Expression of SARS-CoV-2 surface glycoprotein fragment 319-640 in E. coli, and its refolding and purification

Diverse approaches to express recombinant spike protein: A comprehensive review

The spike protein of the Severe Acute Respiratory Syndrome Coronavirus-2 (SARS-CoV-2) is responsible for infecting host cells. It has two segments, S1 and S2. The S1 segment has a receptor-binding domain (RBD) that attaches to the host receptor angiotensin-converting enzyme 2 (ACE2). The S2 segment helps in the fusion of the viral cell membrane by creating a six-helical bundle through the two-heptad repeat domain. To develop effective vaccines and therapeutics against COVID-19, it is critical to express and purify the SARS-CoV-2 Spike protein. Extensive studies have been conducted on expression of a complete recombinant spike protein or its fragments. This review provides an in-depth analysis of the different expression systems employed for spike protein expression, along with their advantages and disadvantages.

Amyloidogenesis of SARS-CoV-2 delta plus and omicron variants receptor-binding domain (RBD): impact of SUMO fusion tag

PURPOSE

The RBD of SARS-CoV-2 mediates viral entry into host cells by binding to the host receptor ACE2. SARS-CoV-2 infection is linked to various health issues resembling amyloid-related problems, persuading us to investigate the amyloidogenicity of the SARS-CoV-2 spike RBD.

METHODS

The FoldAmyloid program was used to assess the amyloidogenic propensities in the RBD of Delta Plus and RBD of the Omicron variant, with and without the SUMO tag. After the expression of RBDs, purification, and dialysis steps were performed, subsequently the ThT assay, FTIR, and TEM were employed to check the RBD ability to form fibrils.

RESULTS

The ThT assay, TEM, and FTIR revealed the ability of RBD to self-assemble into β-sheet-rich aggregates (48.4% β-sheet content). Additionally, the presence of the SUMO tag reduced the formation of RBD amyloid-like fibrils. The amyloidogenic potential of Omicron RBD was higher than Delta Plus, according to both in silico and experimental analyses.

CONCLUSIONS

The SARS-CoV-2 RBD can assemble itself by forming aggregates containing amyloid-like fibrils and the presence of a SUMO tag can significantly decrease the formation of RBD amyloid-like fibrils. In silico analysis suggested that variation in the ThT fluorescence intensity of amyloid accumulations in the two SARS-CoV-2 strains arises from specific mutations in their RBD regions.

Expression and purification of the receptor-binding domain of SARS-CoV-2 spike protein in mammalian cells for immunological assays

The receptor-binding domain (RBD) of the spike glycoprotein of SARS-CoV-2 virus mediates the interaction with the host cell and is required for virus internalization. It is, therefore, the primary target of neutralizing antibodies. The receptor-binding domain soon became the major target for COVID-19 research and the development of diagnostic tools and new-generation vaccines. Here, we provide a detailed protocol for high-yield expression and one-step affinity purification of recombinant RBD from transiently transfected Expi293F cells. Expi293F mammalian cells can be grown to extremely high densities in a specially formulated serum-free medium in suspension cultures, which makes them an excellent tool for secreted protein production. The highly purified RBD is glycosylated, structurally intact, and forms homomeric complexes. With this quick and easy method, we are able to produce large quantities of RBD (80 mg·L-1 culture) that we have successfully used in immunological assays to examine antibody titers and seroconversion after mRNA-based vaccination of mice.

Oral administration of a recombinant modified RBD antigen of SARS-CoV-2 as a possible immunostimulant for the care of COVID-19

BACKGROUND

Developing effective vaccines against SARS-CoV-2 that consider manufacturing limitations, equitable access, and acceptance is necessary for developing platforms to produce antigens that can be efficiently presented for generating neutralizing antibodies and as a model for new vaccines.

