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--helix bundle, where one -helix is contributed by syntaxin 1, one -helix by synaptobrevin and two -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 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--helix bundle. Synaptobrevin and syntaxin contribute one -helix each, while SNAP-25 participates with two -helices (abbreviated as