FIG. 4.

Heat and urea treatments facilitate the infectivity of Mo-MLV. (A and B) 3T3 cell-bound virus was incubated for 2 min at control (37°C) or elevated temperatures in TN-1.8 mM Ca²⁺ (A) or at 30°C in TN-1.8 mM Ca²⁺ containing 0 to 2 M urea (B). Fusion incubations were terminated by washing with a pH 3 buffer, the cultures were further incubated at 37°C for 33 h, and infectivity was measured by scoring Env-positive cells by using immunofluorescence. Infection efficiency is given as a percentage of that of the control virus incubated at 37°C for 2 min (A) or at 30°C for 2 min in the absence of urea (B). (C) Kinetics of cell entry under conditions of protein perturbation. 3T3 cell-bound virus was subjected to a 2-min heat (53°C) or a urea (2 M) pulse and then incubated further at 37°C for 3 to 58 min before Env inactivation treatment. Infection efficiency was measured as described above and given as a percentage of the level of a 60-min control infection without an enhancement pulse.