Structure of Vibrio FliL, a New Stomatin-like Protein That Assists the Bacterial Flagellar Motor Function

Many motile bacteria swim or swarm using a filamentous rotating organelle, the flagellum. FliL, a component protein of the flagellar motor, is known to enhance the motor performance under high-load conditions in some bacteria. Here we determined the structure of the periplasmic region of FliL (FliL(Peri)) of the polar flagellum of Vibrio alginolyticus. FliL(Peri) shows a remarkable structural similarity to the stomatin/prohibitin/flotillin/HflK/C (SPFH) domain of stomatin family proteins, some of which are involved in modulation of ion channel activities in various organisms. FliL(Peri) forms a ring assembly in the crystal with an inner diameter of around 8 nm, which is comparable to the size of the stator unit. Mutational analyses suggest that the FliL ring forms a complex with the stator unit and that the length of the periplasmic linkers of FliL and the stator B-subunit is essential for the complex formation. We propose a model of the FliL-stator complex to discuss how Vibrio FliL modulates stator function in the bacterial flagellar motor under conditions of high viscosity.