Membrane fusion: grappling with SNARE and SM proteins

The two universally required components of the intracellular membrane fusion machinery, SNARE and SM (Sec1/Munc18-like) proteins, play complementary roles in fusion. Vesicular and target membrane-localized SNARE proteins zipper up into an alpha-helical bundle that pulls the two membranes tightly together to exert the force required for fusion. SM proteins, shaped like clasps, bind to trans-SNARE complexes to direct their fusogenic action. Individual fusion reactions are executed by distinct combinations of SNARE and SM proteins to ensure specificity, and are controlled by regulators that embed the SM-SNARE fusion machinery into a physiological context. This regulation is spectacularly apparent in the exquisite speed and precision of synaptic exocytosis, where synaptotagmin (the calcium-ion sensor for fusion) cooperates with complexin (the clamp activator) to control the precisely timed release of neurotransmitters that initiates synaptic transmission and underlies brain function.

Regulation of the endosomal SNARE protein syntaxin 7 by colony-stimulating factor 1 in macrophages

Colony-stimulating factor 1 (CSF-1) is the main growth factor controlling the development of macrophages from myeloid progenitor cells. However, CSF-1 also regulates some of the key effector functions of macrophages (e.g., phagocytosis and cytokine secretion). The endosomal SNARE protein syntaxin 7 (Stx7) regulates vesicle trafficking events involved in phagocytosis and cytokine secretion. Therefore, we investigated the ability of CSF-1 to regulate Stx7. CSF-1 upregulated Stx7 expression in primary mouse macrophages; it also upregulated expression of its SNARE partners Vti1b and VAMP8 but not Stx8. Additionally, CSF-1 induced the rapid serine phosphorylation of Stx7 and enhanced its binding to Vti1b, Stx8, and VAMP8. Bioinformatics analysis and results from experiments with kinase inhibitors suggested the CSF-1-induced phosphorylation of Stx7 was mediated by protein kinase C and Akt in response to phosphatidylinositol 3-kinase activation. Based on mutagenesis studies, CSF-1 appeared to increase the binding of Stx7 to its SNARE partners by inducing the phosphorylation of serine residues in the Habc domain and/or "linker" region of Stx7. Thus, CSF-1 is a key regulator of Stx7 expression and function in macrophages. Furthermore, the effects of CSF-1 on Stx7 may provide a mechanism for the regulation of macrophage effector functions by CSF-1.

The tyrosine phosphorylation of Munc18c induces a switch in binding specificity from syntaxin 4 to Doc2beta

Glucose-stimulated insulin secretion is mediated by syntaxin 4-based SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptor) protein complexes and the Sec1/Munc18 protein Munc18c. Our laboratory recently reported that Munc18c-syntaxin 4 complexes are further regulated by the competitive binding of the double C2 domain protein Doc2beta to Munc18c, although the underlying mechanism for this is unknown. Because the Doc2beta binding region of Munc18c contained residue Tyr-219 and this residue becomes phosphorylated in response to glucose stimulation, we hypothesized that the mechanism would involve Munc18c phosphorylation. Coimmunoprecipitation analyses using detergent lysates prepared from pervanadate-treated MIN6 beta cells revealed that the tyrosine phosphorylation of Munc18c corresponded to a 60% decrease in Munc18c-syntaxin 4 association with a coordinate 2-fold increase in Munc18c-Doc2beta binding. In vitro binding assays identified syntaxin 4 residues 118-194 as sufficient to confer its interaction with Munc18c; residues 118-194 contain the Hc alpha-helix and flexible linker region controlling transition of syntaxins between closed and open conformations. When overexpressed in MIN6 cells, this Hc-linker region functioned as a competitive inhibitor of endogenous syntaxin 4-Munc18c binding, increased syntaxin 4 binding to VAMP2, and significantly enhanced glucose-stimulated secretion. Molecular modeling of these new interactions yielded the predictions 1) that Tyr-219 of Munc18c remains buried under basal conditions in a conformation that is favorable for interaction with "closed" syntaxin 4 and 2) that stimulation leads to changes in syntaxin 4 contacts to facilitate exposure of Munc18c Tyr-219 for phosphorylation and Doc2beta binding.

