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 (Fc epsilon RI), Fc epsilon RI is a tetrameric complex composed of an a chain, a beta-chain, and a dimeric gamma-chain. The extracellular paction of the alpha-chain (alpha-t) is sufficient for the binding of IgE. The Fe portion of IgE contains two copies of the Fc epsilon RI binding sites, In contrast, the binding stoichiometry is 1:1, Previously, it was hypothesized that the binding of Fc epsilon EI to IgE results in a conformational change ill IgE that precludes the binding of a second molecule (Presta, L., Shields, R., O'Connel, L., Lahr, S., Porter, J., German, C., and Jardieu, P. (1994) J. Biol. Chem. 269, 26568-26313), 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 all 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 oil 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.