Kim Jinuk, Ko Saebom, Choi Chulwon, Bae Jungnam, Byeon Hyeonsung, Seok Chaok, Choi Hee-Jung
Department of Biological Sciences, Seoul National University, Seoul, 08826, Republic of Korea.
Division of Biological Science and Technology, Yonsei University, Wonju, 26493, Republic of Korea.
EMBO J. 2025 May;44(10):2803-2826. doi: 10.1038/s44318-025-00429-w. Epub 2025 Apr 7.
The complement system plays crucial roles in innate immunity and inflammatory responses. The anaphylatoxin C3a mediates pro-inflammatory and chemotactic functions through the G protein-coupled receptor C3aR. While the active structure of the C3a-C3aR-G complex has been determined, the inactive conformation and activation mechanism of C3aR remain elusive. Here we report the cryo-EM structure of ligand-free, G protein-free C3aR, providing insights into its inactive conformation. In addition, we determine the structures of C3aR in complex with the synthetic small-molecule agonist JR14a in two distinct conformational states: a G protein-free intermediate, and a fully active G-bound state. The structure of the active JR14a-bound C3aR reveals that JR14a engages in highly conserved interactions with C3aR, similar to the binding of the C-terminal pentapeptide of C3a, along with JR14a-specific interactions. Structural comparison of C3aR in the apo, intermediate, and fully active states provides novel insights into the conformational landscape and activation mechanism of C3aR and defines a molecular basis explaining its high basal activity. Our results may aid in the rational design of therapeutics targeting complement-related inflammatory disorders.
补体系统在固有免疫和炎症反应中发挥着关键作用。过敏毒素C3a通过G蛋白偶联受体C3aR介导促炎和趋化功能。虽然C3a-C3aR-G复合物的活性结构已被确定，但C3aR的非活性构象和激活机制仍不清楚。在此，我们报道了无配体、无G蛋白的C3aR的冷冻电镜结构，为其非活性构象提供了见解。此外，我们确定了C3aR与合成小分子激动剂JR14a在两种不同构象状态下的复合物结构：一种是无G蛋白的中间体，另一种是完全活性的G结合状态。活性JR14a结合的C3aR的结构表明，JR14a与C3aR进行高度保守的相互作用，类似于C3a C末端五肽的结合，同时还有JR14a特异性相互作用。对apo、中间体和完全活性状态下的C3aR进行结构比较，为C3aR的构象变化情况和激活机制提供了新的见解，并定义了解释其高基础活性的分子基础。我们的结果可能有助于合理设计针对补体相关炎症性疾病的治疗药物。
