Properties of a calmodulin-activated Ca2+-dependent protein kinase from wheat germ.

A soluble protein kinase that is largely dependent upon Ca2+ for activity was partially purified from wheat germ. The protein kinase (Mr 90 000) catalyzes the phosphorylation of casein, histones and of endogenous proteins. Calmodulin activates the protein kinase with histone as substrate, half-maximal activation being obtained with 1.4 microM sheep brain calmodulin. The rate of casein phosphorylation is half-maximal at 0.3 microM free Ca2+ and maximal at 2.0 microM free Ca2+. Higher Ca2+ is required for histone phosphorylation, namely 80 microM and 500 microM free Ca2+, respectively, for half-maximal and maximal phosphorylation rates. In addition to Ca2+, millimolar Mg2+ is required for maximal activity of the enzyme; millimolar Mn2+ can substitute for the (Ca2+ + Mg2+) requirement. The Km for ATP is 31 microM; other nucleoside 5'-triphosphates and ADP inhibit phosphoryl transfer from ATP to protein. Serine and threonine residues of casein or histones are phosphorylated by the enzyme. The protein kinase is inhibited by relatively high concentrations of chlorpromazine and fluphenazine. The low free Ca2+ required for activation of the enzyme suggests that this type of Ca2+-dependent protein kinase may be involved in Ca2+-mediated stimulus-response coupling in plants.