Atomic structure of the flagellar filament reveals how ε Proteobacteria escaped Toll-like receptor 5 surveillance.

Vertebrates, from zebra fish to humans, have an innate immune recognition of many bacterial flagellins. This involves a conserved eight-amino acid epitope in flagellin recognized by the Toll-like receptor 5 (TLR5). Several important human pathogens, such as and , have escaped TLR5 activation by mutations in this epitope. When such mutations were introduced into flagellin, motility was abolished. It was previously argued, using very low-resolution cryoelectron microscopy (cryo-EM), that accommodated these mutations by forming filaments with 7 protofilaments, rather than the 11 found in other bacteria. We have now determined the atomic structure of the G508A flagellar filament from a 3.5-Å-resolution cryo-EM reconstruction, and show that it has 11 protofilaments. The residues in the TLR5 epitope have reduced contacts with the adjacent subunit compared to other bacterial flagellar filament structures. The weakening of the subunit-subunit interface introduced by the mutations in the TLR5 epitope is compensated for by extensive interactions between the outer domains of the flagellin subunits. In other bacteria, these outer domains can be nearly absent or removed without affecting motility. Furthermore, we provide evidence for the stabilization of these outer domain interactions through glycosylation of key residues. These results explain the essential role of glycosylation in motility, and show how the outer domains have evolved to play a role not previously found in other bacteria.