Tyrosine 110 of the P protein is a key site for P–STAT1 interaction and inhibition of IFN response. (A) HEK-293T cells were co-transfected with Myc-tagged STAT1 expression plasmids and empty vector, P protein, Y110H mutant or Y110F mutant. Immunoprecipitation (IP) and whole-cell lysates were analyzed by immunoblotting (IB) using anti-Myc, anti-Flag, and anti-β-actin antibodies. (B and C) HEK-293T cells were transfected ISRE (B) or GAS (C) reporter plasmids along with pRL-TK plasmids, and P protein expressing plasmids, Y110H or Y110F. The cells were treated with IFN-β (1000 U/mL) or IFN-γ (100 ng/mL) for 12 h at 24 h post-transfection, and the luciferase activity was determined. (D and E) HEK-293T cells were transfected with empty vector or P protein expressing plasmids, Y110H or Y110F, and the cells were treated with IFN-β (1000 U/mL) (D) or IFN-γ (100 ng/mL) (E) for 12 h at 24 h post-transfection. The relative mRNA expression levels of ISG54, ISG15, IRF1, and STAT1 were measured by qPCR analysis.