Control of eukaryotic membrane fusion by N-terminal domains of SNARE proteins

The STX17-SNAP47-VAMP7/VAMP8 complex is the default SNARE complex mediating autophagosome-lysosome fusion

Autophagosome-lysosome fusion mediated by SNARE complexes is an essential step in autophagy. Two SNAP29-containing SNARE complexes have been extensively studied in starvation-induced bulk autophagy, while the relevant SNARE complexes in other types of autophagy occurring under non-starvation conditions have been overlooked. Here, we found that autophagosome-lysosome fusion in selective autophagy under non-starvation conditions does not require SNAP29-containing SNARE complexes, but requires the STX17-SNAP47-VAMP7/VAMP8 SNARE complex. Further, the STX17-SNAP47-VAMP7/VAMP8 SNARE complex also functions in starvation-induced autophagy. SNAP47 is recruited to autophagosomes following concurrent detection of ATG8s and PI(4,5)P2 via its Pleckstrin homology domain. By contrast, SNAP29-containing SNAREs are excluded from selective autophagy due to inactivation by O-GlcNAcylation under non-starvation conditions. These findings depict a previously unknown, default SNARE complex responsible for autophagosome-lysosome fusion in both selective and bulk autophagy, which could guide research and therapeutic development in autophagy-related diseases.

Phosphorylation of Syntaxin 4 by the Insulin Receptor Drives Exocytic SNARE Complex Formation to Deliver GLUT4 to the Cell Surface

A major consequence of insulin binding its receptor on fat and muscle cells is the stimulation of glucose transport into these tissues. This is achieved through an increase in the exocytic trafficking rate of the facilitative glucose transporter GLUT4 from intracellular stores to the cell surface. Delivery of GLUT4 to the cell surface requires the formation of functional SNARE complexes containing Syntaxin 4, SNAP23, and VAMP2. Insulin stimulates the formation of these complexes and concomitantly causes phosphorylation of Syntaxin 4. Here, we use a combination of biochemistry and cell biological approaches to provide a mechanistic link between these observations. We present data to support the hypothesis that Tyr-115 and Tyr-251 of Syntaxin 4 are direct substrates of activated insulin receptors, and that these residues modulate the protein's conformation and thus regulate the rate at which Syntaxin 4 forms SNARE complexes that deliver GLUT4 to the cell surface. This report provides molecular details on how the cell regulates SNARE-mediated membrane traffic in response to an external stimulus.

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.

Exocytosis Proteins: Typical and Atypical Mechanisms of Action in Skeletal Muscle

Insulin-stimulated glucose uptake in skeletal muscle is of fundamental importance to prevent postprandial hyperglycemia, and long-term deficits in insulin-stimulated glucose uptake underlie insulin resistance and type 2 diabetes. Skeletal muscle is responsible for ~80% of the peripheral glucose uptake from circulation via the insulin-responsive glucose transporter GLUT4. GLUT4 is mainly sequestered in intracellular GLUT4 storage vesicles in the basal state. In response to insulin, the GLUT4 storage vesicles rapidly translocate to the plasma membrane, where they undergo vesicle docking, priming, and fusion via the high-affinity interactions among the soluble N-ethylmaleimide sensitive factor attachment protein receptor (SNARE) exocytosis proteins and their regulators. Numerous studies have elucidated that GLUT4 translocation is defective in insulin resistance and type 2 diabetes. Emerging evidence also links defects in several SNAREs and SNARE regulatory proteins to insulin resistance and type 2 diabetes in rodents and humans. Therefore, we highlight the latest research on the role of SNAREs and their regulatory proteins in insulin-stimulated GLUT4 translocation in skeletal muscle. Subsequently, we discuss the novel emerging role of SNARE proteins as interaction partners in pathways not typically thought to involve SNAREs and how these atypical functions reveal novel therapeutic targets for combating peripheral insulin resistance and diabetes.

The Habc domain of syntaxin 3 is a ubiquitin binding domain

Syntaxins are a family of membrane-anchored SNARE proteins that are essential components required for membrane fusion in eukaryotic intracellular membrane trafficking pathways. Syntaxins contain an N-terminal regulatory domain, termed the Habc domain that is not highly conserved at the primary sequence level but folds into a three-helix bundle that is structurally conserved among family members. The syntaxin Habc domain has previously been found to be structurally very similar to the GAT domain present in GGA family members and related proteins that are otherwise completely unrelated to syntaxins. Because the GAT domain has been found to be a ubiquitin binding domain we hypothesized that the Habc domain of syntaxins may also bind to ubiquitin. Here, we report that the Habc domain of syntaxin 3 (Stx3) indeed binds to monomeric ubiquitin with low affinity. This domain binds efficiently to K63-linked poly-ubiquitin chains within a narrow range of chain lengths but not to K48-linked poly-ubiquitin chains. Other syntaxin family members also bind to K63-linked poly-ubiquitin chains but with different chain length specificities. Molecular modeling suggests that residues of the GGA3-GAT domain known to be important for ionic and hydrophobic interactions with ubiquitin may have equivalent, conserved residues within the Habc domain of Stx3. We conclude that the syntaxin Habc domain and the GAT domain are both structurally and functionally related, and likely share a common ancestry despite sequence divergence. Binding of Ubiquitin to the Habc domain may regulate the function of syntaxins in membrane fusion or may suggest additional functions of this protein family.

