Na+-ATPase from the plasma membrane of the marine alga Tetraselmis (Platymonas) viridis forms a phosphorylated intermediate.

Plasma membranes isolated from the marine unicellular alga Tetraselmis (Platymonas) viridis were phosphorylated by [gamma-32P]ATP, and membrane proteins were then analyzed by PAGE in SDS, under acidic conditions. Three radioactive components with apparent molecular masses of 100 kDa, 76 kDa, and 26 kDa were detected. The phosphorylation of one of them, the 100 kDa polypeptide, was specifically stimulated by Na+. Vanadate almost completely inhibited the Na+-mediated phosphorylation of the peptide. The phosphate bound to this peptide underwent rapid turnover and was discharged by hydroxylamine. The 100 kDa phosphopeptide was sensitive to ADP. The conclusion is drawn that the 100 kDa phosphopeptide is a phosphorylated intermediate of the Na+-transporting ATPase in the T. viridis plasma membrane.