Figure 4.

pH dependence of fusion induced by WT and chimeric HA. Fusion between HA-expressing CV-1 cells and RBC ghosts colabeled with CF and R18 was triggered by application of an acidic buffer of the indicated pH for 2 min at 37°C followed by incubation in PBS supplemented with 20 mM raffinose for 5 min at room temperature. For the HHP* construct, the incubation at neutral pH was extended to 15 min. Every point on the graph corresponds to a mean value obtained from four to eight independent experiments (bars indicate SE). The steep pH dependence of WT HA-induced fusion (open circles) was found for all HA chimeras, although the pH curve for the chimeras was shifted by ∼0.2 units toward more acidic pH. Because of the virtual absence of CF transfer for the HHP* construct, its pH dependence is shown for R18 mixing (circles with cross).