The bacterial flagellar motor is a huge bidirectional rotary nanomachine that drives rotation of the flagellum for bacterial motility. The cytoplasmic C ring of the flagellar motor functions as the switch complex for the rotational direction switching from counterclockwise to clockwise. However, the structural basis of the rotational switching and how the C ring is assembled have long remained elusive. Here, we present two high-resolution cryo-electron microscopy structures of the C ring-containing flagellar basal body–hook complex from Salmonella Typhimurium, which are in the default counterclockwise state and in a constitutively active CheY mutant-induced clockwise state, respectively. In both complexes, the C ring consists of four subrings, but is in two different conformations. The CheY proteins are bound into an open groove between two adjacent protomers on the surface of the middle subring of the C ring and interact with the FliG and FliM subunits. The binding of the CheY protein induces a significant upward shift of the C ring towards the MS ring and inward movements of its protomers towards the motor center, which eventually remodels the structures of the FliG subunits and reverses the orientations and surface electrostatic potential of the α_torque helices to trigger the counterclockwise-to-clockwise rotational switching. The conformational changes of the FliG subunits reveal that the stator units on the motor require a relocation process in the inner membrane during the rotational switching. This study provides unprecedented molecular insights into the rotational switching mechanism and a detailed overall structural view of the bacterial flagellar motors.

细菌鞭毛马达是一个巨大的双向旋转纳米机器，它驱动鞭毛的旋转以实现细菌运动。鞭毛马达的细胞质 C 环充当旋转方向从逆时针切换到顺时针的开关复合体。然而，旋转开关的结构基础以及 C 环的组装方式长期以来一直难以捉摸。在这里，我们提出了来自鼠伤寒沙门氏菌的含有 C 环的鞭毛基体-钩状复合物的两种高分辨率冷冻电子显微镜结构，它们分别处于默认逆时针状态和组成型活性 CheY 突变体诱导的顺时针状态。在这两个复合物中，C 环由四个子环组成，但有两种不同的构象。CheY 蛋白结合到 C 环中间亚环表面两个相邻原聚体之间的开放凹槽中，并与 FliG 和 FliM 亚基相互作用。CheY 蛋白的结合诱导 C 环向 MS 环显着向上移动，其原聚体向内向运动中心移动，最终重塑 FliG 亚基的结构并反转 α_转矩螺旋的方向和表面静电势，以触发逆时针到顺时针的旋转切换。FliG 亚基的构象变化表明，在旋转切换过程中，电机上的定子单元需要在内膜中进行重新定位过程。这项研究为旋转切换机制提供了前所未有的分子见解，并提供了细菌鞭毛马达的详细整体结构视图。