The complement system plays an important role in the innate immune response to invading pathogens. The complement fragment C5a is one of its important effector components and exerts diverse physiological functions through activation of the C5a receptor 1 (C5aR1) and associated downstream G protein and β-arrestin signaling pathways. Dysfunction of the C5a-C5aR1 axis is linked to numerous inflammatory and immune-mediated diseases, but the structural basis for activation and biased signaling of C5aR1 remains elusive. Here, we present cryo-electron microscopy structures of the activated wild-type C5aR1–G_i protein complex bound to each of the following: C5a, the hexapeptidic agonist C5a^pep, and the G protein-biased agonist BM213. The structures reveal the landscape of the C5a–C5aR1 interaction as well as a common motif for the recognition of diverse orthosteric ligands. Moreover, combined with mutagenesis studies and cell-based pharmacological assays, we deciphered a framework for biased signaling using different peptide analogs and provided insight into the activation mechanism of C5aR1 by solving the structure of C5aR1^I116A mutant–G_i signaling activation complex induced by C089, which exerts antagonism on wild-type C5aR1. In addition, unusual conformational changes in the intracellular end of transmembrane domain 7 and helix 8 upon agonist binding suggest a differential signal transduction process. Collectively, our study provides mechanistic understanding into the ligand recognition, biased signaling modulation, activation, and G_i protein coupling of C5aR1, which may facilitate the future design of therapeutic agents.

补体系统在对入侵病原体的先天免疫反应中起着重要作用。补体片段 C5a 是其重要的效应成分之一，通过激活 C5a 受体 1 (C5aR1) 和相关的下游 G 蛋白和 β-arrestin 信号通路发挥多种生理功能。C5a-C5aR1 轴的功能障碍与许多炎症和免疫介导的疾病有关，但 C5aR1 激活和偏向信号传导的结构基础仍然难以捉摸。在这里，我们展示了与以下各项结合的活化野生型 C5aR1–G _i蛋白复合物的低温电子显微镜结构：C5a，六肽激动剂 C5a ^pep和 G 蛋白偏向激动剂 BM213。这些结构揭示了 C5a–C5aR1 相互作用的概况以及识别不同正构配体的共同基序。^{此外，结合诱变研究和基于细胞的药理学测定，我们破译了使用不同肽类似物的偏向信号传导框架，并通过解析 C5aR1 I116A}突变体-G _i的结构提供了对 C5aR1 激活机制的深入了解C089 诱导的信号激活复合物，对野生型 C5aR1 发挥拮抗作用。此外，激动剂结合后跨膜结构域 7 和螺旋 8 的细胞内端异常构象变化表明存在差异信号转导过程。总的来说，我们的研究提供了对C5aR1 的配体识别、偏向信号调节、激活和 G _{i蛋白偶联的机制理解，这可能有助于未来治疗剂的设计。}