Protein G

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Immunoglobulin G-binding protein G ( Streptococcus sp. Group G)
Mass / length primary structure 384 amino acids
Identifier
Gene name (s) spg
External IDs

Protein G is a protein found in the cell wall of bacteria of the genus Streptococcus . Depending on the Streptococcus strain, it has a molecular mass of around 58 to 65 kilodaltons and has two or three homologous binding domains at the C terminus with high affinity for the Fc region of immunoglobulins of the IgG isotype . In addition, it binds to albumin proteins via three homologous domains N-terminal to the IgG binding region . At the N terminus , protein G has another binding region (region E) for human alpha-2 globulin in the native conformation, also known as the s-form.

use

Protein G, like Protein A or Protein A / G , is used in biochemistry for protein purification and the measurement of antibodies. Recombinantly produced protein G is used in immunological research for the purification of IgG antibodies by means of affinity chromatography and for the detection of IgG in immunassays . It is also used, coupled with Sepharose , in immunoprecipitation . In order to increase the specificity for these applications, in contrast to the natural form, recombinant protein G has no binding sites for albumin. The molecular mass of the recombinant form is 30 to 35 kilodaltons.

Columns filled with a gel matrix are used for cleaning, in which the protein G is firmly bound to the gel. The solution to be purified, for example culture supernatant from hybridoma cultures, is passed through the column by means of gravity or pressure. The immunoglobulins bind to the protein G in the column and, in contrast to the rest of the solution, are retained. After the column has been washed, the immunoglobulins are dissolved from protein G by a buffer with a pH value in the acidic range (2.0 to 2.8).

Protein G is used in immunoassays to detect immunoglobulins. Most often, protein G is labeled with enzymes , with biotin or with fluorescent dyes. In this application it is an alternative to anti-IgG antibodies. In addition, microtiter plates coated with protein G are also used for certain immunoassay applications , since in this way an orderly alignment of the antibodies on the plate is achieved.

Since the three-dimensional structure of protein G was elucidated in detail by means of nuclear magnetic resonance spectroscopy and protein crystallography, protein G is used, in addition to the aforementioned applications in immunology, as a model substance for the development of new methods in the structure elucidation of proteins.

Affinity for immunoglobulins

The binding strength for the subclasses IgG1, IgG2 and IgG4 of human IgG is comparable to that of protein A , in contrast to this, however, protein G also binds to IgG3. For mouse IgG antibodies , the binding for IgG2a and IgG2b is comparable to protein A and for IgG1 and IgG3 stronger for protein G. Rat immunoglobulins of all subclasses with the exception of IgG3 are more strongly bound by protein G, since protein A with the exception of IgG3 has no appreciable affinity for rat immunoglobulins. For rat IgG3, the binding strength of the two proteins is comparable.

IgG from guinea pigs and rabbits is less bound by protein G than by protein A; the binding strength for immunoglobulins from pigs and monkeys is comparable. For immunoglobulins from hamsters , horses , cows , sheep and goats , the affinity of protein G is stronger than that of protein A.

Protein G has no notable affinity for immunoglobulins of the IgA , IgD , IgE , IgM or IgY classes .

literature