DISTINCT SATURABLE PATHWAYS FOR THE ENDOCYTOSIS OF DIFFERENT TYROSINEMOTIFS

Endocytosis of surface proteins through clathrin-coated pits requires an internalization signal in the cytoplasmic domain. Two types of inte rnalization signal have been described: one requiring a tyrosine as th e critical residue (tyrosine-based motif), and the other consisting of either two consecutive leucines or an isoleucine and leucine (dileuci ne motif), Although it seems that these signals are necessary and suff icient for endocytic targeting, the mechanism of recognition is not we ll understood. To examine this question, tetracycline-repressible cell lines were used to overexpress one of several receptors bearing a tyr osine-based internalization signal. By measuring the rates of endocyto sis for either the over expressed receptor,or that of other en recepto rs, we were able to show that the endocytosis of identical receptors c ould be saturated, but a complete lack of competition exists between t he transferrin receptor (TfR), the low-density lipoprotein receptor, a nd the epidermal growth factor receptor. Overexpression of any one of these receptors resulted in its redistribution toward the cell surface , implying that entry into coated pits is limited. During high levels of TfR expression, however, a significant increase in the amount of su rface Lamp1, but not low-density lipoprotein receptor, epidermal growt h factor receptor, or Lamp2, is detected. This suggests that Lamp1 and TfR compete for the same endocytic sites. Together, these results sup port the idea that there are at least three distinct saturable compone nts involved in clathrin-mediated endocytosis.