Cantino M E, Brown L D, Chew M, Luther P K, Squire J M
Department of Physiology and Neurobiology, University of Connecticut, Storrs 06269-2131, USA.
J Muscle Res Cell Motil. 2000;21(7):681-90. doi: 10.1023/a:1005661123914.
Despite extensive knowledge of many muscle A-band proteins (myosin molecules, titin, C-protein (MyBP-C)), details of the organization of these molecules to form myosin filaments remain unclear. Recently the myosin head (crossbridge) configuration in a relaxed vertebrate muscle was determined from low-angle X-ray diffraction (Hudson et al. (1997), J Mol Biol 273: 440-455). This showed that, even without C-protein, the myosin head array displays a characteristic polar pattern with every third 143 A-spaced crossbridge level particularly prominent. However, X-ray diffraction cannot determine the polarity of the crossbridge array relative to the neighbouring actin filaments; information crucial to a proper understanding of the contractile event. Here, electron micrographs of negatively-stained goldfish A-segments and of fast-frozen, freeze-fractured plaice A-bands have been used to determine the resting myosin head polarity relative to the M-band. In agreement with the X-ray data, the prominent 429 A-spaced striations are seen outside the C-zone, where no non-myosin proteins apart from titin are thought to be located. The head orientation is with the concave side of the curved myosin heads (containing the entrance to the ATP-binding site) facing towards the M-band and the convex surface (containing the actin-binding region at one end) facing away from the M-band.
尽管对许多肌A带蛋白（肌球蛋白分子、肌联蛋白、C蛋白（肌球蛋白结合蛋白C））已有广泛了解，但这些分子如何组织形成肌球蛋白丝的细节仍不清楚。最近，通过低角度X射线衍射确定了松弛的脊椎动物肌肉中肌球蛋白头部（横桥）的构型（Hudson等人，（1997年），《分子生物学杂志》273：440 - 455）。这表明，即使没有C蛋白，肌球蛋白头部阵列也呈现出一种特征性的极性模式，每隔第三个143 Å间隔的横桥水平尤为突出。然而，X射线衍射无法确定横桥阵列相对于相邻肌动蛋白丝的极性；而这一信息对于正确理解收缩事件至关重要。在这里，已使用负染色金鱼A段和快速冷冻、冷冻断裂的鲽鱼A带的电子显微镜照片来确定静止肌球蛋白头部相对于M带的极性。与X射线数据一致，在C区之外可见突出的429 Å间隔的条纹，据认为除了肌联蛋白外，该区域没有其他非肌球蛋白蛋白。头部的取向是，弯曲的肌球蛋白头部的凹面（包含ATP结合位点的入口）朝向M带，而凸面（一端包含肌动蛋白结合区域）背离M带。
