Immunoglobulin E (IgE) exhibits a uniquely high affinity for its receptor, Fc
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RI, on the surfaceof mast cells and basophils. Previous work has implicated the third domain of the constant region of the
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-heavy chain (C
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3) in binding to Fc
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RI, but the smallest fragment of IgE that is known to bind with fullaffinity is a covalent dimer of the C
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3 and C
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4 domains. We have expressed the isolated C
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3 in
Escherichiacoli, measured its affinity for Fc
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RI, and examined its conformation alone and in the complex with Fc
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RI.Sedimentation equilibrium in the analytical centrifuge reveals that this product is a monomer. The kineticsof binding to an immobilized fragment of the Fc
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RI
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-chain, measured by surface plasmon resonance,yields an affinity constant
Ka = 5 × 10
6 M
-1, as compared with 4 × 10
9 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 Fc
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RI
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-chainfragment, there is no change in the circular dichroism spectrum, indicating that the conformation of C
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3is unchanged in the complex. Thus the isolated C
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3 domain is sufficient for binding to Fc
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RI, but withlower affinity than IgE. This may be due to the loss of its native immunoglobulin domain structure or tothe requirement for two C
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3 domains to constitute the complete binding site for Fc
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RI or to a combinationof these factors.