Fig. 2. Immuno-EM shows that binding of EGF mainly occurs outside caveolae. Ultrathin cryosections of cells incubated with EGF (10 nM) for 15 minutes on ice were double-labeled to localize bound EGF (large gold particles) and caveolin (small gold particles). In both A431 cells (A,B) and HEp-2 cells (C,D) bound EGF was found at smooth noninvaginated areas of the plasma membrane (small arrowheads), and only small amounts colocalized with caveolin in caveolae (large arrowheads). Bars, 100 nm.