A study of the interactions between an IgG-binding domain based on the B domain of staphylococcal protein A and rabbit IgC

The nonantigenic interaction between a recombinant immunoglobulin G (IgG)-binding protein based on the B domain of Protein A from Staphylococcus aureus (termed SpA(1)) and the Fe fragment of rabbit IgG has been investigated. The contribution to binding of four putative hydrogen bond contacts between SpA(1) and IgG-Fc were examined by the individual substitution of the residues in SpA(1) involved in these interactions by others unable to form hydrogen bonds. It was found that the most important of the hydrogen bonds involved Tyr 18 which, when replaced by Phe, resulted in a twofold decrease in IgG-binding affinity. The residues of SpA(1) proposed to make close, mainly hydrophobic, contacts with Pc were replaced by residues with potential electrostatic charge to establish the importance of the hydrophobic interaction in the complex. The IgG-binding affinities of the mutant proteins were compared to the wild-type protein by a competitive enzyme-linked immunosorbant assay. The replacement of individual hydrophobic residues by His generated a number of novel IgG-binding proteins with reduced binding affinity at pH 5.0 but which maintained strong binding affinities at pH 8.0. The elution profile of human IgG(1)-Fe (Fc fragment of human IgG(1)) from a column made from an immobilized two-domain mutant protein shows that the complex dissociates at a higher pH relative to that of the non-mutated protein thus offering favorable elution characteristics.