The Use of Mouse/Human IgE Chimeras to Map the FcϵR Binding Site of IgE

Abstract

The binding of IgE antibodies to their specific receptors on mast cells is a crucial step in the allergic response and can serve as a paradigm for the study of receptor–ligand interactions. Intense efforts have been made to identify amino acid sequences and structural motifs on the IgE molecule that may be involved in the binding to the Fcϵ receptor. Studies using short IgE peptides or fragments are restricted by the conformation of these polypeptides, as a sequence that corresponds to the receptor-binding epitope on the IgE molecule may adopt a different, nonbinding conformation in solution. To avoid this difficulty, we have used exon shuffling between mouse and human IgE to identify the domain involved in binding to the mouse and human low- and high-affinity Fcϵ receptors. The results obtained using such human–mouse IgE chimeras strongly suggest that the Cϵ3 domain is sufficient for species-specific binding to both the low- and the high-affinity Fcϵ receptors. Binding is observed even upon deletion of the Cϵ2 domain. Within the Cϵ3 domain, conformational determinants composed of residues from throughout the domain are needed to form the binding site for the FcϵRI. For the low-affinity receptor, the binding site appears to reside in the C-terminal part of Cϵ3.