated in receptor-mediated endocytic pathway in fibroblasts.

Fusion pores and fusion machines in Ca2+-triggered exocytosis

Exocytosis is initiated within a highly localized region of contact between two biological membranes. Small areas of these membranes draw close, molecules on the two surfaces interact, and structural transformations take place. Membrane fusion requires the action of proteins specialized for this task, and these proteins act as a fusion machine. At a critical point in this process, a fusion pore forms within the membrane contact site and then expands as the spherical vesicle merges with the flat target membrane. Hence, the operation of a fusion machine must be realized through the formation and expansion of a fusion pore. Delineating the relation between the fusion machine and the fusion pore thus emerges as a central goal in elucidating the mechanisms of membrane fusion. We summarize present knowledge of fusion machines and fusion pores studied in vitro, in neurons, and in neuroendocrine cells, and synthesize this knowledge into some specific and detailed hypotheses for exocytosis.

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

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

Transition from hemifusion to pore opening is rate limiting for vacuole membrane fusion

Fusion pore opening and expansion are considered the most energy-demanding steps in viral fusion. Whether this also applies to soluble N-ethyl-maleimide sensitive fusion protein attachment protein receptor (SNARE)- and Rab-dependent fusion events has been unknown. We have addressed the problem by characterizing the effects of lysophosphatidylcholine (LPC) and other late-stage inhibitors on lipid mixing and pore opening during vacuole fusion. LPC inhibits fusion by inducing positive curvature in the bilayer and changing its biophysical properties. The LPC block reversibly prevented formation of the hemifusion intermediate that allows lipid, but not content, mixing. Transition from hemifusion to pore opening was sensitive to guanosine-5'-(gamma-thio)triphosphate. It required the vacuolar adenosine triphosphatase V0 sector and coincided with its transformation. Pore opening was rate limiting for the reaction. As with viral fusion, opening the fusion pore may be the most energy-demanding step for intracellular, SNARE-dependent fusion reactions, suggesting that fundamental aspects of lipid mixing and pore opening are related for both systems.

SNARE-mediated lipid mixing depends on the physical state of the vesicles

Reconstitution experiments have suggested that N-ethylmaleimide sensitive factor attachment protein receptor (SNARE) proteins constitute a minimal membrane fusion machinery but have yielded contradictory results, and it is unclear whether the mechanism of membrane merger is related to the stalk mechanism that underlies physiological membrane fusion. Here we show that reconstitution of solubilized neuronal SNAREs into preformed 100 nm liposomes (direct method) yields proteoliposomes with more homogeneous sizes and protein densities than the standard reconstitution method involving detergent cosolubilization of proteins and lipids. Standard reconstitutions yield slow but efficient lipid mixing at high protein densities and variable amounts of lipid mixing at moderate protein densities. However, the larger, more homogenous proteoliposomes prepared by the direct method yield almost no lipid mixing at moderate protein densities. These results suggest that the lipid mixing observed for standard reconstitutions is dominated by the physical state of the membrane, perhaps due to populations of small vesicles (or micelles) with high protein densities and curvature stress created upon reconstitution. Accordingly, changing membrane spontaneous curvature by adding lysophospholipids inhibits the lipid mixing observed for standard reconstitutions. Our data indicate that the lipid mixing caused by high SNARE densities and/or curvature stress occurs by a stalk mechanism resembling the mechanism of fusion between biological membranes, but the neuronal SNAREs are largely unable to induce lipid mixing at physiological protein densities and limited curvature stress.

