What was Protein G named after?

What was Protein G named after?

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Protein G (the bacterial antibody binding protein) is often used to pulldown antibodies, for example in chromatin immunoprecipitation (ChIP) experiments.

However, I was unable to find a site describing the history of the said protein, and how the name of Protein G came about, due to the fact that the similarly named (but completely different) G protein came up higher in search results.

What was Protein G named after? For example, Protein A was named after the antibody fraction in which it was found, and G protein was named because it bound GDP and GTP.

As @canadianer mentions in his comment, Protein G is more than likely named after the human group G strain of Streptococci, G148.

Protein G, a bacterial cell wall protein with affinity for immunoglobulin G (IgG), has been isolated from a human group G streptococcal strain (G148).

Purification and some properties of streptococcal protein G, a novel IgG-binding reagent. - Björck

Protein G is a Streptococcus protein that can bind the the Fc portion of the Immunoglobulin G protein. While it is similar to protein A, it was found to have better binding affinities to the different varieties of heavy chain constant regions of IgG molecules.

The G comes from Lancefield grouping. Lancefield grouping was a method that used to be used to classify bacterium. Group G Streptococci could be identified using Lancefield serum type G and that is how they received their classification.


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Protein G HP SpinTrap

Protein G HP SpinTrap is a prepacked, single-use spin column containing Protein G Sepharose High Performance.

Select Model

Protein G HP SpinTrap is a prepacked, single-use spin column containing Protein G Sepharose High Performance.

  • For simple, small-scale purification of antibodies or enrichment of specific proteins in prepacked spin columns that are ready to use.
  • Classic and cross-link protocols provide flexibility.
  • Elution conditions formatted for both electrophoresis and LC-MS analysis workflows.
  • Fast binding kinetics and high capacity provides high yield.
  • Prepacked with Protein G Sepharose High Performance for coupling of antibodies of IgG subclasses.
  • Easy scale-up with HiTrap Protein G HP prepacked columns.

Protein G Sepharose HP has high affinity for the Fc region of IgG from a variety of species. The two options in the protein enrichment protocol allow the attached antigen to be eluted separately (cross-link protocol) or together with the antibody (classic protocol).

There is also a protocol for efficient, small-scale purification of monoclonal and polyclonal antibodies from serum and cell culture supernatants in less than 20 min. No pretreatment of the sample is required.

For larger scale please use Protein G HP MultiTrap and HiTrap Protein G HP

Protein A, G and A/G MagBeads

GenScript Protein A, G and A/G MagBeads are developed for quick, convenient and reproducible immunoprecipitation(IP) and small scale purification of immunoglobulin. By covalently pre-coupling recombinant Protein A, G and A/G to the surface of superparamagnetic beads, these MagBeads provide users simple, easy and time-saving procedures for immunomagnetic separation.

Due to the elimination of non-IgG binding domains and other unknown function fragments, the recombinant Protein A, G and A/G both display high affinity for subclasses of IgG from many species including human, rabbit and mouse etc. This permits the purification of IgGs from cell culture supernatants, serum and ascites or other starting samples. In addition, Protein A, G and A/G MagBeads can also be used to immunoprecipitate target proteins from crude samples using a selected primary antibody. To prevent co-elution of the antibody, it is necessary to chemically cross-link the antibody to protein A, G and A/G coated beads prior to the immunoprecipitation steps. Read more »

By applying magnetic separation technology, the Protein A, G and A/G MagBeads supply researchers with fast, easy and efficient IP/isolation procedures. A magnetic separation rack is also needed to use the MagBeads. All steps take place in a single tube, without the need for columns, centrifugation, or pre-treatment. Extremely gentle magnetic handling prevents loss of your target IgGs or protein and keeps them intact throughout the procedure. Protein A, G, and G/G MagBeads are the perfect tools for immunoprecipitation and small-scale IgG purification.

