Protein A as a molecular probe for the detection of antigen induced conformational change in Fc region of rabbit antibody

Rabbit IgG antibody which reacts with protein A of Staphylococcus aureus (SpA) and forms a soluble complex with molar composition (IgG₂-SpA₁)₂ is not able to further bind SpA or to attach to SpA-Sepharose 4B thus proving that the unengaged SpA reactive sites of IgG are shielded by the already bound SpA.On the contrary the (IgG₂-SpA₁)₂ complex was able to bind a small fragment of SpA (fSpA) clearly showing that SpA-reactive sites are present in the rabbit IgG molecules of the complex but that they are not available for the intact SpA molecule.Immune complexes contaning (IgGaFER₂-SpA₁)₂ and ferritin attach to an SpA-Sepharose 4B column showing that SpA binding sites exist on the IgG molecules and became exposed after the conformational change of the Fcγ region induced by the antigen. Therefore SpA can be used for the direct detection of conformational changes induced by antigen in the Fcγ region of rabbit antibody.