INACTIVATION OF CA2+ CALMODULIN-DEPENDENT PROTEIN-KINASE-IV BY CA2+ CALMODULIN AND RESTORATION OF THE ACTIVITY BY MG2+/EGTA

The activity of calmodulin-dependent protein kinase IV (CaM-kinase IV) was progressively decreased by incubation at 30 degrees C with calmodulin in the presence of Ca2+, becoming one-half to one-fifth of the original activity within several minutes. The amount of calmodulin necessary to produce the maximal inactivation was approximately 1 mol for 1 mol of the enzyme, The inactivation of CaM-kinase IV by Ca2+/calmodulin was prevented by ATP in the presence of Mg2+, but such protection was not observed with either of the two alone or with a peptide substrate such as syntide-2. The activity of the calmodulin-inactivated enzyme was increased by incubation at 30 degrees C with Mg2+ in the presence of EGTA, being completely restored to the original level within several minutes, indicating that the Ca2+/calmodulin-induced inactivation of the enzyme was not due to irreversible denaturation of the enzyme, Both the inactivation of CaM-kinase IV by Ca2+/calmodulin and the restoration of its activity by Mg2+/EGTA were time- and temperature-dependent reactions. Kinetic analysis revealed that the alterations of the enzyme activity were due mainly to changes in V-max of the enzyme.