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Effect of phosphorylation in the motor domain of human myosin IIIA on its ATP hydrolysis cycle

机译：人肌球蛋白IIIA的运动域中的磷酸化对其ATP水解周期的影响

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Previous findings suggested that the motor activity of human myosin IIIA (HM3A) is influenced by phosphorylation [Kambara, T., et al. (2006) J. Biol. Chem. 281, 37291-37301]; however, how phosphorylation controls the motor activity of HM3A is obscure. In this study, we clarify the kinetic basis of the effect of phosphorylation on the ATP hydrolysis cycle of the motor domain of HM3A (huM3AMD). The affinity of human myosin IIIA for filamentous actin in the presence of ATP is more than 100-fold decreased by phosphorylation, while the maximum rate of ATP turnover is virtually unchanged. The rate of release of ADP from acto-phosphorylated huM3AMD is 6-fold greater than the overall cycle rate, and thus not a rate-determining step. The rate constant of the ATP hydrolysis step of the actin-dissociated form is markedly increased by phosphorylation by 30-fold. The dissociation constant for dissociation of the ATP-bound form of huM3AMD from actin is greatly increased by phosphorylation, and this result agrees well with the significant increase in the K_(actin) value of the steady-state ATPase reaction. The rate constant of the P_i off step is greater than 60 s~(-1), suggesting that this step does not limit the overall ATP hydrolysis cycle rate. Our kinetic model indicates that phosphorylation induces the dissociation of huM3AMD from actin during the ATP hydrolysis cycle, and this is due to the phosphorylation-dependent marked decrease in the affinity of huM3AMD·ATP for actin and the increase in the ATP hydrolysis rate of huM3AMD in the actin-dissociated state. These results suggest that the phosphorylation of myosin IIIA significantly lowers the duty ratio, which may influence the cargo transporting ability of the native form of myosin IIIA that contains the ATP-independent actin binding site in the tail.
机译：先前的发现表明，人肌球蛋白IIIA（HM3A）的运动活性受磷酸化的影响[Kambara，T.，等人。 （2006）J.Biol。化学281，37291-37301];但是，磷酸化如何控制HM3A的运动活性尚不清楚。在这项研究中，我们阐明了磷酸化作用对HM3A（huM3AMD）的运动域的ATP水解循环的动力学基础。在磷酸存在下，人肌球蛋白IIIA对丝状肌动蛋白的亲和力由于磷酸化作用而降低了100倍以上，而ATP转换的最大速率实际上没有变化。活性磷酸化的huM3AMD释放ADP的速率比整个循环速率高6倍，因此不是速率决定步骤。肌动蛋白解离形式的ATP水解步骤的速率常数通过磷酸化显着增加了30倍。 huM3AMD的ATP结合形式从肌动蛋白上解离的解离常数通过磷酸化大大增加，并且该结果与稳态ATPase反应的K_（actin）值的显着增加完全吻合。 P_i off步骤的速率常数大于60 s〜（-1），表明该步骤不限制总的ATP水解循环速率。我们的动力学模型表明，磷酸化诱导huM3AMD从肌动蛋白的ATP水解周期中解离，这是由于huM3AMD·ATP对肌动蛋白的亲和力的磷酸化依赖性显着降低以及huM3AMD的ATP水解速率的增加。肌动蛋白解离状态。这些结果表明，肌球蛋白IIIA的磷酸化显着降低了占空比，这可能会影响尾部含有ATP独立肌动蛋白结合位点的肌球蛋白IIIA天然形式的货物运输能力。

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