Evidence of allosteric conformational changes in the antibody constant region upon antigen binding

We have addressed the question of whether antigen binding induces a conformational change in the heavy chain constant (CH) domain of antibodies using staphylococcal protein A or streptococcal protein G as probes, since these proteins are known to bind to IgG domains such as CH1 and CH2–CH3 domains. Biosensor assays on interactions between these proteins and mouse IgG specific to (4‐hydroxy‐3‐nitrophenyl)acetyl (NP) or their enzymatic fragments conducted in the presence or absence of the hapten, NP‐ϵ‐aminocaproic acid (NP‐Cap), showed that the binding of IgG to these proteins was inhibited by the binding of NP‐Cap. The results of isothermal titration calorimetry also revealed that the association constant for the interaction of protein A with IgG2b decreased by the addition of NP‐Cap. These results suggested that antigen binding induced conformational changes in binding sites for protein G or protein A located at CH1 and CH2–CH3 domains, respectively.