Accumulation of undegraded autophagosomes by expression of dominant-negative STX17 (syntaxin 17) mutants

Macroautophagy/autophagy, which is one of the main degradation systems in the cell, is mediated by a specialized organelle, the autophagosome. Purification of autophagosomes before fusion with lysosomes is important for both mechanistic and physiological studies of the autophagosome. Here, we report a simple method to accumulate undigested autophagosomes. Overexpression of the autophagosomal Qa-SNARE STX17 (syntaxin 17) lacking the N-terminal domain (NTD) or N-terminally tagged GFP-STX17 causes accumulation of autophagosomes. A HeLa cell line, which expresses GFP-STX17ΔNTD or full-length GFP-STX17 under the control of the tetracycline-responsive promoter, accumulates a large number of undigested autophagosomes devoid of lysosomal markers or early autophagy factors upon treatment with doxycycline. Using this inducible cell line, nascent autophagosomes can be easily purified by OptiPrep density-gradient centrifugation and immunoprecipitation. This novel method should be useful for further characterization of nascent autophagosomes.

Lipid Regulated Intramolecular Conformational Dynamics of SNARE-Protein Ykt6

Cellular informational and metabolic processes are propagated with specific membrane fusions governed by soluble N-ethylmaleimide sensitive factor attachment protein receptors (SNARE). SNARE protein Ykt6 is highly expressed in brain neurons and plays a critical role in the membrane-trafficking process. Studies suggested that Ykt6 undergoes a conformational change at the interface between its longin domain and the SNARE core. In this work, we study the conformational state distributions and dynamics of rat Ykt6 by means of single-molecule Förster Resonance Energy Transfer (smFRET) and Fluorescence Cross-Correlation Spectroscopy (FCCS). We observed that intramolecular conformational dynamics between longin domain and SNARE core occurred at the timescale ~200 μs. Furthermore, this dynamics can be regulated and even eliminated by the presence of lipid dodecylphoshpocholine (DPC). Our molecular dynamic (MD) simulations have shown that, the SNARE core exhibits a flexible structure while the longin domain retains relatively stable in apo state. Combining single molecule experiments and theoretical MD simulations, we are the first to provide a quantitative dynamics of Ykt6 and explain the functional conformational change from a qualitative point of view.

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.

Structure and function of longin SNAREs

Soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) proteins constitute the core membrane fusion machinery of intracellular transport and intercellular communication. A little more than ten years ago, it was proposed that the long N-terminal domain of a subset of SNAREs, henceforth called the longin domain, could be a crucial regulator with multiple functions in membrane trafficking. Structural, biochemical and cell biology studies have now produced a large set of data that support this hypothesis and indicate a role for the longin domain in regulating the sorting and activity of SNAREs. Here, we review the first decade of structure-function data on the three prototypical longin SNAREs: Ykt6, VAMP7 and Sec22b. We will, in particular, highlight the conserved molecular mechanisms that allow longin domains to fold back onto the fusion-inducing SNARE coiled-coil domain, thereby inhibiting membrane fusion, and describe the interactions of longin SNAREs with proteins that regulate their intracellular sorting. This dual function of the longin domain in regulating both the membrane localization and membrane fusion activity of SNAREs points to its role as a key regulatory module of intracellular trafficking.

The Exocyst Subunit Sec6 Interacts with Assembled Exocytic SNARE Complexes

In eukaryotic cells, membrane-bound vesicles carry cargo between intracellular compartments, to and from the cell surface, and into the extracellular environment. Many conserved families of proteins are required for properly localized vesicle fusion, including the multisubunit tethering complexes and the SNARE complexes. These protein complexes work together to promote proper vesicle fusion in intracellular trafficking pathways. However, the mechanism by which the exocyst, the exocytosis-specific multisubunit tethering complex, interacts with the exocytic SNAREs to mediate vesicle targeting and fusion is currently unknown. We have demonstrated previously that the Saccharomyces cerevisiae exocyst subunit Sec6 directly bound the plasma membrane SNARE protein Sec9 in vitro and that Sec6 inhibited the assembly of the binary Sso1-Sec9 SNARE complex. Therefore, we hypothesized that the interaction between Sec6 and Sec9 prevented the assembly of premature SNARE complexes at sites of exocytosis. To map the determinants of this interaction, we used cross-linking and mass spectrometry analyses to identify residues required for binding. Mutation of residues identified by this approach resulted in a growth defect when introduced into yeast. Contrary to our previous hypothesis, we discovered that Sec6 does not change the rate of SNARE assembly but, rather, binds both the binary Sec9-Sso1 and ternary Sec9-Sso1-Snc2 SNARE complexes. Together, these results suggest a new model in which Sec6 promotes SNARE complex assembly, similar to the role proposed for other tether subunit-SNARE interactions.

