Characterization of complexes containing protein A and rabbit immunoglobulin G or Fc gamma fragments

Soluble complexes prepared over a range of molar ratios of [125I] protein A (SpA) and rabbit [131I]immunoglobulin G (IgG) or [131I]Fc gamma fragments were separated by ultracentrifugation in a sucrose gradient and selectively radioimmunoprecipitated with chicken antibodies that bind free or complexed SpA. With excess [125I]SpA the only complexes formed were composed of one molecule of labeled ligand and one molecule of [125I]SpA (1:1 complexes). With excess ligand the only complexes formed corresponded to [(IgG)2SpA]2 or [(Fc gamma)2SpA]2 (4:2 complexes). The 4:2 complex likely is responsible for several of the biological activities associated with SpA, including activation of humoral and cellular immune mechanisms, which are optimal under conditions of excess IgG.