Abstract Activation of the complement cascade is a critical part of our innate immune response against invading pathogens, and it operates in a concerted fashion with the antibodies and phagocytic cells towards the clearance of pathogens. The complement peptide C5a, generated during the activation of complement cascade, is a potent inflammatory molecule, and increased levels of C5a are implicated in multiple inflammatory disorders including the advanced stages of COVID-19 pathophysiology. The proximal step in C5a-mediated cellular and physiological responses is its interaction with two different seven transmembrane receptors (7TMRs) known as C5aR1 and C5aR2. Despite a large body of functional data on C5a-C5aR1 interaction, direct visualization of their interaction at high-resolution is still lacking, and it represents a significant knowledge gap in our current understanding of complement receptor activation and signaling. Here, we present cryo-EM structures of C5aR1 activated by its natural agonist C5a, and a G-protein-biased synthetic peptide ligand C5a pep , in complex with heterotrimeric G-proteins. The C5a-C5aR1 structure reveals the ligand binding interface involving the N-terminus and extracellular loops of the receptor, and we observe that C5a exhibits a significant conformational change upon its interaction with the receptor compared to the basal conformation. On the other hand, the structural details of C5a pep -C5aR1 complex provide a molecular basis to rationalize the ability of peptides, designed based on the carboxyl-terminus sequence of C5a, to act as potent agonists of the receptor, and also the mechanism underlying their biased agonism. In addition, these structural snapshots also reveal activation-associated conformational changes in C5aR1 including outward movement of TM6 and a dramatic rotation of helix 8, and the interaction interface for G-protein-coupling. In summary, this study provides previously lacking molecular basis for the complement C5a recognition and activation of C5aR1, and it should facilitate structure-based discovery of novel lead molecules to target C5aR1 in inflammatory disorders.

Structural insights into ligand-recognition, activation, and signaling-bias at the complement C5a receptor, C5aR1