The bacterial flagellar motor can rotate in counterclockwise (CCW) or clockwise (CW) senses, and transitions are controlled by the phosphorylated form of the response regulator CheY (CheY-P). To dissect the mechanism underlying flagellar rotational switching, we use Borrelia burgdorferi as a model system to determine high-resolution in situ motor structures in cheX and cheY3 mutants, in which motors are locked in either CCW or CW rotation. The structures showed that CheY3-P interacts directly with a switch protein, FliM, inducing a major remodeling of another switch protein, FliG2, and altering its interaction with the torque generator. Our findings lead to a model in which the torque generator rotates in response to an inward flow of H⁺ driven by the proton motive force, and conformational changes in FliG2 driven by CheY3-P allow the switch complex to interact with opposite sides of the rotating torque generator, facilitating rotational switching.

细菌鞭毛马达可以逆时针 (CCW) 或顺时针 (CW) 方向旋转，转换由响应调节剂 CheY (CheY-P) 的磷酸化形式控制。为了剖析鞭毛旋转切换的机制，我们使用伯氏疏螺旋体作为模型系统来确定cheX和cheY3突变体中的高分辨率原位电机结构，其中电机被锁定在 CCW 或 CW 旋转中。结构表明，CheY3-P 直接与开关蛋白 FliM 相互作用，诱导另一个开关蛋白 FliG2 的主要重塑，并改变其与扭矩发生器的相互作用。我们的发现导致了一个模型，其中扭矩发生器响应 H ^{+的内向流动而旋转}由质子动力驱动，以及由 CheY3-P 驱动的 FliG2 的构象变化允许开关复合物与旋转扭矩发生器的相对侧相互作用，从而促进旋转切换。