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 transport vesicles to the plasma membrane. We previously have cloned two yeast genes encoding syntaxins,SSO1 andSSO2 , as suppressors of the temperature-sensitivesec1–1 mutation. We now describe a third suppressor ofsec1–1 , which we callMSO1 . UnlikeSSO1 andSSO2 ,MSO1 is specific forsec1 and does not suppress mutations in any otherSEC genes.MSO1 encodes a small hydrophilic protein that is enriched in a microsomal membrane fraction. Cells that lackMSO1 are viable, but they accumulate secretory vesicles in the bud, indicating that the terminal step in secretion is partially impaired. Moreover, loss ofMSO1 shows synthetic lethality with mutations inSEC1 ,SEC2 , andSEC4 , and other synthetic phenotypes with mutations in several other late-actingSEC genes. We further found that Mso1p interacts with Sec1p bothin vitro and in the two-hybrid system. These findings suggest that Mso1p is a component of the secretory vesicle docking complex whose function is closely associated with that of Sec1p.