Phosphorylation of Myosin Light Chain by a Protease‐Activated Kinase from Rabbit Skeletal Muscle

A protease‐activated protein kinase that phosphorylates the P light chain of myosin in the absence of Ca²⁺ and calmodulin has been isolated from rabbit skeletal muscle. The enzyme has properties similar to protease‐activated kinase I from rabbit reticulocytes [S. M. Tahara and J. A. Traugh (1981) J. Biol. Chem. 256, 11588–11564], which has been shown to phosphorylate the P light chain of myosin [P. T. Tuazon, J. T.Stull, and J. A. Traugh (1982) Biochem. Biophys. Res. Commun. 108, 910–917]. The protease‐activated kinase from skeletal muscle has been partially purified by chromatography on DEAE‐cellulose, phosphocellulose and hydroxyapatite. The enzyme phosphorylates histone as well as the P light chain of myosin following activation by proteolysis. Stoichiometric phosphorylation of myosin light chain was observed with the protease‐activated kinase and myosin light chain kinase. The sites phosphorylated by the protease‐activated kinase and myosin light chain kinase were examined by two‐dimensional peptide mapping following chymotryptic digestion. The phosphopeptides observed with the protease‐activated kinase were different from those obtained with the Ca²⁺ ‐dependent myosin light chain kinase, indicating that the two enzymes phosphorylated different sites on the P light chain of skeletal muscle myosin. When actomyosin from skeletal muscle was examined as substrate, the P light chain was phosphorylated following activation of the protease‐activated kinase by limited proteolysis.