snare (protein) snare (protein) snare proteins – "snap receptor" – are a large protein family consisting of at least 24 members in yeasts, more than 60 members in mammalian cells, and some numbers in plants. the primary role of snare proteins is to mediate vesicle fusion – the fusion of vesicles with the target membrane; this notably mediates exocytosis, but can also mediate the fusion of vesicles with membrane-bound compartments (such as a lysosome). the best studied snares are those that mediate the neurotransmitter release of synaptic vesicles in neurons. these neuronal snares are the targets of the neurotoxins responsible for botulism and tetanus produced by certain bacteria. molecular machinery driving vesicle fusion in neuromediator release. the core snare complex is formed by four α-helices contributed by synaptobrevin, syntaxin and snap-25, synaptotagmin serves as a calcium sensor and closely regulates the snare zipping. snare-fusion membrane complex proteinsidentifierssymbolsnareinterproipr010989scop21kil / scope / supfamtcdb1.f.1opm superfamily197opm protein3hd7membranome198 types snares can be divided into two categories: vesicle or v-snares, which are incorporated into the membranes of transport vesicles during budding, and target or t-snares, which are associated with nerve terminal membranes. evidence suggests that t-snares form stable subcomplexes which serve as guides for v-snare, incorporated into the membrane of a protein-coated vesicle, binding to complete the formation of the snare complex. several snare proteins are located on both vesicles and target membranes, therefore, a more recent classification scheme takes into account structural features of snares, dividing them into r-snares and q-snares. often, r-snares act as v-snares and q-snares act as t-snares. r-snares are proteins that contribute an arginine (r) residue in the formation of the zero ionic layer in the assembled core snare complex. one particular r-snare is synaptobrevin, which is located in the synaptic vesicles. q-snares are proteins that contribute a glutamine (q) residue in the formation of the zero ionic layer in the assembled core snare complex. q-snares include syntaxin and snap-25. q-snares are further classified as qa-, qb-, or qc-snares depending on their location in the four-helix bundle. occurrence variants are known from yeasts, mammals drosophila, and caenorhabditis elegans. structure snares are small, abundant, sometimes tail-anchored proteins which are often post-translationally inserted into membranes via a c-terminal transmembrane domain. seven of the 38 known snares, including snap-25, do not have a transmembrane domain and are instead attached to the membrane via lipid modifications such as palmitoylation. tail-anchored proteins can be inserted into the plasma membrane, endoplasmic reticulum, mitochondria, and peroxisomes among other membranes, though any particular snare is targeted to a unique membrane. the targeting of snares is accomplished by altering either the composition of the c-terminal flanking amino acid residues or the length of the transmembrane domain. replacement of the transmembrane domain with lipid anchors leads to an intermediate stage of membrane fusion where only the two contacting leaflets fuse and not the two distal leaflets of the two membrane bilayer. although snares vary considerably in structure and size, they all share a segment in their cytosolic domain called a snare motif that consists of 60-70 amino acids and contains heptad repeats that have the ability to form coiled-coil structures. v- and t-snares are capable of reversible assembly into tight, four-helix bundles called "trans"-snare complexes. in synaptic vesicles, the readily-formed metastable "trans" complexes are composed of three snares: syntaxin 1 and snap-25 resident in cell membrane and synaptobrevin (also referred to as vesicle-associated membrane protein or vamp) anchored in the vesicle membrane. in neuronal exocytosis, syntaxin and synaptobrevin are anchored in respective membranes by their c-terminal domains, whereas snap-25 is tethered to the plasma membrane via several cysteine-linked palmitoyl chains. the core trans-snare complex is a four- α -helix bundle, where one α -helix is contributed by syntaxin 1, one α -helix by synaptobrevin and two α -helices are contributed by snap-25. the plasma membrane-resident snares have been shown to be present in distinct microdomains or clusters, the integrity of which is essential for the exocytotic competence of the cell. membrane fusion layering of the core snare complex. in the center is the zero hydrophilic ionic layer, flanked by hydrophobic leucine-zipper layers. during membrane fusion, v-snare and t-snare proteins on separate membranes combine to form a trans-snare complex, also known as a "snarepin". depending on the stage of fusion of the membranes, these complexes may be referred to differently. during fusion of trans-snare complexes, the membranes merge and snare proteins involved in complex formation after fusion are then referred to as a "cis"-snare complex, because they now reside in a single (or cis) resultant membrane. after fusion, the cis-snare complex is bound and disassembled by an adaptor protein, alpha-snap. then, the hexameric atpase (of the aaa type) called nsf catalyzes the atp-dependent unfolding of the snare proteins and releases them into the cytosol for recycling. snares are thought to be the core required components of the fusion machinery and can function independently of additional cytosolic accessory proteins. this was demonstrated by engineering "flipped" snares, where the snare domains face the extracellular space rather than the cytosol. when cells containing v-snares contact cells containing t-snares, trans-snare complexes form and cell-cell fusion ensues. components the core snare complex is a 4- α -helix bundle. synaptobrevin and syntaxin contribute one α -helix each, while snap-25 participates with two α -helices (abbreviated as sn1 and sn2). the interacting amino acid residues that zip the snare complex can be grouped into layers. each layer has 4 amino acid residues – one residue per each of the 4 α -helices. in the center of the complex is the zero ionic layer composed of one arginine (r) and three glutamine (q) residues, and it is flanked by leucine zippering. layers '-1', '+1' and '+2' at the centre of the complex most closely follow ideal leucine-zipper geometry and aminoacid composition. the zero ionic layer is composed of r56 from vamp-2, q226 from syntaxin-1a, q53 from sn1 and q174 from sn2, and is completely buried within the leucine-zipper layers. the positively charged guanidino group of the arginine (r) residue interact with the carboxyl groups of each of the three glutamine (q) residues. the