Interaction of the low-affinity receptor CD23/Fc epsilon RII lectin domain with the Fc epsilon 3-4 fragment of human immunoglobulin E

CD23/Fc epsilon RII, the low-affinity receptor for IgE, is a multifunctional protein of importance in blood cell development and the immune system. We have studied the interaction of CD23 with IgE in solution using hydrodynamic methods applied to recombinant fragments of both ligands: sCD23, corresponding to the soluble lectin domain of CD23, and Fc epsilon 3-4, a dimer of the C epsilon 3-C epsilon 4 sequence of IgE. The hydrodynamic, spectroscopic, and biological properties of these fragments suggest that they have a fully native structure. Sedimentation equilibrium studies on mixtures of sCD23 and Fc epsilon 3-4 indicate that IgE has two binding sites for CD23, each characterized by affinities of approximately 10(5) M(-1). Analysis of the sedimentation as a function of temperature allows conclusions to be drawn about the thermodynamics of binding at the two sites. Binding at the first site is characterized by large changes in enthalpy (Delta H degrees(T0) = -2.1 +/- 3.3 kcal mol(-1)) and heat capacity (Delta Cp degrees = -320 +/- 320 cal mol(-1) K-1), whereas binding at the second site is characterized by small changes in enthalpy (Delta H degrees(T0) = 0.1 +/- 5.6 kcal mol(-1)) and heat capacity (Delta Cp degrees = -140 +/- 550 cal mol(-1) K-1). In native CD23, there are two or three lectin domains, associated through an alpha-helical coiled-coil stalk. The predicted structure of the CD23 oligomers and symmetry considerations rule out the possibility of two lectin domains from one oligomer binding to identical sites in IgE. The notion of two types of interaction in the 2:1 complex between CD23 and IgE is consistent with the thermodynamic data presented.