A novel SNARE N-terminal domain revealed by the crystal structure of Sec22b

Intra-cellular membrane fusion is facilitated by the association of SNAREs from opposite membranes into stable alpha -helical bundles. Many SNAREs, in addition to their alpha -helical regions, contain N-terminal domains that likely have essential regulatory functions. To better understand this regulation, we have determined the 2.4-Angstrom crystal structure of the 130-amino acid N-terminal domain of mouse Sec22b (mSec22b), a SNARE involved in endoplasmic reticulum/Golgi membrane trafficking. The domain consists of a mixed alpha -helical/beta -sheet fold that resembles a circular permutation of the actin/polyproline binding protein, profilin, and the GAF/PAS family of regulatory modules. The structure is distinct from the previously characterized N-terminal domain of syn taxin 1A, and, unlike syntaxin 1A, the N-terminal domain of mSec22b has no effect on the rate of SNARE assembly in vitro. An analysis of surface conserved residues reveals a potential protein interaction site. Key residues in this site are distinct in two mammalian Sec22 variants that lack SNARE domains, Finally, sequence analysis indicates that a similar domain is likely present in the endosomal/lysosomal SNARE VAMP7.