Structural basis of bacterial flagellar motor rotation and switching.

The bacterial flagellar motor, a remarkable rotary machine, can rapidly switch between counterclockwise (CCW) and clockwise (CW) rotational directions to control the migration behavior of the bacterial cell. The flagellar motor consists of a bidirectional spinning rotor surrounded by torque-generating stator units. Recent high-resolution in vitro and in situ structural studies have revealed stunning details of the individual components of the flagellar motor and their interactions in both the CCW and CW senses. In this review, we discuss these structures and their implications for understanding the molecular mechanisms underlying flagellar rotation and switching. The bidirectional flagellar motor consists of a rotor ring surrounded by multiple stator units and is controlled by a chemotaxis system that enables bacteria to move to favorable environments. The C-ring, also called the switch complex, can spin in both CCW and CW directions. It has a different diameter and symmetry in different bacteria but shares similar building blocks and conformational changes during directional switching. The stator units form a ring structure surrounding the C-ring. The diameter and symmetry of the stator ring vary across bacterial species to accommodate different C-ring diameters. The stator unit, powered by an ion gradient across the inner membrane, spins in the clockwise sense, driving C-ring rotation. The C-ring alters its interaction sites with the stator ring to enable the directional switch of the flagellar motor. [ABSTRACT FROM AUTHOR]