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 alp ha-chain (sFc epsilon RI alpha) binds to the Fc fragment of IgE (IgE-F c) 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. T his region of IgE has been modelled on the crystal structure of the Fc region of IgG(1), which exhibits twofold rotational symmetry. This im plies that IgE should be divalent with respect to its ligands. X-ray s cattering studies reveal however that the twofold rotational symmetry of IgE-Fc is perturbed by a bend in the linker region between the C ep silon 2 and C epsilon 3 domains. The 1:1 stoichiometry could then aris e from the conformational asymmetry or from steric occlusion of one of the sites by the overhanging C epsilon 2 domains. To test this hypoth esis we have expressed a recombinant epsilon-chain fragment containing C epsilon 3 and C epsilon 4. This product, Fc epsilon 3-4, is secrete d from cells as a disulphide linked dimer and binds with higher affini ty than either IgE or IgE-Fc to cell surface Fc epsilon RI. Titration experiments, together with molecular mass measurements of the Fc epsil on 3-4/sFc epsilon RI alpha complex, reveal that Fc epsilon 3-4 binds only a single receptor molecule. This excludes the possibility that st eric hindrance by C epsilon 2 accounts for the unexpected stoichiometr y.