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Ca2+-dependent structural changes in the B-cell receptor CD23 increase its affinity for human Immunoglobulin E

DOI: 10.1074/jbc.M113.480657 DOI Help
PMID: 23775083 PMID Help

Authors: Daopeng Yuan (King's College London and the Medical Research Council and Asthma UK Centre in Allergic Mechanisms of Asthma) , Anthony H. Keeble (King's College London and the Medical Research Council and Asthma UK Centre in Allergic Mechanisms of Asthma) , Richard G. Hibbert (University of Oxford) , Stella Fabiane (King's College London) , Hannah J. Gould (Kings College London) , James M. Mcdonnell (Kings College London) , Andrew J Beavil (King's College London) , Balvinder Dhaliwal (King's College London) , Brian J. Sutton (King's College London)
Co-authored by industrial partner: No

Type: Journal Paper
Journal: Journal Of Biological Chemistry

State: Published (Approved)
Published: June 2013

Open Access Open Access

Abstract: Immunoglobulin E (IgE) antibodies play a fundamental role in allergic disease and are a target for therapeutic intervention. IgE functions principally through two receptors, Fc?RI and CD23 (Fc?RII). Minute amounts of allergen trigger mast cell or basophil degranulation by cross-linking IgE-bound Fc?RI, 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 C?3 and C?4 domains (Fc?3-4). 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.

Journal Keywords: B-Lymphocytes; Binding; Binding; Competitive; Calcium; Calorimetry; Crystallography; X-Ray; Cyclophilin; Entropy; Humans; Immunoglobulin; Kinetics; Models; Molecular; Mutagenesis; Site-Directed; Mutation; Missense; Protein; Tertiary; Receptors; IgE; Sequence; Amino; Surface Plasmon Resonance

Subject Areas: Biology and Bio-materials, Medicine

Instruments: I02-Macromolecular Crystallography , I03-Macromolecular Crystallography , I04-1-Macromolecular Crystallography (fixed wavelength)

Other Facilities: Daresbury SRS (Cheshire, U.K.) synchrotron.

Added On: 27/06/2013 15:40

Discipline Tags:

Non-Communicable Diseases Health & Wellbeing Structural biology Drug Discovery Life Sciences & Biotech Allergic Diseases

Technical Tags:

Diffraction Macromolecular Crystallography (MX)