The H subunit (Vma13p) of the yeast V-ATPase inhibits the ATPase activity of cytosolic V1 complexes

V-ATPases are composed of a peripheral complex containing the ATP-binding sites, the V-1 sector, attached to a membrane complex containing the proton pore, the V-o sector. In vivo, free, inactive V-1 and V-o sectors exist in dynamic equilibrium with fully assembled, active V-1 V-o complexes, and this equilibrium can be perturbed by changes in carbon source. Free V-1 complexes were isolated from the cytosol of wild-type yeast cells and mutant strains lacking V-o subunit c (Vma3p) or V-1 subunit H (Vma13p). V-1 complexes from wild-type or vma3 Delta mutant cells were very similar, and contained all previously identified yeast V-1 subunits except subunit C (Vma5p). These V-1 complexes hydrolyzed CaATP but not MgATP, and CaATP hydrolysis rapidly decelerated with time. V-1 complexes from vma13 Delta cells contained all V-1 subunits except C and H, and had markedly different catalytic properties. The initial rate of CaATP hydrolysis was maintained for much longer. The complexes also hydrolyzed MgATP, but showed a rapid deceleration in hydrolysis. These results indicate that the H subunit plays an important role in silencing unproductive ATP hydrolysis by cytosolic V-1 complexes, but suggest that other mechanisms, such as product inhibition, may also play a role in silencing in vivo.