Protein G

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

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.

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. The folding events are as follows:

The protein G B1 domain is (aka. GB1) often used as part of a fusion protein to keep other domains in solution during experiments in solution (e.g. NMR). Many previously insoluble domains have become soluble with the fusion of the GB1 domain. The domain is 56 residues (approx 8kDa) long. On SDS-PAGE gels the GB1 domain runs at roughly 13.5kDa despite being only 8kDa.

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