Regulation of postendocytic trafficking of the epidermal growth factor receptor through endosomal retention.

Little is known about the regulation of EGF receptor (EGF-R) trafficking following endocytosis. We investigated this by using a series of EGF-R with altered cytoplasmic tails and comparing their ability to undergo recycling and lysosomal targeting in both the occupied and empty state. We found that 2-3% of empty EGF-R are internalized each minute, but rapidly recycle (t1/2 approximately 5 min). This constitutive internalization and recycling of empty receptors was independent of cytoplasmic receptor sequences. Occupied EGF-R, in contrast, displayed a much slower rate of recycling (t1/2 between 10-23 min) due to retention within recycling endosomes. Endosomal retention of different EGF-R correlated with lysosomal targeting of EGF. Intrinsic receptor tyrosine kinase activity had no discernible effect on postendocytic trafficking of EGF. Although sequences within the cytoplasmic tail of the EGF-R appear to be required for occupancy-dependent endosomal retention, they are distinct from those required for ligand-induced endocytosis. Our studies indicate that intracellular trafficking of the EGF-R is regulated by endosomal components that preferentially recognize occupied receptors. Down-regulation of the EGF-R thus involves two distinct regulatory processes: one at the level of internalization and one at the level of recycling.