The giant protein titin is expressed in vertebrate striated muscle where it spans half a sarcomere from the Z-disc to the M-band and is essential for muscle organisation, activity and health. The C-terminal portion of titin is closely associated with the thick, myosin-containing filament and exhibits a complex pattern of immunoglobulin and fibronectin domains. This pattern reflects features of the filament organisation suggesting that it acts as a molecular ruler and template, but the exact axial disposition of the molecule has not been determined. Here, we present data that allow us to precisely locate titin domains axially along the thick filament from its tip to the edge of the bare zone. We find that the domains are regularly distributed along the filament at 4-nm intervals and we can determine the domains that associate with features of the filament, such as the 11 stripes of accessory proteins. We confirm that the nine stripes ascribed to myosin binding protein-C are not related to the titin sequence previously assumed; rather, they relate to positions approximately 18 domains further towards the C terminus along titin. This disposition also allows a subgroup of titin domains comprising two or three fibronectin domains to associate with each of the 49 levels of myosin heads in each half filament. The results strongly support the role of titin as a blueprint for the thick filament and the arrangement of the myosin motor domains.

巨大的肌联蛋白在脊椎动物横纹肌中表达，它跨越从 Z 盘到 M 带的半个肌节，对于肌肉组织、活动和健康至关重要。肌联蛋白的 C 端部分与含有肌球蛋白的粗丝密切相关，并表现出免疫球蛋白和纤连蛋白结构域的复杂模式。这种图案反映了细丝组织的特征，表明它充当分子标尺和模板，但分子的确切轴向配置尚未确定。在这里，我们提供的数据使我们能够沿着粗丝从其尖端到裸露区域边缘的轴向精确定位肌动蛋白域。我们发现这些结构域以 4 nm 的间隔沿着细丝规则分布，并且我们可以确定与细丝特征相关的结构域，例如辅助蛋白的 11 个条纹。我们确认，归因于肌球蛋白结合蛋白-C 的九个条纹与之前假设的肌联蛋白序列无关；相反，它们涉及沿着肌联蛋白进一步靠近 C 末端约 18 个结构域的位置。这种布置还允许包含两个或三个纤连蛋白结构域的肌联蛋白结构域亚组与每个半丝中的49个肌球蛋白头水平中的每一个相关联。结果有力地支持了肌联蛋白作为粗丝和肌球蛋白运动域排列蓝图的作用。