PR activity assay. Reaction products were analyzed by 19% SDS-PAGE. 10 μM GB1-GFP substrate harboring a FV PR cleavage site between GB1 and GFP was incubated with 10 μM SFVmac PR-RT or PFV PR-RT, respectively, at 37°C for 16 h in reaction buffer (50 mM Na2HPO4/NaH2PO4 pH 7.4, 0.5 mM DTT, 3 M NaCl). C, control, substrate cleavage with TEV protease; (-), uncleaved substrate; M, molecular weight standard. The sizes of the standard proteins are indicated on the left.