The yeast Saccharomyces cerevisiae vacuolar ATPase (V-ATPase) is a multisubunit complex divided into two sectors: the V1 sector catalyzes ATP hydrolysis and the V0 sector translocates protons, resulting in acidification of its resident organelle. Four protein factors participate in V0 assembly. We have discovered a fifth V0 assembly factor, Voa1p (YGR106C); an endoplasmic reticulum (ER)-localized integral membrane glycoprotein. The role of Voa1p in V0 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. V0 assembly was severely compromised in voa1 vma21QQ double mutants. Isolation of V0–Vma21p complexes indicated that Voa1p associates most strongly with Vma21p and the core proteolipid ring of V0 subunits c, c′, and c″. On assembly of the remaining three V0 subunits (a, d, and e) into the V0 complex, Voa1p dissociates from the now fully assembled V0–Vma21p complex. Our results suggest Voa1p functions with Vma21p early in V0 assembly in the ER, but then it dissociates before exit of the V0–Vma21p complex from the ER for transport to the Golgi compartment.
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