SNARE-mediated fusion typically results in the formation a complex consisting of three Q-SNAREs and one R-SNARE. Concerning R-SNAREs, we can distinguish between two subfamilies of short VAMPs or brevins, and of long VAMPs or longins because only the latter share a N-terminus extension, the so-called Longin domain, LD. VAMPS are characterised by the presence of a C-terminal coiled-coil/SNARE domain, together with variable N-terminal domains that are used for their classification, as described PMID:15145528.

Longins are the only type of VAMP protein found in all eukaryotes, suggesting that their longin domain (LD) is essential. Structural domains comprising this superfamily share the structure of this domain, which has profilin-like structural fold and is thought to exert a regulatory function. The best characterised longins are those encoded by the yeast Saccharomyces cerevisiae: Sec22p, Ytk6p, and Nyv1p PMID:16855025.