Protein G is an

immunoglobulin An antibody (Ab), also known as an immunoglobulin (Ig), is a large, Y-shaped protein used by the immune system to identify and neutralize foreign objects such as pathogenic bacteria and viruses. The antibody recognizes a unique molecule of the ...

-binding

protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, respo ...

expressed in group C and G

Streptococcal ''Streptococcus'' is a genus of gram-positive ' (plural ) or spherical bacteria that belongs to the family Streptococcaceae, within the order Lactobacillales (lactic acid bacteria), in the phylum Bacillota. Cell division in streptococci occur ...

bacteria much like

Protein A Protein A is a 42 kDa surface protein originally found in the cell wall of the bacteria ''Staphylococcus aureus''. It is encoded by the ''spa'' gene and its regulation is controlled by DNA topology, cellular osmolarity, and a two-component system ...

but with differing binding specificities. It is a ~60-kDA (65 kDA for strain G148 and 58 kDa for strain C40) 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 Albumin is a family of globular proteins, the most common of which are the serum albumins. All the proteins of the albumin family are water-soluble, moderately soluble in concentrated salt solutions, and experience heat denaturation. Albumins ...

, 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.

Other antibody binding proteins

In addition to Protein G, other immunoglobulin-binding bacterial proteins such as

Protein A/G Protein A/G is a recombinant fusion protein that combines IgG binding domains of both Protein A and Protein G. Protein A/G contains four Fc binding domains from Protein A and two from Protein G, yielding a final mass of 50,460 daltons. The binding ...

and

Protein L Protein L was first isolated from the surface of bacterial species '' Peptostreptococcus magnus'' and was found to bind immunoglobulins through L chain interaction, from which the name was suggested. It consists of 719 amino acid residues. The mo ...

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 In thermodynamics, nucleation is the first step in the formation of either a new thermodynamic phase or structure via self-assembly or self-organization within a substance or mixture. Nucleation is typically defined to be the process that deter ...

event in the

hydrophobic In chemistry, hydrophobicity is the physical property of a molecule that is seemingly repelled from a mass of water (known as a hydrophobe). In contrast, hydrophiles are attracted to water. Hydrophobic molecules tend to be nonpolar and, th ...

core residues followed by small adjustments. The folding events are as follows: # a β-hairpin is formed, stabilized by residues W43, Y45, and F52. # Residue contacts between residue F30, in an

α-helix The alpha helix (α-helix) is a common motif in the secondary structure of proteins and is a right hand-helix conformation in which every backbone N−H group hydrogen bonds to the backbone C=O group of the amino acid located four residues e ...

, and the β-hairpin strengthen. # Nucleation of the

β-sheet The beta sheet, (β-sheet) (also β-pleated sheet) is a common motif of the regular protein secondary structure. Beta sheets consist of beta strands (β-strands) connected laterally by at least two or three backbone hydrogen bonds, forming a gen ...

starting from residues L5 and F52, occurs. # The last nucleation residue, Y3, assists in forming the central part of the β-sheet resulting in a

globular protein In biochemistry, globular proteins or spheroproteins are spherical ("globe-like") proteins and are one of the common protein types (the others being fibrous, disordered and membrane proteins). Globular proteins are somewhat water-soluble (formi ...

. The protein G B1 domain is (aka. GB1) often used as part of a

fusion protein Fusion proteins or chimeric (kī-ˈmir-ik) proteins (literally, made of parts from different sources) are proteins created through the joining of two or more genes that originally coded for separate proteins. Translation of this ''fusion gene'' r ...

to keep other domains in solution during experiments in solution (e.g.

NMR Nuclear magnetic resonance (NMR) is a physical phenomenon in which nuclei in a strong constant magnetic field are perturbed by a weak oscillating magnetic field (in the near field) and respond by producing an electromagnetic signal with ...

). 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 SDS-PAGE (sodium dodecyl sulfate–polyacrylamide gel electrophoresis) is a discontinuous electrophoretic system developed by Ulrich K. Laemmli which is commonly used as a method to separate proteins with molecular masses between 5 and 250 kDa. T ...

gels the GB1 domain runs at roughly 13.5kDa despite being only 8kDa.

References

{{reflist, refs= {{cite journal , vauthors=Sjobring U, Bjorck L, Kastern W, etal , title=Streptococcal protein G. Gene structure and protein binding properties , journal=J Biol Chem , volume=266 , issue=1 , pages=399–405 , year=1991 , doi=10.1016/S0021-9258(18)52448-0 , pmid=1985908, doi-access=free {{cite journal , vauthors= Kmiecik S, Kolinski A , title= Folding pathway of the b1 domain of protein G explored by multiscale modeling , journal= Biophys J , volume=94 , issue=3, pages=726–36 , date=Feb 2008 , pmid= 17890394, pmc=2186257, doi=10.1529/biophysj.107.116095, bibcode= 2008BpJ....94..726K {{Cite journal, doi = 10.1016/j.bbrc.2004.03.068, title = An efficient system for small protein expression and refolding, year = 2004, last1 = Cheng, first1 = Yuan, last2 = Patel, first2 = Dinshaw J., journal = Biochemical and Biophysical Research Communications, volume = 317, issue = 2, pages = 401–405, pmid = 15063772, pmc = 4693640 {{cite journal, author=Hartl MJ, Mayr F, Rethwilm A, Wöhrl BM, title=Biophysical and enzymatic properties of the simian and prototype foamy virus reverse transcriptases. , journal=Retrovirology , year= 2010 , volume= 7 , pages= 5 , pmid=20113504 , doi=10.1186/1742-4690-7-5 , pmc=2835651 Proteins

Other antibody binding proteins

Folding of Protein G, B1 Domain

See also

References