Protein A and Protein G
Protein A is derived from Staphylococcus aureus. Protein G is derived from a Streptococcus species. Both have binding sites for the Fc portion of mammalian IgG. The affinity of these proteins for IgG varies with the animal species. Protein G has a higher affinity for rat, goat, sheep, and bovine IgG, as well as for mouse IgG1 and human IgG3. Protein A has a higher affinity for cat and guinea pig IgG (see Table A). In addition to IgG Fc binding sites, native Protein G contains binding sites for albumin, the Fab region of Igs, and membrane binding regions, which can lead to nonspecific staining. These problems have been addressed by creating recombinant forms of the protein. Recombinant Protein G has been engineered to eliminate the albumin binding region, and recombinant Protein G′ is a truncated protein which lacks the albumin, Fab, and membrane binding sites while retaining the Fc binding site, making it more specific for IgG than the native form.
Use as a general, non-species-specific reagent for binding primary antibodies or surface IgG in mammalian tissues. Protein G is recommended for most species, including mouse and rat. Protein A is recommended for cat and guinea pig. Neither is recommended for detection of IgA or IgM, for detection of Fab fragments, or for detection of avian IgG. When bound to a resin like agarose, Protein A and Protein G may be used to affinity purify immunoglobulins from serum or ascites fluid.
Protein L, from Peptostreptococcus magnus, has an affinity for kappa light chains (see Table B) from various species. It will detect monoclonal or polyclonal IgG, IgA, and IgM as well as Fab, F(ab′)2, and recombinant single-chain Fv (scFv) fragments that contain kappa light chains. It will also bind chicken IgG. Note: Species such as bovine, goat, sheep, and horse whose Igs contain almost exclusively lambda chains will not bind well, if at all, to Protein L.
Use as a general reagent for binding primary mammalian or avian antibodies or surface Igs of all classes. Especially useful for detection of Fab, F(ab′)2 fragments, and recombinant scFv fragments, for detection of Igs bound to Fc receptors, or for detection of monoclonal antibodies in the presence of bovine Igs. Use is limited to detection of Igs bearing kappa light chains.