Manganese enhances the phosphorylation of membrane-associated proteins isolated from barley roots

Microsomal membranes isolated from barley roots (Hordeum vulgare L. cv. CM72) contained endogenous protein phosphorylation activities that were greatly enhanced by Mn²⁺. Mg²⁺ions also stimulated protein phosphorylation, but to a lesser extent than Mn²⁺. Ca²⁺ enhanced Mg²⁺, but not Mn²⁺-dependent phosphorylation. It is proposed that this strong enhancement by Mn²⁺ may be due to a greater affinity of Mn²⁺ than either Ca²⁺ or Mg²⁺ for both the Ca²⁺ and Mg²⁺ binding sites of certain kinases. Some Mn²⁺ stimulated kinase activity was eliminated from the membrane by washing with 0.2 mol/L KCl. The KCl extract contained histone and casein kinase activities, and 4 major phosphoproteins that were phosphorylated on serine and threonine residues. Phosphorylation of a 52 kDa polypeptide corresponded with the characteristics of the histone kinase activity and may represent the autophosphorylation of a CDPK-type kinase. Phosphorylation of a 36 kDa polypeptide was Ca²⁺ stimulated and may represent the autophosphorylation of a different type of unknown kinase. Polypeptides of 18 and 15 kDa had characteristics that suggest they were autophosphorylating subunits of a membrane bound nucleotide di-phosphokinase.