Structural analysis of S-ring composed of FliFG fusion proteins in marine Vibrio polar flagellar motors

The marine bacterium Vibrio alginolyticus possesses a polar flagellum driven by a sodium ion flow. The main components of the flagellar motor are the stator and rotor. The C-ring and MS-ring which are composed of FliG and FliF, respectively, are parts of the rotor. Here, we purified an MS-ring composed of FliF-FliG fusion proteins and solved the near-atomic resolution structure of the S-ring--the upper part of the MS-ring--using cryo-electron microscopy. This is the first report of an S-ring structure from Vibrio whereas, previously, only those from Salmonella have been reported. The Vibrio S-ring structure reveals novel features compared to that of Salmonella such as tilt angle differences of the core domain and the {beta}-collar region, the decrease of the inter-subunit interaction between core domains, and altered electrostatic inner-surface. The residues potentially interact with other flagellar components, such as FliE and FlgB, are well structurally conserved in Vibrio S-ring. These comparisons clarified the conserved and non-conserved structural features of the MS-ring across different species. IMPORTANCEUnderstanding the structure and function of the flagellar motor in bacterial species is essential for uncovering the mechanisms underlying bacterial motility and pathogenesis. Our study revealed the structure of the Vibrio S-ring, a part of its polar flagellar motor, and highlighted its unique features compared with the well-studied Salmonella S-ring. The observed differences in the inter-subunit interactions and in the tilt angles between the Vibrio and Salmonella S-rings highlighted the species-specific variations in the flagellar assembly. By concentrating on the region where the S-ring and the rod proteins interact, we uncovered conserved residues essential for the interaction. Our research contributes to advancing of bacterial flagellar biology.