The dynamics of hinge flexibility in receptor bound immunoglobulin E revealed by electron microscopy

Immunoglobulin E is a mammal specific antibody isotype supporting the immune response against parasites and venoms, but also a driver of allergic responses. Prior studies have defined the conformation of the IgE Fc fragment bound to the cell surface receptor Fc{varepsilon}RI 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. Here we show that in a cryo-EM structure of the IgE Fc{varepsilon}RI complex, IgE adopts a T-like conformation where the antigen binding Fab arms may be parallel to the cell membrane. Two additional conformations are captured in negative stain EM (ns-EM) where the arrangements of the Fab arms differ from the cryo-EM conformation. Small angle scattering data favors the Fc{varepsilon}RI bound IgE conformation observed by cryo-EM, but the major IgE conformation observed by ns-EM possibly may also occur. In all observed conformations of Fc{varepsilon}RI bound IgE, one Fab arm is fixed relative to the IgE Fc moiety whereas the second Fab may alternate its position. Introduction of flexibility in the Fab-Fc hinge diminishes the biological activity of IgE demonstrating a functional role for the observed defined Fab-Fc hinge conformations. Our data show the organization of a full size antibody on its receptor and reveal a new layer of dynamics in Fc{varepsilon}RI bound IgE on top of the well established spectrum of IgE Fc conformations. Development of novel anti-IgE therapeutics may take into account these distinct Fc{varepsilon}RI bound IgE conformations. Significance statementIgE represents a canonical antibody isotype and is a key molecule for the allergic immune response to environmental triggers driven by mast cells and basophils. The requirements for efficient mediation of IgEs effects are not fully understood. Here we elucidate the structure of the entire IgE in complex with its high affinity receptor and identify two clearly distinct and dominant conformations, in which one of the Fab arms is fixed relative to the Fc domains. Enforcing IgE flexibility impacts the biological function with potential consequences for the allergic response. This unique behavior makes IgE different from all other isotypes and its understanding sheds light on the allergenic activation of the immune response.