FIG 8.

IBV HA prefers a temperature of 33°C for protein expression, explaining temperature-restricted fusion of B/Vic HA. (A) HeLa cells were transfected with the indicated HA plasmids and incubated during 48 h at four different temperatures. The graphs show quantitative Western blot data for HA0 band intensity, normalized to that of clathrin and expressed relative to the HA0 band seen at 33°C. Data are the means ± SEM (n = 3). *, P < 0.05; **, P < 0.01; ****, P ≤ 0.0001; ns, not significant (ordinary one-way ANOVA, followed by Dunnett’s test). (B to D) Polykaryon formation in IBV HA-transfected HeLa cells exposed to different temperatures during the stages of HA expression (starting 24 h posttransfection) or cell-cell fusion. Panel B shows fusion induced by a pH 5.0 buffer. Panels C and D show the number of polykaryons as a function of pH, after HA expression at 33°C or 37°C, as indicated. The Western blots show the corresponding HA levels after trypsin activation.