FIG. 6.

Model for membrane fusion mediated by SOS-Env. The SS-bonds between gp120 (light purple) and gp41 (dark purple) of SOS-Env are shown by red starbursts. CD4 is green, and CCR5 is dark cyan. The fusion peptides (red arrows) insert into the target cell membrane at the committed state. The amino-terminal trimeric coiled coil of gp41 is shown as three abutted red cylinders, and the carboxy-terminal heptad repeat regions are shown by blue cylinders. The gp120 subunits, after disengagement from gp41, are shown as being paler than before disengagement. Reduction of the SS-bonds allows gp41 to fold into a prebundle conformation and induce hemifusion at a temperature that is not permissive for fusion or to fold into a six-helix bundle and induce fusion pore formation at the optimal temperature.