Syntaxin 6: the promiscuous behaviour of a SNARE protein

Vesicle flow within the cell is responsible for the dynamic maintenance of and communication between intracellular compartments. In addition, vesicular transport is crucial for communication between the cell and its surrounding environment. The ability of a vesicle to recognise and fuse with an appropriate compartment or vesicle is determined by its protein and lipid composition as well as by proteins in the cytosol. SNARE proteins present on both vesicle as well as target organelle membranes provide one component necessary for the process of membrane fusion. While in mammalian cells the main focus of interest about SNARE function has centred on those involved in exocytosis, recent data on SNAREs involved in intracellular membrane-trafficking steps have provided a deeper insight into the properties of these proteins. We take, as an example, the promiscuous SNARE syntaxin 6, a SNARE involved in multiple membrane fusion events. The properties of syntaxin 6 reveal similarities but also differences in the behaviour of intracellular SNAREs and the highly specialised exocytotic SNARE molecules.     

Munc18-2/syntaxin3 complexes are spatially separated from syntaxin3-containing SNARE complexes

Exocytosis of mast cell granules requires a vesicular- and plasma membrane-associated fusion machinery. We examined the distribution of SNARE membrane fusion and Munc18 accessory proteins in lipid rafts of RBL mast cells. SNAREs were found either excluded (syntaxin2), equally distributed between raft and non-raft fractions (syntaxin4, VAMP-8, VAMP-2), or selectively enriched in rafts (syntaxin3, SNAP-23). Syntaxin4-binding Munc18-3 was absent, whereas small amounts of the syntaxin3-interacting partner Munc18-2 consistently distributed into rafts. Cognate SNARE complexes of syntaxin3 with SNAP-23 and VAMP-8 were enriched in rafts, whereas Munc18-2/syntaxin3 complexes were excluded. This demonstrates a spatial separation between these two types of complexes and suggests that Munc18-2 acts in a step different from SNARE complex formation and fusion.     

Sec1p binds to SNARE complexes and concentrates at sites of secretion.

Proteins of the Sec1 family have been shown to interact with target-membrane t-SNAREs that are homologous to the neuronal protein syntaxin. We demonstrate that yeast Sec1p coprecipitates not only the syntaxin homologue Ssop, but also the other two exocytic SNAREs (Sec9p and Sncp) in amounts and in proportions characteristic of SNARE complexes in yeast lysates. The interaction between Sec1p and Ssop is limited by the abundance of SNARE complexes present in sec mutants that are defective in either SNARE complex assembly or disassembly. Furthermore, the localization of green fluorescent protein (GFP)-tagged Sec1p coincides with sites of vesicle docking and fusion where SNARE complexes are believed to assemble and function. The proposal that SNARE complexes act as receptors for Sec1p is supported by the mislocalization of GFP-Sec1p in a mutant defective for SNARE complex assembly and by the robust localization of GFP-Sec1p in a mutant that fails to disassemble SNARE complexes. The results presented here place yeast Sec1p at the core of the exocytic fusion machinery, bound to SNARE complexes and localized to sites of secretion.     

Evidence that late-endosomal SNARE multimerization complex is promoted by transmembrane segments

Assembly of SNARE proteins into quaternary complexes is a critical step in membrane docking and fusion. Here, we have studied the influence of the transmembrane segments on formation of the late endosomal SNARE complex. The complex was assembled in vitro from full-length recombinant SNAREs and from mutants, where the transmembrane segments were either deleted or replaced by oligo-alanine sequences. We show that endobrevin, syntaxin 7, syntaxin 8, and vti1b readily form a complex. This complex forms a dimer as well as multimeric structures. Interestingly, the natural transmembrane segments accelerate the conversion of the quaternary complex to the dimeric form and are essential for multimerization. These in vitro results suggest that the transmembrane segments are responsible for supramolecular assembly of the endosomal SNARE complex.     

