Protein G is an immunoglobulin-binding protein expressed in group C and G Streptococcal bacteria much like Protein A but with differing specificities. It is a 65-kDa (G148 protein G) and a 58 kDa (C40 protein G)^[1] cell surface protein that has found application in purifying antibodies through its binding to the Fc region. The native molecule also binds albumin, however, because serum albumin is a major contaminant of antibody sources, the albumin binding site has been removed from recombinant forms of Protein G.

[edit] Other antibody binding proteins

In addition to Protein G, other immunoglobulin-binding bacterial proteins such as Protein A, Protein A/G and Protein L are all commonly used to purify, immobilize or detect immunoglobulins. Each of these immunoglobulin-binding proteins has a different antibody binding profile in terms of the portion of the antibody that is recognized and the species and type of antibodies it will bind.

[edit] Folding of Protein G, B1 Domain

An ab initio simulation of the protein G B1 domain demonstrates that, as earlier results suggested, this protein initiates folding via a nucleation event in the hydrophobic core residues followed by small adjustments^[2] . The folding events are as follows:

1. a β-hairpin is formed, stabilized by residues W43, Y45, and F52.
2. Residue contacts between residue F30, in an α-helix, and the β-hairpin strengthen.
3. Nucleation of the β-sheet starting from residues L5 and F52, occurs.
4. The last nucleation residue, Y3, assists in forming the central part of the β-sheet resulting in a globular protein.

[edit] References

http://www.jbc.org/cgi/reprint/266/1/399 Sjobring U, Bjorck L, Kastern W, Streptococcal protein G. Gene structure and protein binding properties, J Biol Chem. 1991 Jan 5;266(1):399-405