Staphylococcal protein A in immunoferritin techniques.

Protein A (pA) of Staphylococcus aureus, by virtue of its reactivity with the Fc portion of a variety of mammalian immumoglobulins (IgG), can be used for electron-microscopical immunoferritin techniques. The conjugation of pA with ferritin (pA-F) by means of bis(4-fluoro-3-nitrophenyl) sulfone results in a highly specific marker for IgG in indirect labeling experiments. Native pA can be used as a noncovalent bridging agent between specific antibodies and antiferritin capturing the marker molecule. The use of pA and pA-F is demonstrated with a model system involving human erythrocytes and two rabbit antisera directed against different surface antigens of the human erythrocyte membrane.