Src up-regulates EGFR expression by inhibiting c-Cbl-induced receptor ubiquitylation. (A) Chinese hamster ovary cells coexpressing EGFR and the indicated Src proteins were untreated or treated with EGF (100 ng/ml) for 15 min; thereafter, cell extracts were analyzed by immunoprecipitation (IP) and immunoblotting (IB) with the indicated antibodies. (B) SYF cells coexpressing EGFR, HA-tagged ubiquitin, c-Cbl, and the indicated forms of c-Src were untreated or treated with EGF (100 ng/ml) for 15 min; thereafter, cell extracts were analyzed as indicated. (C) Down-regulation of EGFR was tested in SYF cells ectopically expressing EGFR alone (●), in combination with c-Cbl (■), or in combination with both c-Cbl and an active c-Src (Y529F; ▴). Cells were incubated at 37°C with an unlabeled EGF (100 ng/ml) for the indicated time intervals, and the residual level of surface EGFR was determined by binding of a radioactive EGF at 4°C. Averages and the corresponding SDs of triplicates (bars) are shown.