Protein A

Protein A
File:Protein A 1DEE 1L6X.png Structure of a domain of protein A as a three-helix bundle binding to the heavy variable chain of a VH3 human Fab left. Minimized protein A bound to Fc fragment of Rituximab.
Identifiers
Symbol	SpA
SCOP	1DEE
SUPERFAMILY	1DEE

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.

History

As a by-product of his work on type-specific staphylococcus antigens, Verwey reported 1940 that a protein fraction prepared from extracts of these bacteria non-specifically precipitated rabbit antisera raised against different staphylococcus types. 1958, Jensen confirmed Verwey’s finding and showed that rabbit pre-immunization sera as well as normal human sera bound to the active component in the staphylococcus extract; he designated this component Antigen A (because it was found in fraction A of the extract) but thought it was a polysaccharide. The misclassification of the protein was the result of faulty tests but it was not long thereafter (1962) that Löfkvist and Sjöquist corrected the error and confirmed that Antigen A was in fact a surface protein on the bacterial wall of certain strains of S. Aureus. The Bergen group from Norway named the protein "Protein A" after the antigen fraction isolated by Jensen.

Protein A antibody binding

Protein A can bind with strong affinity the Fc portion of immunoglobulin of certain species:

Immunoglobulin Species - Protein A Binding

Human IgG, IgG₂, IgG₄ Strong
Human IgG₃, IgD None
Human IgM, IgA Weak

Mouse IgG₁ Weak
Mouse IgG_2A, IgG_2B, IgG₃ Strong
Mouse IgM None

Rat IgG₁ Weak
Rat IgG_2A, IgG_2B None
Rat IgG_2C Strong

Goat IgG₁ Weak
Goat IgG₂ Strong

Bovine IgG₁ None
Bovine IgG₂ Strong

Guinea Pig IgG Strong

Rabbit IgG Strong

Chicken IgG (IgY) None

Other antibody binding proteins

In addition to Protein A, other immunoglobulin-binding bacterial proteins such as Protein G, Protein A/G and Protein L are all commonly used to purify, immobilize or detect immunoglobulins.

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.

Research

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.