A Brain - specific Ca 2 + / Calmodulin - dependent Protein Kinase ( CaM Kinase - Gr ) Is Regulated by Autophosphorylation

A neuronal Ca2+/calmodulin-dependent protein kinase (CaM kinase-Gr) undergoes autophosphorylation on a serine residue(s) in response to Ca2+ and calmodulin. Phosphate incorporation leads to the formation of a Ca2+-independent (autonomous) activity state, as well as potentiation of the Ca’+/calmodulin-dependent response. The autonomous enzyme activity of the phosphorylated enzyme approximately equals the Ca2+/ca1modulin-stimulated activity of the unphosphorylated enzyme, but displays diminished affinity toward ATP and the synthetic substrate, syntide-2. The Km(app) for ATP and syntide-2 increased 4.3and 1.7-fold, respectively. Further activation of the autonomous enzyme by Ca2+/calmodulin yields a marked increase in the affinity for ATP and peptide substrate such that the for ATP and syntide-2 decreased by 14and 8fold, respectively. Both autophosphorylation and the addition of Caz+/calmodulin are required to produce the maximum level of enzyme activation and to increase substrate affinity. Unlike Ca2+/calmodulin-dependent protein kinase type I1 that is dephosphorylated by the Mg2+-independent phosphoprotein phosphatases 1 and 2A, CaM kinase-Gr is dephosphorylated by a Mg2+-dependent phosphoprotein phosphatase that may be related to the type 2C enzyme. Dephosphorylation of CaM kinase-Gr reverses the effects of autophosphorylation on enzyme activity. A comparison between the autophosphorylation and dephosphorylation reactions of CaM kinaseGr and Ca2+/calmodulin-dependent protein kinase type I1 provides useful insights into the operation of Ca2+sensitive molecular switches.