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Schematic representation of CoV S and IAV HA. CoV S (A) and IAV HA (B) are synthesized as the inactive precursor proteins S0 and HA0, respectively, which require proteolytic processing for activation. Cleavage sites are indicated by arrows. Recombinant S was expressed with a C-terminal FLAG-tag in this study. FP = fusion peptide, TM = transmembrane domain.