Accessory proteins stabilize the acceptor complex for synaptobrevin, the 1:1 syntaxin/SNAP-25 complex

Syntaxin/SNAP-25 interactions precede assembly of the ternary SNARE complex that is essential for neurotransmitter release. This binary complex has been difficult to characterize by bulk methods because of the prevalence of a 2:1 dead-end species. Here, using single-molecule fluorescence, we find the structure of the 1:1 syntaxin/SNAP-25 binary complex is variable, with states changing on the second timescale. One state corresponds to a parallel three-helix bundle, whereas other states show one of the SNAP-25 SNARE domains dissociated. Adding synaptobrevin suppresses the dissociated helix states. Remarkably, upon addition of complexin, Munc13, Munc18, or synaptotagmin, a similar effect is observed. Thus, the 1:1 binary complex is a dynamic acceptor for synaptobrevin binding, and accessory proteins stabilize this acceptor. In the cellular environment the binary complex is actively maintained in a configuration where it can rapidly interact with synaptobrevin, so formation is not likely a limiting step for neurotransmitter release.

Structural and biochemical studies of botulinum neurotoxin serotype C1 light chain protease: implications for dual substrate specificity

Clostridial neurotoxins are the causative agents of the neuroparalytic disease botulism and tetanus. They block neurotransmitter release through specific proteolysis of one of the three soluble N-ethylmaleimide-sensitive-factor attachment protein receptors (SNAREs) SNAP-25, syntaxin, and synaptobrevin, which constitute part of the synaptic vesicle fusion machinery. The catalytic component of the clostridial neurotoxins is their light chain (LC), a Zn2+ endopeptidase. There are seven structurally and functionally related botulinum neurotoxins (BoNTs), termed serotype A to G, and tetanus neurotoxin (TeNT). Each of them exhibits unique specificity for their target SNAREs and peptide bond(s) they cleave. The mechanisms of action for substrate recognition and target cleavage are largely unknown. Here, we report structural and biochemical studies of BoNT/C1-LC, which is unique among BoNTs in that it exhibits dual specificity toward both syntaxin and SNAP-25. A distinct pocket (S1') near the active site likely achieves the correct register for the cleavage site by only allowing Ala as the P1' residue for both SNAP-25 and syntaxin. Mutations of this SNAP-25 residue dramatically reduce enzymatic activity. The remote alpha-exosite that was previously identified in the complex of BoNT/A-LC and SNAP-25 is structurally conserved in BoNT/C1. However, mutagenesis experiments show that the alpha-exosite of BoNT/C1 plays a less stringent role in substrate discrimination in comparison to that of BoNT/A, which could account for its dual substrate specificity.

Structure of the Munc18c/Syntaxin4 N-peptide complex defines universal features of the N-peptide binding mode of Sec1/Munc18 proteins

Sec1/Munc18 proteins (SM proteins) bind to soluble NSF attachment protein receptors (SNAREs) and play an essential role in membrane fusion. Divergent modes of regulation have been proposed for different SM proteins indicating that they can either promote or inhibit SNARE assembly. This is in part because of discrete modes of binding that have been described for various SM/SNARE complexes. One mode suggests that SM proteins bind only to Syntaxins (Stx) preventing SNARE assembly, whereas in another they facilitate SNARE assembly and bind to SNARE complexes. The mammalian cell surface SM protein Munc18c binds to an N-peptide in Stx4, and this is compatible with its interaction with SNARE complexes. Here we describe the crystal structure of Munc18c in complex with the Stx4 N-peptide. This structure shows remarkable similarity with a yeast complex indicating that the mode of binding, which can accommodate SNARE complexes, is highly conserved throughout evolution. Modeling reveals the presence of the N-peptide binding mode in most but not all yeast and mammalian SM/Stx pairs, suggesting that it has coevolved to fulfill a specific regulatory function. It is unlikely that the N-peptide interaction alone accounts for the specificity in SM/SNARE binding, implicating other contact surfaces in this function. Together with other data, our results support a sequential two-state model for SM/SNARE binding involving an initial interaction via the Stx N-peptide, which somehow facilitates a second, more comprehensive interaction comprising other contact surfaces in both proteins.

Crystallization and preliminary X-ray diffraction of the Munc18c-syntaxin4 (1-29) complex

The production of diffraction-quality crystals of Munc18c, a protein involved in regulating vesicular exocytosis in mammals, is reported. The diffraction resolution of Munc18c crystals was optimized by (i) cocrystallizing with a peptide fragment of the Munc18c functional binding partner syntaxin4, (ii) using nanolitre free-interface diffusion crystallization-screening chips and microlitre hanging-drop vapour diffusion and (iii) applying a post-crystallization dehydration treatment. Crystals belonging to the cubic space group P2(1)3, with unit-cell parameters a = b = c = 170.8 A, alpha = beta = gamma = 90 degrees , were generated that diffract to 3.7 A resolution on a laboratory X-ray source.

