Mechanisms of ligand induced downregulation and intracellular trafficking of the epidermal growth factor receptor.

Occupancy-induced downregulation of cell surface epidermal growth factor receptors (EGF-R) attenuates signal transduction. To define mechanisms through which downregulation of this class of growth factor receptors occurs, I have investigated the relative roles of ligand-induced internalization and recycling in this process. Occupied, kinase-active EGF receptors were internalized through a ligand induced, high affinity, endocytic system at rates up to 10-fold faster than empty receptors. In contrast, full length EGF receptors lacking tyrosine kinase activity underwent internalization at a rate independent of occupancy. This "kinase-independent" internalization rate appeared to reflect constitutive receptor internalization since it was similar to the internalization rate of both receptors lacking a cytoplasmic domain and of antibodies bound to empty receptors. EGF internalized by either kinase-active or kinase-inactive receptors was efficiently recycled and was found within endosomes containing recycling transferrin receptors. However, targeting of internalized receptors to lysosomes did not require receptor kinase activity. Targeting of internalized receptors to lysosomes was also independent of the domain required for ligand-induced internalization since receptors truncated at amino acid (a.a.) 958 were efficiently targeted to the lysosomes. The truncation at a.a. 958 eliminates a domain of the receptor required for ligand induced internalization. However, targeting to the lysosomes was dependent upon a domain between a.a.647-688 of the receptor. Receptors truncated at a.a.647 were unable to target to the lysosomes while those receptors extended to amino acid 688 were capable of lysosomal transfer. Only receptors that displayed ligand-induced internalization underwent downregulation, indicating that the proximal cause of downregulation is occupancy-induced endocytosis.