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The Journal of Biochemistry
Vol. 91 (1982) No. 5 P 1809-1812

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1. The actin-activated Mg2+-ATPase activity of gizzard HMM increased in proportion to the square of the extent of LC phosphorylation. This result indicates that the LCs of HMM are randomly phosphorylated, and the phosphorylation of both heads of HMM is required for the activation of HMM Mg2+-ATPase by F-actin.
2. In 75mM KCl, the Mg2+-ATPase activity of gizzard myosin was activated by F-actin only slightly when a half of the total LC was phosphorylated. From 1 to
2 mol LC phosphorylation, the activity was enhanced by F-actin almost linearly. In 30mM KCl, the activity of acto-gizzard myosin increased sigmoidally with increase in the extent of LC phosphorylation. On electron microscopy, side-by-side aggregates of myosin filaments were observed in 30mM KCl, but not in 75mM KCl. It was suggested that the activation of the Mg2+-ATPase activity of acto-gizzard myosin LC phosphorylation is modified by formation of myosin filaments and their aggregates.
3. The relationship between the actin-activated Mg2+-ATPase activity of HMM or myosin and the extent of LC phosphorylation was unaffected by tropomyosin.

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