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In the theory of regulation of smooth-muscle actomyosin involving phosphorylation, it is generally accepted that the Mg2+-ATPase activity of phosphorylated myosin is activated by actin, whereas the activity of the unphosphorylated myosin is not. The role of Ca2+, however, is not clear and here there are basically two possibilities; i.e., either Ca2+ acts only to activate the myosin light-chain kinase or, that in addition to the sites on the myosin light-chain kinase, other regulatory sites are involved, presumably on the myosin molecule. To distinguish between these possibilities, three types of experiments were performed: (1) Gizzard myosin was phosphorylated and then separated from the myosin light-chain kinase and phosphatase by chromatography on Sepharose 4-B.

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