FliL ring enhances the function of periplasmic flagella

Bacterial flagellar motors are rotary machines that can power motility in various fluid and surface environments, including within hosts. Activation of the stator complex MotA/MotB is required for torque generation and motor rotation. During activation, the stator complex is expected to undergo an extensive conformational change to allow ions to flow through its transmembrane channels to generate torque. However, the detailed mechanism underlying stator activation remains poorly understood. Here, we use the Lyme disease–causing spirochete Borrelia burgdorferi as the model system to reveal the stator complex and its interaction with the FliL ring, using cryo-electron tomography and subtomogram averaging of flagellar motors from wild-type, ΔmotB, ΔfliL, and ΔfliLmotAB mutants. Upon recruitment of stator units to the motor, FliL oligomerizes from a partial ring into a full ring, which wraps around the MotB periplasmic linkers and stabilizes the stator complex in an extended, active conformation, thus enabling a continuous influx of ions to generate higher torque. Furthermore, we provide evidence that FliL can mediate the assembly of stator complexes around the motor, thereby regulating stator and motor function. Given that FliL and the stator complex are ubiquitous in flagellated bacteria, these mechanisms may be utilized by various bacteria to modulate torque and motility in response to changing environmental conditions.