Protein A

• UniProt: P38507

Protein A is a protein of 40-60 kDa size, which originated from the cell wall of the bacterium Staphylococcus aureus. It is commonly used in biochemical research due to its ability to bind immunoglobulins. Here, proteins of different mammalian species are bound, especially IgG.

Protein A binds to the Fc region of immunoglobulins through interaction with the heavy chain. The bacterium protects itself in the body in front of the defense mechanisms of the immune system; by the antibodies bind to the cell wall ( in comparison to their normal function) "false" around, it is protected against opsonization and phagocytosis.

Antibody binding by protein A

Protein A binds with high affinity to human IgG1 and IgG2 as well as IgG2a, IgG2b and IgG3 mouse. Human IgM, IgA, and IgE and mouse IgG1 bind with moderate affinity. In contrast it does not react with human IgG3 or IgD nor with IgM, IgA or IgE mouse.

Role in the pathogenesis of

As a pathogen Staphylococcus aureus uses the protein A, together with several other proteins and surface factors in order to ensure its survival. Protein A is prevented by the binding of the host 's own antibodies, the recognition by the host immune system and phagocytosis by phagocytic cells such as macrophages. Mutants of S. aureus that do not express protein A can be phagocytosed more rapidly in vitro and in infection models show a reduced virulence.

In a recent paper it is shown that protein A also efficiently kill B lymphocytes, which play an important role in the immune defense against bacterial infections. This could be of pharmaceutical value, because B lymphocytes play a crucial role in some autoimmune diseases such as rheumatoid arthritis and lupus erythematosus; the suppression of B -cell response could thus lead to new therapies for autoimmune diseases.

Research

Recombinant Staphylococcal Protein A is usually produced in E. coli, for use in immunological or biological research. Often the protein is labeled with to other substances, such as fluorescent dyes, enzymes, biotin, colloidal gold, radioactive iodine ( 125 I ). Frequently protein A is also coupled to microparticles ( beads ), whether magnetic beads, latex or Sepharose. Protein A is well suited for the purification of IgG from protein mixtures such as Serum, or coupled to any of the above markers, for detection of antibodies. Protein A coupled to Sepharose is used in the immunoprecipitation.