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.