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.

Molecular Identification of 26 Syntaxin Genes and their Assignment to the Different Trafficking Pathways in Paramecium

SNARE proteins have been classified as vesicular (v)- and target (t)-SNAREs and play a central role in the various membrane interactions in eukaryotic cells. Based on the Paramecium genome project, we have identified a multigene family of at least 26 members encoding the t-SNARE syntaxin (PtSyx) that can be grouped into 15 subfamilies. Paramecium syntaxins match the classical build-up of syntaxins, being 'tail-anchored' membrane proteins with an N-terminal cytoplasmic domain and a membrane-bound single C-terminal hydrophobic domain. The membrane anchor is preceded by a conserved SNARE domain of approximately 60 amino acids that is supposed to participate in SNARE complex assembly. In a phylogenetic analysis, most of the Paramecium syntaxin genes were found to cluster in groups together with those from other organisms in a pathway-specific manner, allowing an assignment to different compartments in a homology-dependent way. However, some of them seem to have no counterparts in metazoans. In another approach, we fused one representative member of each of the syntaxin isoforms to green fluorescent protein and assessed the in vivo localization, which was further supported by immunolocalization of some syntaxins. This allowed us to assign syntaxins to all important trafficking pathways in Paramecium.

Mechanism of arachidonic acid action on syntaxin-Munc18

Syntaxin and Munc18 are, in tandem, essential for exocytosis in all eukaryotes. Recently, it was shown that Munc18 inhibition of neuronal syntaxin 1 can be overcome by arachidonic acid, indicating that this common second messenger acts to disrupt the syntaxin-Munc18 interaction. Here, we show that arachidonic acid can stimulate syntaxin 1 alone, indicating that it is syntaxin 1 that undergoes a structural change in the syntaxin 1-Munc18 complex. Arachidonic acid is incapable of dissociating Munc18 from syntaxin 1 and, crucially, Munc18 remains associated with syntaxin 1 after arachidonic-acid-induced syntaxin 1 binding to synaptosomal-associated protein 25 kDa (SNAP25). We also show that the same principle operates in the case of the ubiquitous syntaxin 3 isoform, highlighting the conserved nature of the mechanism of arachidonic acid action. Neuronal soluble N-ethyl maleimide sensitive factor attachment protein receptors (SNAREs) can be isolated from brain membranes in a complex with endogenous Munc18, consistent with a proposed function of Munc18 in vesicle docking and fusion.

Self-interaction of a SNARE Transmembrane Domain Promotes the Hemifusion-to-fusion Transition

SNARE proteins mediate intracellular fusion of eukaryotic membranes. Some SNAREs have previously been shown to dimerise via interaction of their transmembrane domains. However, the functional significance of these interactions had remained unclear. Here, we show that mutating alternate faces of the transmembrane helix of the yeast vacuolar Q-SNARE Vam3p reduces the ability of the full-length protein to induce contents mixing in yeast vacuole fusion to different extents. Examination of liposome fusion induced by synthetic transmembrane domains revealed that inner leaflet mixing is delayed relative to outer leaflet mixing, suggesting that fusion transits through a hemifusion intermediate. Interestingly, one of the mutations impaired inner leaflet mixing in the liposome system. This suggests that the defect seen in vacuolar contents mixing is due to partial arrest of the reaction at hemifusion. Since covalent dimerisation of this mutant recovered wild-type behaviour, homodimerisation of a SNARE transmembrane domain appears to control the transition of a hemifusion intermediate to complete lipid mixing.

