Greene L
J Biol Chem. 1982 Dec 10;257(23):13993-9.
Previous studies have shown that in the presence of ADP, myosin subfragment 1 (S-1) binds with positive cooperativity to the troponin-tropomyosin-actin complex (regulated actin). This binding is much more cooperative in the absence than in the presence of Ca2+. Based on these data, we proposed a model (Hill, T. L., Eisenberg, E., and Greene, L. E. (1980) Proc. Natl. Acad. Sci. U. S. A. 77, 7209-7213) in which the tropomyosin-actin units in regulated actin exist in two states, a state which binds S-1 weakly and a state which binds S-1 strongly. This model predicts that the equilibrium constant between the weak and strong states of the tropomyosin-actin units is an intrinsic property of the regulated actin filament. Therefore, its value should not change when measured with different nucleotides bound to S-1. To test this prediction, the equilibrium constant was measured in the presence of S-1, S-1 . ADP, and S-1 . AMP-PNP, with and without Ca2+. The results show that, in all cases, S-1 binds with positive cooperativity to regulated actin. The observed cooperativity is consistent with the model since it was possible to fit the data obtained in the presence of different nucleotides with the same value for the equilibrium constant between the weak and strong states of the tropomyosin-actin units. However, the nucleotide bound to S-1 appears to affect the observed cooperativity by changing the affinity of S-1 for the strong and weak states.
先前的研究表明，在二磷酸腺苷（ADP）存在的情况下，肌球蛋白亚片段1（S-1）以正协同性与肌钙蛋白-原肌球蛋白-肌动蛋白复合物（调节型肌动蛋白）结合。在没有钙离子（Ca2+）存在时，这种结合的协同性比有Ca2+时要强得多。基于这些数据，我们提出了一个模型（希尔，T.L.，艾森伯格，E.，以及格林，L.E.（1980年）《美国国家科学院院刊》77卷，7209 - 7213页），其中调节型肌动蛋白中的原肌球蛋白-肌动蛋白单元存在两种状态，一种是与S-1弱结合的状态，另一种是与S-1强结合的状态。该模型预测，原肌球蛋白-肌动蛋白单元的弱态和强态之间的平衡常数是调节型肌动蛋白丝的固有属性。因此，当用与S-1结合的不同核苷酸进行测量时，其值不应改变。为了验证这一预测，在有S-1、S-1·ADP和S-1·AMP-PNP存在的情况下，分别在有和没有Ca2+的条件下测量了平衡常数。结果表明，在所有情况下，S-1都以正协同性与调节型肌动蛋白结合。观察到的协同性与该模型一致，因为可以用原肌球蛋白-肌动蛋白单元的弱态和强态之间相同的平衡常数来拟合在不同核苷酸存在时获得的数据。然而，结合到S-1上的核苷酸似乎通过改变S-1对强态和弱态的亲和力来影响观察到的协同性。
