Mapping the ligand-binding site on the C5a receptor: Arginine(74) of C5a contacts aspartate(282) of the C5a receptor

The interaction between the anaphylatoxin C5a and its receptor involves two distinct sites. One site is formed by acidic residues at the receptor N-te rminus and contributes to only ligand binding. The second site, responsible for activation, is less well defined. In this study, we demonstrate that t he receptor residue D-282, near the extracellular face of transmembrane dom ain VII, is a component of the second ligand-binding site. Mutation of D282 to A decreases the sensitivity of the receptor to activation by intact C5a but not by its less potent metabolite, C5adR(74), which lacks the C-termin al arginine(74). The mutation of the R-74 residue of C5a to A causes a 60-f old decrease in wild-type receptor sensitivity, but only a 2-fold decrease for the receptor mutated at D-282. In contrast, the mutation of R-74 to D m akes C5a. completely inactive on both wild-type and A(282) C5a receptors. T he mutation of D-282 to R partly restores the response to C5a[D-74], which is a more effective ligand than C5a, at the mutant receptor. A peptide mimi c of the C5a activation domain with a C-terminal R potently activates the w ild type but is only a weak agonist at the mutant (DR)-R-282-C5a receptor. Conversely, a peptide with D at the C-terminus is a more effective activato r of (DR)-R-282 than wild-type C5a receptors. These data indicate that the R-74 side chain of C5a makes an interaction with receptor D-282 that is res ponsible for the higher potency of intact C5a versus that of C5adR(74).