菌鞭毛细丝的转动是细菌运动的原因。转动的方向决定细菌是会平稳向前运动还是会改变其运动轨迹。鞭毛“马达”向顺时针方向或逆时针方向驱动这种转动，转动方向由鞭毛开关复合物调控。
它的组成部分之一（一种被称为FliG的环状蛋白）施加使该“马达”能够切换方向的扭动运动或扭矩——这是一个引人注目的本领，因为鞭毛以每秒几百转的速度转动，但能够在不到一毫秒时间内逆转方向。现在，FIiG的全长度结构已被确定，在转动方向的切换中所涉及的构形变化也被识别了出来。（生物谷Bioon.com）
生物谷推荐原始出处：
Nature doi:10.1038/nature09300
Structure of the torque ring of the flagellar motor and the molecular basis for rotational switching
Lawrence K. Lee,Michael A. Ginsburg,Claudia Crovace,Mhairi Donohoe& Daniela Stock
The flagellar motor drives the rotation of flagellar filaments at hundreds of revolutions per second1, 2, efficiently propelling bacteria through viscous media3. The motor uses the potential energy from an electrochemical gradient of cations4, 5 across the cytoplasmic membrane to generate torque. A rapid switch from anticlockwise to clockwise rotation determines whether a bacterium runs smoothly forward or tumbles to change its trajectory6, 7. A protein called FliG forms a ring in the rotor of the flagellar motor that is involved in the generation of torque8, 9, 10, 11, 12, 13 through an interaction with the cation-channel-forming stator subunit MotA12. FliG has been suggested to adopt distinct conformations that induce switching but these structural changes and the molecular mechanism of switching are unknown. Here we report the molecular structure of the full-length FliG protein, identify conformational changes that are involved in rotational switching and uncover the structural basis for the formation of the FliG torque ring. This allows us to propose a model of the complete ring and switching mechanism in which conformational changes in FliG reverse the electrostatic charges involved in torque generation.
