Texas 2-Step: A new Model for YcgR::c-di-GMP Action at the Flagellar Motor
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YcgR is a c-di-GMP effector that inhibits both chemotaxis and swimming speed in E. coli and Salmonella . Genetic, biochemical and structural data suggest that YcgR interacts with the bidirectional flagellar rotor as well as the stator to alter rotor bias to CCW and reduce motor speed, but how both feats are achieved remain unclear. Recent cryo-EM structures showing changes in disposition of the rotor and stator units during directional changes suggested to us a mechanism by which YcgR might bring about its action. We call this the Texas 2-Step model. In the first step, YcgR interacts with a MotA subunit in the CCW conformation of the rotor where the stators are largely outside the C ring. In the second step, the rotating MotA pentamer delivers YcgR to FliG, affecting bias and speed. We provide evidence for the first step of this model, which generates testable new insights.
Importance
To date, the mechanism of YcgR action has been investigated by multiple labs using a variety of techniques, but no consensus has emerged. Some studies have favored action at the rotor, others at the stator. The complicating factor has been the involvement of four proteins - MotA, FliG, FliM, YcgR – in cross-interactions, with multiple regions in some of these proteins participating in the interaction. The availability of rotor-stator cryo-EM structures has clarified the picture, leading us to propose and test a new model that explains most, if not all the available data.
