King's College London BHF Research Excellence Centre, Randall Division for Cell and Molecular Biophysics and Cardiovascular Division, New Hunt's House, Guy's Campus, London SE1 1UL, United Kingdom.
Proc Natl Acad Sci U S A. 2010 Feb 16;107(7):2908-13. doi: 10.1073/pnas.0913736107. Epub 2010 Feb 1.
In the sarcomeric M-band, the giant ruler proteins titin and obscurin, its small homologue obscurin-like-1 (obsl1), and the myosin cross-linking protein myomesin form a ternary complex that is crucial for the function of the M-band as a mechanical link. Mutations in the last titin immunoglobulin (Ig) domain M10, which interacts with the N-terminal Ig-domains of obscurin and obsl1, lead to hereditary muscle diseases. The M10 domain is unusual not only in that it is a frequent target of disease-linked mutations, but also in that it is the only currently known muscle Ig-domain that interacts with two ligands--obscurin and obsl1--in different sarcomeric subregions. Using x-ray crystallography, we show the structural basis for titin M10 interaction with obsl1 in a novel antiparallel Ig-Ig architecture and unravel the molecular basis of titin-M10 linked myopathies. The severity of these pathologies correlates with the disruption of the titin-obsl1/obscurin complex. Conserved signature residues at the interface account for differences in affinity that direct the cellular sorting in cardiomyocytes. By engineering the interface signature residues of obsl1 to obscurin, and vice versa, their affinity for titin can be modulated similar to the native proteins. In single-molecule force-spectroscopy experiments, both complexes yield at forces of around 30 pN, much lower than those observed for the mechanically stable Z-disk complex of titin and telethonin, suggesting why even moderate weakening of the obsl1/obscurin-titin links has severe consequences for normal muscle functions.
在肌节的 M 带上，巨大的调节蛋白肌联蛋白和 obscurin，其小同源物 obscurin-like-1（obsl1），以及肌球蛋白交联蛋白 myomesin 形成一个三元复合物，对于 M 带作为机械连接的功能至关重要。与 obscurin 和 obsl1 的 N 端免疫球蛋白（Ig）结构域相互作用的肌联蛋白最后一个 Ig 结构域 M10 的突变导致遗传性肌肉疾病。M10 结构域不仅因其是疾病相关突变的常见靶点而不寻常，而且它是目前唯一已知的与两个配体—— obscurin 和 obsl1——在不同的肌节亚区相互作用的肌 Ig 结构域。我们使用 X 射线晶体学显示了肌联蛋白 M10 与 obsl1 在新型反平行 Ig-Ig 结构中的相互作用的结构基础，并揭示了肌联蛋白-M10 相关肌病的分子基础。这些病理学的严重程度与肌联蛋白-obsl1/ obscurin 复合物的破坏相关。界面上的保守特征残基解释了指导心肌细胞中细胞分拣的亲和力差异。通过工程改造 obsl1 的界面特征残基到 obscurin，反之亦然，可以类似地调节它们与肌联蛋白的亲和力，类似于天然蛋白。在单分子力谱实验中，两个复合物在约 30 pN 的力下产生，远低于肌联蛋白和 telethonin 的机械稳定 Z 盘复合物观察到的力，这表明为什么 obsl1/ obscurin-肌联蛋白连接的即使是适度的弱化也会对正常肌肉功能产生严重后果。
