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

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Abstract
A protease-activated protein kinase that phosphorylates the P L chain of myosin in the absence of Ca2+ and calmodulin was isolated from rabbit skeletal muscle. The enzyme has properties similar to protease-activated kinase I from rabbit reticulocytes which phosphorylates the P L chain of myosin. The protease-activated kinase from skeletal muscle was partially purified by chromatography on DEAE-cellulose, phosphocellulose and hydroxyapatite. The enzyme phosphorylates histone and the P L chain of myosin following activation by proteolysis. Stoichiometric phosphorylation of myosin L chain was observed with the protease-activated kinase and myosin L chain kinase. The sites phosphorylated by the protease-activated kinase and myosin L chain kinase were examined by 2-dimensional peptide mapping following chymotryptic digestion. The phosphopeptides observed with the protease-activated kinase were different from those obtained with the Ca2+-dependent myosin L chain kinase, indicating that the 2 enzymes phosphorylated different sites on the P L chain of skeletal muscle myosin. When actomyosin from skeletal muscle was examined as substrate, the P L chain was phosphorylated following activation of the protease-activated kinase by limited proteolysis.