RESULTS

This work presents the development of an applicable technology through the oral administration of the SARS-CoV-2 RBD antigen fused with a peptide to improve its antigenic presentation. We focused on the development and production of the recombinant receptor binding domain (RBD) produced in E. coli modified with the addition of amino acids extension designed to improve antigen presentation. The production was carried out in shake flask and bioreactor cultures, obtaining around 200 mg/L of the antigen. The peptide-fused RBD and peptide-free RBD proteins were characterized and compared using SDS-PAGE gel, high-performance chromatography, and circular dichroism. The peptide-fused RBD was formulated in an oil-in-water emulsion for oral mice immunization. The peptide-fused RBD, compared to RBD, induced robust IgG production in mice, capable of recognizing the recombinant RBD in Enzyme-linked immunosorbent assays. In addition, the peptide-fused RBD generated neutralizing antibodies in the sera of the dosed mice. The formulation showed no reactive episodes and no changes in temperature or vomiting.

CONCLUSIONS

Our study demonstrated the effectiveness of the designed peptide added to the RBD to improve antigen immunostimulation by oral administration.

Large-Scale Purification and Characterization of Recombinant Receptor-Binding Domain (RBD) of SARS-CoV-2 Spike Protein Expressed in Yeast

SARS-CoV-2 spike protein is an essential component of numerous protein-based vaccines for COVID-19. The receptor-binding domain of this spike protein is a promising antigen with ease of expression in microbial hosts and scalability at comparatively low production costs. This study describes the production, purification, and characterization of RBD of SARS-CoV-2 protein, which is currently in clinical trials, from a commercialization perspective. The protein was expressed in Pichia pastoris in a large-scale bioreactor of 1200 L capacity. Protein capture and purification are conducted through mixed-mode chromatography followed by hydrophobic interaction chromatography. This two-step purification process produced RBD with an overall productivity of ~21 mg/L at >99% purity. The protein's primary, secondary, and tertiary structures were also verified using LCMS-based peptide mapping, circular dichroism, and fluorescence spectroscopy, respectively. The glycoprotein was further characterized for quality attributes such as glycosylation, molecular weight, purity, di-sulfide bonding, etc. Through structural analysis, it was confirmed that the product maintained a consistent quality across different batches during the large-scale production process. The binding capacity of RBD of spike protein was also assessed using human angiotensin-converting enzyme 2 receptor. A low binding constant range of KD values, ranging between 3.63 × 10-8 to 6.67 × 10-8, demonstrated a high affinity for the ACE2 receptor, revealing this protein as a promising candidate to prevent the entry of COVID-19 virus.

Recombinant Salmonella enterica OmpX protein expression and its potential for serologically diagnosing Salmonella abortion in mares

Background and Aim

Salmonella abortion in mares is caused by Salmonella enterica subspecies enterica serovar abortus equi infection and is characterized by premature (abortion) or non-viable fetus birth. Although all horses are susceptible to infection, the condition is more often clinically manifested in pregnant mares, with most abortions recorded in young females. In addition, nonspecific clinical disease signs and poorly sensitive and effective bacteriological diagnostic methods hinder rapid and reliable infection diagnoses. Immunochemical methods such as enzyme-linked immunosorbent assay (ELISA) and immunochromatography assays can facilitate effective and rapid diagnoses. However, they require highly specific and active antigens and antibodies. This study aimed to generate a recombinant S. enterica outer membrane protein X (OmpX) and evaluate its suitability for serological diagnosis of Salmonella abortion in mares.

Materials and Methods

Outer membrane protein X from the S. enterica antigen was synthesized de novo and expressed in Escherichia coli using the pET28 vector. Transformed E. coli cells were cultured under different conditions to detect recombinant OmpX (rOmpX) expression, and rOmpX purification and refolding were both conducted using metal affinity chromatography. Refolded and purified rOmpX was characterized by western blotting, liquid chromatography with tandem mass spectrometry, and ELISA.

Results

After optimized rOmpX expression, a 23 kDa molecular weight protein was identified. Amino acid sequence analysis using Mascot program suggested that these peptides were the OmpX protein from S. enterica. High specificity and diagnostic efficiency were recorded when rOmpX was used in ELISA against 89 serum samples from aborted and contact mares.

