Voa1p Functions in V-ATPase Assembly in the Yeast Endoplasmic Reticulum

The yeast Saccharomyces cerevisiae vacuolar ATPase (V-ATPase) is a multisubunit complex divided into two sectors: the V-1 sector catalyzes ATP hydrolysis and the V-0 sector translocates protons, resulting in acidification of its resident organelle. Four protein factors participate in V-0 assembly. We have discovered a fifth V-0 assembly factor, Voa1p (YGR106C); an endoplasmic reticulum (ER)-localized integral membrane glycoprotein. The role of Voa1p in V-0 assembly was revealed in cells expressing an ER retrieval-deficient form of the V-ATPase assembly factor Vma21p (Vma21pQQ). Loss of Voa1p in vma21QQ yeast cells resulted in loss of V-ATPase function; cells were unable to acidify their vacuoles and exhibited growth defects typical of cells lacking V-ATPase. V-0 assembly was severely compromised in voa1 vma21QQ double mutants. Isolation of V-0-Vma21p complexes indicated that Voa1p associates most strongly with Vma21p and the core proteolipid ring of V-0 subunits c, c', and c ''. On assembly of the remaining three V-0 subunits (a, d, and e) into the V0 complex, Voa1p dissociates from the now fully assembled V-0-Vma21p complex. Our results suggest Voa1p functions with Vma21p early in V-0 assembly in the ER, but then it dissociates before exit of the V-0-Vma21p complex from the ER for transport to the Golgi compartment.