Fig. 4. HEp2 cells were incubated overnight in serum-free medium in the absence (a and c) or the presence (b and d) of FTI. Cells were then stimulated with EGF in the presence of anti-EGF receptor-gold (10nm) for 45 minutes (a and b) or 90 minutes (c and d) in the continued absence or presence of inhibitor. Multivesicular late endosomes (MVBs) and lysosomes (Lys) are indicated. Anti-EGF receptor-gold in MVBs is monodisperse while that in lysosomes is aggregated owing to proteolysis of the anti-EGF receptor antibody (arrows). In the absence of FTI, gold can be found in both MVBs and lysosomes 45 minutes after EGF treatment, whereas in the presence of FTI the majority of gold is in MVBs. Ninety minutes after EGF stimulation, the majority of gold is in lysosomes in the absence of FTI, whereas gold is still frequently found in MVBs in the presence of inhibitor. There is no evidence that FTI affects sorting of EGF receptor onto internal vesicles of MVB or internal vesicle formation. Scale bar, 100 nm.