Structural insights into the assembly and evolution of a complex bacterial flagellar motor

Knowledge of bacterial flagella has largely come from studies of the simpler motors of Escherichia coli and Salmonella enterica. However, many bacteria harbour more complex motors. The function, mechanisms and evolution associated with such auxiliary motor structures are unclear. Here we deploy structural, genetic, biochemical and functional approaches to characterize complex adaptations of the flagellar motor in Campylobacter jejuni. We observed an E ring formed by 17 FlgY homodimers around the MS ring, a cage-like structure made of FcpMNO and PflD, and PflA-PflB interactions in a spoke-rim formation between the E ring and cage. These scaffolds stabilized the 17 torque-generating stator complexes. Phylogenetic analyses suggest an ancient origin and widespread prevalence of the E ring and spokes across diverse flagellated bacteria, and co-option of type IV pilus components in the ancestral motor of phylum Campylobacterota. Collectively, these data provide insight into the assembly, function and evolution of complex flagellar motors.