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We have characterized the biosynthesis and processing of a 91 amino acid hydrophobic integral membrane protein encoded by human group C adenoviruses which down-regulates the EGF receptor (Carlin, C. R., Tollefson, A. E., Brady, H. A., Hoffman, B. L., and Wold, W. S. M. (1989) Cell 57, 135-144). Previous studies have shown that two immunologically related proteins are produced in vivo, a 13.7-kDa protein encoded by E3 message f and a 11.3-kDa protein derived from 13.7 kDa by proteolysis (Hoffman, B. L., Ullrich, A., Wold, W. S. M., and Carlin, C. R. (1990) Mol. Cell. Biol. 10, 5521-5524; Tollefson, A. E., Krajcsi, P., Yei, S., Carlin, C. R., and Wold, W. S. M. (1990) J. Virol. 64, 794-801). We report here that the 13.7- and 11.3-kDa proteins form intermolecular disulfide bonds cotranslationally at Cys-31 and tend to migrate as high molecular weight aggregates under nonreducing conditions. Both proteins are also present at the cell surface, as evidenced by specific immunoprecipitation from intact monolayers enzymatically labeled with 125I. Moreover, an antiserum specific for a putative extracellular epitope recognizes the same viral proteins as antibodies directed against a C-terminal synthetic 15-mer. The 13.7- and 11.3-kDa proteins are detected at early time points during pulse-chase radiolabeling of infected cells, do not undergo any further changes in molecular weight, and focus at their predicted isoelectric points (7.4 and 7.2, respectively). Identical results are obtained in stable transfectants constitutively expressing only 13.7 and 11.3 kDa, suggesting that biosynthesis and processing is not dependent on other viral proteins. These results have been incorporated into a computer-based model to predict the orientation of 13.7 and 11.3 kDa in the lipid bilayer. This model provides a basis for testing predictions regarding the topology of the viral proteins, as well as putative interactions with heterologous proteins in the microenvironment of the plasma membrane that cause down-regulation of the epidermal growth factor receptor. |