Ternary SNARE complexes are enriched in lipid rafts during mast cell exocytosis

Lipid rafts are membrane microdomains rich in cholesterol and glycosphingolipids that have been implicated in the regulation of intracellular protein trafficking. During exocytosis, a class of proteins termed SNAREs mediate secretory granule-plasma membrane fusion. To investigate the role of lipid rafts in secretory granule exocytosis, we examined the raft association of SNARE proteins and SNARE complexes in rat basophilic leukemia (RBL) mast cells. The SNARE protein SNAP-23 co-localized with a lipid raft marker and was present in detergent-insoluble lipid raft microdomains in RBL cells. By contrast, only small amounts (<20%) of the plasma membrane SNARE syntaxin 4 or the granule-associated SNARE vesicle-associated membrane protein (VAMP)-2 were present in these microdomains. Despite this, essentially all syntaxin 4 and most of VAMP-2 in these rafts were present in SNARE complexes containing SNAP-23, while essentially none of these complexes were present in nonraft membranes. Whereas SNAP-23 is membrane anchored by palmitoylation, the association of the transmembrane protein syntaxin 4 with lipid rafts was because of its binding to SNAP-23. After stimulating mast cells exocytosis, the amount of syntaxin 4 and VAMP-2 present in rafts increased twofold, and these proteins were now present in raft-associated phospho-SNAP-23/syntaxin 4/VAMP-2 complexes, revealing differential association of SNARE fusion complexes during the process of regulated exocytosis.     

Sly1 protein bound to Golgi syntaxin Sed5p allows assembly and contributes to specificity of SNARE fusion complexes

Fusion of transport vesicles with their target organelles involves specific membrane proteins, SNAREs, which form tight complexes bridging the membranes to be fused. Evidence from yeast and mammals indicates that Sec1 family proteins act as regulators of membrane fusion by binding to the target membrane SNAREs. In experiments with purified proteins, we now made the observation that the ER to Golgi core SNARE fusion complex could be assembled on syntaxin Sed5p tightly bound to the Sec1-related Sly1p. Sly1p also bound to preassembled SNARE complexes in vitro and was found to be part of a vesicular/target membrane SNARE complex immunoprecipitated from yeast cell lysates. This is in marked contrast to the exocytic SNARE assembly in neuronal cells where high affinity binding of N-Sec1/Munc-18 to syntaxin 1A precluded core SNARE fusion complex formation. We also found that the kinetics of SNARE complex formation in vitro with either Sly1p-bound or free Sed5p was not significantly different. Importantly, several presumably nonphysiological SNARE complexes easily generated with Sed5p did not form when the syntaxin was first bound to Sly1p. This indicates for the first time that a Sec1 family member contributes to the specificity of SNARE complex assembly.     

Lysophosphatidylcholine inhibits membrane-associated SNARE complex disassembly

In cells, N-ethylmaleimide-sensitive factor (NSF) attachment protein receptors called SNAREs are involved in membrane fusion. In neurons, for example, target membrane proteins SNAP-25 and syntaxin called t-SNAREs present at the pre-synaptic membrane, and a synaptic vesicle-associated membrane protein (VAMP) or v-SNARE, is part of the conserved protein complex involved in neurotransmission. Cholesterol and LPC (L-α-lysophosphatidylcholine) are known to contribute to the negative and positive curvature respectively of membranes. In this study, using purified recombinant neuronal membrane-associated SNAREs, we demonstrate for the first time that membrane-curvature-influencing lipids profoundly influence SNARE complex disassembly. Exposure of cholesterol-associated t-SNARE and v-SNARE liposome mixtures to NSF-ATP results in dissociated vesicles. In contrast, exposure of LPC-associated t-SNARE and v-SNARE liposome mixtures to NSF-ATP, results in inhibition of t-/v-SNARE disassembly and the consequent accumulation of clustered vesicles. Similarly, exposure of isolated rat brain slices and pancreas to cholesterol or LPC, also demonstrates LPC-induced inhibition of SNARE complex disassembly. Earlier studies demonstrate a strong correlation between altered plasma LPC levels and cancer. The altered plasma LPC levels observed in various cancers may in part contribute to defects in SNARE assembly-disassembly and membrane fusion, consequently affecting protein maturation and secretion in cancer cells.     

All three components of the neuronal SNARE complex contribute to secretory vesicle docking

Before exocytosis, vesicles must first become docked to the plasma membrane. The SNARE complex was originally hypothesized to mediate both the docking and fusion steps in the secretory pathway, but previous electron microscopy (EM) studies indicated that the vesicular SNARE protein synaptobrevin (syb) was dispensable for docking. In this paper, we studied the function of syb in the docking of large dense-core vesicles (LDCVs) in live PC12 cells using total internal reflection fluorescence microscopy. Cleavage of syb by a clostridial neurotoxin resulted in significant defects in vesicle docking in unfixed cells; these results were confirmed via EM using cells that were prepared using high-pressure freezing. The membrane-distal portion of its SNARE motif was critical for docking, whereas deletion of a membrane-proximal segment had little effect on docking but diminished fusion. Because docking was also inhibited by toxin-mediated cleavage of the target membrane SNAREs syntaxin and SNAP-25, syb might attach LDCVs to the plasma membrane through N-terminal assembly of trans-SNARE pairs.     

