Partial characterization of a phosphorylated intermediate associated with the plasma membrane ATPase of corn roots [Zea mays]

The phosphorylated protein associated with a deoxycholate-extracted plasma membrane fraction from corn (Zea mays L. var WF9 × Mol7) roots was characterized in order to correlate its properties with those of plasma membrane ATPase. Its phosphorylation, like that of plasma membrane ATPase, was dependent on Mg2+, substrate specific for ATP, insensitive to azide, oligomycin, or molybdate, and sensitive to N,N′-dicyclohexylcarbodiimide, diethylstilbestrol, or vanadate. Monovalent cations affected the phosphorylation of the protein in a manner consistent with their stimulatory effects on ATPase. For K+, this was shown to occur through an increase in the turnover of the phosphoenzyme. Analysis of the phosphorylated protein by NaDodSO4/polyacrylamide gel electrophoresis revealed the presence of a single labeled polypeptide with a molecular weight of about 100,000. Phosphorylation of this polypeptide was dependent on Mg2+, sensitive to K+, and inhibited by vanadate. It is concluded that this polypeptide represents the catalytic subunit of the plasma membrane ATPase. These results are discussed in terms of a model for the coupling of metabolic energy to H+and K+transport in higher plants.