Activation of Ca²⁺-calmodulin-dependent protein kinase Ia is due to direct phosphorylation by its activator

Ca²⁺-Calmodulin-dependent protein kinase Ia (CaM kinase Ia) is phosphorylated, and its activity enhanced up to 50-fold, in the presence of a protein purified from pig brain termed CaM kinase Ia activator. We report here that phosphorylation of CaM kinase Ia in the presence of the activator occurs primarily on threonine (87%) and slightly on serine (13%) residues. Treatment of CaM kinase Ia with the irreversible ATP affinity analogue, 5'-p-fluorosulfonylbenzoyl adenosine (FSBA), reduces its activity by 86% but has no effect on its ability to be phosphorylated, whereas FSBA-treatment of the activator reduces its ability to activate and phosphorylate CaM kinase Ia by 92 and 93%, respectively. Thus, CaM kinase Ia activator is a protein Thr/Ser kinase which activates by phosphorylating CaM kinase Ia rather than by enhancing the latter's autophosphorylation.