Summary
A bioRxiv preprint reports cryo-EM structures of the flagellar motor in Borrelia burgdorferi, revealing stators that remain attached to the motor while disengaged from its rotor. The authors propose that this arrangement lets the bacterium regulate torque by switching individual stators in and out of engagement.
A bioRxiv preprint reports near-atomic structures of the flagellar motor in Borrelia burgdorferi, the spirochete that causes Lyme disease. Using in situ single-particle cryo-electron microscopy, the researchers describe a state in which torque-generating stator complexes remain incorporated in the motor but are disengaged from its rotor. They propose that this works like a molecular clutch, allowing torque generation to be adjusted without removing stators from the motor.
Stators can stay in the motor without driving it
Bacterial flagella are rotating structures that help cells move. In the flagellar motor, stator complexes interact with the rotor and generate the forces that turn it. The new finding separates two states that might otherwise be treated as one: whether a stator is retained in the motor, and whether it is engaged with the rotor.
The preprint identifies the stator as a MotA5B2 complex. In the disengaged state, it adopts an extended, tilted conformation. The authors report that FliL and FlcA form a transmembrane scaffold that retains 16 stator complexes, while a newly identified multidomain protein, FapA, binds to the stator’s cytoplasmic base and stabilises its rotor-disengaged form.
Crucially, the structures show rotor-engaged and rotor-disengaged stators coexisting within individual motors. The motor therefore need not switch every stator together: some can engage the rotor while others remain present but disengaged.
A structural model for adjusting torque
The researchers describe this arrangement as a molecular clutch. In the analogy, a stator can remain mounted in the motor while its connection to the rotor is released, then contribute to torque when engaged. This gives the motor a possible way to regulate the contribution of individual stators rather than relying only on adding or removing them.
The proposed mechanism offers a structural explanation for how B. burgdorferi could tune flagellar torque as its motility needs change. The evidence reported is structural; the suggested rapid control of motility is a mechanistic interpretation, not a quantified change in swimming speed or torque in the abstract.
The work is posted as a bioRxiv preprint. Its findings provide a model of stator engagement in this bacterium, while further research will be needed to establish how the mechanism operates during movement in different environments.