Conclusion

Recombinant outer membrane protein, in comparison to the O antigen, demonstrated better diagnostic characteristics against sera from mares who aborted and contact horses.

The main protease 3CLpro of the SARS-CoV-2 virus: how to turn an enemy into a helper

Despite the long history of use and the knowledge of the genetics and biochemistry of E. coli, problems are still possible in obtaining a soluble form of recombinant proteins in this system. Although, soluble protein can be obtained both in the cytoplasm and in the periplasm of the bacterial cell. The latter is a priority strategy for obtaining soluble proteins. The fusion protein technology followed by detachment of the fusion protein with proteases is used to transfer the target protein into the periplasmic space of E. coli. We have continued for the first time to use the main viral protease 3CL of the SARS-CoV-2 virus for this purpose. We obtained a recombinant 3CL protease and studied its complex catalytic properties. The authenticity of the resulting recombinant enzyme, were confirmed by specific activity analysis and activity suppression by the known low-molecular-weight inhibitors. The catalytic efficiency of 3CL (0.17 ± 0.02 µM-1-s-1) was shown to be one order of magnitude higher than that of the widely used tobacco etch virus protease (0.013 ± 0.003 µM-1-s-1). The application of the 3CL gene in genetically engineered constructs provided efficient specific proteolysis of fusion proteins, which we demonstrated using the receptor-binding domain of SARS-CoV-2 spike protein and GST fusion protein. The solubility and immunochemical properties of RBD were preserved. It is very important that in work we have shown that 3CL protease works effectively directly in E. coli cells when co-expressed with the target fusion protein, as well as when expressed as part of a chimeric protein containing the target protein, fusion partner, and 3CL itself. The results obtained in the work allow expanding the repertoire of specific proteases for researchers and biotechnologists.

A Novel Fusion Protein System for the Production of Nanobodies and the SARS-CoV-2 Spike RBD in a Bacterial System

Antibodies are key proteins of the immune system, and they are widely used for both research and theragnostic applications. Among them, camelid immunoglobulins (IgG) differ from the canonical human IgG molecules, as their light chains are completely missing; thus, they have only variable domains on their heavy chains (VHHs). A single VHH domain, often called a nanobody, has favorable structural, biophysical, and functional features compared to canonical antibodies. Therefore, robust and efficient production protocols relying on recombinant technologies are in high demand. Here, by utilizing ecotin, an Escherichia coli protein, as a fusion partner, we present a bacterial expression system that allows an easy, fast, and cost-effective way to prepare nanobodies. Ecotin was used here as a periplasmic translocator and a passive refolding chaperone, which allowed us to reach high-yield production of nanobodies. We also present a new, easily applicable prokaryotic expression and purification method of the receptor-binding domain (RBD) of the SARS-CoV-2 S protein for interaction assays. We demonstrate using ECD spectroscopy that the bacterially produced RBD is well-folded. The bacterially produced nanobody was shown to bind strongly to the recombinant RBD, with a K_d of 10 nM. The simple methods presented here could facilitate rapid interaction measurements in the event of the appearance of additional SARS-CoV-2 variants.

Utilization of Receptor-Binding Domain of SARS-CoV-2 Spike Protein Expressed in Escherichia coli for the Development of Neutralizing Antibody Assay

