Conserved structural features of the synaptic fusion complex: SNARE proteins reclassified as Q- and R-SNAREs

SNARE [soluble NSF (N-ethylmaleimide-sensitive fusion protein) attachment p rotein receptor] proteins are essential for membrane fusion and are conserv ed from yeast to humans. Sequence alignments of the most conserved regions were mapped onto the recently solved crystal structure of the heterotrimeri c synaptic fusion complex. The association of the four a-helices in the syn aptic fusion complex structure produces highly conserved layers of interact ing amino acid side chains in the center of the four-helix bundle. Mutation s in these layers reduce complex stability and cause defects in membrane tr affic even in distantly related SNAREs. When syntaxin-4 is modeled into the synaptic fusion complex as a replacement of syntaxin-1A, no major steric c lashes arise and the most variable amino acids localize to the outer surfac e of the complex. We conclude that the main structural features of the neur onal complex are highly conserved during evolution. On the basis of these f eatures we have reclassified SNARE proteins into Q-SNAREs and R-SNAREs, and we propose that fusion-competent SNARE complexes generally consist of four -helix bundles composed of three Q-SNAREs and one R-SNARE.