We have used a high-resolution small angle X-ray scattering system, together with a high-performance CCD camera, on the BioCAT beamline at the APS synchrotron radiation facility at the Argonne National Laboratory, to study X-ray interference effects in the meridional reflections generated by the arrays of myosin crossbridges in contracting muscle. These give information about axial movements of the myosin heads during contraction with sub-nanometer resolution. Using whole intact muscle preparations (frog sartorius) we have been able to record the detailed behavior of M3 (the first order meridional reflection from the myosin crossbridges, at 14.56 nm) at each of a number of quick releases of increasing magnitude, on the same specimen, and at the same time make similar measurements on higher order myosin meridional reflections, particularly M6. The latter provides information about the dispersion of lever arm angles of the actin-attached myosin heads. The observations show that in isometric contraction the lever arm angles are dispersed through +/- 20-25 degrees on either side of a mean orientation that is about 60 degrees away from their orientation at the end of the working stroke: and that they move towards that orientation in synchronized fashion, with constant dispersion, during quick releases. The relationship between the shift in the interference fringes (which measures the shift of the myosin heads scattering mass towards the center of the sarcomere, and the changes in the total intensity of the reflections, which measures the changes in the axial profile of the heads, is consistent with the tilting lever arm mechanism of muscle contraction. Significant fixed contributions to the meridional reflections come from unattached myosin heads and from backbone components of the myosin filaments, and the interaction of these with the contributions from actin-attached myosin heads determines the behavior of these reflections.

译文

我们在阿贡国家实验室APS同步加速器辐射设施的BioCAT束线上使用了高分辨率小角度x射线散射系统以及高性能CCD相机,研究收缩肌肉中肌球蛋白交叉桥阵列产生的子午反射中的x射线干扰效应。这些以亚纳米分辨率提供了有关收缩过程中肌球蛋白头部轴向运动的信息。使用完整的完整肌肉制剂 (frog sartorius),我们已经能够记录M3的详细行为 (肌球蛋白横桥的一阶子午反射,在14.56 nm处),在相同的标本上,同时对高阶肌球蛋白子午反射,特别是m6进行类似的测量。后者提供了有关肌动蛋白附着的肌球蛋白头的杠杆臂角度分散的信息。观察结果表明,在等距收缩中,杠杆臂角在平均方向的两侧分散了20-25度,该方向在工作行程结束时与它们的方向相距约60度: 并且它们以同步方式向该方向移动,并具有恒定的色散,在快速释放期间。干涉条纹的位移 (测量肌球蛋白头部散射质量向肌节中心的位移) 与反射总强度的变化 (测量头部轴向轮廓的变化) 之间的关系,与肌肉收缩的倾斜杠杆臂机制一致。对子午反射的显着固定贡献来自未附着的肌球蛋白头和肌球蛋白细丝的骨干成分,而这些与肌动蛋白附着的肌球蛋白头的贡献的相互作用决定了这些反射的行为。

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