Cells were plated at 10 6 /well in a 6-well plate 18-24 h prior to infection. Virus was allowed to adsorb to cells for 1.5 h at 4°C. At this temperature, virus binds to cells but is not internalized [70] . After adsorption, the cultures were incubated at 37°C in fresh medium. Prior to some infections, cells were pre-treated for 3 h with 300 µM E64, 100 nM Baf, 25 µM monensin (Sigma), or 20 mM NH 4 Cl.