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 Helicobacter pylori and Campylobacter jejuni, have escaped TLR5 activation by mutations in this epitope. When such mutations were introduced into Salmonella flagellin, motility was abolished. It was previously argued, using very low-resolution cryoelectron microscopy (cryo-EM), that C. jejuni 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 C. jejuni G508A flagellar filament from a 3.5-Å-resolution cryo-EM reconstruction, and show that it has 11 protofilaments. The residues in the C. jejuni 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 C. jejuni motility, and show how the outer domains have evolved to play a role not previously found in other bacteria.

从斑马鱼到人类的脊椎动物都具有许多细菌鞭毛蛋白的先天免疫识别能力。这涉及鞭毛蛋白中由Toll样受体5（TLR5）识别的保守的八个氨基酸表位。几种重要的人类病原体，例如幽门螺杆菌和空肠弯曲菌，已通过此表位的突变逃脱了TLR5激活。当将此类突变引入沙门氏菌鞭毛蛋白时，活力就消失了。以前有人认为，使用超低分辨率的冷冻电子显微镜（cryo-EM），空肠弯曲杆菌通过形成具有7条原丝的细丝来适应这些突变，而不是其他细菌中发现的11条。现在我们确定了空肠弯曲杆菌的原子结构G508A鞭毛丝由3.5Å分辨率的冷冻EM重建而成，并显示它具有11个原型丝。与其他细菌鞭毛细丝结构相比，空肠弯曲杆菌TLR5表位中的残基与相邻亚基的接触减少。通过鞭毛蛋白亚基的外部结构域之间的广泛相互作用，可以补偿由TLR5表位突变引起的亚基-亚基界面的弱化。在其他细菌中，几乎不存在或除去这些外部结构域而不会影响运动性。此外，我们提供了通过关键残基的糖基化稳定这些外部域相互作用的证据。这些结果解释了糖基化在空肠弯曲杆菌中的重要作用。 活力，并显示外域如何进化以发挥以前在其他细菌中未发现的作用。