CHIMERIC FC-RECEPTORS IDENTIFY IMMUNOGLOBULIN-BINDING REGIONS IN HUMAN FC-GAMMA-RII AND FC-EPSILON-RI

FcgammaRII and FcepsilonRI are functionally distinct cell surface receptors for immunoglobulin (Ig); FcgammaRII binds IgG with low affinity, whereas FcepsilonRI binds IgE with high affinity, yet they are homologous in structure and sequence having extracellular regions containing two Ig-like domains with 38% amino acid identity. Chimeric receptors derived from human FcgammaRII and FcepsilonRI were produced by exchanging homologous regions of the two receptors to define binding region(s) for IgG in FcgammaRII and IgE in FcepsilonRI. Firstly, a chimeric form of the FcepsilonRI a chain was produced by replacing the transmembrane region and cytoplasmic tail with that of FcgammaRII. This mutant a chain could be expressed on the cell surface independently of associated beta and gamma subunits, and retained high-affinity IgE binding, indicating that the extracellular region of the FcepsilonRI alpha chain is sufficient for high-affinity IgE binding. Secondly, to identify the role of the individual domains in Fc binding of both FcgammaRII and FcepsilonRI, chimeric receptors were generated by exchanging the first extracellular domains between FcgammaRII and the a chain mutant and used to demonstrate that the second extracellular domain of both receptors contains region(s) directly involved in Ig binding. Additional chimeric receptors were constructed to localize the Ig interactive regions in domain two of FcgammaRII and FcepsilonRI; these identified a single region of IgG binding in FcgammaRII located between residues Ser136 to Val169, and at least three independent IgE binding regions in the FcepsilonRI alpha chain, between residues Trp87 to Lys128, Tyr129 to Asp145, and Ser146 to Val169.