1. Ca2+-dependent Structural Changes in the B-cell Receptor CD23 Increase Its Affinity for Human Immunoglobulin E*
- Author
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Andrew J. Beavil, Anthony H. Keeble, Daopeng Yuan, Brian J. Sutton, Stella M. Fabiane, B. Dhaliwal, James M. McDonnell, Hannah J. Gould, and Richard G. Hibbert
- Subjects
Models, Molecular ,Allergy ,Entropy ,Immunoglobulin E ,Crystallography, X-Ray ,Biochemistry ,0302 clinical medicine ,X-ray Crystallography ,immune system diseases ,hemic and lymphatic diseases ,Receptor ,0303 health sciences ,B-Lymphocytes ,biology ,CD23 ,Mast cell ,3. Good health ,medicine.anatomical_structure ,030220 oncology & carcinogenesis ,Cyclophilin A ,Protein Binding ,B-cell receptor ,Immunology ,Molecular Sequence Data ,Mutation, Missense ,FC Receptors ,Calorimetry ,Basophil degranulation ,Binding, Competitive ,03 medical and health sciences ,stomatognathic system ,medicine ,Humans ,Amino Acid Sequence ,Binding site ,Molecular Biology ,030304 developmental biology ,Binding Sites ,Sequence Homology, Amino Acid ,Receptors, IgE ,Isothermal titration calorimetry ,Cell Biology ,Surface Plasmon Resonance ,Protein Structure, Tertiary ,Kinetics ,biology.protein ,Biophysics ,Mutagenesis, Site-Directed ,Calcium ,Protein Multimerization - Abstract
Background: Immunoglobulin E (IgE) plays a central role in allergic disease. The B-cell receptor CD23 is pivotal in regulating IgE synthesis. Results: Ca2+-dependent structural changes in CD23 enable additional interactions with IgE. Conclusion: Mechanism for Ca2+-induced increase in affinity is revealed. Significance: Ca2+ binding brings an extra degree of modulation to CD23 function., Immunoglobulin E (IgE) antibodies play a fundamental role in allergic disease and are a target for therapeutic intervention. IgE functions principally through two receptors, FcϵRI and CD23 (FcϵRII). Minute amounts of allergen trigger mast cell or basophil degranulation by cross-linking IgE-bound FcϵRI, leading to an inflammatory response. The interaction between IgE and CD23 on B-cells regulates IgE synthesis. CD23 is unique among Ig receptors in that it belongs to the C-type (calcium-dependent) lectin-like superfamily. Although the interaction of CD23 with IgE is carbohydrate-independent, calcium has been reported to increase the affinity for IgE, but the structural basis for this activity has previously been unknown. We have determined the crystal structures of the human lectin-like head domain of CD23 in its Ca2+-free and Ca2+-bound forms, as well as the crystal structure of the Ca2+-bound head domain of CD23 in complex with a subfragment of IgE-Fc consisting of the dimer of Cϵ3 and Cϵ4 domains (Fcϵ3-4). Together with site-directed mutagenesis, the crystal structures of four Ca2+ ligand mutants, isothermal titration calorimetry, surface plasmon resonance, and stopped-flow analysis, we demonstrate that Ca2+ binds at the principal and evolutionarily conserved binding site in CD23. Ca2+ binding drives Pro-250, at the base of an IgE-binding loop (loop 4), from the trans to the cis configuration with a concomitant conformational change and ordering of residues in the loop. These Ca2+-induced structural changes in CD23 lead to additional interactions with IgE, a more entropically favorable interaction, and a 30-fold increase in affinity of a single head domain of CD23 for IgE. Taken together, these results suggest that binding of Ca2+ brings an extra degree of modulation to CD23 function.
- Published
- 2013