Ochala Julien, Iwamoto Hiroyuki, Larsson Lars, Yagi Naoto
Department of Neuroscience, Clinical Neurophysiology, Uppsala University, Uppsala, Sweden.
Proc Natl Acad Sci U S A. 2010 May 25;107(21):9807-12. doi: 10.1073/pnas.1001733107. Epub 2010 May 10.
Human point mutations in beta- and gamma-tropomyosin induce contractile deregulation, skeletal muscle weakness, and congenital myopathies. The aim of the present study was to elucidate the hitherto unknown underlying molecular mechanisms. Hence, we recorded and analyzed the X-ray diffraction patterns of human membrane-permeabilized muscle cells expressing a particular beta-tropomyosin mutation (R133W) associated with a loss in cell force production, in vivo muscle weakness, and distal arthrogryposis. Upon addition of calcium, we notably observed less intensified changes, compared with controls, (i) in the second (1/19 nm(-1)), sixth (1/5.9 nm(-1)), and seventh (1/5.1 nm(-1)) actin layer lines of cells set at a sarcomere length, allowing an optimal thin-thick filament overlap; and (ii) in the second actin layer line of overstretched cells. Collectively, these results directly prove that during activation, switching of a positive to a neutral charge at position 133 in the protein partially hinders both calcium- and myosin-induced tropomyosin movement over the thin filament, blocking actin conformational changes and consequently decreasing the number of cross-bridges and subsequent force production.
β-和γ-原肌球蛋白中的人类点突变会导致收缩调节异常、骨骼肌无力和先天性肌病。本研究的目的是阐明迄今为止未知的潜在分子机制。因此，我们记录并分析了表达特定β-原肌球蛋白突变（R133W）的人膜通透肌肉细胞的X射线衍射图谱，该突变与细胞力产生丧失、体内肌肉无力和远端关节挛缩有关。加入钙后，与对照组相比，我们显著观察到在以下方面变化不那么强烈：（i）在设定为肌节长度的细胞的第二条（1/19 nm⁻¹）、第六条（1/5.9 nm⁻¹）和第七条（1/5.1 nm⁻¹）肌动蛋白层线处，这允许细肌丝和粗肌丝实现最佳重叠；（ii）在过度拉伸细胞的第二条肌动蛋白层线处。总体而言，这些结果直接证明，在激活过程中，蛋白质中第133位的正电荷转换为中性电荷会部分阻碍钙和肌球蛋白诱导的原肌球蛋白在细肌丝上的移动，阻止肌动蛋白构象变化，从而减少横桥数量和随后的力产生。
