of c subunits past the a subunit, which drives proton transport across the membrane. A stoichiometry of two protons translocated for each ATP hydrolyzed has been proposed by Johnson. In addition to the structural subunits of yeast V-ATPase, associated proteins that are necessary for assembly have been identified. These associated proteins are essential for Vo domain assembly and are termed Vma12p, Vma21p, and Vma22p. Two of the three proteins, Vma12p and Vma22p, form a complex that binds transiently to Vph1p (subunit a) to aid its assembly and maturation. Vma21p coordinates assembly of the Vo subunits as well as escorting the Vo domain into vesicles for transport to the Golgi. V1 The V1 domain of the V-ATPase is the site of ATP hydrolysis. Unlike Vo, the V1 domain is hydrophilic. This soluble domain consists of a hexamer of alternating A and B subunits, a central rotor D, peripheral stators G and E, and regulatory subunits C and H. Hydrolysis of ATP drives a conformational change in the six A|B interfaces and with it rotation of the central rotor D. Unlike with the ATP synthase, the V1 domain is not an active ATPase when dissociated. Subunit C V-ATPase (Vacuolar-ATPase) C represents the C terminal subunit that is part of the V1 complex, and is localised to the interface between the V1 and Vo complexes. Subunit C function The C subunit plays an essential role in controlling the assembly of V-ATPase, acting as a flexible stator that holds together the catalytic (V1) and membrane (VO) sectors of the enzyme . The release of subunit C from the ATPase complex results in the dissociation of the V1 and Vo subcomplexes, which is an important mechanism in controlling V-ATPase activity in cells. Essentially, by creating a high electrochemical gradient and low pH, this powers the enzyme to create more ATP. Subunits E, G These related subunits make up the stalk(s) of A/V-ATPase. They are important in assembly, and may function as pushrods in activity. E has a cap to connect to A/B, while G does not. They likely evolved from a single protein by gene duplication. Subunit H Subunit H, is only involved in activity and not in assembly. This subunit also acts as an inhibitor of free V1 subunits; it stops ATP hydrolysis when V1 and Vo are dissociated. Vo The Vo domain is responsible for proton translocation. Unlike the F-type ATP synthase, the