Protein A is a 56 kDa surface protein originally found in the cell wall of the bacterium Staphylococcus aureus. It is encoded by the spa gene and its regulation is controlled by DNA topology, cellular osmolarity, and a two-component system called ArlS-ArlR. It has found use in biochemical research because of its ability to bind immunoglobulins. It is composed of five homologous Ig-binding domains that fold into a three-helix bundle. Each domain is able to bind proteins from many mammalian species, most notably IgGs. It binds the heavy chain within the Fc region of most immunoglobulins and also within the Fab region in the case of the human VH3 family. Through these interactions in serum, where IgG molecules are bound in the wrong orientation (in relation to normal antibody function), the bacteria disrupts opsonization and phagocytosis.
Protein A antibody binding
Protein A binds with high affinity to human IgG1 and IgG2 as well as mouse IgG2a and IgG2b. Protein A binds with moderate affinity to human IgM, IgA and IgE as well as to mouse IgG3 and IgG1. It does not react with human IgG3 or IgD, nor will it react to mouse IgM, IgA or IgE.
The capacity of protein A to bind antibodies with such high affinity is the driving motivation for its industrial scale use in biologic pharmaceuticals. The protein A used for production of antibodies in bio-pharmaceuticals is most commonly bound to a stationary phase chromatography resin.
Other antibody binding proteins
Role in pathogenesis
As a pathogen Staphylococcus aureus utilizes Protein A, along with a host of other proteins and surface factors to aid its survival and, thus, virulence. Protein A helps inhibit phagocytic engulfment and acts as an immunological disguise. Mutants of S. aureus lacking protein A are more efficiently phagocytosed in vitro, and mutants in infection models have diminished virulence.  Higher levels of Protein A in different strains of S. aureus have been associated with nasal carriage of this bacteria.
Recombinant Staphylococcal Protein A is often produced in E. coli for use in immunology and other biological research. One recombinant form of Protein A is called MabSelect. Protein A is often coupled to other molecules such as a fluorescent dye, enzymes, biotin, colloidal gold or radioactive iodine without affecting the antibody binding site. It is also widely utilized coupled to magnetic, latex and agarose beads.
Protein A is often immobilized onto a solid support and used as reliable method for purifying total IgG from crude protein mixtures such as serum or ascites fluid, or coupled with one of the above markers to detect the presence of antibodies. Immunoprecipitation studies with protein A conjugated to beads are also commonly used to purify proteins or protein complexes indirectly through antibodies against the protein or protein complex of interest.
- Graille M, Stura EA, Corper AL, Sutton BJ, Taussig MJ, Charbonnier JB, Silverman GJ. (May 2000). "Crystal structure of a Staphylococcus aureus protein A domain complexed with the Fab fragment of a human IgM antibody: structural basis for recognition of B-cell receptors and superantigen activity.". Proc Natl Acad Sci U S A 97 (10): 5399–404. doi:10.1073/pnas.97.10.5399. PMC 25840. PMID 10805799.
- Idusogie EE, Presta LG, Gazzano-Santoro H, Totpal K, Wong PY, Ultsch M, Meng YG, Mulkerrin MG. (April 2000). "Mapping of the C1q binding site on rituxan, a chimeric antibody with a human IgG1 Fc". J Immunol 164 (8): 4178–84. PMID 10754313.
- Goodyear CS, Silverman GJ (May 2003). "Death by a B cell superantigen: In vivo VH-targeted apoptotic supraclonal B cell deletion by a Staphylococcal Toxin". J. Exp. Med. 197 (9): 1125–39. doi:10.1084/jem.20020552. PMC 2193973. PMID 12719481.
- Gowrishankar Muthukrishnan, Gerry A. Quinn, Ryan P. Lamers, Carolyn Diaz, Amy L. Cole, Sixue Chen, and Alexander M. Cole (April 2011). "Exoproteome of Staphylococcus aureus reveals putative determinants of nasal carriage". J Proteome Res 10 (4): 2064–78. doi:10.1021/pr200029r. PMC 3070068. PMID 21338050.