PHOSPHORYLATION OF CALMODULIN BY THE EPIDERMAL-GROWTH-FACTOR-RECEPTORTYROSINE KINASE

An epidermal-grow th-factor(EGF)-receptor preparation is elated by cal modulin-affinity chromatography from rat liver plasma membranes is abl e to phosphorylate calmodulin. Calmodulin phosphorylation was enhanced 3-8-fold by EGF, was dependent on the presence of a polycation or bas ic protein and was inhibited by micromolar concentrations of Ca2+. Pho sphate incorporation into calmodulin occurs predominantly on tyrosine residues. Partial proteolysis of phosphocalmodulin by thrombin identif ies Tyr99, located in the third calcium-binding domain of calmodulin, as the phosphorylated residue. Stoichiometric measurements show a P-32 /calmodulin molar ratio of approximately 1 when optimal phosphorylatio n conditions are used.