[W1-2] A small ring contributes to the mechanical response of the bacterial flagellar motor
Many motile bacteria swim or swarm using a filamentous rotating organelle, the flagellum. 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, a component protein of the flagellar motor and is known to enhance the motor performance under high load condition in some bacteria. We determined the crystal structure of the periplasmic region of FliL (FliLPeri), and FliLPeri reveals striking structural similarity to the SPFH domain, a conserved domain of stomatin family proteins some of which are involved in modulation of ion channel activities in various organisms including mammals. FliLPeri forms a ring assembly with an inner diameter of around 8 nm, which is a comparable size to the stator unit. The mutational analyses suggest that the ring-like assembly of FliL around the stator unit would enhance the surface swarming of bacterial cells, and 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 FliL modulates stator function in the bacterial flagellar motor under high viscosity. Our study also implies that the structural element for the ion channel regulation is conserved from bacteria to mammals.