Ion selectivity and rotor coupling of the Vibrio flagellar sodium-driven stator unit.

Bacteria swim using a flagellar motor that is powered by stator units. Vibrio spp. are highly motile bacteria responsible for various human diseases, the polar flagella of which are exclusively driven by sodium-dependent stator units (PomAB). However, how ion selectivity is attained, how ion transport triggers the directional rotation of the stator unit, and how the stator unit is incorporated into the flagellar rotor remained largely unclear. Here, we have determined by cryo-electron microscopy the structure of Vibrio PomAB. The electrostatic potential map uncovers sodium binding sites, which together with functional experiments and molecular dynamics simulations, reveal a mechanism for ion translocation and selectivity. Bulky hydrophobic residues from PomA prime PomA for clockwise rotation. We propose that a dynamic helical motif in PomA regulates the distance between PomA subunit cytoplasmic domains, stator unit activation, and torque transmission. Together, our study provides mechanistic insights for understanding ion selectivity and rotor incorporation of the stator unit of the bacterial flagellum.

Structure and Function of Stator Units of the Bacterial Flagellar Motor.

Many bacteria use the flagellum for locomotion and chemotaxis. Its bidirectional rotation is driven by a membrane-embedded motor, which uses energy from the transmembrane ion gradient to generate torque at the interface between stator units and rotor. The structural organization of the stator unit (MotAB), its conformational changes upon ion transport, and how these changes power rotation of the flagellum remain unknown. Here, we present ~3 Å-resolution cryoelectron microscopy reconstructions of the stator unit in different functional states. We show that the stator unit consists of a dimer of MotB surrounded by a pentamer of MotA. Combining structural data with mutagenesis and functional studies, we identify key residues involved in torque generation and present a detailed mechanistic model for motor function and switching of rotational direction.