The oligomeric nature of the murine Fc epsilon RII/CD23. Implications for function

The low affinity receptor for IgE (Fc epsilon RII/CD23) is a type II integral membrane protein with an extracellular C-terminal region homologous to C-type animal lectins. Immediately adjacent to this lectin homology region is a sequence that is predicted to form an alpha-helical coiled-coil stalk leading to dimer or trimer formation. This provides an explanation for the known self-associative capacity for the Fc epsilon RII. In this study the self-association to a trimer or tetramer is shown with rFc epsilon RII by chemical cross-linking and affinity purification on IgE columns. The data indicate that only the oligomeric form of Fc epsilon RII has sufficient affinity/avidity to bind to an IgE adsorbent. In contrast, Fc epsilon RII that is purified using anti-Fc epsilon RII mAb adsorbents has largely lost its capacity to bind IgE, as well as its capacity to self-associate, indicating that IgE recognizes the oligomeric form of the Fc epsilon RII. This phenomenon was further examined by performing detailed binding analysis of the mouse IgE/Fc epsilon RII interaction. A biphasic binding curve with high (2-7 x 10(7) M-1) and low (2-7 x 10(6) M-1) affinity binding was seen. Fc epsilon RII mutants were prepared that lack one or more of the 21 amino acid homologous repeat domains in the stalk region of the molecule. These mutant Fc epsilon RII molecules bound IgE with only a single low affinity (5-10 x 10(6) M-1). In addition, cross-linking analysis of one of these mutants demonstrated that it does not exhibit the receptor self-association seen for the intact Fc epsilon RII. Two chimeric Fc epsilon RII molecules were prepared having the mouse Fc epsilon RII lectin homology (carboxyl-terminal) region and the stalk region of either the related human Fc epsilon RII or the corresponding domain of Ly-49. Chimeric molecules using the former (alpha-helical coiled-coil) stalk supported normal binding of IgE although the Ly-49/Fc epsilon RII chimera failed to bind IgE. Taken together, the results indicate that high (approximately 10(8) M-1) affinity IgE binding results from interaction of multiple lectin domains with (presumably) symmetrical sites on the IgE molecule. Specificity for IgE is determined by the lectin domain although the binding avidity is determined by oligomerization through the coiled coil stalk.