Cryo-EM Structure of FcεRI Bound IgE Reveals Multiple Defined Conformations of the Fab-Fc Hinge.

Abstract

IgE is the central driver of allergic responses. Prior studies have defined the conformation of the IgE Fc fragment bound to the FcεRIα ectodomain and the dynamic properties of the IgE Fc. It remains unknown, how these prior studies translate to the complex of a full antibody including the Fab arms with the receptor. For structural analysis, crystallography, cryo-EM and negative stain EM (ns-EM) were combined. IgE variants were analyzed by mediator release and CD23 binding assays. An ensemble of 10 cryo-EM structures of the full-size IgE FcεRIα complex was obtained revealing that the receptor bound IgE adopts a pronounced T-like conformation. Either Fab arm may rotate up to 40°. Two additional conformations with different arrangements of the Fab arms were captured in ns-EM. The introduction of additional flexibility into the Fab-Fc hinge does not compromise the biological activity of IgE, suggesting that the observed conformations of the IgE Fab-Fc hinge exhibit equivalent biological function. Comparison of the full IgE receptor complex with recent cryo-EM structures of the intact receptor reveals that FcεRI conformations differ markedly by the orientation of the ectodomain. Hence, our ensemble of IgE FcεRIα structures including the Fab arms enabled critical evaluation of FcεRI conformations. Our data reveal the architecture of a full-size IgE antibody bound to its receptor and a new layer of dynamics in FcεRIα bound IgE on top of the well-established IgE Fc conformations. Development of novel anti-IgE therapeutics may take into account these properties of FcεRIα bound IgE.