Time-dependent effects of bacitracin. (A) Binding assay. Virus was bound to cells in medium with or without 5 mM Bac for 1 h at 4°C. Cells were then thoroughly washed and either trypsin or mock treated before harvesting of total protein by the addition of SDS-PAGE loading buffer. Cell-associated L1 was detected by Western blotting with an anti-L1 monoclonal antibody. The asterisk denotes a nonspecific band of cellular origin. (B) Quantification of the band intensities from three separate binding assays. (C and D) Virus uptake assay in the absence (C) or presence (D) of 5 mM Bac. Cells were mock infected (M, lanes 1 and 10) or prebound with virus and either processed immediately (0 h, lanes 2 and 11, the bound input virus) or trypsinized after various lengths of time at 37°C (lanes 3 to 9 and 12 to 18) before processing for nonreducing SDS-PAGE and Western blotting for L1. Molecular masses (kDa) are shown to the left, and positions of the L1 forms are denoted on the right. The asterisk again denotes the nonspecific band. (E) Bac-induced binding and entry occur through the infectious pathway. Infections were set up with or without Bac pretreatment and with or without Bac during virus binding as described in the text. Cells were washed and switched to 37°C, and infection was measured at 48 h by luciferase assay. (F) Time of addition/withdrawal experiments. Cells were bound with virus in medium with or without Bac and washed, and medium with or without Bac was replaced. After various times at 37°C, medium was changed to add Bac (time of addition, solid line) or to remove Bac (time of withdrawal, dashed line). The highest levels of infection for each experiment were normalized to 100%. All infections were performed in triplicate, with error bars representing standard errors of the means.