Role of the second extracellular loop of human C3a receptor in agonist binding and receptor function

Abstract

The C3a anaphylatoxin receptor (C3aR) is a G protein-coupled receptor with an unusually large second extra-cellular loop (e2 loop,;172 amino acids). To determinethe function of this unique structure, chimeric and de-letion mutants were prepared and analyzed in trans-fected RBL-2H3 cells. Whereas replacement of the C3aRN-terminal segment with that from the human C5a re-ceptor had minimal effect on C3a binding, substitutionof the e2 loop with a smaller e2 loop from the C5a recep-tor (C5aR) abolished binding of125I-C3a and C3a-stimu-lated calcium mobilization. However, as much as 65% ofthe e2 loop sequence (amino acids 198 –308) may be re-moved without affecting C3a binding or calcium re-sponses. The e2 loop sequences adjacent to the trans-membrane domains contain multiple aspartate residuesand are found to play an important role in C3a bindingbased on deletion mutagenesis. Replacement of five as-partate residues in the e2 loop with lysyl residues sig-nificantly compromised both the binding and functionalcapabilities of the C3a receptor mediated by intact C3aor by two C3a analog peptides. These data suggest atwo-site C3a-C3aR interaction model similar to that es-tablished for C5a/C5aR. The anionic residues near the Nand C termini of the C3aR e2 loop constitute a non-effector secondary interaction site with cationic resi-dues in the C-terminal helical region of C3a, whereas theC3a C-terminal sequence LGLAR engages the primaryeffector site in C3aR.