The binding site on human immunoglobulin E for its high affinity receptor

Abstract

Immunoglobulin (Ig) E antibodies mediate allergic responses by binding to specific high affinity receptors, FcdtI, on mast cells and basophils.Previous studies have shown that the principal FceRI binding site is located on the third constant domain, Fce3, of IgE.Based on a model of the IgE Fce3 (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 FccRI.The amino acids are localized in three loops, which form a putative ridge on the most exposed side of the Fee3 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-FceRI binding is mediated primarily by electrostatic interaction.Furthermore, it is possible to confer FccRI binding to an IgG molecule by introducing these three IgE loops into the I& Cy2 domain.Immunoglobulin (Ig)' E antibodies bind to specific high affinity receptors, FceRI, on mast cells, basophils, and Langerhans cells (1-4) via their constant, or Fc, region in a one-to-one complex (5).Degranulation of mast cells and basophils triggered by binding of antigen to IgE-loaded FceRI leads to release of mediators which produce symptoms associated with allergy (6,7).IgE also binds to low affinity receptors (FceRII, CD23) on B-lymphocytes and eosinophils (8,Q).A more in-depth review of IgE and its receptors can be gleaned fmm Ref. 10.Because of the central role of cell-bound IgE in initiation of the allergic cascade, much attention has been focused on localizing the sites on IgE required for receptor binding.Over the last decade a variety of techniques have been used to determine the FcrRI binding site on IgE, including synthetic peptides corresponding to portions of the IgE Fc (11-14), anti-IgE antibodies IgE variants (18), and truncated forms (23).However, the exact IgE residues involved in binding have remained undefined.Helm et a2.(13, 14) reported mapping the FceRI binding site on human IgE to a sequence of 76 amino acids at the junction of the second (Fcr2) and third (Fce3) constant domains.In contrast, a recent study reported that only