Ordered phosphorylation of the two 20 000 molecular weight light chains of smooth muscle myosin.

The time courses of phosphorylation of the Mr 20 000 light chains by purified myosin light chain kinase plus calmodulin were determined. In confirmation of an earlier report [Persechini, A., & Hartshorne, D. J. (1981) Science (Washington, D.C.) 213, 1383-1385], a steady-state kinetic analysis indicates that the phosphorylation occurs in an ordered manner; i.e., at a phosphorylation level of 0.5 mol of 32P incorporated per mol of bound Mr 20 000 light chain, each myosin molecule would have one phosphorylated head. The kinetic parameters obtained for the phosphorylation of the more reactive myosin head are similar to those determined by using isolated light chains. It is suggested that the ordered, or sequential, phosphorylation, and the different reactivities of the two Mr 20 000 light chains, is the result of preexisting asymmetry of the myosin molecule. Similar patterns of myosin phosphorylation are obtained in both the absence and presence of skeletal muscle actin.