Sec9 is a SNAP-25-like Component of a Yeast SNARE Complex That May Be the Effector of Sec4 Function in Exocytosis

To identify potential Sec4 effectors, we isolated high copy suppressors of a Sec4 effector domain mutant. The most potent of these was found to be SEC9, a gene required for post-Golgi transport. The sole essential domain of Sec9 has significant sequence similarity to the neuronal protein SNAP-25, a component of the SNARE complex, that is implicated in vesicle targeting and fusion. Analogous to SNAP-25, Sec9 is bound to the yeast plasma membrane and is absent from post-Golgi vesicles. Furthermore, Sec9 is physically associated with two proteins that are homologous to components of the neuronal SNARE complex. Our results identify Sec9 as the yeast cognate of SNAP-25 and suggest that SNARE complexes acting at specific stages of vesicular transport serve as the ultimate targets of regulation by members of the Sec4/Ypt1/Rab family of GTPases.