Functions of SNAREs in intracellular membrane fusion and lipid bilayer mixing

Intracellular membrane fusion occurs with exquisite coordination and specificity. Each fusion event requires three basic components: Rab-GTPases organize the fusion site; SNARE proteins act during fusion; and N-ethylmaleimide-sensitive factor (NSF) plus its cofactor alpha-SNAP are required for recycling or activation of the fusion machinery. Whereas Rab-GTPases seem to mediate the initial membrane contact, SNAREs appear to lie at the center of the fusion process. It is known that formation of complexes between SNAREs from apposed membranes is a prerequisite for lipid bilayer mixing; however, the biophysics and many details of SNARE function are still vague. Nevertheless, recent observations are shedding light on the role of SNAREs in membrane fusion. Structural studies are revealing the mechanisms by which SNARES form complexes and interact with other proteins. Furthermore, it is now apparent that the SNARE transmembrane segment not only anchors the protein but engages in SNARE-SNARE interactions and plays an active role in fusion. Recent work indicates that the fusion process itself may comprise two stages and proceed via a hemifusion intermediate.

Dynamics of SNARE assembly and disassembly during sperm acrosomal exocytosis

The dynamics of SNARE assembly and disassembly during membrane recognition and fusion is a central issue in intracellular trafficking and regulated secretion. Exocytosis of sperm's single vesicle—the acrosome—is a synchronized, all-or-nothing process that happens only once in the life of the cell and depends on activation of both the GTP-binding protein Rab3 and of neurotoxin-sensitive SNAREs. These characteristics make acrosomal exocytosis a unique mammalian model for the study of the different phases of the membrane fusion cascade. By using a functional assay and immunofluorescence techniques in combination with neurotoxins and a photosensitive Ca²⁺ chelator we show that, in unactivated sperm, SNAREs are locked in heterotrimeric cis complexes. Upon Ca²⁺ entry into the cytoplasm, Rab3 is activated and triggers NSF/α-SNAP-dependent disassembly of cis SNARE complexes. Monomeric SNAREs in the plasma membrane and the outer acrosomal membrane are then free to reassemble in loose trans complexes that are resistant to NSF/α-SNAP and differentially sensitive to cleavage by two vesicle-associated membrane protein (VAMP)–specific neurotoxins. Ca²⁺ must be released from inside the acrosome to trigger the final steps of membrane fusion that require fully assembled trans SNARE complexes and synaptotagmin. Our results indicate that the unidirectional and sequential disassembly and assembly of SNARE complexes drive acrosomal exocytosis.

Unidirectional and sequential disassembly and assembly of SNARE complexes drive sperm acrosomal exocytosis.

PKA-dependent ENaC trafficking requires the SNARE-binding protein complexin

Acute regulation of epithelial sodium channel (ENaC) function at the apical surface of polarized kidney cortical collecting duct (CCD) epithelial cells occurs in large part by changes in channel number, mediated by membrane vesicle trafficking. Several soluble N-ethyl-maleimide-sensitive factor attachment protein receptors (SNARE) have been implicated in this process. A novel SNARE-binding protein, complexin, has been identified in nervous tissue which specifically binds to and stabilizes SNARE complexes at synaptic membranes to promote vesicle fusion. To test whether this protein is present in mouse CCD (mCCD) cells and its possible involvement in acute ENaC regulation, we cloned complexin (isoform II) from a mouse kidney cDNA library. Complexin II mRNA coexpressed with alpha-, beta-, and gamma-ENaC subunits in Xenopus laevis oocytes reduced sodium currents to 16 +/- 3% (n = 19) of control values. Short-circuit current (I(sc)) measurements on mCCD cell lines stably over- or underexpressing complexin produced similar results. Basal I(sc) was reduced from 12.0 +/- 1.0 (n = 15) to 2.0 +/- 0.4 (n = 15) and 1.8 +/- 0.3 (n = 17) microA/cm(2), respectively. Similarly forskolin-stimulated I(sc) was reduced from control values of 20.0 +/- 2 to 2.7 +/- 0.5 and 2.3 +/- 0.4 microA/cm(2) by either increasing or decreasing complexin expression. Surface biotinylation demonstrated that the complexin-induced reduction in basal I(sc)was due to a reduction in apical membrane-resident ENaC and the inhibition in forskolin stimulation was due to the lack of ENaC insertion into the apical membrane to increase surface channel number. Immunofluorescent localization of SNARE proteins in polarized mCCD epithelia detected the presence of syntaxins 1 and 3 and synaptosomal-associated protein of 23 kDa (SNAP-23) at the apical membrane, and vesicle-associated membrane protein (VAMP2) was localized to intracellular compartments. These findings identify SNAREs that may mediate ENaC-containing vesicle insertion in mCCD epithelia and suggest that stabilization of SNARE interactions by complexin is an essential aspect of the regulated trafficking events that increase apical membrane ENaC density either by constitutive or regulated trafficking pathways.

