The Binding Site on Human Immunoglobulin E for Its High Affinity Receptor

Immunoglobulin (Ig) E antibodies mediate allergic responses by binding to specific high affinity receptors, Fc epsilon RI, on mast cells and basophils. Previous studies have shown that the principal Fc epsilon RI binding site is located on the third constant domain, Fc epsilon 3, of IgE. Based on a model of the IgE Fc epsilon 3 (which is homologous to the second constant domain of IgG), homology scanning mutagenesis and replacement of individual residues were used to determine the specific amino acids of human IgE involved in binding to human Fc epsilon RI. The amino acids are localized in three loops, which form a putative ridge on the most exposed side of the Fc epsilon 3 domain of IgE and include Arg-408, Ser-411, Lys-415, Glu-452, Arg-465, and Met-469. The preponderance of charged residues suggests that IgE-Fc epsilon RI binding is mediated primarily by electrostatic interaction. Furthermore, it is possible to confer Fc epsilon RI binding to an IgG molecule by introducing these three IgE loops into the IgG C gamma 2 domain.