flanking leucine-zipper layers act as a water-tight seal to shield the ionic interactions from the surrounding solvent. exposure of the zero ionic layer to the water solvent by breaking the flanking leucine zipper leads to instability of the snare complex and is the putative mechanism by which α -snap and nsf recycle the snare complexes after the completion of synaptic vesicle exocytosis. mechanism of membrane fusion assembly depiction of the formation of a trans-snare complex. shows how munc18 interacts with the snare proteins during complex formation. snare proteins must assemble into trans-snare complexes to provide the force that is necessary for vesicle fusion. the four α-helix domains (1 each from synaptobrevin and syntaxin, and 2 from snap-25) come together to form a coiled-coil motif. the rate-limiting step in the assembly process is the association of the syntaxin snare domain, since it is usually found in a "closed" state where it is incapable of interacting with other snare proteins. when syntaxin is in an open state, trans-snare complex formation begins with the association of the four snare domains at their n-termini. the snare domains proceed in forming a coiled-coil motif in the direction of the c-termini of their respective domains. snap and nsf also associate with the complex formed by snares during this step and participate in the later events of priming and disassembly. the sm protein munc18 is thought to play a role in assembly of the snare complex, although the exact mechanism by which it acts is still under debate. it is known that the clasp of munc18 locks syntaxin in a closed conformation by binding to its α-helical snare domains, which inhibits syntaxin from entering snare complexes (thereby inhibiting fusion). the clasp is also capable, however, of binding the entire four-helix bundle of the trans-snare complex. one hypothesis suggests that, during snare-complex assembly, the munc18 clasp releases closed syntaxin, remains associated with the n-terminal peptide of syntaxin (allowing association of the syntaxin snare domain with other snare proteins), and then reattaches to the newly formed four-helix snare complex. this possible mechanism of dissociation and subsequent re-association with the snare domains could be calcium-dependent. this supports the idea that munc18 plays a key regulatory role in vesicle fusion; under normal conditions the snare complex will be prevented from forming by munc18, but when triggered the munc18 will actually assist in snare-complex assembly and thereby act as a fusion catalyst. zippering and fusion pore opening this figure provides a simple overview of the interaction of snare proteins with vesicles during exocytosis. shows snare complex assembly, zippering, and disassembly. membrane fusion is an energetically demanding series of events, which requires translocation of proteins in the membrane and disruption of the lipid bilayer, followed by reformation of a highly curved membrane structure. the process of bringing together two membranes requires input energy to overcome the repulsive electrostatic forces between the membranes. the mechanism that regulates the movement of membrane associated proteins away from the membrane contact zone prior to fusion is unknown, but the local increase in membrane curvature is thought to contribute in the process. snares generate energy through protein-lipid and protein-protein interactions which act as a driving force for membrane fusion. one model hypothesizes that the force required to bring two membranes together during fusion comes from the conformational change in trans-snare complexes to form cis-snare complexes. the current hypothesis that describes this process is referred to as snare "zippering." when the trans-snare complex is formed, the snare proteins are still found on opposing membranes. as the snare domains continue coiling in a spontaneous process, they form a much tighter, more stable four-helix bundle. during this "zippering" of the snare complex, a fraction of the released energy from binding is thought to be stored as molecular bending stress in the individual snare motifs. this mechanical stress is postulated to be stored in the semi-rigid linker regions between the transmembrane domains and the snare helical bundle. the energetically unfavorable bending is minimized when the complex moves peripherally to the site of membrane fusion. as a result, relief of the stress overcomes the repulsive forces between the vesicle and the cell membrane and presses the two membranes together. several models to explain the subsequent step – the formation of stalk and fusion pore – have been proposed. however, the exact nature of these processes remains debated. in accordance with the "zipper" hypothesis, as the snare complex forms, the tightening helix bundle puts torsional force on the transmembrane (tm) domains of synaptobrevin and syntaxin. this causes the tm domains to tilt within the separate membranes as the proteins coil more tightly. the unstable configuration of the tm domains eventually causes the two membranes to fuse and the snare proteins come together within the same membrane, which is referred to as a "cis"-snare complex. as a result of the lipid rearrangement, a fusion pore opens and allows the chemical contents of the vesicle to leak into the outside environment. the continuum explanation of stalk formation suggests that membrane fusion begins with an infinitesimal radius until it radially expands into a stalk-like structure. however, such a description fails to take into account the molecular dynamics of membrane lipids. recent molecular simulations show that the close proximity of the membranes allows the lipids to splay, where a population of lipids insert their hydrophobic tails into the neighboring membrane – effectively keeping a "foot" in each membrane. the resolution of the splayed lipid state proceeds spontaneously to form the stalk structure. in this molecular view, the splayed-lipid intermediate state is the rate determining barrier rather than the formation of the stalk, which now becomes the free energy minimum. the energetic barrier for establishment of the splayed-lipid conformation is directly proportional to the intermembrane distance. the snare complexes and their pressing of the two membranes together, therefore, could provide the free energy required to overcome the barrier. disassembly the energy input that is required for snare-mediated fusion to take place comes from snare-complex disassembly. the suspected energy source is n-ethylmaleimide-sensitive factor (nsf), an atpase that is involved with membrane fusion. nsf homohexamers, along with the nsf cofactor α-snap, bind and dissociate the snare complex by coupling the process with atp hydrolysis. this process