Wagner P D, Giniger E
J Biol Chem. 1981 Dec 25;256(24):12647-50.
Regulated actin, F-actin plus troponin-tropomyosin, activates the myosin ATPase in the presence but not in the absence of calcium. Ultracentrifugation has been used to examine the interaction of regulated actin with two proteolytic fragments of myosin, heavy meromyosin (HMM), a two-headed species, and subfragment 1 (S-1), a single head. In the presence of ATP, the association constant (Ka) for the binding of S-1 to regulated actin is approximately 1.5 X 10(4) M-1, whether or not calcium is present. This is true for S-1 with and without the Mr = 19,000 phosphorylatable light chain. These results confirm the observations of Chalovich et al. (Chalovich, J., Chock, P. B., and Eisenberg, E. (1981) J. Biol. Chem. 256, 575-578) and support their suggestion that the inhibition of the regulated actin-activated ATPase of S-1 by removal of calcium does not result from preventing S-1 binding. The binding of HMM to regulated actin in the presence of ATP, however, is calcium sensitive. Removal of calcium results in approximately a 4-fold decrease in Ka, from 1.1 X 10(4) M-1 to 2.5 X 10(3) M-1. The results with HMM are more easily reconciled with the low stiffness of relaxed muscle and suggest a functional difference between S-1 and HMM.
受调控的肌动蛋白，即F - 肌动蛋白加上肌钙蛋白 - 原肌球蛋白，在有钙存在时而非无钙时激活肌球蛋白ATP酶。超速离心已被用于研究受调控的肌动蛋白与肌球蛋白的两个蛋白水解片段的相互作用，双头的重酶解肌球蛋白（HMM）和单头的亚片段1（S - 1）。在ATP存在的情况下，无论有无钙，S - 1与受调控的肌动蛋白结合的缔合常数（Ka）约为1.5×10⁴ M⁻¹。对于有和没有分子量为19,000的可磷酸化轻链的S - 1都是如此。这些结果证实了Chalovich等人的观察结果（Chalovich, J., Chock, P. B., and Eisenberg, E. (1981) J. Biol. Chem. 256, 575 - 578），并支持了他们的观点，即通过去除钙来抑制S - 1的受调控肌动蛋白激活的ATP酶并非由于阻止S - 1结合所致。然而，在ATP存在时，HMM与受调控的肌动蛋白的结合对钙敏感。去除钙会导致Ka大约降低4倍，从1.1×10⁴ M⁻¹降至2.5×1⁰³ M⁻¹。HMM的结果更容易与松弛肌肉的低僵硬度相协调，并表明S - 1和HMM之间存在功能差异。
