Thermodynamics of the interaction of human immunoglobulin E with its high-affinity receptor Fc epsilon RI

We have employed isothermal titration calorimetry (ITC) and circular dichroism (CD) spectroscopy to characterize the binding of soluble fragments of IgE (IgE-Fc and Fc epsilon 3-4) to a soluble fragment of the high-affinity receptor Fc epsilon RI alpha-chain (sFc epsilon RI alpha). The thermodynamic parameters for the interaction of IgE-Fc and Fc epsilon 3-4 with sFc epsilon RI alpha, determined using ITC, confirm the earlier conclusion that the C epsilon 2 domain is not involved in the interaction and that the stoichiometry of both complexes is 1:1. For both IgE-Fc and Fc epsilon 3-4, the value of Delta H degrees is -36.9 +/- 4.6 kcal mol-1 at 37.3 degreesC and Delta Cp degrees is -820 +/- 120 cal mol-1 K-1. The temperature at which DeltaS degrees is zero is 284 +/- 1 K, indicating that the entropy contribution to the thermodynamics of association is unfavorable at physiological temperature. Of particular interest is the large value of Delta Cp degrees. The large surface area of IgE and Fc epsilon RI alpha that is implicated in complex formation from previous mutagenesis studies on the two proteins may account in part for the magnitude of Delta Cp degrees. Additional contributions may arise from hydration within the binding site and changes in tertiary structure of the individual components of the complex. However, the CD spectra of IgE, IgE-Fc, and Fc epsilon 3-4 complexes with sFc epsilon RI alpha are merely the sum of the spectra of their individual components, indicating that the secondary structure of the immunoglobulin domain folds are preserved on complex formation. Thus, any change in tertiary structure must be limited to the relative disposition of the immunoglobulin domains C epsilon 3 and C epsilon 4 in IgE and the two immunoglobulin-like domains in the alpha-chain of Fc epsilon RI.

Interaction of the low-affinity receptor CD23/Fc epsilonRII lectin domain with the Fc epsilon3-4 fragment of human immunoglobulin E

CD23/Fc epsilonRII, 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 epsilon3-4, a dimer of the C epsilon3-C epsilon4 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 epsilon3-4 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(degree)To = -2.1 +/- 3.3 kcal mol(-1)) and heat capacity (delta Cp(degree) = -320 +/- 320 cal mol(-1) K(-1)), whereas binding at the second site is characterized by small changes in enthalpy (delta H(degree)To = 0.1 +/- 5.6 kcal mol(-1)) and heat capacity (delta Cp(degree) = -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.

Basis of the 1:1 stoichiometry of the high affinity receptor Fc epsilon RI-IgE complex

A soluble fragment of the high-affinity IgE receptor Fc epsilon RI alpha-chain (sFc epsilon RI alpha) binds to the Fc fragment of IgE (IgE-Fc) as a 1:1 complex. IgE-Fc consists of a dimer of the C epsilon 2, C epsilon 3 and C epsilon 4 domains of the epsilon-heavy chain of IgE. This region of IgE has been modelled on the crystal structure of the Fc region of IgG1, which exhibits twofold rotational symmetry. This implies that IgE should be divalent with respect to its ligands. X-ray scattering studies reveal however that the twofold rotational symmetry of IgE-Fc is perturbed by a bend in the linker region between the C epsilon 2 and C epsilon 3 domains. The 1:1 stoichiometry could then arise from the conformational asymmetry or from steric occlusion of one of the sites by the overhanging C epsilon 2 domains. To test this hypothesis we have expressed a recombinant epsilon-chain fragment containing C epsilon 3 and C epsilon 4. This product, Fc epsilon 3-4, is secreted from cells as a disulphide linked dimer and binds with higher affinity than either IgE or IgE-Fc to cell surface Fc epsilon RI. Titration experiments, together with molecular mass measurements of the Fc epsilon 3-4/sFc epsilon RI alpha complex, reveal that Fc epsilon 3-4 binds only a single receptor molecule. This excludes the possibility that steric hindrance by C epsilon 2 accounts for the unexpected stoichiometry.

Stoichiometry and thermodynamics of the interaction between the Fc fragment of human IgG1 and its low-affinity receptor Fc gamma RIII

IgG-Fc receptors, cell surface glycoproteins binding the Fc region of antibodies, play a crucial role in the immune system. To better understand the nature of the recognition process, we have examined the interaction between huIgG1-Fc and a soluble fragment of huFc gamma RIII (sCD16). Analytical ultracentrifugation experiments clearly demonstrate that IgG1-Fc and sCD16 interact weakly to form a 1:1 complex with an association constant of 1.7 x 10(5) M-1 in PBS at 22.0 degrees C. The thermodynamic parameters, obtained from the temperature dependence of the equilibrium binding constants, exhibit an enthalpy-entropy compensation with a favorable enthalpy at physiological temperatures. The value of -360 cal mol-1 K-1 for delta Cp zero possibly identifies the process as one in which local folding/rearrangement is coupled to complex formation. The 1:1 stoichiometry and thermodynamic parameters provide a basis for understanding the nature of the Fc gamma R-IgG interactions.

Major allergen Phl p Vb in timothy grass is a novel pollen RNase

A cDNA coding for the major group V allergen Phl p Vb was isolated from a timothy grass pollen cDNA library by immunoscreening with a specific monoclonal antibody. It was discovered for the first time that the recombinant Phl p Vb pollen allergen after expression and purification has ribonuclease activity. High homology of Phl p Vb to other group V allergens in grass pollen indicates similar function. By RNase activity gel of natural pollen extract of timothy grass and consecutive Western blot analysis of the excised proteins, the RNase active bands were shown to be group V allergens. Additionally it was demonstrated that an homologous protein to Phl p Vb in the mother plant could be induced by salicylic acid. This indicates that group Vb allergens may be involved in host-pathogen interactions because in pollen they are quickly exported RNases and in the mother plant they depend on a hormone which is related to expression of plant resistance genes.

Hydrodynamic studies of a complex between the Fc fragment of human IgE and a soluble fragment of the Fc epsilon RI alpha chain

The interaction between immunoglobulin E (IgE) and its high-affinity receptor Fc epsilon RI is central to allergic disease. The binding site for Fc epsilon RI lies in the third constant region domain of the epsilon heavy chain of IgE (C epsilon 3). Identical epitopes on the two C epsilon 3 domains in the IgE-Fc are predicted to be on opposite sides of the structure, and therefore each could bind independently to a receptor molecule. Titrations, however, reveal that the IgE-Fc forms an equimolar complex with a soluble fragment of the Fc epsilon RI alpha chain (sFc epsilon RI alpha), and the molecular weight of the complex, as determined by sedimentation equilibrium, confirms this stoichiometry. The measured sedimentation coefficients of the two ligands are in good agreement with computed values for a compact IgE-Fc and an elongated sFc epsilon RI alpha structure. The calculated sedimentation coefficients for possible models of a 1:1 complex lead to exclusion of all highly extended geometries for the complex. Possible explanations for the paradoxical stoichiometry of the IgE-Fc/sFc epsilon RI alpha complex, in terms of the curved shape of IgE, a conformational change in IgE when the receptor binds, and steric interference between two molecules of Fc epsilon RI binding to identical sites, are discussed.