Texas 2-step: a new model for YcgR::c-di-GMP action at the flagellar motor

YcgR is a c-di-GMP effector that inhibits chemotaxis and swimming speed in <i>Escherichia coli</i> and <i>Salmonella</i>. Genetic, biochemical, and structural studies suggest that YcgR interacts with both the bidirectional flagellar rotor and the stator to bias rotation toward counterclockwise (CCW) and reduce motor speed, but the underlying mechanism remains unresolved. Recent cryo-electron microscopy structures revealing conformational changes in the rotor-stator complex during directional switching suggested to us a mechanism by which YcgR acts. We call this the Texas 2-step model, after the country dance in which partners move smoothly in a CCW arc with quick steps followed by slow ones. In this model, YcgR first binds a MotA subunit when the rotor adopts the CCW conformation, in which stators are largely displaced from the C-ring. In the next step, the rotating MotA pentamer delivers YcgR to the rotor protein FliG, thereby slowing motor speed. We provide evidence for the first step of this model, offering testable predictions for future work.IMPORTANCEThe mechanism of YcgR action has been investigated by multiple laboratories using diverse approaches, yet no consensus has emerged. Some studies implicate the rotor, others the stator. A key complication is the involvement of four interacting proteins-MotA, FliG, FliM, and YcgR-with multiple contact sites in several of them. Recent rotor-stator cryo-electron microscopy structures revealing conformational changes during directional switching suggested a mechanism that we set out to test. Our experiments show that rotor conformation is crucial for YcgR function.