The ongoing COVID-19 pandemic has resulted from widespread infection by the SARS-CoV-2 virus. As new variants of concern continue to emerge, understanding the correlation between the level of neutralizing antibodies (NAb) and clinical protection from SAR-CoV-2 infection could be critical in planning the next steps in COVID-19 vaccine programs. This study explored the potential usefulness of E. coli as an alternative expression system that can be used to produce a SARS-CoV-2 receptor-binding domain (RBD) for the development of an affordable and flexible NAb detection assay. We expressed the RBD of Beta, Delta, and Omicron variants in the E. coli BL21(DE3) strain and purified them from whole bacterial cells using His-tag-mediated affinity chromatography and urea-assisted refolding. Next, we conducted a head-to-head comparison of the binding activity of our E. coli-produced RBD (E-RBD) with commercial HEK293-produced RBD (H-RBD). The results of a direct binding assay revealed E-RBD and H-RBD binding with ACE2-hFc in similar signal strengths. Furthermore, in the NAb detection assay, % inhibition obtained from both E-RBD and H-RBD demonstrated comparable results in all the investigated assays, suggesting that non-glycosylated RBD produced from E. coli may offer a cost-effective alternative to the use of more expensive glycosylated RBD produced from human cells in the development of such an assay.

Designing and Expression of Recombinant Chimeric Spike Protein from SARS-CoV-2 in Escherichia coli and Its Immunogenicity Assessment

Since December 2019, the world has been grappling with an ongoing global COVID-19 pandemic. Various virus variants have emerged over the past two years, each posing a greater threat than its predecessors. The recent appearance of the omicron variant (B.1.1.529) has raised significant alarm within the field of epidemiology due to its highly contagious nature and rapid transmission rate. The omicron variant possessed mutations in the key receptor-binding domain (RBD) region, the S region, and these modifications have shown a notable impact on the strain's susceptibility to neutralizing antibodies. Developing safe and efficient vaccines to prevent a future severe acute respiratory outbreak of coronavirus syndrome 2 (SARS-CoV-2) is significant. Viral surface spike proteins are ideal targets for vaccines. This study aimed to find a multi-subunit chimeric vaccine. After conducting bioinformatics analysis, the recombinant spike (RS) protein of SARS-CoV-2 was deliberately designed and subsequently produced using E. coli expression systems. The immunogenicity of RS and neutralizing antibody responses were evaluated on immunized BALB/c mice. There was a significant difference in antibody titers between RS-immunized mice and control groups. The endpoint of the serum antibody titer of mice immunized with our chimeric protein was 2.5 times higher than that of the negative control. The chimeric construct could present multiple antigens simultaneously, influentially affecting immunization. Sera from mice vaccinated by RS could recognize the SARS-CoV-2 virus and neutralize antibodies. Our chimeric peptide could bind to antibodies in the serum of patients infected with different serotypes of the SARS-CoV-2 virus, such as alpha, delta, and omicron variants. The results indicated that the RS protein would be a potential novel antigenic candidate for subunit vaccine development and could be used as a useful alternative to generate diagnostic serological tests for SARS-CoV-2 infection.

Unglycosylated Soluble SARS-CoV-2 Receptor Binding Domain (RBD) Produced in E. coli Combined with the Army Liposomal Formulation Containing QS21 (ALFQ) Elicits Neutralizing Antibodies against Mismatched Variants

The emergence of novel potentially pandemic pathogens necessitates the rapid manufacture and deployment of effective, stable, and locally manufacturable vaccines on a global scale. In this study, the ability of the Escherichia coli expression system to produce the receptor binding domain (RBD) of the SARS-CoV-2 spike protein was evaluated. The RBD of the original Wuhan-Hu1 variant and of the Alpha and Beta variants of concern (VoC) were expressed in E. coli, and their biochemical and immunological profiles were compared to RBD produced in mammalian cells. The E. coli-produced RBD variants recapitulated the structural character of mammalian-expressed RBD and bound to human angiotensin converting enzyme (ACE2) receptor and a panel of neutralizing SARS-CoV-2 monoclonal antibodies. A pilot vaccination in mice with bacterial RBDs formulated with a novel liposomal adjuvant, Army Liposomal Formulation containing QS21 (ALFQ), induced polyclonal antibodies that inhibited RBD association to ACE2 in vitro and potently neutralized homologous and heterologous SARS-CoV-2 pseudoviruses. Although all vaccines induced neutralization of the non-vaccine Delta variant, only the Beta RBD vaccine produced in E. coli and mammalian cells effectively neutralized the Omicron BA.1 pseudovirus. These outcomes warrant further exploration of E. coli as an expression platform for non-glycosylated, soluble immunogens for future rapid response to emerging pandemic pathogens.

