Figure 4.

Ligand binding does not remove ErbB2 from protrusions or allow it to enter clathrin-coated pits and become internalized. (A and B) Immunogold labeling shows that ErbB2 is associated with protrusions of both a heregulin (A) and a herceptin (B)-treated cell. After stimulation with herceptin, the cell surface has become highly ruffled. Note that ErbB2 is not detected in clathrin-coated pits (CP). (C) Biotinylation assay to detect internalized ErbB2. It is seen that in control cells as well as in cells treated with heregulin or herceptin, the internalization level is very low. In contrast, after antibody cross-linking, a substantial fraction of ErbB2 becomes internalized. Bars, 200 nm (A and B).