Evidence that late-endosomal SNARE multimerization complex is promoted by transmembrane segments

Assembly of SNARE proteins into quaternary complexes is a critical step in membrane docking and fusion. Here, we have studied the influence of the transmembrane segments on formation of the late endosomal SNARE complex. The complex was assembled in vitro from full-length recombinant SNAREs and from mutants, where the transmembrane segments were either deleted or replaced by oligo-alanine sequences. We show that endobrevin, syntaxin 7, syntaxin 8, and vti1b readily form a complex. This complex forms a dimer as well as multimeric structures. Interestingly, the natural transmembrane segments accelerate the conversion of the quaternary complex to the dimeric form and are essential for multimerization. These in vitro results suggest that the transmembrane segments are responsible for supramolecular assembly of the endosomal SNARE complex.     

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.     

Evidence for regulation of ER/Golgi SNARE complex formation by hsc70 chaperones

SNARE proteins control intracellular membrane fusion through formation of membrane-bridging helix bundles of amphipathic SNARE motifs. Repetitive cycles of membrane fusion likely involve repetitive folding/unfolding of the SNARE motif helical structure. Despite these conformational demands, little is known about conformational regulation of SNAREs by other proteins. Here we demonstrate that hsc70 chaperones stimulate in vitro SNARE complex formation among the ER/Golgi SNAREs syntaxin 5, membrin, rbetl and sec22b, under conditions in which assembly is normally inhibited. Thus, molecular chaperones can render the SNARE motif more competent for assembly. Partially purified hsc70 fractions from brain cytosol had higher specific activities than fully purified hsc70, suggesting the involvement of unidentified cofactors. Using chemical crosslinking of cells followed by immunoprecipitation, we found that hsc70 was associated with ER/Golgi SNAREs in vivo. Consistent with a modulatory role for hsc70 in transport, we found that excess hsc70 specifically inhibited ER-to-Golgi transport in permeabilized cells.     

SNARE interactions are not selective. Implications for membrane fusion specificity

The SNARE hypothesis proposes that membrane trafficking specificity is mediated by preferential high affinity interactions between particular v (vesicle membrane)- and t (target membrane)-SNARE combinations. The specificity of interactions among a diverse set of SNAREs, however, is unknown. We have tested the SNARE hypothesis by analyzing potential SNARE complexes between five proteins of the vesicle-associated membrane protein (VAMP) family, three members of the synaptosome-associated protein-25 (SNAP-25) family and three members of the syntaxin family. All of the 21 combinations of SNAREs tested formed stable complexes. Sixteen were resistant to SDS denaturation, and most complexes thermally denatured between 70 and 90 degreesC. These results suggest that the specificity of membrane fusion is not encoded by the interactions between SNAREs.     

Plasma membrane targeting of exocytic SNARE proteins

SNARE proteins play a central role in the process of intracellular membrane fusion. Indeed, the interaction of SNAREs present on two opposing membranes is generally believed to provide the driving force to initiate membrane fusion. Eukaryotic cells express a large number of SNARE isoforms, and the function of individual SNAREs is required for specific intracellular fusion events. Exocytosis, the fusion of secretory vesicles with the plasma membrane, employs the proteins syntaxin and SNAP-25 as plasma membrane SNAREs. As a result, exocytosis is dependent upon the targeting of these proteins to the plasma membrane; however, the mechanisms that underlie trafficking of exocytic syntaxin and SNAP-25 proteins to the cell surface are poorly understood. The intracellular trafficking itinerary of these proteins is particularly intriguing as syntaxins are tail-anchored (or Type IV) membrane proteins, whereas SNAP-25 is anchored to membranes via a central palmitoylated domain-there is no common consensus for the trafficking of such proteins within the cell. In this review, we discuss the plasma membrane targeting of these essential exocytic SNARE proteins.     

p115–SNARE Interactions: A Dynamic Cycle of p115 Binding Monomeric SNARE Motifs and Releasing Assembled Bundles

