Sec1p binds to SNARE complexes and concentrates at sites of secretion

Proteins of the Sec1 family have been shown to interact with target-membran e t-SNAREs that are homologous to the neuronal protein syntaxin. We demonst rate that yeast Sec1p coprecipitates not only the syntaxin homologue Ssop, but also the other two exocytic SNAREs (Sec9p and Sncp) in amounts and in p roportions characteristic of SNARE complexes in yeast lysates. The interact ion between Sec1p and Ssop is limited by the abundance of SNARE complexes p resent in sec mutants that are defective in either SNARE complex assembly o r disassembly. Furthermore, the localization of green fluorescent protein ( GFP)-tagged Sec1p coincides with sites of vesicle docking and fusion where SNARE complexes are believed to assemble and function. The proposal that SN ARE complexes act as receptors for Sec1p is supported by the mislocalizatio n of GFP-Sec1p in a mutant defective for SNARE complex assembly and by the robust localization of GFP-Sec1p in a mutant that fails to disassemble SNAR E complexes. The results presented here place yeast Sec1p at the core of th e exocytic fusion machinery, bound to SNARE complexes and localized to site s of secretion.