Protein A, Protein G, Protein A/G and Protein L are native and recombinant proteins of microbial origin that bind to mammalian immunoglobulin molecules. These proteins are available in purified, salt-free, lyophilized form, as well as coated in microplates and covalently immobilized to various solid supports. The most popular support for column affinity purification of antibodies is crosslinked beaded agarose, although Thermo Scientific™ UltraLink™ Biosupport (polyacrylamide) and other porous media are also available. Magnetic beads are the most popular support material for immunoprecipitation applications.


General characteristics of Ig binding proteins

 Native Protein ARecombinant Protein ARecombinant Protein GRecombinant Protein A/GRecombinant Protein L
Native sourceStaphylococcusaureusStaphylococcusaureusStreptococcusN/APeptostreptococcus magnus
Production sourceS. aureusE. coliE. coliE. coliE. coli
Molecular weight46,70044,60021,60050,46035,800
Apparent mass by SDS-PAGE42kDa45kDa32kDa40 to 45kDa36kDa
# binding sites for Ig5524+24
Albumin binding siteNoNoNoNoNo
Optimal binding pH8.28.255 to 8.27.5
Ig binding targetFcFcFcFcVL-kappa

The four proteins bind almost exclusively with the IgG class of antibodies, but their binding properties differ among species and subclasses of IgG. Protein A is generally preferred for rabbit, pig, dog and cat IgG. Protein G has better binding capacity for a broader range of mouse and human IgG subclasses (IgG1, IgG2, etc.).

Protein A/G is a recombinant fusion protein that includes the IgG-binding domains of both Protein A and Protein G. Therefore, Protein A/G is ideal for binding the broadest range of IgG subclasses from rabbit, mouse, human and other mammalian samples.

Protein L binds to certain immunoglobulin kappa light chains. Because kappa light chains occur in members of all classes of immunoglobulin (i.e., IgG, IgM, IgA, IgE and IgD), Protein L can purify these different classes of antibody. However, only those antibodies within each class that possess the appropriate kappa light chains will bind. Generally, empirical testing is required to determine if Protein L is effective for purifying a particular antibody.