Polycation‐dependent, Ca2+‐antagonized phosphorylation of calmodulin by casein kinase‐2 and a spleen tyrosine protein kinase

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Abstract
Ten distinct protein kinases have been tested for their ability to phosphorylate calmodulin. Only casein kinase‐2 and a spleen tyrosine protein kinase (TPK‐III) proved effective, their phosphorylation efficiency being dramatically enhanced by histones and other polybasic peptides while being depressed by 50 μM Ca2+. Phosphorylation by CK‐2 takes place with a Km of 12 μM calmodulin, leading to the incorporation of more than 1.5 mol P/mol substrate. Ser81 and Thr79 are among the residues affected. On the other hand, the two tyrosyl residues of calmodulin are both phosphorylated by TPK‐III, Tyr99 being preferred over Tyr138.