MSO1P - A YEAST PROTEIN THAT FUNCTIONS IN SECRETION AND INTERACTS PHYSICALLY AND GENETICALLY WITH SEC1P

The yeast Sec1p protein functions in the docking of secretory transpor t vesicles to the plasma membrane. We previously have cloned two yeast genes encoding syntaxins, SSO1 and SSO2, as suppressors of the temper ature-sensitive sec1-1 mutation. We now describe a third suppressor of sec1-1, which we call MSO1. Unlike SSO1 and SSO2, MSO1 is specific fo r sec1 and does not suppress mutations in any other SEC genes. MSO1 en codes a small hydrophilic protein that is enriched in a microsomal mem brane fraction. Cells that lack MSO1 are viable, but they accumulate s ecretory vesicles in the bud, indicating that the terminal step in sec retion is partially impaired. Moreover, loss of MSO1 shows synthetic l ethality with mutations in SEC1, SEC2, and SEC4, and other synthetic p henotypes with mutations in several other late-acting SEC genes. We fu rther found that Mso1p interacts with Sec1p both in vitro and in the t wo-hybrid system. These findings suggest that Mso1p is a component of the secretory vesicle docking complex whose function is closely associ ated with that of Sec1p.