The role of autophosphorylation in activation of the type II calmodulin-dependent protein kinase

Autophosphorylation of the type II calmodulin-dependent protein kinase is known to remove the dependence of this enzyme on Ca2+ and calmodulin. The enzymatic activity in the presence of Ca2+, on the other hand, was reported to be unaffected or decreased by this interconversion. The role of autophosphorylation in the kinase reaction was reinvestigated using short assay times and low ATP concentrations to decrease the extent and rate of this process. Under these conditions, the ATP dependence of the kinase reaction with syntide-2 as the substrate (but not the autophosphorylation reaction) exhibited kinetic cooperativity due to a lag in the progress curve of syntide-2 conversion. Partial autophosphorylation of the protein kinase prior to phosphorylation of the peptide substrate completely abolished this hysteretic response without affecting the final rate of substrate conversion. These observations suggest that autophosphorylation is an obligatory step in the response of this kinase to activation by calmodulin.