Staphylococcal protein A consists of five IgG-binding domains.

A genetic approach is described to clarify the IgG-binding properties of the N-terminal portion of staphylococcal protein A (region E). Several gene fragments, encoding region E or B or protein A, have been cloned and expressed in Escherichia coli. The gene products were purified by IgG-affinity chromatography and subjected to structural and functional analyses. Both fragments can be efficiently purified using this method, suggesting that region B as well as region E has Fc-binding activity. In addition, gene fusions were assembled giving fragments EB and EE, which both showed a divalent Fc-binding. These results demonstrate that protein A consists of five IgG-binding domains. The implications of these findings for the structure of protein-A--immunoglobulin-G complexes are discussed.