The role of non-canonical SNAREs in synaptic vesicle recycling

An increasing number of studies suggest that distinct pools of synaptic vesicles drive specific forms of neurotransmission. Interspersed with these functional studies are analyses of the synaptic vesicle proteome which have consistently detected the presence of so-called “non-canonical” SNAREs that typically function in fusion and trafficking of other subcellular structures within the neuron. The recent identification of certain non-canonical vesicular SNAREs driving spontaneous (e.g., VAMP7 and vti1a) or evoked asynchronous (e.g., VAMP4) release integrates and corroborates existing data from functional and proteomic studies and implies that at least some complement of non-canonical SNAREs resident on synaptic vesicles function in neurotransmission. Here, we discuss the specific roles in neurotransmission of proteins homologous to each member of the classical neuronal SNARE complex consisting of synaptobrevin2, syntaxin-1 and SNAP-25.

Lipid regulated conformational dynamics of the longin SNARE protein Ykt6 revealed by molecular dynamics simulations

The conformation and subcellular localization of R-SNARE protein Ykt6 are regulated by the lipidation state of its C-terminal CCAIM motif. Biochemical and crystallography studies showed that lipid molecules binding at a hydrophobic pocket at the interface between the longin domain and the SNARE core can lock Ykt6 at a closed conformation and mimic the farnesylated state of Ykt6. In this study, we performed in silico farnesylation of Ykt6 and explored the conformational dynamics of Ykt6 using conventional and steered MD simulations. We found that the farnesylated Ykt6 model structure is stable during the 2 μs simulation and the farnesyl group adopts conformations similar to those of the DPC molecule bound to Ykt6. Both DPC binding and farnesylation were found to reduce the conformational flexibility of Ykt6 and hinder the dissociation of SNARE core from the longin domain. The dissociation of the αF-αG segment is the rate-limiting step during the putative closed-to-open conformational transition of Ykt6, and the key residues involved in this process are consistent with the experimental mutagenesis study.

Annexin A7 trafficking to alveolar type II cell surface: possible roles for protein insertion into membranes and lamellar body secretion

A role for annexin A7 (A7) is postulated in the obligatory fusion between lamellar bodies and the plasma membrane during surfactant secretion in alveolar type II cells. This study investigated if surfactant secretagogues increase cell surface A7, which could support A7 insertion into plasma membrane as annexin proteins reportedly lack membrane penetration ability. In vivo trafficking of A7 to cell surface was determined by immuno-staining after non-permeabilizing fixation of alveolar type II cells. Stimulation with various secretagogues increased protein kinase-dependent staining for A7 and ABCA3 in comparison to control cells. Biotin-labeling of surface proteins showed ~4% of total A7 in control cells, which increased ~3-4 folds in stimulated type II cells. Increased cell surface A7 was also observed by protein cross-linking studies showing ~70kDa A7-adduct in the membranes but not in the cytosol fraction of PMA- or A23187-stimulated cells. In vitro phosphorylation increased the Ca(2+)-dependent binding of recombinant A7 to lung plasma membranes; and subsequent cross-linking showed increased levels of ~70kDa A7-adduct. PMA-stimulation of type II cells increased A7 trafficking to lipid rafts suggesting that the latter are involved in A7 trafficking to the cell surface. However, in vitro membrane insertion of recombinant A7 and its tryptophan mutants as determined by fluorescence quenching with doxylPC suggested only shallow membrane insertion by A7. Together, our studies support in vivo association between surfactant secretion and cell surface A7 occurring by insertion into plasma membrane and by fusion of A7 containing lamellar bodies.

Annexin A7 and SNAP23 interactions in alveolar type II cells and in vitro: a role for Ca(2+) and PKC

Lung surfactant secretion involves lamellar body docking and fusion with the plasma membrane in alveolar type II cells. Annexin A7 (A7) is postulated to play a role in membrane fusion during exocytosis. Our recent studies demonstrated increased co-localization of A7 with ABCA3 in lamellar bodies in type II cells stimulated with established secretagogues of lung surfactant. In this study, we investigated in vivo and in vitro interactions of A7 with the t-SNARE protein, SNAP23. Immuno-fluorescence studies showed time-dependent increases in co-localization of A7 with SNAP23 in PMA- and in A23187-stimulated cells. PMA and A23187 also caused a time-dependent increase in co-localization of ABCA3 with SNAP23. The relocation of A7 to SNAP23 domains was inhibited in the presence of PKC inhibitor, similar to that previously reported for co-localization of A7 with ABCA3. The interaction of A7 and SNAP23 was confirmed by affinity binding and by in vitro interaction of recombinant A7 and SNAP23 proteins. The in vitro binding of recombinant A7 (rA7) to GST-SNAP23 fusion protein was calcium-dependent. Phosphorylation of rA7 with PKC increased its in vitro binding to SNAP23 suggesting that a similar mechanism may operate during A7 relocation to t-SNARE domains. Thus, our studies demonstrate that annexin A7 may function in co-ordination with SNARE proteins and that protein kinase activation may be required for annexin A7 trafficking to the interacting membranes (lamellar bodies and plasma membrane) to facilitate membrane fusion during surfactant secretion.

