VAC7P, A NOVEL VACUOLAR PROTEIN, IS REQUIRED FOR NORMAL VACUOLE INHERITANCE AND MORPHOLOGY

During cell division, the vacuole of Saccharomyces cerevisiae partitio ns between mother and daughter cells. A portion of the parental vacuol e membrane moves into the bud, and ultimately membrane scission divide s the vacuole into two separate structures, Here we characterize two y east mutations causing defects in vacuole membrane scission, vac7-1 an d vac14-1. A third mutant, afab1-2 strain, isolated in a nonrelated sc reen (A. Yamamoto et al., Mel. Biol. Cell 6:525-539, 1995) shares the vacuolar phenotypes of the vac7-1 and vac14-1 strains. Unlike the wild type, mutant vacuoles are not multilobed structures; in many cases, a single vacuole spans both the mother and bud, with a distinct gap in the mother-bud neck. Thus, even,where the membranes are closely oppose d, vacuole fission is arrested. Simply enlarging the vacuole does not produce this mutant phenotype. An additional common phenotype of these mutants is a defect in vacuole acidification; however, vacuole scissi on in most other vacuole acidification mutants is normal. An alteratio n in vacuole membrane lipids could account for both the vacuole membra ne scission and acidification defects. Because a directed screen has n ot identified additional class III complementation groups, it is likel y that all three genes are involved in a similar process. Interestingl y, FAB1, was previously shown to encode a putative phosphatidylinosito l-4-phosphate 5-kinase. Moreover, overexpression of FAB1 suppresses th e vac14-1 mutation, which suggests that VAC14 and FAB1 act at a common step, VAC7 encodes a novel 128-kDa protein that is localized at the v acuole membrane. This location of Vac7p is consistent with its involve ment in vacuole morphology and inheritance.