Phosphorylation by casein kinase II alters the biological activity of calmodulin

Calmodulin is the major intracellular Ca²⁺-binding protein, providing Ca²⁺-dependent regulation of numerous intracellular enzymes. The phosphorylation of calmodulin may provide an additional mechanism for modulating its function as a signal transducer. Phosphocalmodulin has been identified in tissues and cells, and calmodulin is phosphorylated both in vitro and in intact cells by various enzymes. Phosphorylation of calmodulin on serine/threonine residues by casein kinase II decreases its ability to activate both myosin-light-chain kinase and cyclic nucleotide phosphodiesterase. For myosin-light-chain kinase the primary effect is an inhibition of the V(max.) of the reaction, with no apparent change in the concentration at which half-maximal velocity is attained (K_0.5) for either Ca²⁺ or calmodulin. In contrast, for phosphodiesterase, phosphorylation of calmodulin significantly increases the K_0.5 for calmodulin without noticeably altering the V(max.) or the K_0.5 for Ca²⁺. The higher the stoichiometry of phosphorylation of calmodulin, the greater the inhibition of calmodulin-stimulated activity for both enzymes. Therefore the phosphorylation of calmodulin by casein kinase II appears to provide a Ca²⁺-independent mechanism whereby calmodulin regulates at least two important target enzymes, myosin-light-chain kinase and cyclic nucleotide phosphodiesterase.