A Range of C∊3–C∊4 Interdomain Angles in IgE Fc Accommodate Binding to Its Receptor CD23

The antibody IgE plays a central role in allergic disease, functioning principally through two cell-surface receptors: Fc epsilon RI and CD23. Fc epsilon RI on mast cells and basophils mediates the immediate hypersensitivity response, whilst the interaction of IgE with CD23 on B cells regulates IgE production. Crystal structures of the lectin-like 'head' domain of CD23 alone and bound to a subfragment of IgE consisting of the dimer of C epsilon 3 and C epsilon 4 domains (Fc epsilon 3-4) have recently been determined, revealing flexibility in the IgE-binding site of CD23. Here, a new crystal form of the CD23-Fc epsilon 3-4 complex with different molecular-packing constraints is reported, which together with the earlier results demonstrates that conformational variability at the interface extends additionally to the IgE Fc and the quaternary structure of its domains.