Bovine brain calmodulin-dependent protein kinase II: molecular mechanisms of autophosphorylation.

A bovine brain calmodulin-dependent protein kinase undergoes autophosphorylation with a maximal incorporation of 10 mol phosphate per mol enzyme. The autophosphorylated enzyme is fully active, independent of Ca2+ and calmodulin. Although the autophosphorylation reaction depends on Ca2+ and calmodulin, it can be rendered Ca(2+)-independent after about 15 seconds when 1 mol phosphate has been incorporated per mol enzyme. This Ca(2+)-independent autophosphorylation is an intramolecular reaction, since mixing of autophosphorylated and non-phosphorylated calmodulin-dependent protein kinase does not result in the Ca(2+)-independent phosphorylation of the non-phosphorylated enzyme. The Ca(2+)-independent autophosphorylation can be inhibited by an exogenous substrate, e.g., casein. The casein phosphorylation activity is approximately proportional to the level of phosphorylation of the enzyme.