Cohen C, Vibert P J, Craig R W, Phillips G N
Prog Clin Biol Res. 1980;40:209-31.
Two types of Ca2+-sensitive protein complexes control the contraction of muscle: Troponin (TN) and tropomyosin (TM) are associated with the thin actin filaments, and a specific light chain is a regulatory subunit of myosin itself. Most muscles have both types of regulation. X-ray diffraction diagrams from whole muscle have shown changes in the position of tropomyosin and changes in the pattern of myosin crossbridge attachment associated with different states of the regulatory switches. The full interpretation of these diagrams is often ambiguous, however, and structural studies of the purified proteins provide essential information. Recent crystallographic results reveal that the TM molecule has unusual local domains of marginal stability, leading to extensive motions of the tropomyosin filaments. Electron microscopy of negatively stained thin filaments decorated with subfragments of scallop myosin yields unusually detailed images that show marked conformational changes in myosin crossbridges that are dependent on the presence or absence of the regulatory light chain. These observations suggest that both the special dynamic design of tropomyosin and the striking structural changes in the myosin crossbridges are significant clues for detailed models for the regulatory mechanism.
肌钙蛋白（TN）和原肌球蛋白（TM）与细肌动蛋白丝相关联，并且特定轻链是肌球蛋白自身的调节亚基。大多数肌肉具有这两种调节类型。来自整块肌肉的X射线衍射图显示，原肌球蛋白位置的变化以及与调节开关不同状态相关的肌球蛋白横桥附着模式的变化。然而，这些图的完整解释常常不明确，对纯化蛋白的结构研究提供了重要信息。最近的晶体学结果表明，TM分子具有边缘稳定性异常的局部结构域，导致原肌球蛋白丝发生广泛运动。用扇贝肌球蛋白亚片段装饰的负染细肌丝的电子显微镜观察产生了异常详细的图像，显示出肌球蛋白横桥中明显的构象变化，这些变化取决于调节轻链的存在与否。这些观察结果表明，原肌球蛋白的特殊动态设计和肌球蛋白横桥中显著的结构变化都是调节机制详细模型的重要线索。
