FIG. 3.

Virus-membrane fusion activities of wt and D188A mutant SFV. Wt and D188A virus particles were prebound to BHK cells on ice. Virus fusion with the plasma membrane was then induced by treatment for 1 min at 37°C with medium of the indicated pH. The cells were cultured overnight in medium containing NH₄Cl to prevent secondary infection, and the infected cells were quantitated by immunofluorescence. The data are shown as the fraction of maximal fusion and represent the average of two experiments; the bars indicate the range. D188A-1 and -2 represent two independent clones of the mutant virus.