Protein A (extracellular) from Staphylococcus aureus

Protein A is a highly stable cell surface receptor produced by several strains of Staphylococcus aureus. This protein is capable of binding to the Fc portion of immunoglobulins, especially IgGs, from a large number of species. One protein A molecule has been shown to bind at least 2 molecules of IgG simultaneously. The IgG binding domain of Protein A consists of three anti-parallel alpha-helicies, the third of which is disrupted when the protein is complexed with the Fc region of the immunoglobulins. Protein A will bind the Fc portion of human IgG subclasses, IgM, IgA and IgE; and mouse IgG1 (weakly), IgG2a, and IgG2b. Protein A also associates with IgGs from other species, including monkey, rabbit, pig, guinea pig, dog, and cat.
Protein A is involved in regulating anti-tumor, toxic, and carcinogenic functions. In addition to acting as an immunomodulator, it also has antifungal and antiparasitic properties.

Protein A may be conjugated with various reporter molecules such as fluorescent dyes (FITC), enzyme markers (peroxidase, beta-galactosidase, alkaline phosphatase), biotin, and colloidal gold without affecting the antibody binding site on the molecule. These conjugates may be used to detect immunoglobulins using immunochemical assays such as western blotting, immunohistochemistry, and ELISA. In addition, protein A may also be immobilized on a solid supports like agarose or acrylic beads for the purification of polyclonal or monoclonal immunoglobulins. It is also routinely used for immunoprecipitation assays. Furthermore, protein A has been used for affinity chromatography and SDS-PAGE assays.

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Protein A: nature′s universal anti-antibody - a review on properties and applications of protein A

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