Essential component of the vacuolar proton pump (V-ATPase), a multimeric enzyme that catalyzes the translocation of protons across the membranes. Required for assembly and activity of the V-ATPase. Required in both the contractile vacuole system and the endosomal/lysosomal system. Also required for cytosolic pH regulation. The V-ATPase is a heteromultimeric enzyme. Found in membrane of contractile vacuole complex, an osmoregulatory organelle. Cells show defects in V-ATPase enzyme assembly, endocytic function and cytosolic pH regulation. Belongs to the V-ATPase 116 kDa subunit family.