Protein G is an immunoglobulin-binding protein expressed in group C and G Streptococcal bacteria much like Protein A but with differing specificities. Protein G binds with greater affinity to most mammalian immunoglobulins than Protein A, including human IgG3 and rat IgG2a, does not bind to human IgM, IgD and IgA. It is a 65-kDa (from group G Streptococcus) cell surface protein that has found application in purifying antibodies through its binding to the Fc region. The native molecule also binds albumin, however, because serum albumin is a major contaminant of antibody sources, the albumin binding site has been removed from this recombinant form of Protein G (a.a. 190-384).
Recombinant protein G is expressed by E. coli and purified by affinity chromatography to give a purity of 95% or more (Coomassie staining SDS-PAGE). The purification process does not use antibody column affinity chromatography to avoid the possibility of incorporation of unrelated IgG in the product.
Protein G affinity chromatography columns have become widely used as affinity columns for purified antibodies from hybridoma ascites, serum, tissue culture supernatants and other biological fluids.