Protein A and Protein G Purification of Antibodies

Protein A and Protein G are immunoglobulin-binding proteins expressed in Staphylococcus aureus and Streptococcus sp., respectively, that have been adapted for use in purifying large amounts of IgG. They are available covalently attached to affinity resins such as 4% cross-linked agarose, making them suitable for low-pressure antibody isolation. Protein A is not recommended for the isolation of mouse mAbs because it lacks affinity for mouse IgG1, or for the isolation of antibodies from sheep, goat, chicken, hamster, or rat. IgGs from most species bind to Protein G at near physiological pH and ionic strength with a higher affinity than IgG binding to Protein A. Therefore, the pH required to dissociate bound IgG is lower, resulting in the loss of activity for some antibodies. If this is observed, Protein A may be an alternative if the IgG from the species being isolated can be purified using Protein A. Neither Protein A nor Protein G can be used for the isolation of chicken antibodies.