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Chicken IgY Binds Its Receptor at the C_H3/C_H4 Interface Similarly as the Human IgA:FcRI Interaction¹

Department of Veterinary Sciences, University of Munich, Munich, Germany

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 Fc IgY domains and mutational analyses of selected residues. Using several fusion proteins that linked the human C2 and C3 domains with the Fc IgY domains, a binding assay revealed that both the Fc3 and Fc4 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 FcRI 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 Fc3 and two in the Fc4 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 C3 to C4 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 C3/C4 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.

The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked advertisement in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

¹ This work was supported by Deutsche Forschungsgemeinschaft Grant 489/3-6 (to T.W.G.).

² Address correspondence and reprint requests to Dr. Thomas Göbel, Dept. of Veterinary Sciences, Institute for Animal Physiology, Veterinärstrasse 13, 80539 Munich, Germany. E-mail address: goebel{at}lmu.de

³ Abbreviations used in this paper: CHIR, chicken Ig-like receptor; LRC, leukocyte receptor complex.