Ordering the final events in yeast exocytosis

In yeast, assembly of exocytic soluble N-ethylmaleimide-sensitive fusion pr otein (NSF) attachment protein receptor (SNARE) complexes between the secre tory vesicle SNARE Sncp and the plasma membrane SNAREs Ssop and Sec9p occur s at a late stage of the exocytic reaction. Mutations that block either sec retory vesicle delivery or tethering prevent SNARE complex assembly and the localization of Sec1p, a SNARE complex binding protein, to sites of secret ion. By contrast, wild-type levels of SNARE complexes persist in the sccl-l mutant after a secretory block is imposed, suggesting a role for Sec1p aft er SNARE complex assembly. In the sec18-1 mutant, cis-SNARE complexes conta ining surface-accessible Sncp accumulate in the plasma membrane. Thus, one function of Sec18p is to disassemble SNARE complexes on the postfusion memb rane.