Functional Expression of the Recombinant Spike Receptor Binding Domain of SARS-CoV-2 Omicron in the Periplasm of Escherichia coli

A new severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) variant known as Omicron has caused a rapid increase in recent global patients with coronavirus infectious disease 2019 (COVID-19). To overcome the COVID-19 Omicron variant, production of a recombinant spike receptor binding domain (RBD) is vital for developing a subunit vaccine or a neutralizing antibody. Although bacterial expression has many advantages in the production of recombinant proteins, the spike RBD expressed in a bacterial system experiences a folding problem related to disulfide bond formation. In this study, the soluble Omicron RBD was obtained by a disulfide isomerase-assisted periplasmic expression system in Escherichia coli. The Omicron RBD purified from E. coli was very well recognized by anti-SARS-CoV-2 antibodies, sotrovimab (S309), and CR3022, which were previously reported to bind to various SARS-CoV-2 variants. In addition, the kinetic parameters of the purified Omicron RBD upon binding to the human angiotensin-converting enzyme 2 (ACE2) were similar to those of the Omicron RBD produced in the mammalian expression system. These results suggest that an E. coli expression system would be suitable to produce functional and correctly folded spike RBDs of the next emerging SARS-CoV-2 variants quickly and inexpensively.

Production of the SARS-CoV-2 Spike protein and its Receptor Binding Domain in plant cell suspension cultures

The COVID-19 pandemic, caused by the worldwide spread of SARS-CoV-2, has prompted the scientific community to rapidly develop efficient and specific diagnostics and therapeutics. A number of avenues have been explored, including the manufacture of COVID-related proteins to be used as reagents for diagnostics or treatment. The production of RBD and Spike proteins was previously achieved in eukaryotic cells, mainly mammalian cell cultures, while the production in microbial systems has been unsuccessful until now. Here we report the effective production of SARS-CoV-2 proteins in two plant model systems. We established transgenic tobacco BY-2 and Medicago truncatula A17 cell suspension cultures stably producing the full-length Spike and RBD recombinant proteins. For both proteins, various glycoforms were obtained, with higher yields in Medicago cultures than BY-2. This work highlights that RBD and Spike can be secreted into the culture medium, which will impact subsequent purification and downstream processing costs. Analysis of the culture media indicated the presence of the high molecular weight Spike protein of SARS-CoV-2. Although the production yields still need improvement to compete with mammalian systems, this is the first report showing that plant cell suspension cultures are able to produce the high molecular weight Spike protein. This finding strengthens the potential of plant cell cultures as production platforms for large complex proteins.

High-yield production in Escherichia coli and convenient purification of a candidate vaccine against SARS-CoV-2

OBJECTIVES

The aim of the present work was to identify a time-saving, effective, and low-cost strategy to produce in Escherichia coli a protein chimera representing a fusion anti-SARS-CoV-2 candidate vaccine, consisting of immunogenic and antigenic moieties.

RESULTS

We overexpressed in E. coli BL21(DE3) a synthetic gene coding for CRM197-RBD, and the target protein was detected in inclusion bodies. CRM197-RBD was solubilized with 1 % (w/v) of the anionic detergent N-lauroylsarcosine (sarkosyl), the removal of which from the protein solution was conveniently accomplished with Amberlite XAD-4. The detergent-free CRM197-RBD was then separated from contaminating DNA using polyethylenimine (PEI), and finally purified from PEI by salting out with ammonium sulfate. Structural (CD spectrum) and functional (DNase activity) assays revealed that the CRM197-RBD chimera featured a native and active conformation. Remarkably, we determined a yield of purified CRM197-RBD equal to 23 mg per litre of culture.