Trans-SNARE pairing can precede a hemifusion intermediate in intracellular membrane fusion

The question concerning whether all membranes fuse according to the same mechanism has yet to be answered satisfactorily. During fusion of model membranes or viruses, membranes dock, the outer membrane leaflets mix (termed hemifusion), and finally the fusion pore opens and the contents mix. Viral fusion proteins consist of a membrane-disturbing 'fusion peptide' and a helical bundle that pin the membranes together. Although SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptor) complexes form helical bundles with similar topology, it is unknown whether SNARE-dependent fusion events on intracellular membranes proceed through a hemifusion state. Here we identify the first hemifusion state for SNARE-dependent fusion of native membranes, and place it into a sequence of molecular events: formation of helical bundles by SNAREs precedes hemifusion; further progression to pore opening requires additional peptides. Thus, SNARE-dependent fusion may proceed along the same pathway as viral fusion: both use a docking mechanism via helical bundles and additional peptides to destabilize the membrane and efficiently induce lipid mixing. Our results suggest that a common lipidic intermediate may underlie all fusion reactions of lipid bilayers.

Sec17p and HOPS, in distinct SNARE complexes, mediate SNARE complex disruption or assembly for fusion

SNARE functions during membrane docking and fusion are regulated by Sec1/Munc18 (SM) chaperones and Rab/Ypt GTPase effectors. These functions for yeast vacuole fusion are combined in the six-subunit HOPS complex. HOPS facilitates Ypt7p nucleotide exchange, is a Ypt7p effector, and contains an SM protein. We have dissected the associations and requirements for HOPS, Ypt7p, and Sec17/18p during SNARE complex assembly. Vacuole SNARE complexes bind either Sec17p or the HOPS complex, but not both. Sec17p and its co-chaperone Sec18p disassemble SNARE complexes. Ypt7p regulates the reassembly of unpaired SNAREs with each other and with HOPS, forming HOPS.SNARE complexes prior to fusion. After HOPS.SNARE assembly, lipid rearrangements are still required for vacuole content mixing. Thus, Sec17p and HOPS have mutually exclusive interactions with vacuole SNAREs to mediate disruption of SNARE complexes or their assembly for docking and fusion. Sec17p may displace HOPS from SNAREs to permit subsequent rounds of fusion.

ATP-independent control of Vac8 palmitoylation by a SNARE subcomplex on yeast vacuoles

Yeast vacuole fusion requires palmitoylated Vac8. We previously showed that Vac8 acylation occurs early in the fusion reaction, is blocked by antibodies against Sec18 (yeast N-ethylmaleimide-sensitive fusion protein (NSF)), and is mediated by the R-SNARE Ykt6. Here we analyzed the regulation of this reaction on purified vacuoles. We show that Vac8 acylation is restricted to a narrow time window, is independent of ATP hydrolysis by Sec18, and is stimulated by the ion chelator EDTA. Analysis of vacuole protein complexes indicated that Ykt6 is part of a complex distinct from the second R-SNARE, Nyv1. We speculate that during vacuole fusion, Nyv1 is the classical R-SNARE, whereas the Ykt6-containing complex has a novel function in Vac8 palmitoylation.

