Evidence for Nucleotide-Dependent Passive H+ Transport Protein in the Plasma Membrane of Barley Roots

Plasma membranes were isolated from barley roots by two-phase partitioning, and octylglucoside-soluble and -insoluble fractions were obtained. The insoluble fractions were reconstituted into liposomes, and the plasma membrane H(+)-ATPase was shown to participate in MgATP-dependent H(+) transport activity. The H(+) transport was decreased when the octylglucoside-soluble fraction was reconstituted together with the insoluble fraction. The decrease was not due to inhibition of the H(+)-ATPase, but rather was likely due to the increased H(+) leakage from the proteoliposome. The octylglucoside-soluble fraction was, therefore, reconstituted in the liposomes and the passive H(+) transport was determined using the pH jump method. A pH gradient across the membranes was generated by the pH jump, and the gradient was found to be dissipated by passive H(+) transport. The H(+) transport required ATP, K(+), and valinomycin. The H(+)-transport also occurred when ADP, AMP, GTP, or ATP-gamma-S was present instead of ATP, and did not occur when the octylglucoside-soluble fraction was boiled before the reconstitution. These findings suggest that nucleotide-dependent H(+ )transport protein is present in the plasma membrane of root cells.