The expression of soluble, full-length, recombinant human TSH receptor in a prokaryotic system

Expression and purification of recombinant G protein-coupled receptors: A review

Given their extensive role in cell signalling, GPCRs are significant drug targets; despite this, many of these receptors have limited or no available prophylaxis. Novel drug design and discovery significantly rely on structure determination, of which GPCRs are typically elusive. Progress has been made thus far to produce sufficient quantity and quality of protein for downstream analysis. As such, this review highlights the systems available for recombinant GPCR expression, with consideration of their advantages and disadvantages, as well as examples of receptors successfully expressed in these systems. Additionally, an overview is given on the use of detergents and the styrene maleic acid (SMA) co-polymer for membrane solubilisation, as well as purification techniques.

Expression, Purification and Characterization of the Human Cannabinoid 1 Receptor

The human cannabinoid 1 receptor (hCB1) is involved in numerous physiological processes and therefore provides a wide scope of potential therapeutic opportunities to treat maladies such as obesity, cardio-metabolic disorders, substance abuse, neuropathic pain, and multiple sclerosis. Structure-based drug design using the current knowledge of the hCB1 receptor binding site is limited and requires purified active protein. Heterologous expression and purification of functional hCB1 has been the bottleneck for ligand binding structural studies using biophysical methods such as mass spectrometry, x-ray crystallography and NMR. We constructed several plasmids for in-cell or in vitro Escherichia coli (E. coli) based expression of truncated and stabilized hCB1 receptor (hΔCB1 and hΔCB1_T4L) variants and evaluated their competency to bind the CP-55,940 ligand. MALDI-TOF MS analysis of in vitro expressed and purified hΔCB1_T4Lhis6 variants, following trypsin digestion, generated ~80% of the receptor sequence coverage. Our data demonstrate the feasibility of a cell-free expression system as a promising part of the strategy for the elucidation of ligand binding sites of the hCB1 receptor using a "Ligand Assisted Protein Structure" (LAPS) approach.

Development and application of an indirect ELISA for the detection of antibodies against encephalomyocarditis virus

Encephalomyocarditis virus (EMCV) can cause acute myocarditis in young pigs or reproductive failure in sows. It has been recognized worldwide as a pathogen infecting many species and causes substantial economic losses. In the present study, an indirect ELISA was developed for the detection of antibodies to EMCV. The VP1 gene of EMCV was amplified by reverse transcription-quantitative polymerase chain reaction and expressed in Escherichia coli with 49.3 kDa under the condition of isopropyl-β-d-thiogalactoside. Following this, the authors obtained the recombinant protein VP1 as a coating antigen. The antigen concentration and serum dilution were optimized using a checkerboard titration. Compared with viral neutralization tests, the sensitivity and specificity of the indirect ELISA was 95.7% and 92.9%, respectively. A total of 265 clinical swine serum samples from different pig farms in China were used to a serological survey. The seropositive rate of the serum samples was 81.9%. In conclusion, the developed indirect ELISA assay is sensitive and specific, which will be useful for large-scale serological survey in EMCV infection and monitoring antibodies titers against EMCV.

Functional expression of thyroid-stimulating hormone receptor on nano-sized bacterial magnetic particles in Magnetospirillum magneticum AMB-1

The measurement of autoantibodies to thyroid-stimulating hormone receptor (TSHR) is important for the diagnosis of autoimmune thyroid disease such as Graves' disease (GD). Although TSHR from porcine thyroid membrane is commonly used for the measurement of TSHR autoantibodies (TRAb), recombinant human TSHR (hTSHR) remains ideal in terms of stable supply and species identity. Here we set out to express recombinant hTSHR on the lipid-bilayer surface of magnetic nanoparticles from a magnetotactic bacterium, Magnetospirillum magneticum AMB-1. Using a tetracycline-inducible expression system, we successfully overexpressed functional hTSHR on bacterial magnetic particles (BacMPs) in AMB-1 via an anchor protein specific for BacMPs. The overexpressed hTSHR was membrane integrated and possessed both ligand and autoantibody binding activity. Our data suggest that hTSHR-displayed BacMPs have potential as novel tools for ligand-receptor interaction analysis or for TRAb immunoassay in GD patients.

