PDF(Pubmed)
Abstract：
Dynamic interactions between the myosin motor head on thick filaments and the actin molecular track on thin filaments drive the myosin-crossbridge cycle that powers muscle contraction. The process is initiated by Ca2+ and the opening of troponin-tropomyosin-blocked myosin-binding sites on actin. The ensuing recruitment of myosin heads and their transformation from pre-powerstroke to post-powerstroke conformation on actin produce the force required for contraction. Cryo-EM-based atomic models confirm that during this process, tropomyosin occupies three different average positions on actin. Tropomyosin pivoting on actin away from a TnI-imposed myosin-blocking position accounts for part of the Ca2+ activation observed. However, the structure of tropomyosin on thin filaments that follows pre-powerstroke myosin binding and its translocation during myosin\'s pre-powerstroke to post-powerstroke transition remains unresolved. Here, we approach this transition computationally in silico. We used the myosin helix-loop-helix motif as an anchor to dock models of pre-powerstroke cardiac myosin to the cleft between neighboring actin subunits along cardiac thin filaments. We then performed targeted molecular dynamics simulations of the transition between pre- and post-powerstroke conformations on actin in the presence of cardiac troponin-tropomyosin. These simulations show Arg 369 and Glu 370 on the tip of myosin Loop-4 encountering identically charged residues on tropomyosin. The charge repulsion between residues causes tropomyosin translocation across actin, thus accounting for the final regulatory step in the activation of the thin filament, and, in turn, facilitating myosin movement along the filament. We suggest that during muscle activity, myosin-induced tropomyosin movement is likely to result in unencumbered myosin head interactions on actin at low-energy cost.
摘要：
粗丝上的肌球蛋白运动头与细丝上的肌动蛋白分子轨迹之间的动态相互作用驱动肌球蛋白交叉桥循环，从而为肌肉收缩提供动力。该过程由Ca2和肌动蛋白上肌钙蛋白-原肌球蛋白阻断的肌球蛋白结合位点的打开启动。肌球蛋白头的随后募集及其在肌动蛋白上从动力前中风到动力后中风构象的转变产生了收缩所需的力。基于Cryo-EM的原子模型证实，在这个过程中，原肌球蛋白在肌动蛋白上占据三个不同的平均位置。原肌球蛋白在肌动蛋白上旋转远离TnI施加的肌球蛋白阻断位置是观察到的Ca2激活的一部分。然而,原肌球蛋白在纤丝上的结构与前发肌球蛋白结合及其在前发肌球蛋白向后发肌球蛋白过渡期间的易位仍未解决。这里,我们在计算机上计算这个过渡。我们使用肌球蛋白螺旋-环-螺旋基序作为锚，将前发心肌肌球蛋白的模型停靠在沿着心脏细丝的相邻肌动蛋白亚基之间的裂缝上。然后，我们在存在心肌肌钙蛋白-原肌球蛋白的情况下，对肌动蛋白的动力中风前后构象之间的转变进行了靶向分子动力学模拟。这些模拟显示肌球蛋白环4的尖端上的Arg369和Glu370遇到原肌球蛋白上相同电荷的残基。残基之间的电荷排斥导致原肌球蛋白易位穿过肌动蛋白，因此，考虑到激活细丝的最终监管步骤，and,反过来,促进肌球蛋白沿着细丝运动。我们建议在肌肉活动期间，肌球蛋白诱导的原肌球蛋白运动可能以低能量成本导致肌动蛋白上无阻碍的肌球蛋白头部相互作用。
