An activation switch in the ligand binding pocket of the C5a receptor

Although agonists are thought to occupy binding pockets within the seven-he lix core of serpentine receptors, the topography of these binding pockets a nd the conformational changes responsible for receptor activation are poorl y understood. To identify the ligand binding pocket in the receptor for com plement factor 5a (C5aR), we assessed binding affinities of hexapeptide lig ands, each mutated at a single position, for seven mutant C5aRs, each mutat ed at a single position in the putative ligand binding site. In ChaW tan an tagonist) and W5Cha tan agonist), the side chains at position 5 are tryptop han and cyclohexylalanine, respectively. Comparisons of binding affinities indicated that the hexapeptide residue at this position interacts with two C5aR residues, IIe-116 (helix III) and Val-286 (helix VII); in a C5aR model these two side chains point toward one another. Both the I116A and the V28 6A mutations markedly increased binding affinity of W5Cha but not that of C haW, Moreover, ChaW, the antagonist hexapeptide, acted as a full agonist on the I116A mutant. These results argue that C5aR residues Ile-116 and Val-2 86 interact with the side chain at position 5 of the hexapeptide ligand to form an activation switch. Based on this and previous work, we present a do cking model for the hexapeptide within the C5aR binding pocket. We propose that agonists induce a small change in the relative orientations of helices III and VII and that these helices work together to allow movement of heli x VI away from the receptor core, thereby triggering G protein activation,