Optimization of purification and refolding of the human chemokine receptor CXCR1 improves the stability of proteoliposomes for structure determination

The human chemokine receptor CXCR1 is a G-protein coupled receptor that has been successfully expressed in E. coli as inclusion bodies, and purified and refolded in multi-milligram quantities required for structural studies. Expression in E. coli enables selective and uniform isotopic labeling with (13)C and (15)N for NMR studies. Long-term chemical and conformational stability and oligomeric homogeneity of CXCR1 in phospholipid bilayers are crucial for structural studies under physiological conditions. Here we describe substantial refinements in our previously described purification and reconstitution procedures for CXCR1 in phospholipid bilayers. These refinements have led to the preparation of highly purified, completely monomeric, proteoliposome samples that are stable for months at 35°C while subject to the high power radiofrequency irradiations of solid-state NMR experiments. The principal changes from the previously described methods include: 1) ensure that CXCR1 is pure and homogeneously monomeric within the limits of detection (>98%); 2) monitor and control the pH at all times especially following the addition of TCEP, which serves as a reducing agent but also changes the pH; 3) slowly refold CXCR1 with the complete removal of all traces of SDS using a KCl precipitation/dialysis method; and 4) ensure that the molar ratio between the CXCR1 and the phospholipids does not change during refolding and detergent removal. NMR samples prepared with these protocols yield reproducible results over a period of many months at 35°C. This purification and refolding protocol is likely to be applicable with minimal changes to other GPCRs as well as other membrane proteins.

Strategies for the cloning and expression of membrane proteins

Despite the determination of thousands of high-resolution structures of soluble proteins, many features of integral membrane proteins render them difficult targets for the structural biologist. Among these, the most important challenge is in expressing sufficient quantities of active protein to support downstream purification and structure determination efforts. Over 190 unique membrane protein structures have now been solved, and noticeable trends in successful expression strategies are beginning to emerge. A number of groups have also explored high-throughput (HTP) methods for membrane protein expression, with varying degrees of success. Here we review the current state of expressing membrane proteins for functional and structural studies. We first survey successful methods that have already yielded levels of membrane protein expression sufficient for structure determination. HTP methods are also examined since these aim to explore large numbers of targets and can predict reasonable starting points for many membrane proteins. Since HTP techniques may fail, particularly for certain classes of eukaryotic targets, detailed strategies for the expression of two prominent classes of eukaryotic protein families, G-protein-coupled receptors and ion channels, are also summarized.

cDNA clone, prokaryotic expression and purification of human interleukin-13 receptor [alpha]2 chain

Despite advances in surgical technology and radiation therapy, the prognosis in the patients with malignant glioma remains poor. Recent studies show that interleukin-13 receptor [alpha]2 chain (IL-13Ra2), a brain tumor-associated receptor for IL-13, may play a role in immunotherapy for glioblastoma. We thus amplified human IL-13Ra2 gene from the human glioblastoma cell line using RT-PCR and cloned the target gene into the pET-28a, a prokaryotic expressing plasmid. After transformation, the recombinant plasmid expressed a soluble protein induced by IPTG. The purified recombinant protein was shown to be a single band on the SDS-PAGE with a predicated molecular weight of human IL-13Ra2 gene, suggesting that the recombinant protein of human IL-13Ra2 was successfully expressed. Recombinant IL-13Ra2 protein can be used as an anti-tumor vaccine, which may provide a promising new strategy for the treatment of brain malignant gliomas.

Efficient production of membrane-integrated and detergent-soluble G protein-coupled receptors in Escherichia coli

G protein-coupled receptors (GPCRs) are notoriously difficult to express, particularly in microbial systems. Using GPCR fusions with the green fluorescent protein (GFP), we conducted studies to identify bacterial host effector genes that result in a general and significant enhancement in the amount of membrane-integrated human GPCRs that can be produced in Escherichia coli. We show that coexpression of the membrane-bound AAA+ protease FtsH greatly enhances the expression yield of four different class I GPCRs, irrespective of the presence of GFP. Using this new expression system, we produced 0.5 and 2 mg/L of detergent-solubilized and purified full-length central cannabinoid receptor (CB1) and bradykinin receptor 2 (BR2) in shake flask cultures, respectively, two proteins that had previously eluded expression in microbial systems.

Purification and refolding of a novel cancer/testis antigen BJ-HCC-2 expressed in the inclusion bodies of Escherichia coli

BJ-HCC-2 is one of the cancer/testis antigens that may be the most promising targets for tumor immunotherapy. To investigate the expression of BJ-HCC-2 protein in tumor cells and its capacity to elicit CTL response, the recombinant protein of BJ-HCC-2 was expressed in the inclusion bodies in Escherichia coli. The inclusion bodies were solubilized effectively with 0.3% N-lauroyl sarcosine in alkaline buffer. Under this denatured form, the BJ-HCC-2 protein carrying 6x histidine tag was purified with Ni-NTA affinity chromatography in a single step with a purity of over 97%. The yield of the purified protein was about 78%. The purified recombinant protein was refolded in a simple way. The correct refolding of the recombinant protein was verified in the recovery of its secondary and tertiary structures as assessed by circular dichroism and fluorescence emission spectra. The recovery rate of refolded protein was 92.1%. The renatured protein displayed its immunoreactivity with the antibodies to BJ-HCC-2 protein by Western blotting. This method of protein purification and refolding is easy to manipulate and may be applicable to the hydrophobic proteins that are unable to be purified by other methods.