Sorting of ligand-activated epidermal growth factor receptor to lysosomes requires its actin-binding domain

Ligand-induced down-regulation of the epidermal growth factor receptor (EGFR) comprises activation of two sequential transport steps. The first involves endocytic uptake by clathrin-coated vesicles, the second transfer of endocytosed EGFR from endosomes to lysosomes. Here we demonstrate that the second transport step requires a domain of the EGFR that encompasses residues 985-996 and was previously found to interact with actin. Deletion of domain 989-994 (Delta989-994 EGFR) did not interfere with EGFR uptake but completely abrogated its degradation. In contrast, both uptake and degradation were affected for K721A EGFR, a kinase-deficient EGFR mutant. To measure intracellular EGFR sorting, we developed a novel cell fractionation assay toward which cells were co-transfected for chicken hepatic lectin, a receptor for agialoglycoproteins. These cells were incubated with agialofetuin-coupled colloidal gold, which was targeted to lysosomes after receptor-mediated endocytosis. Compartments within the lysosomal pathway gained buoyant density because of the presence of colloidal gold and could be isolated from cell homogenates by ultracentrifugation through a high-density sucrose cushion. In contrast to endocytosed wild type EGFR, both Delta989-994 EGFR and K721A EGFR were largely not retrieved in gold-containing endocytic compartments. These results are supported with morphological data. We conclude that sorting of endocytosed EGFR into the degradation pathway requires both its kinase activity and actin-binding domain.