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.