X-Ray Diffraction Studies on Structural Changes of Actin-Containing Thin Filaments during Contraction of Skeletal Muscles

利用同步辐射对青蛙等长破伤风时骨骼肌5.9 nm和5.1 nm的肌动蛋白层线进行了时间分辨x射线衍射研究。在5.9 nm反射时，这些肌动蛋白层线的综合强度增加了30-50%，在5.1 nm反射时增加了约70%。这两种反射强度的增加都大于静止状态到僵硬状态的转变。5.9 nm反射的强度变化先于42.9 nm的离经反射和赤道反射，以及等距张力。收缩时5.9 nm层线的强度分布与严格状态下观察到的强度分布不同。一种概念上的新型积分区x射线探测器（成像板）被用于肌肉收缩的衍射研究。成像板的高灵敏度和宽动态范围导致曝光时间急剧减少，使得使用同步辐射可以在10秒内记录非常清晰的二维x射线衍射图，从收缩的青蛙骨骼肌轴向间距高达2.0 nm。在蛙骨骼肌收缩过程中，肌动蛋白层线的强度大多增加，沿层线和轴向间距的峰值位置没有明显变化。测量肌动蛋白层线的层线强度分布。通过模型计算对结构进行分析，以获得与观测强度的最佳拟合。结果表明，在收缩过程中，肌动蛋白亚基结构域的改变以及原肌凝蛋白分子的位置变化是通过与肌凝蛋白头的相互作用以及钙离子与肌钙蛋白的结合而发生的。

Time-resolved X-ray diffraction studies have been made on the 5.9 nm and 5.1 nm actin layer lines from frog skeletal muscles during an isometric tetanus, using synchrotron radiation. The integrated intensities of these actin layer lines were found to increase during a tetanus by 30-50% for the 5.9 nm reflection and about 70% for the 5.1 nm reflection of the resting values. The intensity increase of both reflections was greater than that taking place in the transition from rest to rigor state. The intensity change of the 5.9 nm reflection preceded those of the myosin 42.9 nm off-meridional reflection and equatorial reflections, as well as the isometric tension. The intensity profile of the 5.9 nm layer line during contraction was found to be different from that observed in the rigor state.A conceptually new type of an integrating area X-ray detector (an imaging plate) was used in a diffraction study of muscle contraction. The high sensitivity and wide dynamic range of the imaging plate resulted in a dramatic reduction in the exposure time, making it possible to record very clear two-dimensional X-ray diffraction patterns of up to 2.0 nm axial spacing from a contracting frog skeletal muscle as little as 10 seconds using synchrotron radiation. During contraction of a frog skeletal muscle, most of actin layer lines increased in intensity without noticeasble changes in their peak positions along the layer line and axial spacing. The layer-line intensity distributions of actin layer lines were measured. The structural analyses were made by model-calculation to obtain the best-fit to the observed intensities. The results indicated that during contraction the changes of the domain structure in the actin subunit as well as the positional change of tropomyosin molecules take place by the interaction with myosin heads as well as the binding of Ca ions to troponin.