The Legionella pneumophila effector DrrA is sufficient to stimulate SNARE-dependent membrane fusion

The intracellular bacterial pathogen Legionella pneumophila subverts host membrane transport pathways to promote fusion of vesicles exiting the endoplasmic reticulum (ER) with the pathogen-containing vacuole. During infection there is noncanonical pairing of the SNARE protein Sec22b on ER-derived vesicles with plasma membrane (PM)-localized syntaxin proteins on the vacuole. We show that the L. pneumophila Rab1-targeting effector DrrA is sufficient to stimulate this noncanonical SNARE association and promote membrane fusion. DrrA activation of the Rab1 GTPase on PM-derived organelles stimulated the tethering of ER-derived vesicles with the PM-derived organelle, resulting in vesicle fusion through the pairing of Sec22b with the PM syntaxin proteins. Thus, the effector protein DrrA stimulates a host membrane transport pathway that enables ER-derived vesicles to remodel a PM-derived organelle, suggesting that Rab1 activation at the PM is sufficient to promote the recruitment and fusion of ER-derived vesicles.

The V-ATPase proteolipid cylinder promotes the lipid-mixing stage of SNARE-dependent fusion of yeast vacuoles

The V-ATPase V(0) sector associates with the peripheral V(1) sector to form a proton pump. V(0) alone has an additional function, facilitating membrane fusion in the endocytic and late exocytic pathways. V(0) contains a hexameric proteolipid cylinder, which might support fusion as proposed in proteinaceous pore models. To test this, we randomly mutagenized proteolipids. We recovered alleles that preserve proton translocation, normal SNARE activation and trans-SNARE pairing but that impair lipid and content mixing. Critical residues were found in all subunits of the proteolipid ring. They concentrate within the bilayer, close to the ring subunit interfaces. The fusion-impairing proteolipid substitutions stabilize the interaction of V(0) with V(1). Deletion of the vacuolar v-SNARE Nyv1 has the same effect, suggesting that both types of mutations similarly alter the conformation of V(0). Also covalent linkage of subunits in the proteolipid cylinder blocks vacuole fusion. We propose that a SNARE-dependent conformational change in V(0) proteolipids might stimulate fusion by creating a hydrophobic crevice that promotes lipid reorientation and formation of a lipidic fusion pore.

Secretagogues of lung surfactant increase annexin A7 localization with ABCA3 in alveolar type II cells

Membrane fusion between the lamellar bodies and plasma membrane is an obligatory event in the secretion of lung surfactant. Previous studies have postulated a role for annexin A7 (A7) in membrane fusion during exocytosis in some cells including alveolar type II cells. However, the intracellular trafficking of A7 during such fusion is not described. In this study, we investigated association of endogenous A7 with lamellar bodies in alveolar type II cells following treatment with several secretagogues of lung surfactant. Biochemical studies with specific antibodies showed increased membrane-association of cell A7 in type II cells stimulated with agents that increase secretion through different signaling mechanisms. Immuno-fluorescence studies showed increased co-localization of A7 with ABCA3, the lamellar body marker protein. Because these agents increase surfactant secretion through activation of PKC and PKA, we also investigated the effects of PKC and PKA inhibitors, bisindolylmaleimideI (BisI) and H89, respectively, on A7 partitioning. Western blot analysis showed that these inhibitors prevented secretagogue-mediated A7 increase in the membrane fractions. These inhibitors also blocked increased co-localization of A7 with ABCA3 in secretagogue-treated cells, as revealed by immuno-fluorescence studies. In vitro studies with recombinant A7 showed phosphorylation with PKC and PKA. The cell A7 was also phosphorylated in cells treated with surfactant secretagogues. Thus, our studies demonstrate that annexin A7 relocates to lamellar bodies in a phosphorylation-dependent manner. We suggest that activation of protein kinase promotes phosphorylation and membrane-association of A7 presumably to facilitate membrane fusion during lung surfactant secretion.

Inclusion membrane proteins of Protochlamydia amoebophila UWE25 reveal a conserved mechanism for host cell interaction among the Chlamydiae

Chlamydiae are a group of obligate intracellular bacteria comprising several important human pathogens. Inside the eukaryotic cell, chlamydiae remain within a host-derived vesicular compartment, termed the inclusion. They modify the inclusion membrane through insertion of unique proteins, which are involved in interaction with and manipulation of the host cell. Among chlamydiae, inclusion membrane proteins have been exclusively found in members of the family Chlamydiaceae, which predominantly infect mammalian and avian hosts. Here, the presence of inclusion membrane proteins in Protochlamydia amoebophila UWE25, a chlamydial endosymbiont of free-living amoebae, is reported. A genome-wide screening for secondary structure motifs resulted in the identification of 23 putative inclusion membrane proteins for this organism. Immunofluorescence analysis demonstrated that five of these proteins were expressed, and four of them could be localized to a halo surrounding the intracellular bacteria. Colocalization studies showed an almost complete overlap of the signals obtained for the four putative inclusion membrane proteins, and immuno-transmission electron microscopy unambiguously demonstrated their location in the inclusion membrane. The presence of inclusion membrane proteins (designated IncA, IncQ, IncR, and IncS) in P. amoebophila shows that this strategy for host cell interaction is conserved among the chlamydiae and is used by chlamydial symbionts and pathogens alike.

