VMA7 encodes a novel 14-kDa subunit of the Saccharomyces cerevisiae vacuolar H(+)-ATPase complex

The Saccharomyces cerevisiae vacuolar proton-translocating ATPase (V-ATPase) is composed of at least 10 subunits belonging to either the peripheral V1 or integral membrane V0 subcomplex. We have characterized a novel 14-kDa V-ATPase subunit (Vma7p), encoded by the VMA7 gene, which exhibits features common to both V1 and V0 subunit proteins. Vma7p is a hydrophilic protein of 118 amino acids with a predicted molecular mass of 13,452 Da. Vacuolar membranes isolated from a vma7 delta null mutant contained no V-ATPase activity. Western analysis of vma7 delta cells revealed greatly reduced levels of the remaining V0 complex V-ATPase subunits, but normal levels of the V1 subunits. However, the V1 subunits failed to associate with the vacuolar membrane. Unlike the integral membrane subunits of the V0 complex, Vma7p was easily stripped from vacuolar membranes. Density gradient fractionation revealed that Vma7p associated only with the fully assembled V-ATPase and did not associate with a separate lower density V0 subcomplex fraction. The unique properties of the Vma7p may reflect a critical role in stabilizing the V0 complex and bridging the V1 and V0 complexes to form a functional V-ATPase complex.