Fine structure analysis of interaction of Fc epsilon RI with IgE

The high affinity receptor for IgE (Fc epsilon RI) plays an integral role i n triggering IgE-mediated hypersensitivity reactions. The IgE-interactive s ite of human Fc epsilon RI has previously been broadly mapped to several la rge regions in the second extracellular domain (D2) of the a-subunit (Fc ep silon RI alpha). In this study, the IgE binding site of human FceRIa has be en further localized to subregions of D2, and key residues putatively invol ved in the interaction with IgE have been identified. Chimeric receptors ge nerated between Fc epsilon RI alpha and the functionally distinct but struc turally 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 ac id segments Tyr(129)-His(134) and Lys(154)-Glu(161). Both regions were capa ble of independently binding IgE upon placement into Fc gamma RIIa, Molecul ar modeling of the three-dimensional structure of Fc epsilon RI alpha-D2 ha s 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 systemati c 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 intera ction with IgE, Substitution of Tyr(131), Glu(132), Val(155) and Asp(159) d ecreased 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 adjace nt to the C'-E and F-G loops, has suggested Trp(113) also contributes to Ig E binding, since the substitution of this residue with alanine dramatically reduces binding. This information should prove valuable in the design of s trategies to intervene in the Fc epsilon RI alpha-IgE interaction for the p ossible treatment of IgE-mediated allergic disease.