# Regulatory Role of a Hydrophobic Core in the FliG C-Terminal Domain in the Rotary Direction of a Flagellar Motor

## TL;DR

This study explores how a hydrophobic core in the FliG protein affects the direction of bacterial flagellar motor rotation.

## Contribution

The study identifies specific FliG mutations that influence rotary direction by affecting the hydrophobic core.

## Key findings

- CheY deletion restored motility in the fliG L259Q mutant.
- CCW-biased fliG G214S mutation restored swimming, but CW-biased G215A did not.
- Mutants D251R and L329Q showed CW-biased rotation, indicating hydrophobic core disruption.

## Abstract

A flagellar motor can rotate either counterclockwise (CCW) or clockwise (CW), and rotational switching is triggered by conformational changes in FliG, although the molecular mechanism is still unknown. Here, we found that cheY deletion, which locks motor rotation in the CCW direction, restored the motility abolished by the fliG L259Q mutation. We found that the CCW-biased fliG G214S mutation also restored the swimming of the L259Q mutant, but the CW-biased fliG G215A mutation did not. Since the L259 residue participates in forming the FliG hydrophobic core at its C-terminal domain, mutations were introduced into residues structurally closer to L259, and their motility was examined. Two mutants, D251R and L329Q, exhibited CW-biased rotation. Our results suggest that mutations in the hydrophobic core of FliGC collapse its conformational switching and/or stator interaction; however, the CCW state of the rotor enables rotation even with this disruption.