We have investigated the capacity of N- and C-terminally truncated and chimeric human (h) IgE-derived peptides to inhibit the binding of I-labeled hIgE, and to engage cell lines expressing high and low affinity receptors (FcRI/II). The peptide sequence Pro-Ser of the hC3 domain is common to all h-chain peptides that recognize hFcRI. This region in IgE is homologous to the A loop in C2 that engages the rat neonatal IgG receptor. Optimum FcRI occupancy by hIgE occurs at pH 6.4, with a second peak at 7.4. N- or C-terminal truncation has little effect on the association rate of the ligands with this receptor. Dissociation markedly increases following C-terminal deletion, and hFcRI occupancy at pH 6.4 is diminished. His residue(s) in the C-terminal region of the -chain may thus contribute to the high affinity of interaction. Grafting the homologous rat -chain sequence into hIgE maintains hFcRI interaction without conferring binding to rat FcRI. hFcRII interaction is lost, suggesting that these residues also contribute to hFcRII binding. h-chain peptides comprising only this sequence do not block hIgE/hFcRI interaction or engage the receptor. Therefore, sequences N- or C-terminal to this core peptide provide structures necessary for receptor recognition.

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