Dissociation and Reassembly of the Vacuolar H⁺-ATPase Complex from Oat Roots

Abstract

Conditions for the dissociation and reassembly of the multi-subunit vacuolar proton-translocating ATPase (H⁺-ATPase) from oat roots (Avena sativa var Lang) were investigated. The peripheral sector of the vacuolar H⁺-ATPase is dissociated from the membrane integral sector by chaotropic anions. Membranes treated with 0.5 molar KI lost 90% of membrane-bound ATP hydrolytic activity; however, in the presence of Mg²⁺ and ATP, only 0.1 molar KI was required for complete inactivation of ATPase and H⁺-pumping activities. A high-affinity binding site for MgATP (dissociation constant = 34 micromolar) was involved in this destabilization. The relative loss of ATPase activity induced by KI, KNO₃, or KCl was accompanied by a corresponding increase in the peripheral subunits in the supernatant, including the nucleotide-binding polypeptides of 70 and 60 kilodaltons. The order of effectiveness of the various ions in reducing ATPase activity was: KSCN > KI > KNO₃ > KBr > K-acetate > K₂SO₄ > KCl. The specificity of nucleotides (ATP > GTP > ITP) in dissociating the ATPase is consistent with the participation of a catalytic site in destabilizing the enzyme complex. Following KI-induced dissociation of the H⁺-ATPase, the removal of KI and MgATP by dialysis resulted in restoration of activity. During dialysis for 24 hours, ATP hydrolysis activity increased to about 50% of the control. Hydrolysis of ATP was coupled to H⁺ pumping as seen from the recovery of H⁺ transport following 6 hours of dialysis. Loss of the 70 and 60 kilodalton subunits from the supernatant as probed by monoclonal antibodies further confirmed that the H⁺-ATPase complex had reassembled during dialysis. These data demonstrate that removal of KI and MgATP resulted in reassociation of the peripheral sector with the membrane integral sector of the vacuolar H⁺-ATPase to form a functional H⁺ pump. The ability to dissociate and reassociate in vitro may have implications for the regulation, biosynthesis, and assembly of the vacuolar H⁺-ATPase in vivo.