An FcγRIIa-binding peptide that mimics the interaction between FcγRIIa and IgG

A disulphide-con strained peptide that binds to the low affinity Fe receptor, Fc gamma RIIa (CD32) has been identified and its structure solved by NMR. Linear (7-mer and 12-mer) and disulphide-constrained (7-mer) phage display peptide libraries were panned on recombinant soluble Fc gamma RIIa genetically fused to HSA (HSA-Fc gamma RIIa). Peptides were isolated only from the constrained peptide library and these contained the consensus sequence, CWPGWxxC. Phage clones displaying variants of the peptide consensus sequence bound to Fc gamma RIIa and the strongest binding clone C7C1 (CWPGWDLNC) competed with IgG for binding to Fc gamma RIIa and was inhibited from binding to Fc gamma RIIa by the Fc gamma RIIa-blocking antibody, IV.3, suggesting that C7C1 and IgG share related binding sites on Fc gamma RIIa. A synthetic disulphide-constrained peptide, pep-C7C1 bound to Fc gamma RIIa by biosensor analysis, albeit with low affinity (K-D similar to 100 mu M). It was significant that the Fc gamma RIIa consensus peptide sequence contained a Proline (Pro(3)), which when substituted with alanine abrogated Fc-yRIIa binding, consistent with Pro 3 contributing to receptor binding. Upon binding of IgG and IgE to their respective Fe receptors (Fc gamma Rs and Fc epsilon RI) Pro(329) in the Fc makes a critical interaction with two highly conserved Trp residues (Trp(90) and Trp(113)) of the FcRs. The NMR structure of pep-C7C1 revealed a stabilizing type II beta-turn between Trp(2) and Trp(5), with Pro(3) solvent exposed. Modelling of the pep-C7C1 structure in complex with Fc gamma RIIa suggests that Pro(3) of C7C1 binds to Fc-yRIIa by inserting between Trp(90) and Trp(113) of Fc gamma RIIa thereby mimicking the molecular interaction made between Fc gamma RIIa and IgG. (c) 2007 Elsevier Ltd. All rights reserved.