The nucleotide-binding domain- and leucine-rich repeat (LRR)-containing proteins (NLRs) function as intracellular immune receptors to detect the presence of pathogen- or host-derived signals. The mechanisms of how NLRs sense their ligands remain elusive. Here we report the structure of a bacterial flagellin derivative in complex with the NLR proteins NAIP5 and NLRC4 determined by cryo-electron microscopy at 4.28 Å resolution. The structure revealed that the flagellin derivative forms two parallel helices interacting with multiple domains including BIR1 and LRR of NAIP5. Binding to NAIP5 results in a nearly complete burial of the flagellin derivative, thus stabilizing the active conformation of NAIP5. The extreme C-terminal side of the flagellin is anchored to a sterically constrained binding pocket of NAIP5, which likely acts as a structural determinant for discrimination of different bacterial flagellins by NAIP5, a notion further supported by biochemical data. Taken together, our results shed light on the molecular mechanisms underlying NLR ligand perception.

中文翻译：

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NLR蛋白NAIP5识别特定鞭毛蛋白的结构基础。

含有核苷酸结合结构域和富含亮氨酸的重复序列（LRR）的蛋白质（NLR）充当细胞内免疫受体，以检测病原体或宿主来源的信号的存在。NLR如何感知其配体的机制仍然难以捉摸。在这里，我们报告了一种细菌鞭毛蛋白衍生物的结构，该蛋白与NLR蛋白NAIP5和NLRC4通过冷冻电子显微镜在4.28Å分辨率下测定。结构表明鞭毛蛋白衍生物形成两个平行的螺旋，与包括NAIP5的BIR1和LRR在内的多个域相互作用。与NAIP5的结合导致鞭毛蛋白衍生物几乎完全被掩埋，从而稳定了NAIP5的活性构象。鞭毛蛋白的C端极端固定在NAIP5的空间受限结合口袋中，它可能是通过NAIP5区分不同细菌鞭毛蛋白的结构决定因素，这一观点得到了生化数据的进一步支持。两者合计，我们的结果阐明了NLR配体感知的分子机制。