Membrane fusion of secretory vesicles of the sea urchin egg in the absence of NSF

The role of cytosolic ATPases such as N-ethylmaleimide (NEM)-sensitive fusion protein (NSF) in membrane fusion is controversial. We examined the physiology and biochemistry of ATP and NSF in the cortical system of the echinoderm egg to determine if NSF is an essential factor in membrane fusion during Ca(2+)-triggered exocytosis. Neither exocytosis in vitro, nor homotypic cortical vesicle (CV) fusion required soluble proteins or nucleotides, and both occurred in the presence of non-hydrolyzable analogs of ATP. While sensitive to thiol-specific reagents, CV exocytosis is not restored by the addition of cytosolic NSF, and fusion and NSF function are differentially sensitive to thiol-specific agents. To test participation of tightly bound, non-exchangeable NSF in CV-CV fusion, we cloned the sea urchin homolog and developed a species-specific antibody for western blots and physiological analysis. This antibody was without effect on CV exocytosis or homotypic fusion, despite being functionally inhibitory. NSF is detectable in intact cortices, cortices from which CVs had been removed and isolated CVs treated with ATP-gamma-S and egg cytosol to reveal NSF binding sites. In contrast, isolated CVs, though all capable of Ca(2+)-triggered homotypic fusion, contain less than one hexamer of NSF per CV. Thus NSF is not a required component of the CV fusion machinery.

Cycling of synaptic vesicles: how far? How fast!

Synaptic transmission is based on the regulated exocytotic fusion of synaptic vesicles filled with neurotransmitter. In order to sustain neurotransmitter release, these vesicles need to be recycled locally. Recent data suggest that two tracks for the cycling of synaptic vesicles coexist: a slow track in which vesicles fuse completely with the presynaptic plasma membrane, followed by clathrin-mediated recycling of the vesicular components, and a fast track that may correspond to the transient opening and closing of a fusion pore. In this review, we attempt to provide an overview of the components involved in both tracks of vesicle cycling, as well as to identify possible mechanistic links between these two pathways.

Functional links between mucolipin-1 and Ca2+-dependent membrane trafficking in mucolipidosis IV

Most of the membrane trafficking phenomena including those involving the interactions between endosomes and lysosomes are regulated by changes in intracellular Ca2+ (Cai). These processes are disturbed in some types of mucolipidoses and other lysosomal storage disorders, such as mucolipidosis IV (MLIV), a neurological disorder that usually presents during the first year of life with blindness, cognitive impairment, and psychomotor delays. It is caused by mutations in MCOLN1, the gene encoding mucolipin-1 (MLN1), which we have recently established to represent a Ca2+-permeable cation channel that is transiently modulated by changes in Cai. The cells of MLIV patients contain enlarged lysosomes that are likely associated with abnormal sorting and trafficking of these and related organelles. We studied fibroblasts from MLIV patients and found disturbed Ca2+ signaling and large acidic organelles such as late endosomes and lysosomes (LEL) with altered cellular localization in these cells. The fusion between LEL vesicles in these cells was defective. This is a Ca2+-dependent process related to signaling pathways involved in regulation of Ca2+ homeostasis and trafficking. The MLN1 channels could play a key role in Ca2+ release from LEL vesicles, which triggers the fusion and trafficking of these organelles. The characterization of this MLN1-mediated Ca2+-dependent process should provide new insights into the pathophysiological mechanisms that lead to the development of MLIV and other mucolipidoses associated with similar disturbances in membrane trafficking.

alpha-SNAP and NSF are required in a priming step during the human sperm acrosome reaction

