Regulation of the Actin‐Activated Mg‐ATPase of Brain Myosin via Phosphorylation by the Brain Ca²⁺, Calmodulin‐Dependent Protein Kinases

We have previously isolated two Ca²⁺, calmodulin‐dependent protein kinases with molecular weights of 120,000 (120K enzyme) and 640,000 (640K enzyme), respectively, by gel filtration analysis from rat brain. Chicken gizzard myosin light‐chain kinase and the 120K enzyme phosphorylated two light chains of brain myosin, whereas the 640K enzyme phosphorylated both the two light chains and the heavy chain. The phosphopeptides of the light chains digested by Staphylococcus aureus V8 protease were similar among chicken gizzard myosin light‐chain kinase, the 120K enzyme, and the 640K enzyme. Only the seryl residue in the light chains and the heavy chain was phosphorylated by the enzymes. The phosphorylation of brain myosin by any of these enzymes led to an increase in actin‐activated Mg‐ATPase activity. The results suggest that brain myosin is regulated by brain Ca²⁺, calmodulin‐dependent protein kinases in a similar but distinct mechanism in comparison with that of smooth muscle myosin.