Abstract
Myosin L chain kinase is a protein of MW about 80,000 that is inactive in the absence of calmodulin. In the presence of 1 mol of calmodulin/mol of kinase 80-90% of the maximal activity is obtained. Crude preparations of the whole L chain fraction of rabbit fast-skeletal-muscle myosin contain enough calmodulin to activate the enzyme. A method for the preparation of calmodulin-free P L is described. A procedure is described for the isolation of calmodulin from rabbit fast skeletal muscle. Rabbit fast skeletal muscle calmodulin is indistinguishable from bovine brain calmodulin in its ability to activate myosin L chain kinase. The other properties of these 2 proteins are also very similar. Rabbit fast skeletal muscle troponin C was about 10% as effective as calmodulin as activator for myosin L chain kinase. By chromatography on a Sepharose-calmodulin affinity column evidence was obtained for the formation of a Ca2+-dependent complex between calmodulin and myosin L chain kinase. Troponin I from rabbit fast skeletal muscle and histone IIAS were phosphorylated by fully activated myosin L chain kinase at about 1% of the rate of the P L chain.

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