Protein G

Short description: Immunoglobulin-binding protein expressed in group C and G Streptococcal bacteria

GA-like domain

Identifiers
Symbol| ?
Pfam| PF17573
InterPro| IPR035152
| Available protein structures:

Pfam | structures / ECOD
PDB| RCSB PDB; PDBe; PDBj
PDBsum| structure summary
Immunoglobulin G-binding protein G

Identifiers
Organism| Streptococcus sp. group G
Symbol| spg
UniProt| P06654
| Search for

Structures| Swiss-model
Domains| InterPro

Protein G is an immunoglobulin-binding protein expressed in group C and G streptococcal bacteria much like protein A but with differing binding specificities. It is a ~60-kDA (65 kDA for strain G148 and 58 kDa for strain C40)[1] 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. This recombinant protein G, either labeled with a fluorophore or a single-stranded DNA strand, was used as a replacement for secondary antibodies in immunofluorescence and super-resolution imaging.[2]

## Contents

* 1 Other antibody binding proteins
* 2 Folding of protein G, B1 domain
* 3 See also
* 4 References

## 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.

## 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.[3] 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.

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.[4] The domain is 56 residues (approx 8kDa) long. On SDS-PAGE gels the GB1 domain runs at roughly 13.5kDa despite being only 8kDa.[5]

## See also

* GA module

## References

1. ↑ "Streptococcal protein G. Gene structure and protein binding properties". J Biol Chem 266 (1): 399–405. 1991. doi:10.1016/S0021-9258(18)52448-0. PMID 1985908.
2. ↑ Schlichthaerle, Thomas; Ganji, Mahipal; Auer, Alexander; Wade, Orsolya Kimbu; Jungmann, Ralf (2018). "Bacterial-derived antibody binders as small adapters for DNA-PAINT microscopy" (in en). ChemBioChem 20 (8): 1032–1038. doi:10.1002/cbic.201800743. ISSN 1439-7633. PMID 30589198.
3. ↑ "Folding pathway of the b1 domain of protein G explored by multiscale modeling". Biophys J 94 (3): 726–36. Feb 2008. doi:10.1529/biophysj.107.116095. PMID 17890394. Bibcode: 2008BpJ....94..726K.
4. ↑ Cheng, Yuan; Patel, Dinshaw J. (2004). "An efficient system for small protein expression and refolding". Biochemical and Biophysical Research Communications 317 (2): 401–405. doi:10.1016/j.bbrc.2004.03.068. PMID 15063772.
5. ↑ Hartl MJ, Mayr F, Rethwilm A, Wöhrl BM (2010). "Biophysical and enzymatic properties of the simian and prototype foamy virus reverse transcriptases.". Retrovirology 7: 5. doi:10.1186/1742-4690-7-5. PMID 20113504.

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