Sulfated tyrosines contribute to the formation of the C5a docking site of the human C5a anaphylatoxin receptor

The complement anaphylatoxin C5a and its seven-transmembrane segment (7TMS) receptor play an important role in host defense and in a number of inflamm ation-associated pathologies. The NH2-terminal domain of the C5a receptor ( C5aR/CD88) contributes substantially to its ability to bind C5a. Here we sh ow that the tyrosines at positions 11 and 14 of the C5aR are posttranslatio nally modified by the addition of sulfate groups. The sulfate moieties of e ach of these tyrosines are critical to the ability of the C5aR to bind C5a and to mobilize calcium. A C5aR variant lacking these sulfate moieties effi ciently mobility calcium in response to a small peptide agonist, but not to C5a, consistent with a two-site model of ligand association in which the t yrosine-sulfated region of the C5aR mediates the initial docking interactio n. A peptide based on the NH2 terminal of the C5aR and sulfated at these tw o tyrosines, but not its unsulfated analogue or a doubly sulfated control p eptide, partially inhibited C5a association with its receptor. These observ ations clarify structural and mutagenic studies of the C5a/C5aR association and suggest that related 7TMS receptors are also modified by functionally important sulfate groups on their NH2-terminal tyrosines.