Table 1. Properties of the recombinant Protein A, G and L on Resins

Protein A Protein A
MagBeads MX
Protein G Protein G
MagBeads MX
Protein A/G
Source Staphylococcus aureus Staphylococcus aureus Streptococcus Streptococcus
Molecular weight 34 kDa 34 kDa ≈22 kDa ≈22 kDa 43 kDa
Numbers of IgG(Ig) binding sites 5 5 3 3 4 of Protein A and 2 of Protein G
IgG Binding capacity 10 mg rabbit IgG per ml settled beads ≧ 30 mg human IgG per ml settled beads 10 mg goat IgG per ml settled beads ≧ 25 mg human IgG per ml settled beads 10 mg goat IgG per ml settled beads
Cat NO. L00273 L006724 L00274 L006734 L00277

Lane M, Protein Molecular Weight Standard.
Lane 1, Flowthrough of rabbit serum.
Lane 2, Purified rabbit IgG from serum.
Lane 3, Bacterial lysate expressing His-tagged protein.
Lane 4, Immunoprecipitated His-tagged protein, 32 kDa (Anti-His antibody cross-linked to the MagBeads).

Fig.2. Purification of Rabbit IgG from Rabbit serum and Immunoprecipitation of His-Tagged protein from bacterial lysate using GenScript Protein A MagBeads.

  • Semi-Automated Magnetic Separation System:Shorten the protein & antibody purification time.
  • Epitope Tag Antibody Coated MagBeads: Immunoprecipitation and fast isolation of tagged proteins.
  • Magnetic Separation Racks:well-designed and practicable device in bio-magnetic separation technology.


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Tagged Protein Purification Resins: purification of polyhistidine, GST, biotin and streptavidin tagged proteins.

Protein G column for general-purpose antibody purification

HiTrap Protein G HP columns are prepacked with Protein G Sepharose High Performance resin. This protein G affinity resin is a good first choice for general-purpose capture of antibodies at laboratory scale, because it binds to a broader range of IgG from eukaryotic species and also binds to more subclasses of IgG than protein A-based resins do.

Use these columns for IgG purification from cow, sheep, horse, rat, mouse, and human sources, including ascites, serum, and cell culture supernatants.

HiTrap Protein G HP columns deliver high-performance, high-resolution purifications with a binding capacity of 25 mg IgG/mL resin and fast kinetics. Columns are stable over a wide pH range – operational pH 3 to 9, and pH 2 to 10 for cleaning.

What is Protein A?

Protein A is defined as a surface protein that is in the size of 42 kDa. Protein A is originally found in the cell wall of Staphylococcus aureus. It is encoded by the “spa” gene. Protein A is regulated by DNA topology, cellular osmolarity, and a two-component system. This microbial recombinant protein is highly involved in the biochemical reactions, because of its ability to bind with several types of antibodies such as IgG, IgA, IgE, and IgM. So, this microbial protein is used to purify human antibody types. It is having five homologous “Ig” binding domains that fold into three-helix bundles. Each domain is capable of binding to immunoglobulin proteins from many mammalian species most notably with IgG antibodies. Protein A specifically binds to the heavy chain of Fc region of most of the immunoglobulins.

With regard to human VH3 family proteins, Protein A binds to the Fab region. The recombinant protein A is broader in its ability to bind to other human antibodies (IgA, IgE, IgM) other than IgG antibody. But it is weakly biding with human IgG3 subclass and it is not binding with IgD human antibody. Protein A also capable of binding to IgG antibodies of other species such as horse, rabbit, mouse, dog, monkey, cow etc.

Figure 01: Protein A

Protein A plays a pivotal role in Staphylococcus aureus pathogenesis. This protein facilitates the binding of bacteria to the human Von–Willebrand factor coated surface. Thus, it is increasing the bacterial infection efficiency. Protein A also cripples the human humoral mediated immunity. This microbial recombinant protein is produced through the industrial fermentation process.

