Symmetry mismatch in the MS-ring of the bacterial flagellar rotor explains structural coordination of secretion and rotation

The bacterial flagellum is a complex, self-assembling, nanomachine that confers motility to the cell. Despite great variation across species, all flagella are ultimately constructed from a helical propellor attached to a motor embedded in the inner membrane. The motor consists of a series of stator units surrounding a central rotor made up of two ring complexes, the MS-ring and the C-ring. Despite many studies, high resolution structural information is still completely lacking for the MS-ring of the rotor, and proposed mismatches in stoichiometry between the two rings have long provided a source of confusion for the field. We here present structures of the Salmonella MS-ring, revealing an unprecedented level of inter- and intra-chain symmetry variation that provides a structural explanation for the ability of the MS-ring to function as a complex and elegant interface between the two main functions of the flagellum, protein secretion and rotation.