Replication of A/HK/218847/06 (H1N1) and A/HK/218449/06 (H3N2) influenza viruses depends on ERK activation. (A) MDCK cells were infected at m.o.i. = 1 either with H1N1 ± TPA or with H3N2 ± U0126. After Western blot analysis, ERK activation was analyzed with a mAb specific for phosphorylated ERK (P-ERK). Subsequently, loading was controlled with a mAb against ERK2. Respective bands of three independent experiments were quantified, and relative ERK activation was calculated and normalized to the loading control (mock-infected, white bar). Virus types as well as the time of analysis post-infection (p.i.) are indicated. (B) MDCK cells were infected at m.o.i. = 1 either with H1N1 ± TPA or with H3N2 ± U0126, and the supernatant was harvested 9 h later. The mean virus titers are given in percent as well as plaque forming units/ml. The error bars were derived from three independent experiments.