Domains and Repeats

Feature keyPosition(s)Description Actions Graphical viewLength
<p>This subsection of the <a href="">Family and Domains</a> section describes the position and type of a domain, which is defined as a specific combination of secondary structures organized into a characteristic three-dimensional structure or fold.<p><a href='/help/domain' target='_top'>More. </a></p> Domain i 3 – 46 IgG_binding_B InterPro annotation

<p>Information which has been generated by the UniProtKB automatic annotation system, without manual validation.</p> <p><a href="/manual/evidences#ECO:0000259">More. </a></p> Automatic assertion inferred from signature match i

Automatic assertion inferred from signature match i

Automatic assertion inferred from signature match i

Family and domain databases

Integrated resource of protein families, domains and functional sites

Pfam protein domain database

Molecule processing

Feature keyPosition(s)Description Actions Graphical viewLength
<p>This subsection of the 'PTM / Processing' section describes the extent of a polypeptide chain in the mature protein following processing or proteolytic cleavage.<p><a href='/help/chain' target='_top'>More. </a></p> Chain i PRO_0000127700 1 – 340 Guanine nucleotide-binding protein G(I)/G(S)/G(T) subunit beta-3 Add BLAST 340

Proteomic databases

jPOST - Japan Proteome Standard Repository/Database

MaxQB - The MaxQuant DataBase

PaxDb, a database of protein abundance averages across all three domains of life

PRoteomics IDEntifications database

ProteomicsDB: a multi-organism proteome resource

PTM databases

iPTMnet integrated resource for PTMs in systems biology context

Comprehensive resource for the study of protein post-translational modifications (PTMs) in human, mouse and rat.

Interactions Outside the Antigen-Combining Site


Streptococcal protein G is a cell wall protein with an extracellular part composed of two (or three) small domains that bind serum albumin (GA domains) and two (or three) immunoglobulin binding domains (B domains). According to NMR spectroscopy and x-ray crystallography studies, B domains are composed of a central α-helix packed against a four strand β-sheet ( Gronenborn and Clore, 1993 Sauer-Eriksson et al., 1995 ). The equilibrium constants of the reaction between protein G and human, rabbit, mouse, and goat IgG range between 10 10 M −1 and 10 10 M −1 , which is greater than the corresponding values for protein A binding ( Åckerström and Björck, 1986 ). Protein G binding is optimal at pH 4–5, whereas the optimum for protein A binding is at pH 8.0.

All four human IgG subclasses interact with protein G with high affinity ( Table 22 ). Protein G binds to the Fc as well as to the Fab portions. However, the interactions between protein G and Fab are much weaker than the interaction with Fc. Some immunoglobulins interact with protein G predominantly by Fab, such as mouse IgG1, while the others bind protein G primarily by Fc, such as human IgG ( Lian et al., 1994 ). Fab of human IgG2 is unable to react with protein G ( Perosa et al., 1997 ).

The interactions between protein G and the Fab and Fc regions were studied by NMR and by x-ray crystallography ( Derrick and Wrigley, 1994 Sauer-Eriksson et al., 1995 Lian et al., 1994 Kato et al., 1994 ). The Fc-binding site for B domain is located in the cleft between the CH2 and CH3 domains, similar to the binding site used by protein A ( Fig. 79 ). Three residues of CH2 and four residues of CH3 are involved in the interfacial interactions. Eight residues of the immunoglobulin-binding B domain, including five residues from α-helix, contribute to the interface. Even though the Fc binding sites for protein G and protein A overlap extensively, modes of interactions are quite different in both cases. The protein G-Fc complex mainly involves charged and polar contacts: protein G has 12 polar or charged interactions with Fc and no hydrophobic contacts. Protein A and Fc contacts are stabilized mainly through nonspecific hydrophobic interactions. Five hydrophobic contacts but only six polar interactions occur between protein A and Fc.

The B domain of protein G uses different, nonoverlapping portions for binding to Fab and to Fc. The second β-strand of the B domain binds to Fab, forming an antiparallel β-sheet with the seventh β-strand from the CH1 domain, including residues from Ser-209 to Lys-216 ( Fig. 80 ). In addition, there is a second minor contact between the α-helix of B domain with the first β-strand in CH1 (residues Pro-125 to Tyr-129). Both contacts form an extensive binding surface between the B domain and Fab, which is comparable in size with the antibody-antigen interaction sites. The contacting part of Fab is highly conserved between different subclasses and species and such binding strategy of protein G minimizes the effects of sequence variability on the immunoglobulin. No large changes in conformation were found in either protein on formation of the complex.

FIGURE 80 . Interaction of the protein G domain III with the CH1 domain of a mouse Fab fragment.

( Derrick and Wrigley, 1992 . Reprinted with permission.) Copyright © 1992