Deregulation of Rab5 and Rab4 proteins in p85R274A-expressing cells alters PDGFR trafficking

Activated receptor tyrosine kinases recruit many signaling proteins to activate downstream cell proliferation and survival pathways, including phosphatidylinositol 3-kinase (PI3K) consisting of a p85 regulatory protein and a p110 catalytic protein. We have recently shown the p85alpha protein also has in vitro GTPase activating protein (GAP) activity towards Rab5 and Rab4, small GTPases that regulate vesicle trafficking events for activated receptors. Expression of a GAP-defective mutant, p85R274A, resulted in sustained levels of activated platelet-derived growth factor receptors (PDGFRs) and enhanced downstream signaling. In this report we have characterized Rab5- and Rab4-mediated PDGFR trafficking in cells expressing wild type p85 and GAP-defective mutant p85R274A. Wild type p85 overexpressing cells had slower PDGFR trafficking consistent with enhanced GAP activity deactivating Rab5 and Rab4 to block their vesicle trafficking functions. Mutant p85R274A expression increased the internalization rate of PDGFRs, a Rab5-dependent process, without preventing PDGFR ubiquitination. Immunofluorescence studies further demonstrated that p85R274A-expressing cells showed Rab5 accumulation at intracellular locations. Pull-down and FRAP (fluorescence recovery after photobleaching) experiments indicate this is likely membrane-associated Rab5-GTP, sustained due to decreased p85 GAP activity for the p85R274A mutant. These cells also had substantial amounts of activated PDGFRs in Rab4-positive recycling endosomes, a compartment that usually contains primarily deactivated/dephosphorylated receptors. Our results suggest that the PDGFR-associated GAP activity of p85 regulates both Rab5 and Rab4 functions in cells to influence the movement of activated PDGFR through endosomal compartments. Disruption of this regulation by p85R274A expression impacts PDGFR phosphorylation/dephosphorylation, degradation kinetics and downstream signaling by altering the time receptors spend in specific intracellular endosomal compartments. These results demonstrate that the p85alpha protein is an important regulator of Rab-mediated PDGFR trafficking, which significantly impacts receptor signaling and degradation.

The SNARE protein SNAP-29 interacts with the GTPase Rab3A: Implications for membrane trafficking in myelinating glia

During myelin formation, vast amounts of specialized membrane proteins and lipids are trafficked toward the growing sheath in cell surface-directed transport vesicles. Soluble N-ethylmaleimide-sensitive factor (NSF) attachment proteins (SNAPs) are important components of molecular complexes required for membrane fusion. We have analyzed the expression profile and molecular interactions of SNAP-29 in the nervous system. In addition to its known enrichment in neuronal synapses, SNAP-29 is abundant in oligodendrocytes during myelination and in noncompact myelin of the peripheral nervous system. By yeast two-hybrid screen and coimmunoprecipitation, we found that the GTPases Rab3A, Rab24, and septin 4 bind to the N-terminal domain of SNAP-29. The interaction with Rab24 or septin 4 was GTP independent. In contrast, interaction between SNAP-29 and Rab3A was GTP dependent, and colocalization was extensive both in synapses and in myelinating glia. In HEK293 cells, cytoplasmic SNAP-29 pools were redistributed upon coexpression with Rab3A, and surface-directed trafficking of myelin proteolipid protein was enhanced by overexpression of SNAP-29 and Rab3A. Interestingly, the abundance of SNAP-29 in sciatic nerves was increased during remyelination and in a rat model of Charcot-Marie-Tooth disease, two pathological situations with increased myelin membrane biogenesis. We suggest that Rab3A may regulate SNAP-29-mediated membrane fusion during myelination.

Identification of the neuroblastoma-amplified gene product as a component of the syntaxin 18 complex implicated in Golgi-to-endoplasmic reticulum retrograde transport

Syntaxin 18, a soluble N-ethylmaleimide-sensitive factor (NSF) attachment protein receptor (SNARE) protein implicated in endoplasmic reticulum (ER) membrane fusion, forms a complex with other SNAREs (BNIP1, p31, and Sec22b) and several peripheral membrane components (Sly1, ZW10, and RINT-1). In the present study, we showed that a peripheral membrane protein encoded by the neuroblastoma-amplified gene (NAG) is a subunit of the syntaxin 18 complex. NAG encodes a protein of 2371 amino acids, which exhibits weak similarity to yeast Dsl3p/Sec39p, an 82-kDa component of the complex containing the yeast syntaxin 18 orthologue Ufe1p. Under conditions favoring SNARE complex disassembly, NAG was released from syntaxin 18 but remained in a p31-ZW10-RINT-1 subcomplex. Binding studies showed that the extreme N-terminal region of p31 is responsible for the interaction with NAG and that the N- and the C-terminal regions of NAG interact with p31 and ZW10-RINT-1, respectively. Knockdown of NAG resulted in a reduction in the expression of p31, confirming their intimate relationship. NAG depletion did not substantially affect Golgi morphology and protein export from the ER, but it caused redistribution of Golgi recycling proteins accompanied by a defect in protein glycosylation. These results together suggest that NAG links between p31 and ZW10-RINT-1 and is involved in Golgi-to-ER transport.