Involvement of syntaxin 18, an endoplasmic reticulum (ER)-localized SNARE protein, in ER-mediated phagocytosis

The endoplasmic reticulum (ER) is thought to play an important structural and functional role in phagocytosis. According to this model, direct membrane fusion between the ER and the plasma or phagosomal membrane must precede further invagination, but the exact mechanisms remain elusive. Here, we investigated whether various ER-localized SNARE proteins are involved in this fusion process. When phagosomes were isolated from murine J774 macrophages, we found that ER-localized SNARE proteins (syntaxin 18, D12, and Sec22b) were significantly enriched in the phagosomes. Fluorescence and immuno-EM analyses confirmed the localization of syntaxin 18 in the phagosomal membranes of J774 cells stably expressing this protein tagged to a GFP variant. To examine whether these SNARE proteins are required for phagocytosis, we generated 293T cells stably expressing the Fc gamma receptor, in which phagocytosis occurs in an IgG-mediated manner. Expression in these cells of dominant-negative mutants of syntaxin 18 or D12 lacking the transmembrane domain, but not a Sec22b mutant, impaired phagocytosis. Syntaxin 18 small interfering RNA (siRNA) selectively decreased the efficiency of phagocytosis, and the rate of phagocytosis was markedly enhanced by stable overexpression of syntaxin 18 in J774 cells. Therefore, we conclude that syntaxin 18 is involved in ER-mediated phagocytosis, presumably by regulating the specific and direct fusion of the ER and plasma or phagosomal membranes.

Re-engineering the target specificity of Clostridial neurotoxins - a route to novel therapeutics

The ability to chemically couple proteins to LH(N)-fragments of clostridial neurotoxins and create novel molecules with selectivity for cells other than the natural target cell of the native neurotoxin is well established. Such molecules are able to inhibit exocytosis in the target cell and have the potential to be therapeutically beneficial where secretion from a particular cell plays a causative role in a disease or medical condition. To date, these molecules have been produced by chemical coupling of the LH(N)-fragment and the targeting ligand. This is, however, not a suitable basis for producing pharmaceutical agents as the products are ill defined, difficult to control and heterogeneous. Also, the molecules described to date have targeted neuroendocrine cells that are susceptible to native neurotoxins, and therefore the benefit of creating a molecule with a novel targeting domain has been limited. In this paper, the production of a fully recombinant fusion protein from a recombinant gene encoding both the LH(N)-domain of a clostridial neurotoxin and a specific targeting domain is described, together with the ability of such recombinant fusion proteins to inhibit secretion from non-neuronal target cells. Specifically, a novel protein consisting of the LH(N)-domains of botulinum neurotoxin type C and epidermal growth factor (EGF) that is able to inhibit secretion of mucus from epithelial cells is reported. Such a molecule has the potential to prevent mucus hypersecretion in asthma and chronic obstructive pulmonary disease.

Apical targeting of syntaxin 3 is essential for epithelial cell polarity

In polarized epithelial cells, syntaxin 3 localizes to the apical plasma membrane and is involved in membrane fusion of apical trafficking pathways. We show that syntaxin 3 contains a necessary and sufficient apical targeting signal centered around a conserved FMDE motif. Mutation of any of three critical residues within this motif leads to loss of specific apical targeting. Modeling based on the known structure of syntaxin 1 revealed that these residues are exposed on the surface of a three-helix bundle. Syntaxin 3 targeting does not require binding to Munc18b. Instead, syntaxin 3 recruits Munc18b to the plasma membrane. Expression of mislocalized mutant syntaxin 3 in Madin-Darby canine kidney cells leads to basolateral mistargeting of apical membrane proteins, disturbance of tight junction formation, and loss of ability to form an organized polarized epithelium. These results indicate that SNARE proteins contribute to the overall specificity of membrane trafficking in vivo, and that the polarity of syntaxin 3 is essential for epithelial cell polarization.

Molecular dissection of the Munc18c/syntaxin4 interaction: implications for regulation of membrane trafficking

Sec1p/Munc18 (SM) proteins are believed to play an integral role in vesicle transport through their interaction with SNAREs. Different SM proteins have been shown to interact with SNAREs via different mechanisms, leading to the conclusion that their function has diverged. To further explore this notion, in this study, we have examined the molecular interactions between Munc18c and its cognate SNAREs as these molecules are ubiquitously expressed in mammals and likely regulate a universal plasma membrane trafficking step. Thus, Munc18c binds to monomeric syntaxin4 and the N-terminal 29 amino acids of syntaxin4 are necessary for this interaction. We identified key residues in Munc18c and syntaxin4 that determine the N-terminal interaction and that are consistent with the N-terminal binding mode of yeast proteins Sly1p and Sed5p. In addition, Munc18c binds to the syntaxin4/SNAP23/VAMP2 SNARE complex. Pre-assembly of the syntaxin4/Munc18c dimer accelerates the formation of SNARE complex compared to assembly with syntaxin4 alone. These data suggest that Munc18c interacts with its cognate SNAREs in a manner that resembles the yeast proteins Sly1p and Sed5p rather than the mammalian neuronal proteins Munc18a and syntaxin1a. The Munc18c-SNARE interactions described here imply that Munc18c could play a positive regulatory role in SNARE assembly.

