Basis of the 1:1 stoichiometry of the high affinity receptor Fc epsilon RI-IgE complex.

A soluble fragment of the high-affinity IgE receptor Fc epsilon RI alpha-chain (sFc epsilon RI alpha) binds to the Fc fragment of IgE (IgE-Fc) as a 1:1 complex. IgE-Fc consists of a dimer of the C epsilon 2, C epsilon 3 and C epsilon 4 domains of the epsilon-heavy chain of IgE. This region of IgE has been modelled on the crystal structure of the Fc region of IgG1, which exhibits twofold rotational symmetry. This implies that IgE should be divalent with respect to its ligands. X-ray scattering studies reveal however that the twofold rotational symmetry of IgE-Fc is perturbed by a bend in the linker region between the C epsilon 2 and C epsilon 3 domains. The 1:1 stoichiometry could then arise from the conformational asymmetry or from steric occlusion of one of the sites by the overhanging C epsilon 2 domains. To test this hypothesis we have expressed a recombinant epsilon-chain fragment containing C epsilon 3 and C epsilon 4. This product, Fc epsilon 3-4, is secreted from cells as a disulphide linked dimer and binds with higher affinity than either IgE or IgE-Fc to cell surface Fc epsilon RI. Titration experiments, together with molecular mass measurements of the Fc epsilon 3-4/sFc epsilon RI alpha complex, reveal that Fc epsilon 3-4 binds only a single receptor molecule. This excludes the possibility that steric hindrance by C epsilon 2 accounts for the unexpected stoichiometry.