A molecular clutch governs stator engagement and flagellar torque
Motile bacteria must continuously adjust their motility to navigate and colonize diverse environments, including mammalian hosts, yet the underlying mechanisms remain poorly understood. Here, using in situ single-particle cryo-electron microscopy, we determine near-atomic structures of the flagellar motor in the Lyme disease spirochete Borrelia burgdorferi and uncover a novel mechanism for controlling torque generation. We identify a distinct neutral state in which individual torque-generating MotA5B2 stator complexes remain incorporated in the motor but adopt an extended, tilted conformation disengaged from the rotor. Two conserved flagellar proteins, FliL and FlcA, form a transmembrane scaffold that retains 16 stator complexes, while FapA, a newly identified multidomain flagellar protein, binds the cytoplasmic base of the stator complex and stabilizes its rotor-disengaged conformation. Strikingly, rotor-engaged and rotor-disengaged stators coexist within individual motors, demonstrating that stator retention and torque engagement are separable states. Our findings establish a motor-retained neutral stator state and reveal a molecular-clutch mechanism that independently controls the engagement of individual stator units with the rotor, enabling rapid and precise regulation of flagellar torque and motility.