Purification of IgG Using Protein A or Protein G

Some strains of Staphylococcus aureus synthesize protein A, a group-specific ligand that binds to the Fc region of IgG from many species (1, 2). Protein A does not bind all subclasses of IgG, e.g., human IgG3, mouse IgG3, sheep IgG1, and some subclasses bind only weakly, e.g., mouse IgG1. For some species, IgG does not bind to protein A at all, e.g., rat, chicken, goat, and some MAbs show abnormal affinity for the protein. These properties make the use of protein A for IgG purification limited in certain cases, although it can be used to an advantage in separating IgG subclasses from mouse serum (3). Protein G (derived from groups C and G Streptococci) also binds to IgG Fc with some differences in species specificity from protein A. Protein G binds to IgG of most species, including rat and goat, and recognizes most subclasses (including human IgG3 and mouse IgG1), but has a lower binding capacity. Protein G also has a high affinity for albumin, although recombinant DNA forms now exist in which the albumin-binding site has been spliced out, and are therefore very useful for affinity chro-matography. Other streptococcal immunoglobulin binding proteins are protein H (binds IgG Fc), protein B, which binds IgA and protein Arp, which binds IgG & IgA. These are not generally available for immunochemical use.