Acidification of endomembrane compartments by the vacuolar-type H-translocating ATPase (V-ATPase) is vital to the growth and development of plants. The V-ATPase purified from oat roots is a large complex of 650X10Afr that contains 10 different subunits of 70, 60, 44, 42, 36, 32, 29, 16, Band 12X 10 MT. This set of ten polypeptides is sufficient to couple ATP hydrolysis to proton pumping after reconstitution of the ATPase into liposomes. Unlike some animal V-ATPases, the purified and reconstituted V-ATPase from oat is directly stimulated by Cl~. The peripheral complex of the ATPase includes the nucleotide-binding subunits of 70 and 60X10Mr and polypeptides of 44, 42, 36 and 29X l O ^ r . Six copies of the 16X WfiMr proteolipid together with three other polypeptides are thought to make up the integral sector that forms the H-conducting pathway. Release of the peripheral complex from the native membrane completely inactivates the pump; however, the peripheral subunits can be reassembled with the membrane sector to form a functional H pump. Comparison of V-ATPases from several plants indicates considerable variations in subunit composition. Hence, several forms of the V-ATPase may exist among, and probably within, plant species. At least four distinct cDNAs encode the l6X.[(PMr proteolipid subunit in oat. Multiple genes could encode different subtypes of the H pump that are regulated by the developmental stage and physiological function specific to the cell or tissue type.