Tethering factors regulate the targeting of membrane‐enclosed vesicles under the control of Rab GTPases. p115, a golgin family tether, has been shown to participate in multiple stages of ER/Golgi transport. Despite extensive study, the mechanism of action of p115 is poorly understood. SNARE proteins make up the machinery for membrane fusion, and strong evidence shows that function of p115 is directly linked to its interaction with SNAREs. Using a gel filtration binding assay, we have demonstrated that in solution p115 stably interacts with ER/Golgi SNAREs rbet1 and sec22b, but not membrin and syntaxin 5. These binding preferences stemmed from selectivity of p115 for monomeric SNARE motifs as opposed to SNARE oligomers. Soluble monomeric rbet1 can compete off p115 from coat protein II (COPII) vesicles. Furthermore, excess p115 inhibits p115 function in trafficking. We conclude that monomeric SNAREs are a major binding site for p115 on COPII vesicles, and that p115 dissociates from its SNARE partners upon SNAREpin assembly. Our results suggest a model in which p115 forms a mixed p115/SNARE helix bundle with a monomeric SNARE, facilitates the binding activity and/or concentration of the SNARE at prefusion sites and is subsequently ejected as SNARE complex formation and fusion proceed.      

A Targeted siRNA Screen to Identify SNAREs Required for Constitutive Secretion in Mammalian Cells

The role of SNAREs in mammalian constitutive secretion remains poorly defined. To address this, we have developed a novel flow cytometry‐based assay for measuring constitutive secretion and have performed a targeted SNARE and Sec1/Munc18 (SM) protein‐specific siRNA screen (38 SNAREs, 4 SNARE‐like proteins and 7 SM proteins). We have identified the endoplasmic reticulum (ER)/Golgi SNAREs syntaxin 5, syntaxin 17, syntaxin 18, GS27, SLT1, Sec20, Sec22b, Ykt6 and the SM protein Sly1, along with the post‐Golgi SNAREs SNAP‐29 and syntaxin 19, as being required for constitutive secretion. Depletion of SNAP‐29 or syntaxin 19 causes a decrease in the number of fusion events at the cell surface and in SNAP‐29‐depleted cells causes an increase in the number of docked vesicles at the plasma membrane as determined by total internal reflection fluorescence (TIRF) microscopy. Analysis of syntaxin 19‐interacting partners by mass spectrometry indicates that syntaxin 19 can form SNARE complexes with SNAP‐23, SNAP‐25, SNAP‐29, VAMP3 and VAMP8, supporting its role in Golgi to plasma membrane transport or fusion. Surprisingly, we have failed to detect any requirement for a post‐Golgi‐specific R‐SNARE in this process.     

The Habc domain and the SNARE core complex are connected by a highly flexible linker

Syntaxin 1a is a member of the SNARE superfamily of small, mostly membrane-bound proteins that mediate membrane fusion in all eukaryotic cells. Upon membrane fusion, syntaxin 1 forms a stable complex with its partner SNAREs. Syntaxin contains a C-terminal transmembrane domain, an adjacent SNARE motif that interacts with its partner SNAREs, and an N-terminal Habc domain. The Habc domain reversibly folds back upon the SNARE motif, resulting in a "closed" conformation that is stabilized by binding to the protein munc18. The SNARE motif and the Habc domain are separated by a linker region of about 40 amino acids. When syntaxin is complexed with munc18, the linker is structured and consists of a mix of turns and small alpha-helices. When syntaxin is complexed with its partner SNAREs, the Habc domain is dissociated, but the structure of the linker region is not known. Here we used site-directed spin labeling and EPR spectroscopy to determine the structure of the linker region of syntaxin in the SNARE complex. We found that the entire linker region of syntaxin is unstructured except for three residues at the N-terminal and six residues at the C-terminal boundary whereas the structures of the flanking regions in the Habc domain and the SNARE motif correspond to the high-resolution structures of the isolated fragments. We conclude that the linker region exhibits a high degree of conformational flexibility.     

Structure and function of SNARE and SNARE-interacting proteins

This review focuses on the so-called SNARE (soluble N-ethyl maleimide sensitive factor attachment protein receptor) proteins that are involved in exocytosis at the pre-synpatic plasma membrane. SNAREs play a role in docking and fusion of synaptic vesicles to the active zone, as well as in the Ca2+-triggering step itself, most likely in combination with the Ca2+ sensor synaptotagmin. Different SNARE domains are involved in different processes, such as regulation, docking, and fusion. SNAREs exhibit multiple configurational, conformational, and oliogomeric states. These different states allow SNAREs to interact with their matching SNARE partners, auxiliary proteins, or with other SNARE domains, often in a mutually exclusive fashion. SNARE core domains undergo progressive disorder to order transitions upon interactions with other proteins, culminating with the fully folded post-fusion (cis) SNARE complex. Physiological concentrations of neuronal SNAREs can juxtapose membranes, and promote fusion in vitro under certain conditions. However, significantly more work will be required to reconstitute an in vitro system that faithfully mimics the Ca2+-triggered fusion of a synaptic vesicle at the active zone.     

