An engineered disulfide bond reversibly traps the IgE-Fc3-4 in a closed, nonreceptor binding conformation.

IgE antibodies interact with the high affinity IgE Fc receptor, FcεRI, and activate inflammatory pathways associated with the allergic response. The IgE-Fc region, comprising the C-terminal domains of the IgE heavy chain, binds FcεRI and can adopt different conformations ranging from a closed form incompatible with receptor binding to an open, receptor-bound state. A number of intermediate states are also observed in different IgE-Fc crystal forms. To further explore this apparent IgE-Fc conformational flexibility and to potentially trap a closed, inactive state, we generated a series of disulfide bond mutants. Here we describe the structure and biochemical properties of an IgE-Fc mutant that is trapped in the closed, non-receptor binding state via an engineered disulfide at residue 335 (Cys-335). Reduction of the disulfide at Cys-335 restores the ability of IgE-Fc to bind to its high affinity receptor, FcεRIα. The structure of the Cys-335 mutant shows that its conformation is within the range of previously observed, closed form IgE-Fc structures and that it retains the hydrophobic pocket found in the hinge region of the closed conformation. Locking the IgE-Fc into the closed state with the Cys-335 mutation does not affect binding of two other IgE-Fc ligands, omalizumab and DARPin E2_79, demonstrating selective blocking of the high affinity receptor binding.

Structural changes in the lectin domain of CD23, the low-affinity IgE receptor, upon calcium binding.

CD23, the low-affinity receptor for IgE (Fc epsilonRII), regulates IgE synthesis and also mediates IgE-dependent antigen transport and processing. CD23 is a unique Fc receptor belonging to the C-type lectin-like domain superfamily and binds IgE in an unusual, non-lectin-like manner, requiring calcium but not carbohydrate. We have solved the high-resolution crystal structures of the human CD23 lectin domain in the presence and absence of Ca2+. The crystal structures differ significantly from a previously determined NMR structure and show that calcium binding occurs at the principal binding site, but not at an auxiliary site that appears to be absent in human CD23. Conformational differences between the apo and Ca2+ bound structures suggest how IgE-Fc binding can be both calcium-dependent and carbohydrate-independent.

Structural studies of the parainfluenza virus 5 hemagglutinin-neuraminidase tetramer in complex with its receptor, sialyllactose.

The paramyxovirus hemagglutinin-neuraminidase (HN) functions in virus attachment to cells, cleavage of sialic acid from oligosaccharides, and stimulating membrane fusion during virus entry into cells. The structural basis for these diverse functions remains to be fully understood. We report the crystal structures of the parainfluenza virus 5 (SV5) HN and its complexes with sialic acid, the inhibitor DANA, and the receptor sialyllactose. SV5 HN shares common structural features with HN of Newcastle disease virus (NDV) and human parainfluenza 3 (HPIV3), but unlike the previously determined HN structures, the SV5 HN forms a tetramer in solution, which is thought to be the physiological oligomer. The sialyllactose complex reveals intact receptor within the active site, but no major conformational changes in the protein. The SV5 HN structures do not support previously proposed models for HN action in membrane fusion and suggest alternative mechanisms by which HN may promote virus entry into cells.

Structural insights into the interactions between human IgE and its high affinity receptor FcepsilonRI.

The interaction of IgE antibodies with the high affinity IgE receptor, FcepsilonRI, is a key step in the initiation of anti-parasitic immunity and allergic reactions. Recent structural studies of the receptor, the IgE-Fc and the IgE-Fc:FcepsilonRI complex have revealed how these two proteins interact to prime mast cell responses to antigen. The structures have revealed a novel arrangement for the FcepsilonRI ectodomains that is also observed in homologous members of this antibody receptor family. The crystal structure of the IgE-Fc:FcepsilonRI complex clarified how a 1:1 complex between the antibody and receptor is formed, with the receptor binding each chain of the antibody Fc dimer. The IgE-Fc structure in the absence of the receptor revealed the potential for large conformational rearrangements within the IgE that may affect receptor binding. These studies provide the basis for further investigation of the specificity of antibody:receptor binding and for the development of new treatments for allergic hypersensitivities.

Conformational flexibility in immunoglobulin E-Fc 3-4 revealed in multiple crystal forms.

The structure of immunoglobulin E (IgE)-Fc(3-4) has been solved in three new crystal forms, providing 13 snapshots of the Fc conformation and revealing a diverse range of open-closed motions among subunit chains and dimers. A more detailed analysis of the open-to-closed motion of IgE-Fc(3-4) was possible with so many structures, and the new structures allow a more thorough examination of the flexibility of IgE-Fc and its implications for receptor binding. The existence of a hydrophobic pocket at the elbow region of the Fc appears to be conformation dependent and suggests a means of regulating the IgE-Fc conformation (and potentially receptor binding) with small molecules.