Physiology Laboratory, Department of Evolutionary Biology and Consorzio Nazionale Interuniversitario per le Scienze Fisiche della Materia, Università di Firenze, 50019 Sesto Fiorentino, Italy.
Proc Natl Acad Sci U S A. 2011 Apr 26;108(17):7236-40. doi: 10.1073/pnas.1018330108. Epub 2011 Apr 11.
Muscle contraction is driven by a change in the structure of the head domain of myosin, the "working stroke" that pulls the actin filaments toward the midpoint of the myosin filaments. This movement of the myosin heads can be measured very precisely in intact muscle cells by X-ray interference, but until now this technique has not been applied to physiological activation and force generation following electrical stimulation of muscle cells. By using this approach, we show that the long axes of the myosin head domains are roughly parallel to the filaments in resting muscle, with their center of mass offset by approximately 7 nm from the C terminus of the head domain. The observed mass distribution matches that seen in electron micrographs of isolated myosin filaments in which the heads are folded back toward the filament midpoint. Following electrical stimulation, the heads move by approximately 10 nm away from the filament midpoint, in the opposite direction to the working stroke. The time course of this motion matches that of force generation, but is slower than the other structural changes in the myosin filaments on activation, including the loss of helical and axial order of the myosin heads and the change in periodicity of the filament backbone. The rate of force development is limited by that of attachment of myosin heads to actin in a conformation that is the same as that during steady-state isometric contraction; force generation in the actin-attached head is fast compared with the attachment step.
肌肉收缩是由肌球蛋白头部结构的变化驱动的，这是一种“工作冲程”，它将肌动蛋白丝拉向肌球蛋白丝的中点。通过 X 射线干涉，可以非常精确地测量完整肌肉细胞中肌球蛋白头部的这种运动，但直到现在，这项技术还没有应用于肌肉细胞电刺激后的生理激活和力的产生。通过使用这种方法，我们表明，在静止肌肉中，肌球蛋白头部的长轴大致与纤维平行，其质心相对于头部末端大约偏离 7nm。观察到的质量分布与在分离的肌球蛋白纤维的电子显微镜照片中看到的一致，其中头部向纤维中点折叠。电刺激后，头部向纤维中点移动约 10nm，与工作冲程相反。这一运动的时间过程与力的产生相匹配，但比激活时肌球蛋白纤维的其他结构变化慢，包括肌球蛋白头部的螺旋和轴向有序性的丧失以及纤维骨架周期性的变化。力的发展速度受到肌球蛋白头部与肌动蛋白附着的速度限制，这种构象与稳态等长收缩时相同；在附着步骤中，附着在肌动蛋白上的头部的力生成速度很快。
