Directional Switching Mechanism of the Bacterial Flagellar Motor

Tohru Minamino,Miki Kinoshita,Keiichi Namba

doi:10.1016/j.csbj.2019.07.020

Tohru Minamino, Miki Kinoshita + Show 1 more

Open Access

https://doi.org/10.1016/j.csbj.2019.07.020

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Abstract

Bacteria sense temporal changes in extracellular stimuli via sensory signal transducers and move by rotating flagella towards into a favorable environment for their survival. Each flagellum is a supramolecular motility machine consisting of a bi-directional rotary motor, a universal joint and a helical propeller. The signal transducers transmit environmental signals to the flagellar motor through a cytoplasmic chemotactic signaling pathway. The flagellar motor is composed of a rotor and multiple stator units, each of which acts as a transmembrane proton channel to conduct protons and exert force on the rotor. FliG, FliM and FliN form the C ring on the cytoplasmic face of the basal body MS ring made of the transmembrane protein FliF and act as the rotor. The C ring also serves as a switching device that enables the motor to spin in both counterclockwise (CCW) and clockwise (CW) directions. The phosphorylated form of the chemotactic signaling protein CheY binds to FliM and FliN to induce conformational changes of the C ring responsible for switching the direction of flagellar motor rotation from CCW to CW. In this mini-review, we will describe current understanding of the switching mechanism of the bacterial flagellar motor.

Highlights

Bacteria sense temporal changes in extracellular stimuli via sensory signal transducers and move by rotating flagella towards into a favorable environment for their survival
Since the elementary process of torque generation caused by sequential stator–rotor interactions in the flagellar motor is symmetric in the CCW and CW rotation, HelixTorque is postulated to rotate 180° relative to MotAC in a highly cooperative manner when the motor switches between the CCW and CW states of the C ring [12]
The C ring consisting of a cylindrical wall and inner lobes is formed by FliG, FliM and FliN on the cytoplasmic face of the MS ring with the inner lobes connected to the MS ring (Fig. 1B) [14]

Summary

Structure of the C Ring

FliF assembles into the MS ring within the cytoplasmic membrane [13]. The C ring consisting of a cylindrical wall and inner lobes is formed by FliG, FliM and FliN on the cytoplasmic face of the MS ring with the inner lobes connected to the MS ring (Fig. 1B) [14]. Inter-molecular interactions between FliGN and FliGN and between FliGM and FliGCN are responsible for the assembly of FliG into the ring structure on the cytoplasmic face of the MS ring [26,27,28,29]. The binding of CheY-P to FliMN affects inter-molecular FliMM–FliMM interactions, thereby inducing a conformational change in the C ring responsible for switching the direction of flagellar motor rotation [34]. FliN is composed of an intrinsically disordered N-terminal region (FliNN) and a compactly folded domain (FliNC), which structurally looks similar to FliMC [36]. The binding of CheY-P to FliN affects interactions between FliMC and FliN, inducing the conformational change of the C ring responsible for directional switching of flagellar motor rotation [38]. Val-111, Val-112 and Val-113 of FliN are responsible for the interaction with FliH (Fig. 2D) [39,41]

Subunit Organization in the C Ring Structure

Structural Basis for the Rotational Switching Mechanism

Adaptive remodeling of the C ring

Summary and Perspectives