Size exclusion chromatography
The oligomerization state of the recombinant S protein was evaluated by high-resolution size exclusion chromatography (SEC). Elution fractions from IMAC containing purified S protein were collected and concentrated, and the buffer was exchanged and loaded on a Superdex 200 increase 10/300 (Cytiva, Marlborough, USA) equilibrated in 100 mM sodium phosphate buffer, 150 mM NaCl, pH 7.4. All fractions were collected, and the S protein was revealed by WB using an anti-6x His-tag antibody, as described previously. The calculated molecular weight (MW) of the expressed protein, without considering the glycan moieties, was 138914 Da.