Structural basis of V-ATPase V region assembly by Vma12p, 21p, and 22p.

Vacuolar-type adenosine triphosphatases (V-ATPases) are rotary proton pumps that acidify specific intracellular compartments in almost all eukaryotic cells. These multi-subunit enzymes consist of a soluble catalytic V region and a membrane-embedded proton-translocating V region. V is assembled in the endoplasmic reticulum (ER) membrane, and V is assembled in the cytosol. However, V binds V only after V is transported to the Golgi membrane, thereby preventing acidification of the ER. We isolated V complexes and subcomplexes from bound to V-ATPase assembly factors Vma12p, Vma21p, and Vma22p. Electron cryomicroscopy shows how the Vma12-22p complex recruits subunits a, e, and f to the rotor ring of V while blocking premature binding of V. Vma21p, which contains an ER-retrieval motif, binds the V:Vma12-22p complex, "mature" V, and a complex that appears to contain a ring of loosely packed rotor subunits and the proteins YAR027W and YAR028W. The structures suggest that Vma21p binds assembly intermediates that contain a rotor ring and that activation of proton pumping following assembly of V with V removes Vma21p, allowing V-ATPase to remain in the Golgi. Together, these structures show how Vma12-22p and Vma21p function in V-ATPase assembly and quality control, ensuring the enzyme acidifies only its intended cellular targets.