N- to C-Terminal SNARE Complex Assembly Promotes Rapid Membrane Fusion

Assembly of the soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs) syntaxin 1, SNAP-25, and synaptobrevin 2 is thought to be the driving force for the exocytosis of synaptic vesicles. However, whereas exocytosis is triggered at a millisecond time scale, the SNARE-mediated fusion of liposomes requires hours for completion, which challenges the idea of a key role for SNAREs in the final steps of exocytosis. We found that liposome fusion was dramatically accelerated when a stabilized syntaxin/SNAP-25 acceptor complex was used. Thus, SNAREs do have the capacity to execute fusion at a speed required for neuronal secretion, demonstrating that the maintenance of acceptor complexes is a critical step in biological fusion reactions.

The Sec1p/Munc18 protein Vps45p binds its cognate SNARE proteins via two distinct modes

Sec1p/Munc18 (SM) proteins are essential for SNARE-mediated membrane trafficking. The formulation of unifying hypotheses for the function of the SM protein family has been hampered by the observation that two of its members bind their cognate syntaxins (Sxs) in strikingly different ways. The SM protein Vps45p binds its Sx Tlg2p in a manner analogous to that captured by the Sly1p–Sed5p crystal structure, whereby the NH₂-terminal peptide of the Sx inserts into a hydrophobic pocket on the outer face of domain I of the SM protein. In this study, we report that although this mode of interaction is critical for the binding of Vps45p to Tlg2p, the SM protein also binds Tlg2p-containing SNARE complexes via a second mode that involves neither the NH₂ terminus of Tlg2p nor the region of Vps45p that facilitates this interaction. Our findings point to the possibility that SM proteins interact with their cognate SNARE proteins through distinct mechanisms at different stages in the SNARE assembly/disassembly cycle.

The identification of a novel endoplasmic reticulum to Golgi SNARE complex used by the prechylomicron transport vesicle

Dietary long chain fatty acids are absorbed in the intestine, esterified to triacylglycerol, and packaged in the unique lipoprotein of the intestine, the chylomicron. The rate-limiting step in the transit of chylomicrons through the enterocyte is the exit of chylomicrons from the endoplasmic reticulum in prechylomicron transport vesicles (PCTV) that transport chylomicrons to the cis-Golgi. Because chylomicrons are 250 nm in average diameter and lipid absorption is intermittent, we postulated that a unique SNARE pairing would be utilized to fuse PCTV with their target membrane, cis-Golgi. PCTV loaded with [(3)H]triacylglycerol were incubated with cis-Golgi and were separated from the Golgi by a sucrose step gradient. PCTV-chylomicrons acquire apolipoprotein-AI (apoAI) only after fusion with the Golgi. PCTV became isodense with Golgi upon incubation and were considered fused when their cargo chylomicrons acquired apoAI but docked when they did not. PCTV, docked with cis-Golgi, were solubilized in 2% Triton X-100, and proteins were immunoprecipitated using VAMP7 or rBet1 antibodies. In both cases, a 112-kDa complex was identified in nonboiled samples that dissociated upon boiling. The constituents of the complex were VAMP7, syntaxin 5, vti1a, and rBet1. Antibodies to each SNARE component significantly inhibited fusion of PCTV with cis-Golgi. Membrin, Sec22b, and Ykt6 were not found in the 112-kDa complex. We conclude that the PCTV-cis-Golgi SNARE complex is composed of VAMP7, syntaxin 5, Bet1, and vti1a.

