Fine structure analysis of interaction of FcεRI with IgE

The high affinity receptor for IgE (Fc epsilon RI) plays an integral role in triggering IgE-mediated hypersensitivity reactions. The IgE-interactive site of human Fc epsilon RI has previously been broadly mapped to several large regions in the second extracellular domain (D2) of the a-subunit (Fc epsilon RI alpha). In this study, the IgE binding site of human FceRIa has been further localized to subregions of D2, and key residues putatively involved in the interaction with IgE have been identified. Chimeric receptors generated between Fc epsilon RI alpha and the functionally distinct but structurally homologous low affinity receptor for IgG (Fc gamma RIIa) have been used to localize two IgE binding regions of Fc epsilon RI alpha to amino acid segments Tyr(129)-His(134) and Lys(154)-Glu(161). Both regions were capable of independently binding IgE upon placement into Fc gamma RIIa, Molecular modeling of the three-dimensional structure of Fc epsilon RI alpha-D2 has suggested that these binding regions correspond to the "exposed" C'-E and F-G loop regions at the membrane distal portion of the domain. A systematic site-directed mutagenesis strategy, whereby each residue in the Tyr(129)-His(134) and Lys(154)-Glu(161) regions of Fc epsilon RI alpha was replaced with alanine, has identified key residues putatively involved in the interaction with IgE, Substitution of Tyr(131), Glu(132), Val(155) and Asp(159) decreased the binding of IgE, whereas substitution of Trp(130), Trp(156) Tyr(160), and Glu(161) increased binding. In addition, mutagenesis of residues Trp(113), Val(115), and Tyr(116) in the B-C loop region, which lies adjacent to the C'-E and F-G loops, has suggested Trp(113) also contributes to IgE binding, since the substitution of this residue with alanine dramatically reduces binding. This information should prove valuable in the design of strategies to intervene in the Fc epsilon RI alpha-IgE interaction for the possible treatment of IgE-mediated allergic disease.