Characterisation of C5a receptor agonists from phage display libraries

C5a des-Arg74 has a 10- to 100-fold lower receptor binding affinity than intact C5a and is only a partial agonist. We have used phage display selection from randomly mutated C5a des-Arg74 libraries to isolate variant proteins that can activate C5a receptors with similar potency to C5a. Here we explore the interactions of three variants (V1-3) with C5aR mutated at residues involved in the differential response. The mutant Asp 282Arg-C5aR is preferentially activated by C5a des-Arg74, probably due to repulsion between Arg74 of C5a and the substituent Arg282. In accordance with this hypothesis, V2 (with a polar C-terminus which has no Arg residue) but not V1 (with a C-terminal Arg residue at position 73) could activate Asp282Arg-C5aR. V3, with a very hydrophobic C-terminus, was the most potent agonist at Asp 282Arg-C5aR. Arg175 is a potential counterion for the C-terminal carboxylate of C5a. C5aR mutated to either Ala or Asp at this position lost nearly all responsiveness to both C5a and C5a des-Arg 74, suggesting that mutation of Arg175 caused a non-specific loss of receptor conformation and a loss of signalling capacity. However, V3 could still activate Arg175Asp/Ala-C5aR with the same potency as wild-type C5aR, demonstrating that the mutant receptors retained high signalling capability and showed a specific loss of responsiveness. Thus C5a des-Arg74 variants produced by phage display are potentially useful tools for the dissection of ligand-receptor interactions. © 2003 Elsevier Inc. All rights reserved.