Role of the Vam3p Transmembrane Segment in Homodimerization and SNARE Complex Formation

Intracellular membrane fusion in eukaryotic cells is mediated by SNARE (soluble N-ethylmaleimide sensitive factor (NSF) attachment protein receptor) proteins and is known to involve assembly of cognate subunits to heterooligomeric complexes. For synaptic SNAREs, it has previously been shown that the transmembrane segments drive homotypic and support heterotypic interactions. Here, we demonstrate that a significant fraction of the yeast vacuolar SNARE Vam3p is a homodimer in detergent extracts of vacuolar membranes. This homodimer exists in parallel to the heterooligomeric SNARE complex. A Vam3p homodimer also formed from the isolated recombinant protein. Interestingly, homodimerization depended on the transmembrane segment. In contrast, formation of the quaternary SNARE complex from recombinant Vam3p, Nyv1p, Vti1p, and Vam7p subunits did not depend on the transmembrane segment of Vam3p nor on the transmembrane segments of its partner proteins. We conclude that Vam3p homodimerization, but not quaternary SNARE complex formation, is promoted by TMS-TMS interaction. As the transmembrane segments of Vam3p and other SNARE homologues were previously shown to be critical for membrane fusion downstream of membrane apposition, our results may shed light on the functional significance of SNARE TMS-TMS interactions.

Transmembrane Organization of Yeast Syntaxin-Analogue Sso1p†

Membrane fusion in secretory pathways is thought to be mediated by SNAREs. It is proposed that membrane fusion transits through hemifusion, a condition in which the outer leaflets of the bilayers are mixed, but the inner leaflets are not. Hemifusion then proceeds to the fusion pore that connects the two internal contents. It is believed that the transmembrane domains (TMDs) of the fusion proteins play an essential role in the transition from hemifusion to the fusion pore. In this work, the structure, dynamics, and membrane topology of the TMD of Sso1p, a target membrane (t-) SNARE involved in the trafficking from Golgi to plasma membrane in yeast, was investigated using site-directed spin labeling and EPR spectroscopy. The EPR analysis of spin-labeled mutants showed that the TMD of Sso1p is a well-defined membrane spanning alpha-helix. The results also indicate that there is an equilibrium between the monomers and the oligomers. The oligomerization is mainly mediated through the interaction at the N-terminal half of the TMD, whereas the C-terminal half is free of the tertiary interaction. Additionally, the isotropic hyperfine splitting values were examined for nitroxide-scanning mutants, and it was found that the hyperfine splitting values show a V-shaped profile across the bilayer. Thus, hyperfine splitting may be used as an additional parameter to measure bilayer immersion depths of nitroxide.

SNARE complex zero layer residues are not critical for N-ethylmaleimide-sensitive factor-mediated disassembly

Membrane-anchored SNAREs assemble into SNARE complexes that bring membranes together to promote fusion. SNARE complexes are parallel four-helix bundles stabilized in part by hydrophobic interactions within their core. At the center of SNARE complexes is a distinctive zero layer that consists of one arginine and three glutamines. This zero layer is thought to play a special role in the biology of the SNARE complex. One proposal is that the polar residues of the zero layer enable N-ethylmaleimide-sensitive factor (NSF)-mediated SNARE complex disassembly. Here, we studied the effects of manipulating the zero layer of the well studied synaptic SNARE complex in vitro and in vivo. Using a fluorescence-based assay to follow SNARE complex disassembly in real time, we found that the maximal rate at which NSF disassembles complexes was unaffected by mutations in the zero layer, including single replacement of the syntaxin glutamine with arginine as well as multiple replacement of all four layer residues with non-polar amino acids. To determine whether syntaxin with arginine instead of glutamine in its zero layer can support SNARE function in vivo, we introduced it as a transgene into a Caenorhabditis elegans syntaxin-null strain. Mutant syntaxin rescued viability and locomotory defects similarly to wild-type syntaxin, demonstrating that SNARE complexes with two glutamines and two arginines in the zero layer can support neurotransmission. These findings show that residues of the zero layer do not play an essential role in NSF-mediated disassembly.

The polybasic juxtamembrane region of Sso1p is required for SNARE function in vivo

Exocytosis in Saccharomyces cerevisiae requires the specific interaction between the plasma membrane t-SNARE complex (Sso1/2p;Sec9p)and a vesicular v-SNARE (Snc1/2p). While SNARE proteins drive membrane fusion, many aspects of SNARE assembly and regulation are ill defined. Plasma membrane syntaxin homologs (including Sso1p) contain a highly charged juxtamembrane region between the transmembrane helix and the "SNARE domain" or core complex domain. We examined this region in vitro and in vivo by targeted sequence modification, including insertions and replacements. These modified Sso1 proteins were expressed as the sole copy of Sso in S. cerevisiae and examined for viability. We found that mutant Sso1 proteins with insertions or duplications show limited function, whereas replacement of as few as three amino acids preceding the transmembrane domain resulted in a nonfunctional SNARE in vivo. Viability is also maintained when two proline residues are inserted in the juxtamembrane of Sso1p, suggesting that helical continuity between the transmembrane domain and the core coiled-coil domain is not absolutely required. Analysis of these mutations in vitro utilizing a reconstituted fusion assay illustrates that the mutant Sso1 proteins are only moderately impaired in fusion. These results suggest that the sequence of the juxtamembrane region of Sso1p is vital for function in vivo, independent of the ability of these proteins to direct membrane fusion.

