FIG. 3.

SOS-Env is activated by DTT applied after but not before binding to target cells for 1 h at 37°C. The extent of fusion induced by SOS-Env (hatched bars) and WT-Env (open bars) is shown for different DTT treatment protocols. In all cases, 25 mM DTT was maintained for 10 min at 37°C. In separate experiments, SOS cells were incubated with 25 μg of sCD4 per ml before the SS-bond was reduced with DTT (last two bars).