Phosphorylation-dephosphorylation of membrane proteins controls the microsomal H-ATPase activity of corn roots

The Mg²⁺-dependent H⁺-ATPase activity of a sealed microsomal vesicle fraction isolated from corn (Zea mays L.) roots appears to be controlled by a phosphorylation-dephosphorylation cycle. Phosphorylation of the microsomal fraction is carried out by a Ca²⁺/calmodulin (CaM)-stimulated process. The H⁺-ATPase activity decreases with increasing phosphorylation of the membranes and becomes only slightly uncoupled by ionophores and less inhibited by dicyclohexylcarbodiimide (DCCD), diethylstilbestrol (DES), NO₃^? and vanadate. The inhibitory effect of phosphorylation is greater on the NO₃^?-sensitive H⁺-ATPase activity than on the vanadate-sensitive activity. Restoration of H⁺-ATPase activity is achieved by allowing the phosphorylated membranes to dephosphorylate either in the absence or presence of exogenous alkaline phosphatase. Moreover, the presence of fluphenazine during the Ca²⁺/CaM-stimulated treatment inhibits membrane phosphorylation and protects the H⁺-ATPase activity from inhibition.