Expression and characterization of a tripartite fusion protein consisting of chimeric IgG-binding receptors and β-galactosidase

TRIM21 chimeric protein as a new molecular tool for multispecies IgG detection

Background

The production of monoclonal antibodies for immunoglobulin detection is not cost-effective, while polyclonal antibody production depends on laboratory animals, raising concerns on animal welfare. The widespread use of immunoglobulins in the pharmaceutical industry and the increasing number and variety of new antibodies entering the market require new detection and purification strategies. The Tripartite motif-containing protein 21 is a soluble intracellular immunoglobulin G receptor that binds to the constant region of immunoglobulin G from various species with high affinity. We hypothesized that using this protein as an antibody-binding module to create immunoglobulin detection probes will improve the portfolio of antibody affinity ligands for diagnostic or therapeutic purposes.

Results

We created a chimeric protein containing a mutated form of the C-terminal domain of mouse Tripartite motif-containing protein 21 linked to streptavidin to detect immunoglobulin G from various species of mammals. The protein is produced by heterologous expression and consists of an improved molecular tool, expanding the portfolio of antibody-affinity ligands for immunoassays. We also demonstrate that this affinity ligand may be used for purification purposes since imidazole elution of antibodies can be achieved instead of acidic elution conditions of current antibody purification methods.

Conclusion

Data reported here provides an additional and superior alternative to the use of secondary antibodies, expanding the portfolio of antibodies affinity ligands for detection and purification purposes.

Supplementary Information

The online version contains supplementary material available at 10.1186/s43141-022-00396-3.

IgG-detection devices for the Tus-Ter-lock immuno-PCR diagnostic platform

The number of new Immuno-PCR technologies and applications is steadily growing as a result of a general need for more sensitive immunoassays for early detection of diseases. Although Immuno-PCR has been demonstrated to be superior to its immunoassay counterpart, it is still regarded as a challenging technology due to various problems arising from its increased detection power, such as high background noise as well as substantial batch-to-batch reproducibility issues. Current efforts have intensified to produce homogeneous universal protein-DNA conjugates to simplify this technology and render it more robust. We have recently developed a new quantitative Immuno-PCR (qIPCR) technology using the Tus-Ter-lock (TT-lock) interaction to produce homogeneous protein-DNA conjugates that can detect very small numbers of disease-related antibodies. We now report the further development of the TT-lock Immuno-PCR platform for the quasi universal quantitative detection of antigens and mammalian IgG. For this, Tus was fused to various IgG-binding proteins--i.e. protein G, protein L and their LG chimera--and self-assembled to the TT-lock-T template. These detection devices were then evaluated and applied in various direct and indirect Immuno-PCR formats. The direct TT-lock qIPCR could detect goat anti-GFP IgG at concentrations as low as 0.3 pM and total human IgG in serum samples with great sensitivity. Further indirect TT-lock qIPCR systems were developed that could detect 1 pM of GFP and 10 pM of measles nucleoprotein. In all cases, the superiority of the TT-lock Immuno-PCR was demonstrated in terms of sensitivity over an analogous Protein G-Peroxidase ELISA.

Use of genetically prepared enzyme conjugates in enzyme immunoassay

Enzyme immunoassay, using enzymes crosslinked to either antibodies or antigens, has proved a valuable immunological tool for many years. Recently, gene fusion techniques have been used to prepare these enzyme conjugates. This method may be especially advantageous in cases where (1) the antigen is difficult and costly to obtain in large quantities or (2) when the activity of the marker enzyme or the affinity of the antibody or antigen is severely reduced or even destroyed by use of conventional linking methods such as chemical crosslinking. This article uses specific examples to illustrate the potential of gene fusion as a conjugation method.

Partitioning of β-galactosidase fusion proteins in PEG/potassium phosphate aqueous two-phase systems

Four different beta-galactosidase fusion proteins have been partitioned in poly(ethylene glycol) (PEG) 4000/potassium phosphate aqueous two-phase systems. The partition coefficients (K) of staphylococcal protein A-beta-galactosidase (SpA beta gal) (K = 3.5) and staphylococcal protein A-streptococcal protein G-beta-galactosidase (AG beta gal) (K = 2.8) were compared with the partition coefficients of their constituent molecules, beta-galactosidase, SpA, and protein AG. It was found that by fusing beta-galactosidase to the smaller proteins SpA and protein AG, their partition coefficients were increased four to five times. Experimental data were fitted into, and found to agree with, the Albertsson partition model of interacting molecules. The compatibility with PEG and potassium phosphate of beta-galactosidase, SpA, and two different versions of the SpA beta gal protein, displayed as precipitation curves, showed a relationship to the protein partition coefficients in PEG/potassium phosphate systems. High solubility in one phase component was accompanied by preferential partitioning to the phase rich in the same component in the PEG/potassium phosphate system. Also, a changed linker region in SpA beta gal resulted in a more soluble protein. This, together with the improved K values of the target proteins by fusion, shows that it is possible to use beta-galactosidase as an affinity handle.