A DUAL REGULATION OF THE ACTIN-MYOSIN INTERACTION IN ADRENAL MEDULLARY ACTOMYOSIN BY ACTIN-LINKED AND MYOSIN-LINKED SYSTEMS

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Actomyosin isolated from the bovine adrenal medulla was capable of Ca²⁺-dependent superprecipitation, and retained the actin-stimulated Mg²⁺-ATPase activity. A half maximum activation was attained with Ca²⁺ at about 4 μM. The superprecipitation was inhibited by agents interacting with calmodulin, i.e. trifluoperazine, chlorpromazine, W-7 and promethazine with I₅₀ values of 15, 70, 85 and 440 μM, respectively, suggesting the involvement of calmodulin in the Ca²⁺-dependent actin-myosin interaction in medullary actomyosin. About 8-10 mg of myosin was obtained from 200 mg of medullary actomyosin isolated from 100g of the bovine adrenal medulla. Caldesmons (caldesmon₁₅₀ and caldesmon₇₇), actin-linked regulatory proteins, were identified in our actomyosin preparation by immunoblotting using anti-gizzard caldesmon IgG. The medullary actinmyosin interaction was inhibited by caldesmons in a dose-dependent manner. Myosin light chain kinase, a myosin-linked regulatory protein, was also identified in our medullary actomyosin preparation by immunoblotting. Actin-stimulated Mg²⁺-ATPase activity of medullary myosin was enhanced 2.5-fold when the myosin light chain was phosphorylated in the presence of Ca²⁺ and calmodulin. We suggest that the actin-myosin interaction in medullary actomyosin is dually regulated by Ca²⁺ via the actin-linked and myosin-linked systems.