The flagellar motor of Vibrio alginolyticus undergoes major structural remodeling during rotational switching.

Jun Liu,Tatsuro Nishikino,Michio Homma,Wangbiao Guo,Shiwei Zhu,Seiji Kojima,Brittany L Carroll

doi:10.7554/elife.61446

Abstract

The bacterial flagellar motor switches rotational direction between counterclockwise (CCW) and clockwise (CW) to direct the migration of the cell. The cytoplasmic ring (C-ring) of the motor, which is composed of FliG, FliM, and FliN, is known for controlling the rotational sense of the flagellum. However, the mechanism underlying rotational switching remains elusive. Here, we deployed cryo-electron tomography to visualize the C-ring in two rotational biased mutants in Vibrio alginolyticus. We determined the C-ring molecular architectures, providing novel insights into the mechanism of rotational switching. We report that the C-ring maintained 34-fold symmetry in both rotational senses, and the protein composition remained constant. The two structures show FliG conformational changes elicit a large conformational rearrangement of the rotor complex that coincides with rotational switching of the flagellum. FliM and FliN form a stable spiral-shaped base of the C-ring, likely stabilizing the C-ring during the conformational remodeling.

Highlights

Many bacteria navigate complex environments by controlling the flagellar rotational switch between counterclockwise (CCW) and clockwise (CW) in response to chemical stimuli
Spanning from the cytosol through the outer membrane, the motor consists of a series of rings, with the L-ring at the outer membrane, the P-ring located within the periplasmic space, the MS-ring embedded in the cytoplasmic membrane, and the cytoplasmic ring (C-ring) inside the cytoplasm (Francis et al, 1992; Francis et al, 1994; Homma et al, 1987; Ueno et al, 1992)
Visualization of intact flagellar motors in CCW and CW rotation states To address the mechanism of rotational switching, we utilized cryo-ET to visualize the C-rings of on two FliG mutants (G214S and G215A) of V. alginolyticus

Summary

Introduction

Many bacteria navigate complex environments by controlling the flagellar rotational switch between counterclockwise (CCW) and clockwise (CW) in response to chemical stimuli. Escherichia coli and Salmonella enterica use a “run-and-tumble” approach for controlling movement, where flagella rotating in a CCW sense drives the cell body forward and when the rotation sense switches to CW the bacterium tumbles through the medium in an attempt to change direction (Berg, 2003; Chevance and Hughes, 2008; Terashima et al, 2008). The motor is the most intricate part of the flagellum that is responsible for flagellar assembly and rotation, and essential for rotational switch. The O-ring is located on the outside of the outer membrane (Zhu et al, 2017), the H-ring facilitates the outer membrane penetration of the flagellum (Terashima et al, 2010; Zhu et al, 2018), and the Tring contacts the H-ring and stators, presumably acting as a scaffold to hold the stators (Terashima et al, 2006; Zhu et al, 2019)

Methods

Results

Conclusion