FIG.7.

Wild-type and pseudotyped ALV-B were unable to fuse at the cell surface upon low-pH treatment. Pseudotyped SFV (10 ng of p24) (A), pseudotyped ALV-B (10 ng of p24) (B), and wild-type ALV-B (MOI = 10) (C) were prebound to HEK 293-TVB^S3 cells at 4°C for 1 h. Cells were treated with a medium, pH 5.3 or 7.0, for 2 min at 37°C. Subsequently, cells were incubated in fresh medium for 4 h in the presence or absence of 50 mM NH₄Cl. Supernatants were replaced with fresh medium, and luciferase activity was measured 48 h postinfection (A and B). Flow cytometry analysis for GFP-positive cells was used to determine infection by wild-type ALV-B (C). Pseudotyped ALV-B (10 ng of p24) was prebound to HEK 293-TVB^S3 cells at 4°C for 1 h. Cells were treated with a medium, pH 5.3, containing or lacking 50 mM NH₄Cl for 5, 15, or 30 min at 37°C. Subsequently, cells were incubated with medium containing 50 mM NH₄Cl for 4 h. Cells were grown in fresh medium, and extracts were prepared 48 h postinfection to measure luciferase activity (D). Experiments were performed in triplicate, and standard deviations are shown.