CONCLUSIONS

To produce CRM197-RBD, we devised the use of sarkosyl as an alternative to urea to solubilize the target protein from E. coli inclusion bodies, and the easy removal of sarkosyl by means of Amberlite XAD-4.

Production and characterization of chimeric SARS-CoV-2 antigens based on the capsid protein of cowpea chlorotic mottle virus

The COVID-19 pandemic has highlighted the need for new vaccine platforms to rapidly develop solutions against emerging pathogens. In particular, some plant viruses offer several advantages for developing subunit vaccines, such as high expression rates in E. coli, high immunogenicity and safety, and absence of pre-immunity that could interfere with the vaccine's efficacy. Cowpea chlorotic mottle virus (CCMV) is a model system that has been extensively characterized, with key advantages for its use as an epitope carrier. In the present study, three relevant epitopes from the SARS-CoV-2 Spike protein were genetically inserted into the CCMV CP and expressed in E. coli cultures, resulting in the CCMV1, CCMV2, and CCMV3 chimeras. The recombinant CP mutants were purified from the formed inclusion bodies and refolded, and their immunogenicity as a subunit vaccine was assessed in BALB/c mice. The three mutants are immunogenic as they induce high IgG antibody titers that recognize the recombinant full-length S protein. This study supports the application of CCMV CP as an attractive carrier for the clinical evaluation of vaccine candidates against SARS-CoV-2. Furthermore, it suggests that VLPs assembled from these chimeric proteins could result in antigens with better immunogenicity.

A magnetic bead immunoassay to detect high affinity human IgG reactive to SARS-CoV-2 Spike S1 RBD produced in Escherichia coli

Immunological assays to detect SARS-CoV-2 Spike Receptor Binding Domain (RBD) antigen seroconversion in humans are important tools to monitor the levels of protecting antibodies in the population in response to infection and/or immunization. Here we describe a simple, low cost, and high throughput Ni²⁺ magnetic bead immunoassay to detect human IgG reactive to Spike S1 RBD Receptor Binding Domain produced in Escherichia coli. A 6xHis-tagged Spike S1 RBD was expressed in E. coli and purified by affinity chromatography. The protein was mobilized on the surface of Ni²⁺ magnetic beads and used to investigate the presence of reactive IgG in the serum obtained from pre-pandemic and COVID-19 confirmed cases. The method was validated with a cohort of 290 samples and an area under the receiver operating characteristic curve of 0.94 was obtained. The method was operated with > 82% sensitivity at 98% specificity and was also able to track human IgG raised in response to vaccination with Comirnaty at > 85% sensitivity. The IgG signal obtained with the described method was well-correlated with the signal obtained when pre fusion Spike produced in HEK cell lines was used as antigen. This novel low-cost and high throughput immunoassay may act as an important tool to investigate protecting IgG antibodies against SARS-CoV-2 in the human population.

Escherichia coli recombinant expression of SARS-CoV-2 protein fragments

We have developed a method for the inexpensive, high-level expression of antigenic protein fragments of SARS-CoV-2 proteins in Escherichia coli. Our approach uses the thermophilic family 9 carbohydrate-binding module (CBM9) as an N-terminal carrier protein and affinity tag. The CBM9 module was joined to SARS-CoV-2 protein fragments via a flexible proline–threonine linker, which proved to be resistant to E. coli proteases. Two CBM9-spike protein fragment fusion proteins and one CBM9-nucleocapsid fragment fusion protein largely resisted protease degradation, while most of the CBM9 fusion proteins were degraded at some site in the SARS-CoV-2 protein fragment. All of the fusion proteins were highly expressed in E. coli and the CBM9-ID-H1 fusion protein was shown to yield 122 mg/L of purified product. Three purified CBM9-SARS-CoV-2 fusion proteins were tested and found to bind antibodies directed to the appropriate SARS-CoV-2 antigenic regions. The largest intact CBM9 fusion protein, CBM9-ID-H1, incorporates spike protein amino acids 540–588, which is a conserved region overlapping and C-terminal to the receptor binding domain that is widely recognized by human convalescent sera and contains a putative protective epitope.

