HYDRODYNAMIC STUDIES OF A COMPLEX BETWEEN THE FC FRAGMENT OF HUMAN IGE AND A SOLUBLE FRAGMENT OF THE FC-EPSILON-RI ALPHA-CHAIN

The interaction between immunoglobulin E (IgE) and its high-affinity r eceptor Fc epsilon RI is central to allergic disease. The binding site for Fc epsilon RI lies in the third constant region domain of the E h eavy chain of IgE (C epsilon 3). Identical epitopes on the two C epsil on 3 domains in the IgE-Fc are predicted to be on opposite sides of th e structure, and therefore each could bind independently to a receptor molecule. Titrations, however, reveal that the IgE-Fc forms an equimo lar complex with a soluble fragment of the Fc epsilon RI alpha chain ( sFc epsilon RI alpha), and the molecular weight of the complex, as det ermined by sedimentation equilibrium, confirms this stoichiometry. The measured sedimentation coefficients of the two ligands are in good ag reement with computed values for a compact IgE-Fc and an elongated sFc epsilon RI alpha structure. The calculated sedimentation coefficients for possible models of a 1:1 complex lead to exclusion of all highly extended geometries for the complex. Possible explanations for the par adoxical stoichiometry of the IgE-Fc/sFc epsilon RI alpha complex, in terms of tile curved shape of IgE, a conformational change in IgE when the receptor binds, and steric interference between two molecules of Fc epsilon RI binding to identical sites, are discussed.