Structure of the molecular bushing of the bacterial flagellar motor

The bacterial flagellum is a motility organelle, consisting of the basal body acting as a rotary motor, the filament as a helical propeller and the hook connecting these two as a universal joint1,2. The basal body contains three rings: the MS ring as the transmembrane core of the rotor; the C ring essential for torque generation and switching regulation; and the LP ring as a bushing supporting the distal rod for its rapid, stable rotation without much friction. The negatively charged surface of the distal rod suggested electrostatic repulsive force in supporting high-speed rotation of the rod as a drive shaft3, but the LP ring structure was needed to see the actual mechanisms of its bushing function and assembly against the repulsive force. Here we report the LP ring structure by electron cryomicroscopy at 3.5 [A] resolution, showing 26-fold rotational symmetry and intricate intersubunit interactions of each subunit with up to six partners that explains the structural stability. The inner surface is charged both positively and negatively, and positive charges on the P ring presumably play important roles in its initial assembly around the rod in the peptidoglycan layer followed by the L ring assembly in the outer membrane.