The ancient E-ring in bacterial flagellar motors

The bacterial flagellum is an elaborate nanomachine that powers motility in a variety of environments. While recent cryo-electron tomography studies have revealed great complexity as well as diversity in flagellar motor structures, less is known about the components that constitute the auxiliary structures observed in the periplasm for several species. One example is the E-ring, which was first observed in 1979 in Caulobacter crescentus but whose composition has only recently been shown to be a single protein, FlgY and its homologs. Multiple FlgY dimers form a conserved ring-spoke structure encircling the MS-ring, although the impact of the E-ring on motility seems to differ across bacterial phyla. Remarkably, the E-ring is widely present in flagellated species in the Bacteria domain except β- and γ-proteobacteria, suggesting an ancient origin that likely traces back to the last bacterial common ancestor. Future investigation is required to determine the exact role of this conserved structure in motor function, which may reveal mechanisms distinct from the current working model based on Escherichia coli and Salmonella enterica, which lack the E-ring, and also shed light on the architecture and function of the ancestral motor.