SNARE-mediated fusion typically results in the formation a complex consisting of three Q-SNAREs and one R-SNARE. R-SNAREs that contain a profilin-like fold in their N-terminal domains have been termed longins. The genome of the yeast Saccharomyces cerevisiae encodes five R-SNAREs of which three (Sec22p, Ytk6p, and Nyv1p) are longins. It is important to note that the given longin domain (LD) is not restricted to R-SNAREs and has been identified in a variety of proteins.

Structural domains comprising this superfamily share the structure of the N-terminal longin domain of the Vacuolar R-SNARE Nyv1 protein. Studies on this domain have shown that it contains a YXXomega-like sorting motif (Y31GTI34), responsible for the AP3-dependent vacuolar localization of Nyv1p PMID:16855025.