Structure of the torque ring of the flagellar motor and the molecular basis for rotational switching

The bacterial flagellar motor drives the rotation of flagellar filaments, propelling bacteria through viscous media. The rotation can switch from an anticlockwise to a clockwise direction, determining a smooth or tumbling motion. A protein called FliG forms a ring in the motor's rotor, and has been proposed to adopt distinct conformations that induce switching. Here, the full-length structure of FliG is presented, and conformational changes are identified that are involved in switching between clockwise and anticlockwise rotations.