Changes in cytosolic magnesium levels can regulate the activity of the plasma membrane H+-ATPase in maize

Plant plasma membrane (PM) H+-ATPase, a major consumer of cellular ATP, is driven by the magnesium ATP complex which may dissociate at low cytosolic Mg2+ activity. We investigated whether hydrolytic activity of PM H+-ATPase is inhibited at ATP concentrations exceeding the Mg2+ concentration. Activity in isolated maize plasma membranes was measured at pH 6.5 in the presence of 5 mM Mg2+ (high) or 2 mM Mg2+ (low), while K+ was applied at concentrations of 155 mM (high) or 55 mM (low). In all experiments, with membrane vesicles either from roots or leaves, the enzyme activity decreased in the presence of Mg-free ATP. At inhibitory ATP concentrations, the activity was not influenced by the K+ concentration. The activity was restored after increasing the Mg2+ concentration. ATP inhibition also occurred at pH 7.5. Kinetic modeling shows that Mg-free ATP acted as a competitive inhibitor with a Ki in the range of the Km. Ki decreased by 75% at low K+ concentration. Ki was one order of magnitude lower at pH 7.5 compared to pH 6.5. The observed inhibition is consistent with a concept in which down-regulation of the cytosolic Mg2+ activity is involved in (phyto)hormonal stress responses.