Full Activation of Brain Calmodulin-Dependent Protein Kinase IV Requires Phosphorylation of the Amino-Terminal Serine-Rich Region by Calmodulin-Dependent Protein Kinase IV Kinase1

Rat brain calmodulin-dependent protein kinase IV (CaM-kinase IV) was activated approximately 20 to 30-fold by incubation with CaM-kinase IV kinase purified from rat brain under the Ca2+/calmodulin-dependent phosphorylation conditions. When CaM-kinase IV was incubated without CaM-kinase IV kinase, no significant activation was observed, indicating that the marked activation of CaM-kinase IV occurred as a result of the action of CaM-kinase IV kinase. More than 3 mol of phosphate were incorporated into 1 mol of the enzyme after incubation with CaM-kinase IV kinase at 30 degrees C for 20 min, but the activation occurred upon the initial incorporation of 1 mol of phosphate. The rate of the initial phosphorylation increased when the amount of CaM-kinase IV kinase added into the reaction mixture increased, but the rate of phosphorylation following the initial phosphorylation did not increase, suggesting that the initial phosphorylation was catalyzed by CaM-kinase IV kinase, and that subsequent phosphorylation was catalyzed by CaM-kinase IV itself activated by CaM-kinase IV kinase. The initial phosphorylation occurred in the amino-terminal serine-rich region of CaM-kinase IV. Kinetic analysis revealed that the increase in the activity upon phosphorylation was due mainly to an increase in the Vmax values.