Immunochemical demonstration of ubiquitin conjugates in fibroblasts treated with cysteine proteinase inhibitors

in the denser lysosome fractions of cells treated with E-64 is due to an increase in ubiquitin conjugates in these fractions. Ubiquitin has been suggested to be involved in the marking of proteins for non-lysosomal degradation [4]. Our results suggest that ubiquitination and/or aggregation may: ( a ) serve to ‘isolate’ polypeptides whose degradation by the lysosomal system has been prevented; ( 6 ) ‘mark’ polypeptides for lysosomal degradation; ( c ) be a feature of some normally ubiquitinated proteins, e.g. cell surface proteins [5 , 6, 71 which are degraded lysosomally and accumulate in the lysosome when lysosomal proteolysis is blocked. 3. Hershko, A,, Eytan, E. & Ciechanover, A. & Haas, A. L. (1982) J. Biol. Chem. 251,13964-13970 4. Ciechanover, A,, Finley, D. & Varshavsky, A. ( 1 984) J. Cell. Biochem. 24,27-58 5. Leung, D. W., Spencer, S. A,, Cachianes, G., Hammonds, R. G., Collins, C., Henzel, W. J., Barnard, R., Waters, M. J. & Wood, W. I. (1987) Nulure( London) 330,537-543 6. Yarden, Y., Escobedo, J. A., Kuang, W.-J., Yang-Feng, T. L., Tremble, D. P. M., Chen, E. Y., Ando, M. E., Harkins, R. N., Francke, U., Fried, V. A,, Ullrich, A. & Williams, L. T. (1986) Nature(London) 323,226-232 7. Meyer, E. M., West, C. M. & Chau, V. (1986) J. Biol. Chem. 261, 14365-14368