The PB1 gene of the A/HK/218449/06 (H3N2) influenza virus contributes to ERK activation and enhanced nuclear RNP export. (A) MDCK cells were infected at m.o.i. = 1 with reverse genetics (rg)-rescued rgH1N1, rgH3N2, or rgH1N1(H3N2-PB1). After Western blot analysis, ERK activation was analyzed with a P-ERK-specific mAb. Subsequently, loading was controlled with an anti-ERK2 mAb. Respective bands of three independent experiments were quantified, and relative ERK activation was calculated and normalized to the loading control (mock, white bar). Virus types and the time of analysis post infection (p.i.) are indicated. (B) MDCK cells were infected at m.o.i. = 1 with rgH1N1, rgH3N2, or rgH1N1(H3N2-PB1). RNPs were stained with anti-NP mAb and Alexa488-coupled goat anti-mouse Abs (green). The nucleus was counterstained with TO-PRO-3 (blue). Intracellular RNP localization was analyzed by confocal laser scanning microscopy at 6 h p.i. The merger of both channels is shown.