REGULATION OF POSTENDOCYTIC TRAFFICKING OF THE EPIDERMAL GROWTH-FACTOR RECEPTOR THROUGH ENDOSOMAL RETENTION

Little is known about the regulation of EGF receptor (EGF-R) trafficki ng following endocytosis. We investigated this by using a series of EG F-R with altered cytoplasmic tails and comparing their ability to unde rgo recycling and lysosomal targeting in both the occupied and empty s tate. We found that 2-3% of empty EGF-R are internalized each minute, but rapidly recycle (t(1/2) approximately 5 min). This constitutive in ternalization and recycling of empty receptors was independent of cyto plasmic receptor sequences. Occupied EGF-R, in contrast, displayed a m uch slower rate of recycling (t(1/2) between 10-23 min) due to retenti on within recycling endosomes. Endosomal retention of different EGF-R correlated with lysosomal targeting of EGF. Intrinsic receptor tyrosin e kinase activity had no discernible effect on postendocytic trafficki ng of EGF. Although sequences within the cytoplasmic tail of the EGF-R appear to be required for occupancy-dependent endosomal retention, th ey are distinct from those required for ligand-induced endocytosis. Ou r studies indicate that intracellular trafficking of the EGF-R is regu lated by endosomal components that preferentially recognize occupied r eceptors. Down-regulation of the EGF-R thus involves two distinct regu latory processes: one at the level of internalization and one at the l evel of recycling.