MECHANISM OF REGULATION OF THE ACTOMYOSIN ATPASE

在我们的调节模型中，原肌球蛋白可以存在于 细丝，形成弱结合状态或强结合状态 受调控的肌动蛋白的状态。处于弱结合状态，该状态发生在 钙缺乏时肌钙蛋白原抑制S-1和S-1的结合 腺苷二磷酸对肌动蛋白的作用，而不是S-1.ATP。因此，ATPase的抑制似乎是 由于调节肌动蛋白处于弱结合状态，抑制PI的释放。 首先，得到了与模型后半部分一致的结果。 使用与肌动蛋白交联的S-1。发现了肌钙蛋白-原肌球蛋白 显著抑制交联型肌动蛋白S-1的ATPase活性 低离子强度和高离子强度。由于交联型S-1似乎表现出 就像S-1在无限浓度的肌动蛋白存在下的运动学一样， 通过交联型S-1获得广泛的调控不是由于 肌钙蛋白原肌球蛋白阻断交联型S-1与肌动蛋白的结合。 其次，我们考察了pPDM.S-1的能力，它是 S-1.ATP，开启或增强调节的ACTO的ATPase活性 S以1比1获胜。与我们的绑定数据一致，显示没有明显的 S-1、三磷酸腺苷或pPDM.S-1三磷酸腺苷与调节肌动蛋白的协同结合， PPDM.S-1三磷酸腺苷不能显著激活细胞的ATPase活性 根据S-1的调节。这些结果表明原肌球蛋白仍然存在于 PPDM S-1ATP与调节肌动蛋白结合时的抑制位置。第三，我们 研究发现，与骨骼肌实验结果相反， 肌动球蛋白ATPase循环中PI释放的抑制 不伴随肌球蛋白与肌动蛋白结合的抑制。这些 结果表明，虽然抑制PI释放似乎是一种普遍现象 肌肉调节机制，它并不总是与抑制相结合 肌球蛋白ADP与肌动蛋白的结合。

In our model of regulation, tropomyosin can exist in two positions on the thin filament, forming either the weak-binding state, or the strong-binding state of regulated actin. In the weak-binding state, which occurs in the absence of Ca2+, troponin-tropomyosin inhibits the binding of S-1 and S-1 ADP to actin, but not S-1.ATP. Therefore, ATPase inhibition seems to be due to regulated actin in the weak-binding state inhibiting Pi release. First, results were obtained consistent with the latter part of our model using S-1 which was cross-linked to actin. Troponin-tropomyosin was found to markedly inhibit the ATPase activity of cross-linked actin S-1 both at low and high ionic strength. Since cross-linked S-1 appears to behave kinetically like S-1 in the presence of infinite actin concentration, the extensive regulation obtained with cross-linked S-1 is not due to troponin-tropomyosin blocking the binding of cross-linked S-1 to actin. Second, we examined the ability of pPDM.S-1, which is a stable analog of S-1.ATP, to turn on or potentiate the ATPase activity of regulated acto S-1. Consistent with our binding data, which shows no apparent cooperatively in the binding of S-1.ATP or pPDM.S-1.ATP to regulated actin, pPDM.S-1 ATP does not significantly turn on the ATPase activity of regulated acto S-1. These results indicate that tropomyosin remains in the inhibitory position when pPDM S-1 ATP binds to regulated actin. Third, we found that in contrast to the results obtained with skeletal muscle, the inhibition of Pi release in the actomyosin ATPase cycle of smooth muscle is not accompanied by inhibition of the binding of myosin.ADP to actin. These results show that although inhibition of Pi release appears to be a general mechanism of muscle regulation, it is not always coupled to inhibition in the binding of myosin ADP to actin.