Conserved alpha-helical segments on yeast homologs of the synaptobrevin/VAMP family of V-SNAREs mediate exocytic function

We are studying yeast homologs of the synaptobrevin/VAMP family of vesicle-associated membrane proteins, which act as vesicular compartment-soluble N-ethylmaleimide-sensitive factor attachment protein receptors (v-SNAREs) in cells having a capacity for stimulus-coupled secretion, as well as in other cell types. The yeast homologs, Snc1 and Snc2, localize to secretory vesicles and are required for normal bulk secretion in Saccharomyces cerevisiae. Here we have used Snc deletion mutants and chimeric Snc-VAMP proteins to demonstrate that these v-SNAREs can be dissected into regions that are either indispensable or dispensable for exocytic function in vivo. We have found that a region encompassing two predicted amphipathic alpha-helices (helix 1 and helix 2) (residues 32-85), which are thought to form coiled-coil structures, is essential for conferring exocytosis in yeast. Deletions in either the helix 1 or helix 2 segments result in a complete loss in the ability of the protein to confer secretion competence to snc cells and to interact genetically with components of the proposed fusion complex: the Sec9 and Sso2 t-SNAREs and the Sec17 alpha-SNAP homolog. In contrast, deletions in either the variable (residues 2-27) or putative intravesicular (residues 115-117) regions have no deleterious effect upon v-SNARE function. This makes it unlikely that sequences in either the amino or carboxyl terminus act in an exocytic capacity. Along with additional studies utilizing chimeric Snc-VAMP proteins, we suggest that although the Snc and synaptobrevin/VAMP proteins have evolved to mediate vastly different exocytic programs, their structural requirements and actions have remained remarkably well-conserved in evolution.