Human platelet myosin light chain kinase requires the calcium-binding protein calmodulin for activity.

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Abstract
In an actomyosin fraction isolated from human platelets, phosphorylation of the 20,000 dalton L chain of myosin was stimulated by Ca and the Ca-binding protein calmodulin. The enzyme catalyzing this phosphorylation was isolated by using calmodulin-affinity chromatography. Platelet myosin L chain kinase activity was monitored throughout the isolation procedures by using the 20,000 dalton smooth muscle myosin L chain purified from turkey gizzards as substrate. The partially purified myosin kinase required Ca and calmodulin for activity and had a specific activity of 3.1 .mu.mol of phosphate transferred to the 20,000 dalton L chain/mg of kinase per min under optimal assay conditions. Km values determined for ATP and myosin L chains were 121 .mu.M and 18 .mu.M, respectively. Of several substrates surveyed as phosphate acceptors (.alpha.-casein, histone II-A, phosphorylase b, protamine, histone V-S and phosvitin), only the 20,000 dalton myosin L chain was phosphorylated at a significant rate. Platelet myosin L chain kinase is apparently a Ca-dependent enzyme, and the requirement for Ca is apparently mediated by the Ca-binding protein calmodulin.

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