FIG. 6.

Activated MVs induce cell-cell fusion only after a postadsorption low-pH treatment. Purified WR MVs were treated before adsorption (Pre-adsorption) with either pH 5 or pH 7.4 buffer for 3 min at 37°C or were digested with 50 μg/ml of papain for 30 min at 37°C and then adsorbed to BS-C-1 cells at a multiplicity of 300 PFU for 1 h at 4°C. The cells were then immersed in either pH 5 or pH 7.4 buffer at 37°C (Post-adsorption). After 3 min, the buffers were replaced with EMEM containing 300 μg of cycloheximide per ml and incubated for 3 h at 37°C. The cells were fixed and stained with Alexa Fluor 568-phalloidin and DAPI to display actin filaments and DNA, respectively.