The influence of Lys68 in decapeptide agonists of C5a on C5a receptor binding, activation and selectivity

The potent, conformationally biased C5a agonist peptide YSFKPMPLaR (C5a(65- 74), Y65, F67, P69, P71, D-Ala73) was used as a template to gain insight in to the nature and importance of lysine at position 68 in the peptide-recept or interaction. A panel of YSFKPMPLaR analogs with systematic substitutions for Lys68 was evaluated for C5a receptor (C5aR) binding affinity and activ ation in two well-characterized assay systems: human polymorphonuclear leuk ocytes (PMNs) and human fetal artery. In addition, we determined the activi ty of these new analogs in transfected rat basophilic leukemia (RBL) cells in which the Glu at position 199 of the C5aR (wtGlu199) was replaced by a G in (C5aR-Gln199) or a Lys (C5aR-Lys199). Our results indicated that Lys68 i n YSFKPMPLaR plays an important role in binding the C5aR expressed on PMNs and RBL cells. Furthermore, the data indicated that Lys68 interacted with G lu199 of the C5aR in PMNs and RBL cells. In human fetal artery, however, Ly s68 substitutions had little or no effect on activity, which suggested that the receptor conformation may be different in this tissue. Thus, the inter action between Lys68 of the decapeptide agonist and Glu199 of the C5aR may be cell type-specific and may form the molecular basis for tissue-specific responses to C5a agonists.