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

Sensitive and specific serology tests are essential for epidemiological and public health studies of COVID-19 and for vaccine efficacy testing. The presence of antibodies to SARS-CoV-2 surface glycoprotein (Spike) and, specifically, its receptor-binding domain (RBD) correlates with inhibition of SARS-CoV-2 binding to the cellular receptor and viral entry into the cells. Serology tests that detect antibodies targeting RBD have high potential to predict COVID-19 immunity and to accurately determine the extent of the vaccine-induced immune response. Cost-effective methods of expression and purification of Spike and its fragments that preserve antigenic properties are essential for development of such tests. Here we describe a method of production of His6-tagged S319-640 fragment containing RBD in E. coli. It includes expression of the fragment, solubilization of inclusion bodies, and on-the-column refolding. The antigenic properties of the resulting product are similar, but not identical to the RBD-containing fragment expressed in human cells.