Effect of Mg 2+, Triton X-100 and temperature on basal and FC-stimulated plasma membrane ATPase activity

The H +-pumping driven by the plasma membrane H +-ATPase in membrane vesicles from 24-h-old radish seedlings was earlier shown to be stimulated by pretreatment of the membranes with fusicoccin (FC) [see Plant Physiology (1986) 82, 121-125]. FC-pretreatment also stimulated the ATPase activity, but to a lesser extent than H +-pumping. More than 80% of the ATPase activity was inhibited by 100 muM vanadate or by 3 mM Ca 2+. Preincubation of diluted membranes in the presence of 5 mM MgSO 4 without ATP lowered both ATPase and H +-pumping activity by 20-30% without affecting FC-stimulated activities. After preincubation with MgSO 4, ATPase activity of membranes pretreated with or without FC was differently affected by Triton X-100 and by temperature: Triton X-100 activated FC-stimulated ATPase more than that of the controls, whereas an increase of temperature (13 -> 23 -> 33C) enhanced ATPase activity to a greater extent in the controls. It is suggested that, while H +-pumping in this membrane fraction is driven only by the plasma membrane H +-ATPase, ATP-hydrolysis is catalyzed by two different enzymes (or forms of the same enzyme).