The interaction between different domains of staphylococcal protein A and human polyclonal IgG, IgA, IgM and F(ab')2: separation of affinity from specificity.

Binding properties of staphylococcal protein A (SpA) to different human immunoglobulins have been investigated. In this analysis, intact SpA as well as SpA-derived fragments containing one to five IgG-binding domains of different compositions, were used. The affinity binding constants of the different proteins to human polyclonal IgG, IgA, IgM and F(ab')2-fragments as well as their binding capacity to the immunoglobulin molecules were determined. The results show that although all the proteins bound to IgG, regardless of size or composition, the binding strength differed significantly. Proteins containing five domains have a stronger affinity for IgG than those containing one or two. There were no marked differences in binding strength between different domains. However, the binding ability to IgA and IgM showed a marked difference between the various SpA-derived proteins of different compositions. This discrepancy was correlated to differences in their relative binding properties to isolated F(ab')2-fragments of IgG. Hence, we conclude that the binding affinity is mainly affected by the number of domains, whereas the binding specificity is to a large extent determined by which domains are selected.