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We have investigated the capacity of N- and C-terminally truncated and chimeric human (h) IgE-derived peptides to inhibit the binding of ¹²⁵I-labeled hIgE, and to engage cell lines expressing high and low affinity receptors (FcεRI/II). The peptide sequence Pro³⁴³-Ser³⁵³ of the hCε3 domain is common to all hε-chain peptides that recognize hFcεRI. This region in IgE is homologous to the A loop in Cγ2 that engages the rat neonatal IgG receptor. Optimum FcεRI 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 hFcεRI occupancy at pH 6.4 is diminished. His residue(s) in the C-terminal region of the ε-chain may thus contribute to the high affinity of interaction. Grafting the homologous rat ε-chain sequence into hIgE maintains hFcεRI …