Fig. 4.

Effect of an observed compensatory mutation on cell-cell fusion of the mutant A117F. (A) A cell-cell fusion assay (previously described for Fig. 2) was used to determine the effect of the E76K mutation. The single E76K mutation was able to mediate fusion at a pH of >6.2. A117F is unable to mediate cell-cell fusion on its own; however, when combined with E76K it regains fusion activity with a pH profile similar to E76K. (B) Titration of sensitivity to NH₄Cl. Cells were pretreated with NH₄Cl at the indicated concentrations, and eGFP-expressing viruses were allowed to infect in the presence of NH₄Cl. The neutralization was maintained throughout the infection, and at 5 hpi eGFP expression was monitored via FACS analysis. The wild type is more sensitive to this neutralization than viruses that contain the E76K mutation in G. All experiments were performed in triplicate; error bars indicate standard deviations.