The acrosome is a membrane-limited granule that overlies the nucleus of the mature spermatozoon. In response to physiological or pharmacological stimuli it undergoes a special type of Ca2+-dependent exocytosis termed the acrosome reaction (AR), which is an absolute prerequisite for fertilization. Aided by a streptolysin-O permeabilization protocol developed in our laboratory, we have previously demonstrated requirements for Rab3A, N-ethylmaleimide-sensitive factor (NSF), several soluble NSF-attachment protein receptor (SNARE) proteins, and synaptotagmin VI in the human sperm AR. Here, we show that alpha-soluble NSF-attachment protein (alpha-SNAP), a protein essential for most fusion events through its interaction with NSF and the SNARE complex, exhibits a direct role in the AR. First, the presence of alpha-SNAP is demonstrated by the Western blot of human sperm protein extracts. Immunostaining experiments reveal an acrosomal localization for this protein. Second, the Ca2+ and Rab3A-triggered ARs are inhibited by anti-alpha-SNAP antibodies. Third, bacterially expressed alpha-SNAP abolishes exocytosis in a fashion that depends on its interaction with NSF. Fourth, we show a requirement for alpha-SNAP/NSF in a prefusion step early in the exocytotic pathway, after the tethering of the acrosome to the plasma membrane and before the efflux of intra-acrosomal Ca2+. These results suggest a key role for alpha-SNAP/NSF in the AR, and strengthen our understanding of the molecular players involved in the vesicle-to-plasma membrane fusion taking place during exocytosis.

Mutual Control of Membrane Fission and Fusion Proteins

Membrane fusion and fission are antagonistic reactions controlled by different proteins. Dynamins promote membrane fission by GTP-driven changes of conformation and polymerization state, while SNAREs fuse membranes by forming complexes between t- and v-SNAREs from apposed vesicles. Here, we describe a role of the dynamin-like GTPase Vps1p in fusion of yeast vacuoles. Vps1p forms polymers that couple several t-SNAREs together. At the onset of fusion, the SNARE-activating ATPase Sec18p/NSF and the t-SNARE depolymerize Vps1p and release it from the membrane. This activity is independent of the SNARE coactivator Sec17p/alpha-SNAP and of the v-SNARE. Vps1p release liberates the t-SNAREs for initiating fusion and at the same time disrupts fission activity. We propose that reciprocal control between fusion and fission components exists, which may prevent futile cycles of fission and fusion.

Single molecule observation of liposome-bilayer fusion thermally induced by soluble N-ethyl maleimide sensitive-factor attachment protein receptors (SNAREs)

A single molecule fluorescence assay is presented for studying the mechanism of soluble N-ethyl maleimide sensitive-factor attachment protein receptors (SNAREs)-mediated liposome fusion to supported lipid bilayers. The three neuronal SNAREs syntaxin-1A, synaptobrevin-II (VAMP), and SNAP-25A were expressed separately, and various dye-labeled combinations of the SNAREs were tested for their ability to dock liposomes and induce fusion. Syntaxin and synaptobrevin in opposing membranes were both necessary and sufficient to dock liposomes to supported bilayers and to induce thermally activated fusion. As little as one SNARE interaction was sufficient for liposome docking. Fusion of docked liposomes with the supported bilayer was monitored by the dequenching of soluble fluorophores entrapped within the liposomes. Fusion was stimulated by illumination with laser light, and the fusion probability was enhanced by raising the ambient temperature from 22 to 37 degrees C, suggesting a thermally activated process. Surprisingly, SNAP-25 had little effect on docking efficiency or the probability of thermally induced fusion. Interprotein fluorescence resonance energy transfer experiments suggest the presence of other conformational states of the syntaxin*synaptobrevin interaction in addition to those observed in the crystal structure of the SNARE complex. Furthermore, although SNARE complexes involved in liposome docking preferentially assemble into a parallel configuration, both parallel and antiparallel configurations were observed.

