Structures of the stator complex that drives rotation of the bacterial flagellum

SummaryThe bacterial flagellum is the proto-typical protein nanomachine and comprises a rotating helical propeller attached to a membrane-embedded motor complex1. The motor consists of a central rotor surround by stator units that couple ion flow across the cytoplasmic membrane to torque generation. Here we present the structures of stator complexes from multiple bacterial species, allowing interpretation of the extensive body of data on stator mechanism. The structures reveal an unexpected asymmetric A5B2 subunit assembly in which the five A subunits enclose the two B subunits. Comparison to novel structures of other ion-driven motors indicates that this A5B2 architecture is fundamental to bacterial systems that couple energy from ion-flow to generate mechanical work at a distance, and suggests that such events involve rotation in the motor structures.