All individual domains of staphylococcal protein A show Fab binding

The interactions between the individual domains (E, D, A, B and C) of staphylococcal protein A (SPA) and Fc and Fab regions of human immunoglobulins were studied using real-time biospecific interaction analysis. An engineered domain Z, similar to fragment B but with a single glycine to alanine amino acid substitution, was also included in the study. The domains were expressed in Escherichia coli, affinity purified and immobilised onto sensor chip surfaces in a directed manner using a unique C-terminal cysteine residue engineered into the recombinant proteins. All domains bound to a recombinant human IgG1 Fc fragment with similar strength. For the first time, binding to human Fab was demonstrated for all native SPA domains, using both polyclonal F(ab′)₂ and a recombinant scFv fragment as reagents. Interestingly, the engineered Z domain showed a considerably lower affinity for Fab as compared to the native domains.