Molecular basis of high-torque transmission of the Vibrio polar flagellar motor

The bacterial flagellar motors are huge protein nanomachines that drive rotation of the flagellum for bacterial motility, and have considerable diversity in structure among bacterial species, which enables the transmission of different torques to the flagellar filaments to propel bacteria and renders various swimming abilities. Relative to the bacterial peritrichous flagellar motors, the polar flagellar motors are the faster rotational machines and transmit high torque to drive bacterial high-speed motility in liquid and empower swimming in viscous environments. However, the structural basis of high-torque transmission of the polar flagellar motors is still unclear. Here we present an atomic-resolution cryo-electron microscope structure of the polar flagellar motor in complex with the hook from Vibrio alginolyticus, comprising 295 subunits from 18 proteins. Extensive inter-subunit interactions and additional phospholipids generate the higher rigidity of the rod. The LP ring utilizes more electrostatic charges on the inner surface and less physical contacts to facility the higher-speed rotation of the rod. The additional HT ring tightly binds to the outer surface of the LP ring to enhance the LP-ring stability. A previously function-unknown component FlrP enhances the interactions of 15 FliF peptides of the MS ring with the rod and stabilizes the LPHT and MS rings. The hook has two different states, L- and R-state, which provides structural flexibility of the hook to drive the unique forward-reverse-flick motility of the bacteria in the flicking process. This study provides unprecedented molecular insights into evolution and structural adaptions of the bacterial polar flagellar motors for high-torque transmission.