The key role of protein flexibility in modulating IgE interactions

The interaction between IgE and its high affinity receptor (FcepsilonRI) is a critical step in the development of allergic responses. Detailed characterization of the IgE-FcepsilonRI interaction may offer insights into possible modes of inhibiting the interaction, which could thereby act as a potential therapy for allergy. In this study, NMR, CD, and fluorescence spectroscopies have been used to characterize structurally the Cepsilon3 domain of IgE and its interaction with other protein ligands, namely, Cepsilon2, Cepsilon4, sFcepsilonRIalpha, and CD23. We have shown that the recombinant Cepsilon3 domain exists alone in solution as a "molten globule." On interaction with sFcepsilonRIalpha, Cepsilon3 adopts a folded tertiary structure, as shown by the release of the fluorescent probe 8-anilinonaphthalene-1-sulfonate and by characteristic changes in the H-1, N-15 heteronuclear single quantum coherence NMR spectrum. However, the interactions between the Cepsilon3 domain and Cepsilon2, Cepsilon4, or CD23 do not induce such folding and would therefore be expected to involve only local interaction surfaces. The conformational flexibility of the Cepsilon3 domain of the whole IgE molecule may play a role in allowing fine tuning of the affinity and specificity of IgE for a variety of different physiological ligands and may be involved in the conformational change of IgE postulated to occur on interaction with FcepsilonRI.