Mg/KCl-ATPase of plant plasma membranes is an electrogenic pump.
The function of the Mg(2+)-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) Proc. Natl. Acad. Sci. USA 77, 5904-5908). A transmembrane electrical potential (+30 to +44 mV), monitored by uptake of a permeant anion ((35)SCN(-)), was generated specifically by ATP in purified microsomal vesicles of tobacco callus. ATP-dependent (35)SCN(-) uptake required Mg(2+), was optimal at pH 6.75, and showed similar ATP concentration dependence as the Mg(2+)-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. The results provide in vitro evidence that the Mg(2+)/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.
Department of Botany, University of Kansas, Lawrence, Kansas 66045.
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Churchill