Vesicle-associated membrane protein 8 (VAMP8) is a SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptor) selectively required for sequential granule-to-granule fusion

Compound exocytosis is found in many cell types and is the major form of regulated secretion in acinar and mast cells. Its key characteristic is the homotypic fusion of secretory granules. These then secrete their combined output through a single fusion pore to the outside. The control of compound exocytosis remains poorly understood. Although soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs) such as syntaxin 2, SNAP23 (synaptosome-associated protein of 23 kDa), and SNAP25 have been suggested to play a role, none has been proven. Vesicle-associated membrane protein 8 (VAMP8) is a SNARE first associated with endocytic processes but more recently has been suggested as an R-SNARE in regulated exocytosis. Secretion in acinar cells is reduced when VAMP8 function is inhibited and is less in VAMP8 knock-out mice. Based on electron microscopy experiments, it was suggested that VAMP8 may be involved in compound exocytosis. Here we have tested the hypothesis that VAMP8 controls homotypic granule-to-granule fusion during sequential compound exocytosis. We use a new assay to distinguish primary fusion events (fusion with the cell membrane) from secondary fusion events (granule-granule fusion). Our data show the pancreatic acinar cells from VAMP8 knock-out animals have a specific reduction in secondary granule fusion but that primary granule fusion is unaffected. Furthermore, immunoprecipitation experiments show syntaxin 2 association with VAMP2, whereas syntaxin 3 associates with VAMP8. Taken together our data indicate that granule-to-granule fusion is regulated by VAMP8 containing SNARE complexes distinct from those that regulate primary granule fusion.     

Structure and Dynamics of a Two‐Helix SNARE Complex in Live Cells

SNAREs are clustered membrane proteins essential for intracellular fusion steps. During fusion, three to four SNAREs with a Q_a‐, Q_b‐, Q_c‐ and R‐SNARE‐motif form a complex. The core complex represents a Q_aQ_bQ_cR‐SNARE‐motif bundle, most certainly assembling in steps. However, to date it is unknown which intermediate SNARE complex observed in vitro also exists in vivo. Here we have applied comparative fluorescence recovery after photobleaching (FRAP)‐studies as a novel approach for studying in intact cells a SNARE interaction involved in synaptic vesicle fusion [catalyzed by syntaxin 1A (Q_a), SNAP25 (Q_b/Q_c) and synaptobrevin 2 (R)]. We find that the Q_b‐SNARE‐motif of SNAP25 interacts reversibly with clustered syntaxin. The interaction requires most of the alpha helical Q_b‐SNARE‐motif and depends on its position within the molecule. We conclude that a zippered Q_aQ_b‐SNARE complex represents a short‐lived SNARE intermediate in intact cells, most likely providing an initial molecular platform toward membrane fusion.     

Multiple SNARE interactions of an SM protein: Sed5p/Sly1p binding is dispensable for transport

Sec1/Munc18 (SM) proteins are central to intracellular transport and neurotransmitter release but their exact role is still elusive. Several SM proteins, like the neuronal N-Sec1 and the yeast Sly1 protein, bind their cognate t-SNAREs with high affinity. This has been thought to be critical for their function. Here, we show that various mutant forms of Sly1p and the Golgi-localized syntaxin Sed5p, which abolish their high-affinity interaction, are fully functional in vivo, indicating that the tight interaction of the two molecules per se is not relevant for proper function. Mutant Sly1p unable to bind Sed5p is excluded from core SNARE complexes, also demonstrating that Sly1p function is not directly coupled to assembled SNARE complexes thought to execute membrane fusion. We also find that wild-type Sly1p and mutant Sly1p unable to bind Sed5p directly interact with selected ER-to-Golgi and intra-Golgi nonsyntaxin SNAREs. The newly identified, direct interactions of the SM protein with nonsytaxin SNAREs might provide a molecular mechanism by which SNAREs can be activated to engage in pairing and assemble into fusogenic SNARE complexes.