During 2020, the COVID-19 pandemic affected almost 10 individuals. Quite a number of vaccines against COVID-19 were therefore developed, and a few recently received authorization for emergency use. Overall, these vaccines target specific viral proteins by antibodies whose synthesis is directly elicited or indirectly triggered by nucleic acids coding for the desired targets. Among these targets, the receptor binding domain (RBD) of COVID-19 spike protein (SP) does frequently occur in the repertoire of candidate vaccines. However, the immunogenicity of RBD per se is limited by its low molecular mass, and by a structural rearrangement of full-length SP accompanied by the detachment of RBD. Here we show that the RBD of COVID-19 SP can be conveniently produced in Escherichia coli when fused to a fragment of CRM197, a variant of diphtheria toxin currently used for a number of conjugated vaccines. In particular, we show that the CRM197-RBD chimera solubilized from inclusion bodies can be refolded and purified to a state featuring the 5 native disulphide bonds of the parental proteins, the competence in binding angiotensin-converting enzyme 2, and a satisfactory stability at room temperature. Accordingly, our observations provide compulsory information for the development of a candidate vaccine directed against COVID-19.
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