SNARE protein

SNARE proteins (an acronym derived from "SNAP (Soluble NSF Attachment Protein) REceptor") are a large protein superfamily consisting of at least 24 members in yeasts and more than 60 members in mammalian cells. The primary role of SNARE proteins is to mediate vesicle fusion, that is, the fusion of vesicles with their target membrane bound compartments (such as a lysosome). The best studied SNAREs are those that mediate docking of synaptic vesicles with the presynaptic membrane in neurons. These SNAREs are the targets of the bacterial neurotoxins responsible for botulism and tetanus.

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 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 depending on their location in the four-helix bundle.

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, , 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.

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