Mg 2+ /KCl-ATPase of plant plasma membranes is an electrogenic pump

Abstract

The function of the Mg2+-requiring KCl-stimulated ATPase (ATP phosphohydrolase, EC 3.6.1.3) of higher plants in active ion transport was investigated by using a purified microsomal fraction containing sealed plasma membrane vesicles. (Sze, H-1980). A transmembrane electrical potential (+30 to +44 mV), monitored by uptake of a permeant anion (35SCN-), was generated specifically by ATP in purified microsomal vesicles of tobacco [Nicotiana tabacum L. cv. Wisconsin 38] callus. ATP-dependent 35SCN- uptake required Mg2+, was optimal at pH 6.75, and showed similar ATP concentration dependence as the Mg2+-requiring KCl-stimulated ATPase activity. Plasma membrane ATPase inhibitors (N,N''-dicyclohexylcarbodiimide and vanadate) inhibited generation of the ATP-dependent electrical potential. A proton conductor (carbonyl cyanide m-chlorophenylhydrazone), but not a K+ ionophore (valinomycin), completely collapsed the electrical potential. In vitro evidence is provided that the Mg2+/KCl-ATPase of higher plants is an electrogenic pump. These results are consistent with the hypothesis that an electrogenic H+ pump is catalyzed by the plasma membrane ATPase of plants.