Prokaryotic expression, refolding, and purification of fragment 450–650 of the spike protein of SARS-coronavirus

The spike (S) glycoprotein is one of the major structure proteins of SARS-associated coronavirus (CoV). Fragment 450–650 (S450-650) of the S protein contains receptor-binding domain and neutralizing epitopes. In this study, S450-650 was expressed with a histidine tag in Escherichia coli BL21. Bacterial inclusion bodies containing the recombinant S450-650 were solubilized with 8 M urea and then applied onto a Ni–nitrilotriacetic acid column. On-column refolding and purification was performed. Reduced glutathione and oxidized glutathione were included in the refolding buffer. In the wash and elution buffers, glycerol and glucose were necessary additives to prevent protein aggregation during purification. This refolding and purification procedure allowed production of S450-650 at up to 500 μg/ml in soluble form, which maintained appropriate antigenicity and immunogenicity. It was able to induce strong IgG responses in BALB/c mice. In Western blot assays, the recombinant S450-650 was recognized by monoclonal Ab against the His-tag and also sera from a convalescent SARS patient. S450-650-based ELISA system was able to detect anti-SARS-CoV IgG Abs in patient sera.