Protein A is a 42 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
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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.
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