E3-13.7 integral membrane proteins encoded by human adenoviruses alter epidermal growth factor receptor trafficking by interacting directly with receptors in early endosomes

Animal cell viruses provide valuable model systems for studying many normal cellular processes, including membrane protein sorting. The focus of this study is an integral membrane protein encoded by the E3 transcription regio n of human adenoviruses called E3-13.7, which diverts recycling EGF recepto rs to lysosomes without increasing the rate of receptor internalization or intrinsic receptor tyrosine kinase activity. Although E3-13.7 can be found on the plasma membrane when it is overexpressed, its effect on EGF receptor trafficking suggests that the plasma membrane is not its primary site of a ction. Using cell fractionation and immunocytochemical experimental approac hes, we now report that the viral protein is located predominantly in early endosomes and limiting membranes of endosome-to-lysosome transport interme diates called multivesicular endosomes. We also demonstrate that E3-13.7 ph ysically associates with EGF receptors undergoing E3-13.7-mediated down-reg ulation in early endosomes. Receptor-viral protein complexes then dissociat e, and EGF receptors proceed to lysosomes, where they are degraded, while E 3-13.7 is retained in endosomes. We conclude that E3-13.7 is a resident ear ly endocytic protein independent of EGF receptor expression, because it has identical intracellular localization in mouse cells lacking endogenous rec eptors and cells expressing a human cytomegalovirus-driven receptor cDNA. F inally, we demonstrate that EGF receptor residues 675-697 are required for E3-13.7-mediated down-regulation. Interestingly, this sequence includes a k nown EGF receptor leucine-based lysosomal sorting signal used during ligand -induced trafficking, which is also conserved in the viral protein. E3-13.7 , therefore, provides a novel model system for determining the molecular ba sis of selective membrane protein transport in the endocytic pathway. Our s tudies also suggest new paradigms for understanding EGF receptor sorting in endosomes and adenovirus pathogenesis.