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

An unconventional strategy for purifying recombinant SARS-CoV-2 spike protein

The soluble domain of the trimeric SARS-CoV-2 spike protein is a promising candidate for a COVID-19 vaccine. Purification of this protein from mammalian cell culture supernatant using conventional resin-based chromatography is challenging as its large size (∼550 kDa) restricts its access and mobility within the pores of the resin particles. This reduces binding capacity and process robustness very significantly as extremely low flow rates need to be used during purification. Convection-based ion-exchange membrane chromatography has been found to be suitable in this respect. However, the high ionic strength of mammalian cell culture supernatant makes it difficult to bind this protein on charged membranes without dilution with a suitable buffer. An unconventional strategy involving size-exclusion chromatography as the first step, followed by cation exchange membrane chromatography as the second step is proposed in this paper. In the size exclusion chromatography step, the spike protein is excluded from the pores and can therefore be isolated in the void volume fraction. This step removes small molecule impurities and also serves as a desalting and buffer exchange step, making the partially purified material suitable for the cation exchange membrane chromatography step. The proposed process is variant-independent, fast and scalable and addresses some of the challenges associated with the currently used purification methods.

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.