Sterol Modulation of the Plasma Membrane H+-ATPase Activity from Corn Roots Reconstituted into Soybean Lipids

Abstract

A partially purified H+-ATPase from the plasma membrane (PM) of corn (Zea mays L.) roots was inserted into vesicles prepared with soybean (Glycine max L.) phospholipids and various concentrations of individual sterols using either a freeze-thaw sonication or an octylglucoside dilution procedure. Both methods yielded a functional enzyme that retained its native characteristics. We have investigated the effects of typical plant sterols (i.e. sitosterol, stigmasterol, and 24-methylcholesterol) on both ATP hydrolysis and H+ pumping by the reconstituted corn root PM ATPase. We have also checked the influence of cholesterol and of two unusual sterols, 24-methylpollinastanol and 14[alpha],24-dimethylcholest-8-en-3[beta]-ol. Here we present evidence for a sterol modulation of the plant PM H+-ATPase activity. In particular, cholesterol and stigmasterol were found to stimulate the pump, especially when present at 5 mol%, whereas all of the other sterols tested behaved as inhibitors at any concentration in proteoliposomes. In all situations H+ pumping was shown to be more sensitive to a sterol environment than was ATP hydrolysis. Our results suggest the occurrence of binding sites for sterols on the plant PM H+-ATPase.