The bacterial flagellar motor rotates in both counterclockwise (CON) and clockwise (CW) directions. FIiG, 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. FIiG consists of three domains: FliG(N), FIiG(M) and FliG(C). FliG(N) directly binds to FliF. Intermolecular interactions between FIiG(M) and FliG(C) drive FIiG ring formation. FIiG(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 (Delta PEV) in FIiG of Thermotaoga maritia (Tm-FIiG) on FIiG-FliG and FliG-FliM interactions. The PEV deletion stabilized the intramolecular interaction between FIiG(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 FIiG(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. (C) 2017 Elsevier Inc. All rights reserved.