Trans-SNARE interactions elicit Ca2+ efflux from the yeast vacuole lumen

Ca²⁺ transients trigger many SNARE-dependent membrane fusion events. The homotypic fusion of yeast vacuoles occurs after a release of lumenal Ca²⁺. Here, we show that trans-SNARE interactions promote the release of Ca²⁺ from the vacuole lumen. Ypt7p–GTP, the Sec1p/Munc18-protein Vps33p, and Rho GTPases, all of which function during docking, are required for Ca²⁺ release. Inhibitors of SNARE function prevent Ca²⁺ release. Recombinant Vam7p, a soluble Q-SNARE, stimulates Ca²⁺ release. Vacuoles lacking either of two complementary SNAREs, Vam3p or Nyv1p, fail to release Ca²⁺ upon tethering. Mixing these two vacuole populations together allows Vam3p and Nyv1p to interact in trans and rescues Ca²⁺ release. Sec17/18p promote sustained Ca²⁺ release by recycling SNAREs (and perhaps other limiting factors), but are not required at the release step itself. We conclude that trans-SNARE assembly events during docking promote Ca²⁺ release from the vacuole lumen.

Molecular and phylogenetic characterization of syntaxin genes from parasitic protozoa

Vesicular transport is an integral process in eukaryotic cells and the syntaxins, a member of the SNARE protein superfamily, are a critical piece of the vesicular transport machinery. We have obtained syntaxin homologues from diverse protozoan parasites (including Entamoeba, Giardia, Trichomonas and Trypanosoma), determined the paralogue affinity of the homologues by molecular phylogenetics and compared functionally critical amino acid sites identified in other syntaxins. Surprisingly, three sequences deviate at the signature glutamine residue position, conserved in all previously identified syntaxin homologues. It is known that, despite conserved structure and function of both the syntaxins and the proteins of the regulatory SM superfamily, the various syntaxin paralogues bind their respective SM partners at different regions of the syntaxin molecule. These sites of interactions have been identified down to the individual residues. The pattern of conservation at these residues, in our evolutionarily diverse sampling of syntaxin paralogues, is therefore used to gain further insight into the interaction of these proteins. Phylogenetic analysis confirms and extends previous conclusions that the syntaxin families are present in diverse eukaryotes and that the syntaxin sub-families diverged early in eukaryotic evolution. This result is expanded with the inclusion of new homologues for previously sampled taxa, newly sampled taxa, and newly sampled syntaxin sub-families. Because of their integral role in membrane trafficking, the syntaxin genes represent a valuable potential molecular marker for the experimental study of the endomembrane system of disease-causing protists.

A soluble SNARE drives rapid docking, bypassing ATP and Sec17/18p for vacuole fusion

Membrane fusion requires priming, the disassembly of cis-SNARE complexes by the ATP-driven chaperones Sec18/17p. Yeast vacuole priming releases Vam7p, a soluble SNARE. Vam7p reassociation during docking allows trans-SNARE pairing and fusion. We now report that recombinant Vam7p (rVam7p) enters into complex with other SNAREs in vitro and bypasses the need for Sec17p, Sec18p, and ATP. Thus, the sole essential function of vacuole priming in vitro is the release of Vam7p from cis-SNARE complexes. In 'bypass fusion', without ATP but with added rVam7p, there are sufficient unpaired vacuolar SNAREs Vam3p, Vti1p, and Nyv1p to interact with Vam7p and support fusion. However, active SNARE proteins are not sufficient for bypass fusion. rVam7p does not bypass requirements for Rho GTPases,Vps33p, Vps39p, Vps41p, calmodulin, specific lipids, or Vph1p, a subunit of the V-ATPase. With excess rVam7p, reduced levels of PI(3)P or functional Ypt7p suffice for bypass fusion. High concentrations of rVam7p allow the R-SNARE Ykt6p to substitute for Nyv1p for fusion; this functional redundancy among vacuole SNAREs may explain why nyv1delta strains lack the vacuole fragmentation seen with mutants in other fusion catalysts.

Principles of exocytosis and membrane fusion

Exocytosis is a ubiquitous process occurring in every eukaryotic cell including processes as diverse as membrane expansion during growth and the highly regulated release of neurotransmitter from neurons. Work during the past decade has established that exocytotic membrane fusion is mediated by members of conserved protein families including Rab proteins and SNAREs. SNAREs are probably catalyzing membrane fusion, and major progress has been made in unraveling their molecular mechanism. In contrast, less is known about regulatory mechanisms. Here, a brief overview is given about the current state of knowledge, focusing on SNAREs involved in neuronal exocytosis.

