Protein A, derived from the cell wall ofStaphylococcus aureus, and protein G, derived from the cell wall of certain strains of b-hemolyticStreptococcci, binds the Fc region of various immunoglobulines. Protein A is a 42 kDa polypeptide that binds the residues in the CH2 and CH3 domains of immunoglobulin heavy chains. Protein G is an approximatley 30 kDa polypeptide that binds immunoglobulins and albumin. Recombinant protein G is available in which the albumin binding site has been deleted. Agarose beads coated with these proteins will bind antibodies and serve to immunoprecipitate antigens from solution. There are differences in the affinity of these proteins for antibodies from different species and for different isotypes within a species. Tables I may be used to determine relative avidity for polyclonal antibodies of a given species. Table II shows relative affinities for varius isotypes. Table I
| Human | ++++ | ++++ |
| Mouse | ++ | ++ |
| Rat | +/- | ++ |
| Hamster | + | ++ |
| Guinea Pig | ++++ | ++ |
| Rabbit | ++++ | +++ |
| Horse | ++ | ++++ |
| Cow | ++ | ++++ |
| Pig | +++ | +++ |
| Sheep | +/- | ++ |
| Goat | - | ++ |
| Chicken | - | + |
Table II
| Human IgG1 | ++++ | ++++ |
| Human IgG2 | ++++ | ++++ |
| Human IgG3 | - | ++++ |
| Human IgG4 | ++++ | ++++ |
| Mouse IgG1 | + | ++++ |
| Mouse IgG2a | ++++ | ++++ |
| Mouse IgG2b | +++ | +++ |
| Mouse IgG3 | ++ | +++ |
| Rat IgG1 | - | + |
| Rat IgG2a | - | ++++ |
| Rat IgG2b | - | ++ |
| Rat IgG2c | + | ++ |
Sources of information:Akerstrom, B., Brodin, T., Reis, K., and Bjorck, L. 1985. Protein G: A powerful tool for binding and detection of monoclonal and polyclonal antibodies.J. Immunol.135:2589-2592.Akerstrom, B. and Bjorck, L. 1986. A physicochemical study f protein G, a molecule with unique immunoglobulin G-binding properties.J. Biol. Chem.261:10240-10247.Harlow, E. and Lane, D. 1988. Antibodies. A laboratory manual. Cold Spring Harbor Laboratory. Kronvall, G., Seal, U. S., Finstad, J., Williams, Jr., R. C. 1970. Phylogenetic insight into evolution of mammalian Fc fragment of g G globulin using staphylococcal protein A.J. Immunol.104:140-147.Kronvall, G. 1973. A surface component in group A, C, and G streptococci with non-immune reactivity for immunoglobulin G.J. Immunol.111:1401-1406. Richman, D. D., Cleveland, P. H., Oxman, M. N., and Johnson, K. M. 1982. The binding of staphylococcl protein A by the sera of different animal species.J. Immunol.128:2300-2305.
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