Residues 21-30 within the extracellular N-terminal region of the C5a receptor represent a binding domain for the C5a anaphylatoxin.

The functions of the C5a anaphylatoxin are expressed through its interaction with a cell-surface receptor with seven transmembrane helices. The interaction of C5a with the receptor has been explained by a two-site model whereby recognition and effector sites on C5a bind, respectively, to recognition and effector domains on the receptor, leading to receptor activation (Chenoweth, D. E., and Hugli, T. E. (1980) Mol. Immunol. 17, 151-161. In addition, the extracellular N-terminal region of the C5a receptor has been implicated as the recognition domain for C5a, responsible for approximately 50% of the binding energy of the C5a-receptor complex (Mery, L., and Boulay, F. (1994) J. Biol. Chem. 269, 3457-3463; DeMartino, J. A., Van Riper, G., Siciliano, S. J., Molineaux, C. J., Konteatis, Z. D., Rosen, H., and Springer, M. S. (1994) J. Biol. Chem. 269, 14446-14450). In this work, the interactions of C5a with the N-terminal domain of the C5a receptor were examined by use of recombinant human C5a molecules and peptide fragments M1NSFN5YTTPD10YGHYD15DKDTL20DLNTP25VDKTS30NTLR(hC5aRF-1-34), acetyl-HYD15DKDTL20DLNTP25VDKTS30NTLR (hC5aRF-13-34), and acetyl-TL20DLNTP25VDKTS30N-amide (hC5aRF-19-31) derived from human C5a receptor. Binding induced resonance perturbations in the NMR spectra of the receptor fragments and the C5a molecules indicated that the isolated Nterminal domain or residues 1-34 of the C5a receptor retain specific binding to C5a and to a C5a analog devoid of the agonistic C-terminal tail in the intact C5a. Residues of C5a perturbed by the binding of the receptor peptides are localized within the helical core of the C5a structure, in agreement with the results from functional studies employing mutated C5a and intact receptor molecules. All three receptor peptides, hC5aRF-1-34, hC5aRF-13-34, and hC5aRF-19-31, responded to the binding of C5a through the 21-30 region containing either hydrophobic, polar, or positively charged residues such as Thr24, Pro25, Val26, Lys28, Thr29, and Ser30. The 21-30 segment of all three receptor fragments was found to have a partially folded conformation in solution, independent of residues 1-18. These results indicate that a short peptide sequence, or residues 21-30, of the C5a receptor N terminus may constitute the binding domain for the recognition site on C5a.