purification(17) RBD(16) SARS-CoV-2(15) antibody(14) functional(14) expressed(14) Protein(12) expression(12) Antigen(11) eukaryotic(10) medium(10) Receptor binding domain(10) Culture(10) culture medium(10) COVID-19(9) spike(9) the SARS-CoV-2(9) binding(9) protein production(8) SARS-CoV-2 spike(7) element(7) purified(7) protein expression(7) Critical(7) Spike protein(7) demonstrated(7) robust(7) Escherichia coli(6) Receptor-binding domain(6) domain(6)