The platelet release reaction: just when you thought platelet secretion was simple

PURPOSE OF REVIEW

In response to agonists produced at vascular lesions, platelets release a host of components from their three granules: dense core, alpha, and lysosome. This releasate activates other platelets, promotes wound repair, and initiates inflammatory responses. Although widely accepted, the specific mechanisms underlying platelet secretion are only now coming to light. This review focuses on the core machinery required for platelet secretion.

RECENT FINDINGS

Proteomic analyses have provided a catalog of the components released from activated platelets. Experiments using a combination of in-vitro secretion assays and knockout mice have led to assignments of both vesicle-soluble N-ethylmaleimide-sensitive fusion protein attachment protein receptor (v-SNARE) and target membrane SNARE to each of the three secretion events. SNARE knockout mice are also proving to be useful models for probing the role of platelet exocytosis in vivo. Other studies are beginning to identify SNARE regulators, which control when and where SNAREs interact during platelet activation.

SUMMARY

A complex set of protein-protein interactions control the membrane fusion events required for the platelet release reaction. SNARE proteins are the core elements but the proteins that control SNARE interactions represent key points at which platelet signaling cascades could affect secretion and thrombosis.

Membrane fusion by VAMP3 and plasma membrane t-SNAREs

Pairing of SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptor) proteins on vesicles (v-SNAREs) and SNARE proteins on target membranes (t-SNAREs) mediates intracellular membrane fusion. VAMP3/cellubrevin is a v-SNARE that resides in recycling endosomes and endosome-derived transport vesicles. VAMP3 has been implicated in recycling of transferrin receptors, secretion of alpha-granules in platelets, and membrane trafficking during cell migration. Using a cell fusion assay, we examined membrane fusion capacity of the ternary complexes formed by VAMP3 and plasma membrane t-SNAREs syntaxin1, syntaxin4, SNAP-23 and SNAP-25. VAMP3 forms fusogenic pairing with t-SNARE complexes syntaxin1/SNAP-25, syntaxin1/SNAP-23 and syntaxin4/SNAP-25, but not with syntaxin4/SNAP-23. Deletion of the N-terminal domain of syntaxin4 enhanced membrane fusion more than two fold, indicating that the N-terminal domain negatively regulates membrane fusion. Differential membrane fusion capacities of the ternary v-/t-SNARE complexes suggest that transport vesicles containing VAMP3 have distinct membrane fusion kinetics with domains of the plasma membrane that present different t-SNARE proteins.

SNAREs — engines for membrane fusion

Since the discovery of SNARE proteins in the late 1980s, SNAREs have been recognized as key components of protein complexes that drive membrane fusion. Despite considerable sequence divergence among SNARE proteins, their mechanism seems to be conserved and is adaptable for fusion reactions as diverse as those involved in cell growth, membrane repair, cytokinesis and synaptic transmission. A fascinating picture of these robust nanomachines is emerging.

Increased mobility in the membrane targeting PX domain induced by phosphatidylinositol 3-phosphate

Phosphoinositides (PIs) are concentrated in specific subcellular membranes in order to recruit and regulate cytosolic proteins responsible for vesicular trafficking, cytoskeletal rearrangement, and eukaryotic cell growth, differentiation, and survival. Phox homology (PX) domains are found in proteins that are integral players in endocytic pathways. For example, Vam7p is targeted by its PX domain to phosphatidylinositol 3-phosphate [PtdIns(3)P] in the yeast vacuole, where it interacts with other SNARE proteins and GTPases of the vesicular membrane fusion machinery. Although several PX structures have been solved, the role of dynamics in their interactions with membrane lipids is unclear. Here, we present the first detailed characterization of the backbone dynamics of a PX domain, that of Vam7p, in the presence and absence of its ligand. The structure appears to tumble more rapidly in solution upon binding PtdIns(3)P, revealing a conformational change that includes adjustments in the flexible membrane insertion loop (MIL). The flexibilities of the MIL and domain termini are pronounced in both states, while the alpha1 and alpha2 helices are rigid. Dynamic effects are spread across the binding pocket, with PtdIns(3)P inducing altered mobility of different residues on multiple timescales, including a shift in the MIL to slower timescale motions. The bound state is more dynamic overall, particularly in the beta-sheet lobe, which packs against the ligand's 3-phosphate. Thus, the induced dynamic and structural effects are transduced from the buried heart of the binding pocket in the helical lobe through the beta-sheet lobe to the exposed surface of the bilayer-inserted protein.

