Conformation of the Isolated C3 Domain of IgE and Its Complex with the High-Affinity Receptor, Fc 详细信息    查看全文

• 作者：Alistair J. Henry ; James M. McDonnell ; Rodolfo Ghirlando ; Brian J. Sutton ; and Hannah J. Gould
• 刊名：Biochemistry
• 出版年：2000
• 出版时间：June 27, 2000
• 年：2000
• 卷：39
• 期：25
• 页码：7406 - 7413
• 全文大小：113K
• 年卷期：v.39,no.25(June 27, 2000)
• ISSN：1520-4995

文摘

Immunoglobulin E (IgE) exhibits a uniquely high affinity for its receptor, FcRI, on the surfaceof mast cells and basophils. Previous work has implicated the third domain of the constant region of the-heavy chain (C3) in binding to FcRI, but the smallest fragment of IgE that is known to bind with fullaffinity is a covalent dimer of the C3 and C4 domains. We have expressed the isolated C3 in Escherichiacoli, measured its affinity for FcRI, and examined its conformation alone and in the complex with FcRI.Sedimentation equilibrium in the analytical centrifuge reveals that this product is a monomer. The kineticsof binding to an immobilized fragment of the FcRI -chain, measured by surface plasmon resonance,yields an affinity constant K_a = 5 × 10⁶ M^-1, as compared with 4 × 10⁹ M^-1 for IgE. The circulardichroism spectrum and measurements of fluorescence as a function of the concentration of a denaturantdo not reveal any recognizable secondary structure or hydrophobic core. On binding to the FcRI -chainfragment, there is no change in the circular dichroism spectrum, indicating that the conformation of C3is unchanged in the complex. Thus the isolated C3 domain is sufficient for binding to FcRI, but withlower affinity than IgE. This may be due to the loss of its native immunoglobulin domain structure or tothe requirement for two C3 domains to constitute the complete binding site for FcRI or to a combinationof these factors.