POSTENDOCYTIC TRAFFICKING OF EPIDERMAL GROWTH FACTOR-RECEPTOR COMPLEXES IS MEDIATED THROUGH SATURABLE AND SPECIFIC ENDOSOMAL INTERACTIONS

Intracellular trafficking of the epidermal growth factor receptor (EGF -R) is regulated by receptor occupancy. To investigate this, we develo ped an assay to study endosomal sorting under steady-state conditions. Using a cell line transfected with EGF-R variants, we found that the fraction of internalized EGF.EGF-R complexes sorted to lysosomes was a function of the number of intracellular complexes and required sequen ces in the cytoplasmic domain of the receptor. As the number of intrac ellular occupied wild-type receptors increased from 3 x 10(2) to 2 x 1 0(5)/cell, the fraction of internalized EGF that was degraded dropped from 70 to 20%. Transforming growth factor-alpha, which dissociates fr om the EGF-R at endosomal pH, was degraded to a uniform extent of appr oximately 50% at all intracellular ligand concentrations. EGF internal ized by receptors lacking a cytoplasmic domain (c'647) was degraded to an extent of only 5-10% independent of the number of intracellular co mplexes. Mutant receptors truncated either at residues 1022 or 973 dis played sorting patterns intermediate between wild-type and c'647 recep tors. Despite large differences in their internalization rates, the fr actional sorting patterns of c'1022 and c'973 receptors were indisting uishable, Receptor tyrosine kinase activity appeared to have a small e ffect on sorting pattern, but only in the context of full-length recep tors. Our results indicate that the default pathway of internalized re ceptors is rapidly recycling and that lysosomal targeting of occupied EGF-R is due to endosomal retention that is both specific and saturabl e. In addition, internalization and endosomal retention of EGF-R appea r to be mediated by distinct structural elements.