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

摘要

>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 K_m 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.