Molecular cloning and functional characterization ofavaB, a gene encoding Vam6p/Vps39p-like protein inAspergillus nidulans

It has been demonstrated that Saccharomyces cerevisiae Vam6p/Vps39p plays a critical role in the tethering steps of vacuolar membrane fusion by facilitating guanine nucleotide exchange on small guanosine triphosphatase (GTPase) Vam4p/Ypt7p. We report here the identification and characterization of a novel protein in Aspergillus nidulans, AvaB, that exhibits similarity to Vam6p/Vps39p and plays a critical role in vacuolar morphogenesis in A. nidulans. AvaB is comprised of 1058 amino acids with amino-terminal citron homology (CNH) and central clathrin homology (CLH) domains, as observed for other Vam6p/Vps39p family proteins. Disruption of avaB in A. nidulans resulted in the fragmentation of vacuoles and reduced growth rate under various growth conditions, implying its importance in maintaining vacuolar morphology and function. Yeast two-hybrid analysis demonstrated the interaction of AvaB with AvaA, a Vam4p/Ypt7p homolog in A. nidulans, as well as the homooligomer formation of AvaB, suggesting that AvaB performs its function through hetero- or homophilic protein-protein interactions.

The SNARE Ykt6 mediates protein palmitoylation during an early stage of homotypic vacuole fusion

The NSF homolog Sec18 initiates fusion of yeast vacuoles by disassembling cis-SNARE complexes during priming. Sec18 is also required for palmitoylation of the fusion factor Vac8, although the acylation machinery has not been identified. Here we show that the SNARE Ykt6 mediates Vac8 palmitoylation and acts during a novel subreaction of vacuole fusion. This subreaction is controlled by a Sec17-independent function of Sec18. Our data indicate that Ykt6 presents Pal-CoA via its N-terminal longin domain to Vac8, while transfer to Vac8's SH4 domain occurs spontaneously and not enzymatically. The conservation of Ykt6 and its localization to several organelles suggest that its acyltransferase activity may also be required in other intracellular fusion events.

Single-molecule studies of SNARE complex assembly reveal parallel and antiparallel configurations

Vesicle fusion in eukaryotes is thought to involve the assembly of a highly conserved family of proteins termed soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs) into a highly stable parallel four-helix bundle. We have used intermolecular single-molecule fluorescence resonance energy transfer to characterize preassembled neuronal SNARE complexes consisting of syntaxin, synaptobrevin, and synaptosome-associated protein of 25 kDa on deposited lipid bilayers. Surprisingly, we found a mixture of parallel as well as antiparallel configurations involving the SNARE motifs of syntaxin and synaptobrevin as well as those of syntaxin and synaptosome-associated protein of 25 kDa. The subpopulation with the parallel four-helix bundle configuration could be greatly enriched by an additional purification step in the presence of denaturant, indicating that the parallel configuration is the energetically most favorable state. Interconversion between the configurations was not observed. From this observation, we infer the conversion rate to be <1.5 h-1. The existence of antiparallel configurations suggests a regulatory role of chaperones, such as N-ethylmaleimide-sensitive factor, or the membrane environment during SNARE complex assembly in vivo, and it could be a partial explanation for the relatively slow rates of vesicle fusion observed by reconstituted fusion experiments in vitro.

SNARE Protein Structure and Function

The SNARE superfamily has become, since its discovery approximately a decade ago, the most intensively studied element of the protein machinery involved in intracellular trafficking. Intracellular membrane fusion in eukaryotes requires SNARE (soluble N-ethylmaleimide-sensitive-factor attachment protein receptor) proteins that form complexes bridging the two membranes. Although common themes have emerged from structural and functional studies of SNAREs and other components of the eukaryotic membrane fusion machinery, there is still much to learn about how the assembly and activity of this machinery is choreographed in living cells.