A Multigene Family Encoding R-SNAREs in the Ciliate Paramecium tetraurelia

SNARE proteins (soluble N-ethylmaleimide-sensitive factor attachment protein receptors) mediate membrane interactions and are conventionally divided into Q-SNAREs and R-SNAREs according to the possession of a glutamine or arginine residue at the core of their SNARE domain. Here, we describe a set of R-SNAREs from the ciliate Paramecium tetraurelia consisting of seven families encoded by 12 genes that are expressed simultaneously. The complexity of the endomembrane system in Paramecium can explain this high number of genes. All P. tetraurelia synaptobrevins (PtSybs) possess a SNARE domain and show homology to the Longin family of R-SNAREs such as Ykt6, Sec22 and tetanus toxin-insensitive VAMP (TI-VAMP). We localized four exemplary PtSyb subfamilies with GFP constructs and antibodies on the light and electron microscopic level. PtSyb1-1, PtSyb1-2 and PtSyb3-1 were found in the endoplasmic reticulum, whereas PtSyb2 is localized exclusively in the contractile vacuole complex. PtSyb6 was found cytosolic but also resides in regularly arranged structures at the cell cortex (parasomal sacs), the cytoproct and oral apparatus, probably representing endocytotic compartments. With gene silencing, we showed that the R-SNARE of the contractile vacuole complex, PtSyb2, functions to maintain structural integrity as well as functionality of the osmoregulatory system but also affects cell division.

Progressively reduced synaptic vesicle pool size in cultured neurons derived from neuronal ceroid lipofuscinosis-1 knockout mice

The neuronal ceroid lipofuscinoses are a newly-recognized group of lysosomal storage disorders in which neurodegeneration predominates. The pathophysiological basis for this is unknown. In the current paper, we sought to determine whether neurons that lack the enzyme responsible for the infantile form of neuronal ceroid lipofuscinosis (INCL) display abnormalities in culture that could be related to the clinical disorder. Electrophysiological and fluorescent dye studies were performed using cortical neuronal cultures established from postnatal day 2 palmitoyl-protein thioesterase-1 (Ppt1) knockout mice. We found a 30% reduction in synaptic vesicle number per bouton that was progressive with time in culture as well as an elevation in lysosomal pH, whereas a number of passive and active membrane properties of the neurons were normal. The reduction in vesicle pool size was also reflected in a decrease in the frequency of miniature synaptic currents. The progressive and gradual decline in vesicle numbers and miniature event frequency we observed here may be an early indicator of synapse degeneration, in keeping with observations during competitive stimulation at the neuromuscular junction or age-related synapse elimination recently reported by others. PPT1 did not colocalize with synaptic vesicle or synapse markers, suggesting that lysosomal dysfunction leads indirectly to the synaptic abnormalities. We conclude that from an early age, neurons deficient in PPT1 enzyme activity display intrinsically abnormal properties that could potentially explain key features of the clinical disease, such as myoclonus and seizures.

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.

The SNARE Ykt6 is released from yeast vacuoles during an early stage of fusion

The farnesylated SNARE (N-ethylmaleimide-sensitive factor attachment protein receptor) Ykt6 mediates protein palmitoylation at the yeast vacuole by means of its amino-terminal longin domain. Ykt6 is localized equally to membranes and the cytosol, although it is unclear how this distribution is mediated. We now show that Ykt6 is released efficiently from vacuoles during an early stage of yeast vacuole fusion. This release is dependent on the disassembly of vacuolar SNAREs (priming). In recent literature, it had been demonstrated for mammalian Ykt6 that the membrane-bound form is both palmitoylated and farnesylated at its carboxy-terminal CAAX box, whereas soluble Ykt6 is only farnesylated. In agreement with this, we find that yeast Ykt6 becomes palmitoylated in vitro at its C-terminal CAAX motif. Mutagenesis of the potential palmitoylation site in yeast Ykt6 prevents stable membrane association and is lethal. On the basis of these and other findings, we speculate that Ykt6 is released from membranes by depalmitoylation. Such a mechanism could enable recycling of this lipid-anchored SNARE from the vacuole independent of retrograde transport.

SNAREs and traffic

SNAREs (soluble N-ethylmaleimide-sensitive factor attachment protein receptors) are now generally accepted to be the major players in the final stage of the docking and the subsequent fusion of diverse vesicle-mediated transport events. The SNARE-mediated process is conserved evolutionally from yeast to human, as well as mechanistically and structurally across different transport events in eukaryotic cells. In the post-genomic era, a fairly complete list of "all" SNAREs in several organisms (including human) can now be made. This review aims to summarize the key properties and the mechanism of action of SNAREs in mammalian cells.

Ca2+-regulated exocytosis and SNARE function

Secretory vesicles formed at the trans-Golgi network of neuroendocrine and endocrine cells must undergo several steps, such as translocation, docking and priming, before they are ready to fuse with the plasma membrane and deliver their cargo into the extracellular space. This process is called regulated exocytosis and is controlled by Ca(2+) (using synaptotagmin) and mediated by SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptors) proteins. Recent studies from three leading laboratories reveal novel details about the mechanism by which Ca(2+) and SNAREs regulate this complex process. These findings highlight the roles of both SNAP25 (synaptosome-associated protein of 25kD), one of the SNARE proteins, and CAPS (Ca(2+)-dependent activator protein for secretion), a Ca(2+)-sensor protein, in vesicle priming, depriming and fusion.

