Köhler Jan, Winkler Gerhard, Schulte Imke, Scholz Tim, McKenna William, Brenner Bernhard, Kraft Theresia
Molekular- und Zellphysiologie, Medizinische Hochschule, D-30625 Hannover, Germany.
Proc Natl Acad Sci U S A. 2002 Mar 19;99(6):3557-62. doi: 10.1073/pnas.062415899.
Elastic distortion of a structural element of the actomyosin complex is fundamental to the ability of myosin to generate motile forces. An elastic element allows strain to develop within the actomyosin complex (cross-bridge) before movement. Relief of this strain then drives filament sliding, or more generally, movement of a cargo. Even with the known crystal structure of the myosin head, however, the structural element of the actomyosin complex in which elastic distortion occurs remained unclear. To assign functional relevance to various structural elements of the myosin head, e.g., to identify the elastic element within the cross-bridge, we studied mechanical properties of muscle fibers from patients with familial hypertrophic cardiomyopathy with point mutations in the head domain of the beta-myosin heavy chain. We found that the Arg-719 --> Trp (Arg719Trp) mutation, which is located in the converter domain of the myosin head fragment, causes an increase in force generation and fiber stiffness under isometric conditions by 48-59%. Under rigor and relaxing conditions, fiber stiffness was 45-47% higher than in control fibers. Yet, kinetics of active cross-bridge cycling were unchanged. These findings, especially the increase in fiber stiffness under rigor conditions, indicate that cross-bridges with the Arg719Trp mutation are more resistant to elastic distortion. The data presented here strongly suggest that the converter domain that forms the junction between the catalytic and the light-chain-binding domain of the myosin head is not only essential for elastic distortion of the cross-bridge, but that the main elastic distortion may even occur within the converter domain itself.
肌动球蛋白复合体结构元件的弹性变形是肌球蛋白产生运动力能力的基础。弹性元件允许在运动之前在肌动球蛋白复合体（横桥）内产生应变。这种应变的释放随后驱动细丝滑动，或者更一般地说，驱动货物的运动。然而，即使已知肌球蛋白头部的晶体结构，肌动球蛋白复合体中发生弹性变形的结构元件仍然不清楚。为了确定肌球蛋白头部各种结构元件的功能相关性，例如确定横桥内的弹性元件，我们研究了β - 肌球蛋白重链头部结构域存在点突变的家族性肥厚性心肌病患者的肌肉纤维的力学特性。我们发现位于肌球蛋白头部片段转换结构域的Arg - 719→Trp（Arg719Trp）突变，在等长条件下使力的产生和纤维刚度增加了48 - 59%。在僵直和松弛条件下，纤维刚度比对照纤维高45 - 47%。然而，活性横桥循环的动力学没有变化。这些发现，特别是在僵直条件下纤维刚度的增加，表明具有Arg719Trp突变的横桥对弹性变形更具抗性。这里呈现的数据强烈表明，形成肌球蛋白头部催化结构域和轻链结合结构域之间连接的转换结构域不仅对于横桥的弹性变形至关重要，而且主要的弹性变形甚至可能发生在转换结构域本身内部。