An intramolecular t-SNARE complex functions in vivo without the syntaxin NH2-terminal regulatory domain

Membrane fusion in the secretory pathway is mediated by SNAREs (located on the vesicle membrane [v-SNARE] and the target membrane [t-SNARE]). In all cases examined, t-SNARE function is provided as a three-helix bundle complex containing three approximately 70-amino acid SNARE motifs. One SNARE motif is provided by a syntaxin family member (the t-SNARE heavy chain), and the other two helices are contributed by additional t-SNARE light chains. The syntaxin family is the most conformationally dynamic group of SNAREs and appears to be the major focus of SNARE regulation. An NH2-terminal region of plasma membrane syntaxins has been assigned as a negative regulatory element in vitro. This region is absolutely required for syntaxin function in vivo. We now show that the required function of the NH2-terminal regulatory domain (NRD) of the yeast plasma membrane syntaxin, Sso1p, can be circumvented when t-SNARE complex formation is made intramolecular. Our results suggest that the NRD is required for efficient t-SNARE complex formation and does not recruit necessary scaffolding factors.

Alternate interfaces may mediate homomeric and heteromeric assembly in the transmembrane domains of SNARE proteins

The fusion of a vesicle to a target membrane is mediated by temporally and spatially regulated interactions within a set of evolutionarily conserved proteins. Integral to proper fusion is the interaction between proteins originating on both vesicle and target membranes to form a protein bridge between the two membranes, known as the SNARE complex. This protein complex includes the single-pass transmembrane helix proteins: syntaxin and synaptobrevin. Experimental data and amino acid sequence analysis suggest that an interface of interaction is conserved between the transmembrane regions of the two proteins. However, conflicting reports have been presented on the role of the synaptobrevin transmembrane domain in mediating important protein-protein interactions. To address this question, a thermodynamic study was carried out to determine quantitatively the self-association propensities of the transmembrane domains of synaptobrevin and syntaxin. Our results show that the transmembrane domain of synaptobrevin has only a modest ability to self-associate, whereas the transmembrane domain of syntaxin is able to form stable homodimers. Nevertheless, by a single amino acid substitution, synaptobrevin can be driven to dimerize with the same affinity as syntaxin. Furthermore, crosslinking studies show that dimerization of synaptobrevin is promoted by oxidizing agents. Despite the presence of a conserved cysteine residue in the same location as in synaptobrevin, syntaxin dimerization is not promoted by oxidization. This analysis suggests that subtle yet distinct differences are present between the two transmembrane dimer interfaces. A syntaxin/synaptobrevin heterodimer is able to form under oxidizing conditions, and we propose that the interface of interaction for the heterodimer may resemble the homodimer interface formed by the synaptobrevin transmembrane domain. Computational analysis of the transmembrane sequences of syntaxin and synaptobrevin reveal structural models that correlate with the experimental data. These data may provide insight into the role of transmembrane segments in the mechanism of vesicle fusion.

Syntaxin 9 is Enriched in Skin Hair Follicle Epithelium and Interacts With the Epidermal Growth Factor Receptor

We describe a novel syntaxin family member, syntaxin 9 (Syn 9), which does not possess a typical C-terminal hydrophobic tail anchor. Syn 9 has, however, a Q-SNARE domain and an overall homology to syntaxins (with the highest overall homology with mammalian syntaxin 11). Syn 9 is enriched in some epithelial cells, particularly that of the stomach lining and the skin. At the skin, it is found in the epidermal layers as well as structures associated with hair follicles. A biochemical interaction screen revealed that Syn 9 interacts specifically with the epidermal growth factor (EGF) receptor. Overexpression of Syn 9 perturbed EGF receptor endocytosis but does not appear to affect the internalization of the transferrin receptor. Syn 9 may therefore have a role in EGF receptor transport and signaling in certain epithelial cell types.