Expression, purification and characterization of SARS-CoV-2 spike RBD in ExpiCHO cells

Reliable diagnosis is critical to identify infections of SARS-CoV-2 as well as to evaluate the immune response to virus and vaccines. Consequently, it becomes crucial the isolation of sensitive antibodies to use as immunocapture elements of diagnostic tools. The final bottleneck to achieve these results is the availability of enough antigen of good quality. We have established a robust pipeline for the production of recombinant, functional SARS-CoV-2 Spike receptor binding domain (RBD) at high yield and low cost in culture flasks.

RBD was expressed in transiently transfected ExpiCHO cells at 32 °C and 5% CO₂ and purified up to 40 mg/L. The progressive protein accumulation in the culture medium was monitored with an immunobinding assay in order to identify the optimal collection time. Successively, a two-step chromatographic protocol enabled its selective purification in the monomeric state. RBD quality assessment was positively evaluated by SDS-PAGE, Western Blotting and Mass Spectrometry, while Bio-Layer Interferometry, flow cytometer and ELISA tests confirmed its functionality. This effective protocol for the RBD production in transient eukaryotic system can be immediately extended to the production of RBD mutants.

Designed SARS‐CoV‐2 receptor binding domain variants form stable monomers

The receptor binding domain (RBD) of the SARS‐CoV‐2 spike (S)‐protein is a prime target of virus‐neutralizing antibodies present in convalescent sera of COVID‐19 patients and thus is considered a key antigen for immunosurveillance studies and vaccine development. Although recombinant expression of RBD has been achieved in several eukaryotic systems, mammalian cells have proven particularly useful. The authors aimed to optimize RBD produced in HEK293‐6E cells towards a stable homogeneous preparation and addressed its O‐glycosylation as well as the unpaired cysteine residue 538 in the widely used RBD (319‐541) sequence. The authors found that an intact O‐glycosylation site at T323 is highly relevant for the expression and maintenance of RBD as a monomer. Furthermore, it was shown that deletion or substitution of the unpaired cysteine residue C538 reduces the intrinsic propensity of RBD to form oligomeric aggregates, concomitant with an increased yield of the monomeric form of the protein. Bead‐based and enzyme‐linked immunosorbent assays utilizing these optimized RBD variants displayed excellent performance with respect to the specific detection of even low levels of SARS‐CoV‐2 antibodies in convalescent sera. Hence, these RBD variants could be instrumental for the further development of serological SARS‐CoV‐2 tests and inform the design of RBD‐based vaccine candidates.

Purification and characterization of the receptor-binding domain of SARS-CoV-2 spike protein from Escherichia coli

SARS-CoV-2 is responsible for a disruptive worldwide viral pandemic, and renders a severe respiratory disease known as COVID-19. Spike protein of SARS-CoV-2 mediates viral entry into host cells by binding ACE2 through the receptor-binding domain (RBD). RBD is an important target for development of virus inhibitors, neutralizing antibodies, and vaccines. RBD expressed in mammalian cells suffers from low expression yield and high cost. E. coli is a popular host for protein expression, which has the advantage of easy scalability with low cost. However, RBD expressed by E. coli (RBD-1) lacks the glycosylation, and its antigenic epitopes may not be sufficiently exposed. In the present study, RBD-1 was expressed by E. coli and purified by a Ni Sepharose Fast Flow column. RBD-1 was structurally characterized and compared with RBD expressed by the HEK293 cells (RBD-2). The secondary structure and tertiary structure of RBD-1 were largely maintained without glycosylation. In particular, the major β-sheet content of RBD-1 was almost unaltered. RBD-1 could strongly bind ACE2 with a dissociation constant (KD) of 2.98 × 10-8 M. Thus, RBD-1 was expected to apply in the vaccine development, screening drugs and virus test kit.