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

Abstract

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 <i>Vibrio alginolyticus</i> 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 <i>Vibrio</i> FliL modulates stator function in the bacterial flagellar motor under conditions of high viscosity.<b>IMPORTANCE</b> Some flagellated bacteria regulate motor torque in response to the external load change. This behavior is critical for survival, but the mechanism has remained unknown. Here, we focused on a key protein, FliL of <i>Vibrio alginolyticus</i>, and solved the crystal structure of its periplasmic region (FliL_Peri). FliL_Peri reveals striking structural similarity to a conserved domain of stomatin, which is involved in ion channel regulation in some organisms, including mammals. FliL_Peri forms a ring with an inner diameter that is comparable in size to the stator unit. The mutational analyses suggested that the presence of the ring-like assembly of FliL around the stator unit enhances the surface swarming of <i>Vibrio</i> cells. Our study data also imply that the structural element for the ion channel regulation is conserved from bacteria to mammals.