Saccharomyces cerevisiae SNARE protein Snc1p forms a complex with synaptobrevin homolog Sso1p (Qa) and the SNAP-25 homolog Sec9p (Qb/c) which is involved in exocytosis. Snc1 is a member of the R-SNARE subgroup of SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptor) proteins, which consist of coiled-coil helices (called SNARE motifs) that mediate the interactions between SNARE proteins, and a transmembrane domain. The SNARE complexes mediate membrane fusion, important for trafficking of newly synthesized proteins, recycling of pre-existing proteins and organelle formation. SNARE proteins are classified into four groups, Qa-, Qb-, Qc- and R-SNAREs, depending on whether the residue in the hydrophilic center layer of the four-helical bundle is a glutamine (Q) or arginine (R). Qa-, as well as Qb- and Qc-SNAREs, are localized to target organelle membranes, while R-SNARE is localized to vesicle membranes. They form unique complexes consisting of one member of each subgroup, that mediate fusion between a specific type of vesicles and their target organelle.
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