The immunoglobulin E (IgE)-binding site of its high-affinity receptor is localized in the second immunoglobulin-like domain (D2) of the alpha-subunit (FcepsilonRIalpha). In this study. the randomized pentapeptides were introduced between Glu(132) and Ile(131) of FcepsilonRIalpha D2 and displayed on a filamentous phage. After eight rounds of panning, a phage clone having a mutation of Asp(135)Tyr(136)Met(137) in FcepsilonRIalpha D2 was obtained. The binding affinity of the mutant phages to immobilized IgE was approximately 500 times higher than that of the wild type. The mutant phages competitively inhibited the binding of IgE to the soluble receptor at a 50% inhibition (IC50) value of 116 pM. The mutant FcalphaRIalpha D2, which had been expressed as a fusion protein with glutathione S-transferase in Escherichia coli, also showed higher IgE-binding capacity than the wild type. The mutant FcepsilonRIalpha D2 is expected to manifest its improved IgE-binding affinity together with any fusion partner. (C) 2002 Elsevier Science (USA). All rights reserved.