Plasma membrane ATPase of red beet forms a phosphorylated intermediate

When a plasma membrane-enriched fraction isolated from red beet (Beta vulgaris L.) was incubated in the presence of 40 micromolar [γ-³²P] ATP, 40 micromolar MgSO₄ at pH 6.5, a rapidly turning over phosphorylated protein was formed. Phosphorylation of the protein was substrate-specific for ATP, sensitive to diethylstilbestrol and vanadate, but insensitive to azide. When the dephosphorylation reaction was specifically studied, KCl was found to increase the turnover of the phosphorylated protein consistent with its stimulatory effect upon plasma membrane ATPase. The protein-bound phosphate was found to be most stable at a pH between 2 and 3 and under cold temperature, suggesting that the protein phosphate bond was an acyl-phosphate. When the phosphorylated protein was analyzed with lithium dodecyl sulfate gel electrophoresis, a labeled polypeptide with a molecular weight of about 100,000 daltons was observed. Phosphorylation of this polypeptide was rapidly turning over and Mg-dependent. It is concluded that the phosphorylation observed represents a reaction intermediate of the red beet plasma membrane ATPase.