FIG. 3.

DTT rescue of heat-facilitated fusion that has been blocked by alkylation. XC cell-bound Mo-MLV was subjected to fusion incubation for 2 min at 53°C in TN-1.8 mM Ca²⁺ with (columns 3 and 4) or without (columns 1 and 2) 2 mM M135 and then, after a wash, was either further incubated at 37°C for 15 min in TN-1.8 mM Ca²⁺ with (column 4) or without (columns 2 and 3) 25 mM DTT or kept on ice (column 1). The fusion incubations were terminated by treatment with a pH 3 buffer. Fusion efficiencies were determined by monitoring the extent of polykaryon formation as described for Fig. 1D. The efficiencies were expressed as percentages of that obtained by the initial and second incubations under drug-free conditions (column 2; set at 100%).