The contractile proteins of smooth muscle. Properties and components of a Ca2+-sensitive actomyosin from chicken gizzard.

The preparation and characterization of a Ca²⁺-sensitive actomyosin from chicken gizzard is described. The pH curve of the Mg²⁺ ATPase activity of the actomyosin was dominated by the activity of the myosin component, and this gave rise to the acid and alkaline optima. Skeletal muscle myosin showed a similar curve. Both the activation of myosin ATPase by actin, and the Ca²⁺ sensitivity were confined to the neutral pH region. The subunit composition of the Ca²⁺-sensitive actomyosin was interesting in that no components corresponding to skeletal muscle troponin were obvious. It is suggested that the activity of gizzard actomyosin is regulated by a protein on the thin filaments with a subunit weight of ~130,000.