Intramolecular protein-protein and protein-lipid interactions control the conformation and subcellular targeting of neuronal Ykt6

Although the membrane-trafficking functions of most SNAREs are conserved from yeast to humans, some mammalian SNAREs have evolved specialized functions unique to multicellular life. The mammalian homolog of the prenylated yeast SNARE Ykt6p might be one such example, because rat Ykt6 is highly expressed only in brain neurons. Furthermore, neuronal Ykt6 displayed a remarkably specialized, punctate localization that did not overlap appreciably with conventional compartments of the endomembrane system, suggesting that Ykt6 might be involved in a pathway unique to or specifically modified for neuronal function. Targeting of Ykt6 to its unique subcellular location was directed by its profilin-like longin domain. We have taken advantage of high-resolution structural data available for the yeast Ykt6p longin domain to examine mechanisms by which the mammalian longin domain controls Ykt6 conformation and subcellular targeting. We found that the overall tertiary structure of the longin domain, not sequence-specific surface features, drives direct targeting to the Ykt6 punctate structures. However, several sequence-specific surface features of the longin domain indirectly regulate Ykt6 localization through intramolecular interactions that mask otherwise-dominant targeting signals on the SNARE motif and lipid groups. Specifically, two hydrophobic binding pockets, one on each face of the longin domain, and one mixed hydrophobic/charged surface, participate in protein-protein interactions with the SNARE motif and protein-lipid interactions with the lipid group(s) at the molecule's C-terminus. One of the hydrophobic pockets suppresses protein-palmitoylation-dependent mislocalization of Ykt6 to the plasma membrane. The Ykt6 intramolecular interactions would be predicted to create a compact, closed conformation of the SNARE that prevents promiscuous targeting interactions and premature insertion into membranes. Interestingly, both protein-protein and protein-lipid interactions are required for a tightly closed conformation and normal targeting.

Assay and Stimulation of the Rab5 GTPase by the p85α Subunit of Phosphatidylinositol 3‐Kinase

Rab5 is a small monomeric GTPase involved in regulating vesicle fusion events during receptor-mediated endocytosis. During endocytosis of the activated platelet-derived growth factor receptor, phosphatidylinositol 3-kinase (PI3K) remains associated with the receptor. We have found that the p85 alpha subunit of PI3K binds directly to Rab5 and possesses GTPase-activating protein (GAP) activity toward Rab5. We describe two methods used to characterize the GAP activity of p85 toward the Rab5 protein. The first method is a steady-state GAP assay, used to show that the p85 alpha protein has GAP activity toward Rab5. The second method is a single turnover GAP assay and measures changes in the catalytic rate of Rab5 GTP hydrolysis with or without the p85 alpha protein.

Imaging single membrane fusion events mediated by SNARE proteins

Using total internal reflection fluorescence microscopy, we have developed an assay to monitor individual fusion events between proteoliposomes containing vesicle soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs) and a supported planar bilayer containing cognate target SNAREs. Approach, docking, and fusion of individual vesicles to the target membrane were quantified by delivery and subsequent lateral spread of fluorescent phospholipids from the vesicle membrane into the target bilayer. Fusion probability was increased by raising divalent cations (Ca2+ and Mg2+). Fusion of individual vesicles initiated in <100 ms after the rise of Ca2+ and membrane mixing was complete in 300 ms. Removal of the N-terminal H(abc) domain of syntaxin 1A increased fusion probability >30-fold compared to the full-length protein, but even in the absence of the H(abc) domain, vesicle fusion was still enhanced in response to Ca2+ increase. Our observations establish that the SNARE core complex is sufficient to fuse two opposing membrane bilayers at a speed commensurate with most membrane fusion processes in cells. This real-time analysis of single vesicle fusion opens the door to mechanistic studies of how SNARE and accessory proteins regulate fusion processes in vivo.

The specificity of SNARE-dependent fusion is encoded in the SNARE motif

Soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) proteins constitute the core of the fusion machinery, and isolated SNAREs fuse membranes with exquisite specificity by cognate pairing. Most SNAREs have a membrane-spanning region, an N-terminal domain, and a membrane proximal SNARE motif domain. Although the SNARE motif is critical for SNARE complex formation, is it the sole determinant of the specificity of SNARE-dependent fusion? To test this, we make use of a SNARE complex functioning in the late endosomal compartment in yeast. Studying this complex and the previously identified early endosomal SNARE complex, we find that the specificity of fusion resides in the SNARE motifs.

The mechanisms of vesicle budding and fusion

Genetic and biochemical analyses of the secretory pathway have produced a detailed picture of the molecular mechanisms involved in selective cargo transport between organelles. This transport occurs by means of vesicular intermediates that bud from a donor compartment and fuse with an acceptor compartment. Vesicle budding and cargo selection are mediated by protein coats, while vesicle targeting and fusion depend on a machinery that includes the SNARE proteins. Precise regulation of these two aspects of vesicular transport ensures efficient cargo transfer while preserving organelle identity.