A conformational rearrangement upon binding of IgE to its high affinity receptor

One of the critical steps in the allergic reaction is the binding of immunoglobulin E (IgE) to its high affinity receptor (FcepsilonRI). FcepsilonRI is a tetrameric complex composed of an alpha-chain, a beta-chain, and a dimeric gamma-chain. The extracellular portion of the alpha-chain (alpha-t) is sufficient for the binding of IgE. The Fc portion of IgE contains two copies of the FcepsilonRI binding sites. In contrast, the binding stoichiometry is 1:1. Previously, it was hypothesized that the binding of FcepsilonRI to IgE results in a conformational change in IgE that precludes the binding of a second molecule (Presta, L., Shields, R., O'Connel, L., Lahr, S., Porter, J. , Gorman, C., and Jardieu, P.(1994) J. Biol. Chem. 269, 26368-26373). Here we characterize the secondary structure of IgE and alpha-t and analyze their interaction by circular dichroism spectroscopy. Binding experiments show that when IgE interacts with alpha-t there is a 15-26% decrease of the negative ellipticity at 217 nm. Together, the absence of an alpha-helix element in alpha-t and the small contribution of alpha-t to the spectra of the complex indicate that upon binding, a major conformational rearrangement must occur on IgE. In addition, we analyze the thermal unfolding of alpha-t, IgE, and their complex. Despite the several domains that constitute IgE and alpha-t, these molecules unfold cooperatively with two-state kinetics.