Defects in synaptic vesicle docking in unc-18 mutants

Sec1-related proteins function in most, if not all, membrane trafficking pathways in eukaryotic cells. The Sec1-related protein required in neurons for synaptic vesicle exocytosis is UNC-18. Several models for UNC-18 function during vesicle exocytosis are under consideration. We have tested these models by characterizing unc-18 mutants of the nematode Caenorhabditis elegans. In the absence of UNC-18, the size of the readily releasable pool is severely reduced. Our results show that the near absence of fusion-competent vesicles is not caused by a reduction in syntaxin levels, by a mislocalization of syntaxin, by a defect in fusion or by a failure to open syntaxin during priming. Rather, we found a reduction of docked vesicles at the active zone in unc-18 mutants, suggesting that UNC-18 functions, directly or indirectly, as a facilitator of vesicle docking.

Tomosyn interacts with the t-SNAREs syntaxin4 and SNAP23 and plays a role in insulin-stimulated GLUT4 translocation

The Sec1p-like/Munc18 (SM) protein Munc18a binds to the neuronal t-SNARE Syntaxin1A and inhibits SNARE complex assembly. Tomosyn, a cytosolic Syntaxin1A-binding protein, is thought to regulate the interaction between Syntaxin1A and Munc18a, thus acting as a positive regulator of SNARE assembly. In the present study we have investigated the interaction between b-Tomosyn and the adipocyte SNARE complex involving Syntaxin4/SNAP23/VAMP-2 and the SM protein Munc18c, in vitro, and the potential involvement of Tomosyn in regulating the translocation of GLUT4 containing vesicles, in vivo. Tomosyn formed a high affinity ternary complex with Syntaxin4 and SNAP23 that was competitively inhibited by VAMP-2. Using a yeast two-hybrid assay we demonstrate that the VAMP-2-like domain in Tomosyn facilitates the interaction with Syntaxin4. Overexpression of Tomosyn in 3T3-L1 adipocytes inhibited the translocation of green fluorescent protein-GLUT4 to the plasma membrane. The SM protein Munc18c was shown to interact with the Syntaxin4 monomer, Syntaxin4 containing SNARE complexes, and the Syntaxin4/Tomosyn complex. These data suggest that Tomosyn and Munc18c operate at a similar stage of the Syntaxin4 SNARE assembly cycle, which likely primes Syntaxin4 for entry into the ternary SNARE complex.

Involvement of LMA1 and GATE-16 family members in intracellular membrane dynamics

Intracellular membrane fusion is conserved from yeast to man as well as among different intracellular trafficking pathways. This process can be generally divided into several well-defined biochemical reactions. First, an early recognition (or tethering) takes place between donor and acceptor membranes, mediated by ypt/rab GTPases and complexes of tethering factors. Subsequently, a closer association between the two membranes is achieved by a docking process, which involves tight association between membrane proteins termed SNAREs. The formation of such a trans-SNARE complex leads to the final membrane fusion, resulting in an accumulation of cis-SNARE complexes on the acceptor membrane. Thus, multiple rounds of transport and delivery of the donor SNARE back to its original membrane require dissociation of the SNARE complexes. SNARE dissociation, termed priming, is mediated by the AAA ATPase, N-ethylmaleimide-sensitive factor (NSF) and its partner, soluble NSF attachment protein (SNAP), in a reaction that requires ATP hydrolysis. In the present review we focus on LMA1 and GATE-16, two low-molecular-weight proteins, which assist in priming SNARE molecules in the vacuole in yeast and the Golgi complex in mammals, respectively. LMA1 and GATE-16 are suggested to keep the dissociated cis-SNAREs apart from each other, allowing multiple fusion processes to take place. GATE-16 belongs to a novel family of ubiquitin-like proteins conserved from yeast to man. We discuss here the involvement of this family in multiple intracellular trafficking pathways.