Fig. 4.

The effect of alanine residue substitution in positions 581–586 on the cleavage properties of spike protein and viral recovery. Immunoprecipitation of 293 T cells transfected with the expression vector pCAGGS-S or the expression vector encoding the alanine substituted residues in positions 581–586 of the spike protein (pCAGGS-S (581–586)). The cells were metabolically labeled with [³⁵S]-methionine and cysteine for eight hours at 37 °C and the lysate was precipitated with the monoclonal anti-S antibodies A2.1 and A2.3 S uncleaved denotes the 180 kDa uncleaved spike protein, and S cleaved denotes the 90 kDa cleaved S1 and S2 subunits.