Fig. 6. Irreversibility of a low-pH-activated HA conformation with an exposed FP at influenza virus particles. Virus was treated with pH 4.9 at 22°C for 0 (marked as N), 15, 30 or 60 s. FP exposure was assayed by immunoprecipitation. The antiserum was added either immediately after reneutralization or after an additional 60 min incubation at pH 7.4. The intensity of the HA2 band in the presented gel was quantified and plotted in the bar chart, with the subtracted intensity of the band observed in the experiment without low-pH application. The total protein concentration loaded, determined by volume, is comparable in each lane, since we started the experiment with the same amount of virus for each sample.