Insight into adaptive remodeling of the rotor ring complex of the bacterial flagellar motor.

The bacterial flagellar motor rotates in both counterclockwise (CCW) and clockwise (CW) directions. FliG, FliM and FliN form the C ring on the cytoplasmic face of the MS ring made of a transmembrane protein, FliF. The C ring acts not only as a rotor but also as a switch of the direction of motor rotation. FliG consists of three domains: FliG _N , FliG _M and FliG _C . FliG _N directly binds to FliF. Intermolecular interactions between FliG _M and FliG _C drive FliG ring formation. FliG _M is responsible for the interaction with FliM. FliG _C is involved in the interaction with the stator protein MotA. Adaptive remodeling of the C ring occurs when the motor switches between the CCW and CW states. However, it remained unknown how. Here, we report the effects of a CW-locked deletion mutation (ΔPEV) in FliG of Thermotaoga maritia (Tm-FliG) on FliG-FliG and FliG-FliM interactions. The PEV deletion stabilized the intramolecular interaction between FliG _M and FliG _C , thereby suppressing the oligomerization of Tm-FliG _MC in solution. This deletion also induced a conformational change of Helix _MC connecting FliG _M and FliG _C to reduce the binding affinity of Tm-FliG _MC for FliM. We will discuss adaptive remodeling of the C ring responsible for flagellar motor switching.
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