VMA21P IS A YEAST MEMBRANE-PROTEIN RETAINED IN THE ENDOPLASMIC-RETICULUM BY A DI-LYSINE MOTIF AND IS REQUIRED FOR THE ASSEMBLY OF THE VACUOLAR H-ATPASE COMPLEX()

The yeast vacuolar proton-translocating ATPase (V-ATPase) is a multisu bunit complex comprised of peripheral membrane subunits involved in AT P hydrolysis and integral membrane subunits involved in proton pumping . The yeast vma21 mutant was isolated from a screen to identify mutant s defective in V-ATPase function. vma21 mutants fail to assemble the V -ATPase complex onto the vacuolar membrane: peripheral subunits accumu late in the cytosol and the 100-kDa integral membrane subunit is rapid ly degraded. The product of the VMA21 gene (Vma2lp) is an 8.5-kDa inte gral membrane protein that is not a subunit of the purified V-ATPase c omplex but instead resides in the endoplasmic reticulum. Vma2lp contai ns a dilysine motif at the carboxy terminus, and mutation of these lys ine residues abolishes retention in the endoplasmic reticulum and resu lts in delivery of Vma2lp to the vacuole, the default compartment for yeast membrane proteins. Our findings suggest that Vma2lp is required for assembly of the integral membrane sector of the V-ATPase in the en doplasmic reticulum and that the unassembled 100-kDa integral membrane subunit present in Delta vma21 cells is rapidly degraded by nonvacuol ar proteases.