SATURATION OF THE ENDOCYTIC PATHWAY FOR THE TRANSFERRIN RECEPTOR DOESNOT AFFECT THE ENDOCYTOSIS OF THE EPIDERMAL GROWTH-FACTOR RECEPTOR

Cell-surface receptors that undergo clathrin-mediated endocytosis cont ain short amino acid sequences in their cytoplasmic domain that serve as internalization signals. interactions between these sequences and c omponents of the endocytic machinery should become limiting upon overe xpression of the constitutively recycling transferrin receptor (TfR). A tetracycline-responsive system was used to induce overexpression of the TfR up to 20-fold in HeLa cells. Internalization assays indicate t he rate of I-125-transferrin uptake per surface TfR is reduced by a fa ctor of 4 in induced cells. Consistent with endocytosis being the rate -limiting step, TfRs shift from an endosomal to more of a plasma membr ane distribution with TfR overexpression. The clathrin-associated prot ein AP-2 has been proposed to interact directly with the cytoplasmic d omain of many receptors, yet no changes in the amount or distribution of AP-2 were detected in induced cells. The internalization rate for t he epidermal growth factor receptor was also measured, with or without induction of TfR expression. Even though endocytosis of the TfR is sa turated in induced cells, I-125-labeled epidermal growth factor contin ues to be internalized at a rate identical to that seen in uninduced c ells. We propose that there are different limiting steps for the endoc ytosis of these two receptors.