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

Abstract

One of the critical steps in the allergic reaction is the binding of immunoglobulin E (IgE) to its high affinity receptor (FcεRI). FcεRI is a tetrameric complex composed of an α-chain, a β-chain, and a dimeric γ- chain. The extracellular portion of the α-chain (α-t) is sufficient for the binding of IgE. The Fc portion of IgE contains two copies of the FcεRI binding sites. In contrast, the binding stoichiometry is 1:1. Previously, it was hypothesized that the binding of FcεRI 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 α-t and analyze their interaction by circular dichroism spectroscopy. Binding experiments show that when IgE interacts with α-t there is a 15-26% decrease of the negative ellipticity at 217 nm. Together, the absence of an α-helix elements in α- t and the small contribution of α-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 α-t, IgE, and their complex. Despite the several domains that constitute IgE and α-t, these molecules unfold cooperatively with two-state kinetics.