Assembly of the Yeast Vacuolar H⁺-ATPase Occurs in the Endoplasmic Reticulum and Requires a Vma12p/Vma22p Assembly Complex

Three previously identified genes from Saccharomyces cerevisiae, VMA12, VMA21, and VMA22, encode proteins localized to the endoplasmic reticulum (ER). These three proteins are required for the biogenesis of a functional vacuolar ATPase (V-ATPase), but are not part of the final enzyme complex. Subcellular fractionation and chemical cross-linking studies have revealed that Vma12p and Vma22p form a stable membrane associated complex. Cross-linking analysis also revealed a direct physical interaction between the Vma12p/Vma22p assembly complex and Vph1p, the 100-kD integral membrane subunit of the V-ATPase. The interaction of the Vma12p/Vma22p complex with Vph1p was transient (half-life of ∼5 min), reflecting trafficking of this V-ATPase subunit through the ER en route to the vacuolar membrane. Analysis of these protein–protein interactions in ER-blocked sec12 mutant cells indicated that the Vph1p-Vma12p/Vma22p interactions are quite stable when transport of the V-ATPase out of the ER is blocked. Fractionation of solubilized membrane proteins on a density gradient revealed comigration of Vma22p and Vma12p, indicating that they form a complex even in the absence of cross-linker. Vma12p and Vma22p migrated to fractions separate from Vma21p. Loss of Vph1p caused the Vma12p/Vma22p complex to sediment to less dense fractions, consistent with association of Vma12p/ Vma22p with nascent Vph1p in ER membranes. This is the first evidence for a dedicated assembly complex in the ER required for the assembly of an integral membrane protein complex (V-ATPase) as it is transported through the secretory pathway.