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Chicken IgY Binds Its Receptor at the CH3/CH4 Interface Similarly as the Human IgA:FcαRI Interaction1

Chicken IgY, the ancestral form of mammalian IgE and IgG, is recognized by the high-affinity FcY receptor CHIR-AB1, a member of the leukocyte receptor family. In this study, we have characterized the receptor ligand interaction site by consecutive truncations of the Fcυ IgY domains and mutational analyses of selected residues. Using several fusion proteins that linked the human Cγ2 and Cγ3 domains with the Fcυ IgY domains, a binding assay revealed that both the Fcυ3 and Fcυ4 domains were essential for the IgY CHIR-AB1 interaction. Sequence comparisons of chicken IgY with human IgA demonstrated that 11 of the 19 contact residues important for the IgA FcαRI interaction have been conserved in chicken IgY, although the overall amino acid identity is only 34%. Among the 19 amino acids at respective positions in IgY, the mutation of two residues in the Fcυ3 and two in the Fcυ4 domain completely abolished the IgY to CHIR-AB1 binding revealed by two independent assays. Three further mutations substantially altered the interaction. Molecular modeling on the Cυ3 to Cυ4 crystal structure revealed that all critical residues, although on two domains, are in close proximity. The importance of N-linked carbohydrates was demonstrated by the failure of the CHIR-AB1 interaction after mutation of the glycosylation site. The identification of the IgY Cυ3/Cυ4 interdomain region as critical for binding to CHIR-AB1 significantly enhances our understanding of the IgY receptor interaction and allows further conclusions regarding the FcR phylogeny.