The role of ubiquitin conjugation in glucose-induced proteolysis of Saccharomyces maltose permease

In Saccharomyces, the addition of glucose induces a rapid degradation of ma ltose permease that is dependent on endocytosis and vacuolar proteolysis (M edintz, I., Jiang, H, Wan, E, K., Cui, W., and Michels, C. A. (1996) J. Bac teriol. 178, 2245-2254). Here we report on the role of ubiquitin conjugatio n in this process. Deletion of DOA4, which causes decreased levels of avail able ubiquitin, severely decreases the rate of glucose-induced proteolysis, and this is suppressed by the overproduction of ubiquitin. Overexpression of ubiquitin in an endocytosis-deficient end3-ts strain results in the gluc ose-stimulated accumulation of a larger molecular weight species of maltose permease, which we demonstrate is a ubiquitin-modified form of the protein by utilizing two ubiquitin alleles with different molecular weights. The s ize of this ubiquitinated species of maltose permease is consistent with mo noubiquitination. A promoter mutation that reduces expression of RSP5/NPI1, a postulated ubiquitin-protein ligase, dramatically reduces the rate of gl ucose-induced proteolysis of maltose permease. The role of various ubiquiti n-conjugating enzymes was investigated using strains carrying mutant allele s ubc1 Delta ubc4 Delta, ubc4 Delta ubc5 Delta, cdc34-ts2/ubc3, and ubc9-ts . Loss of these functions was not shown to effect glucose-induced proteolys is of maltose permease, but loss of Ubc1, -4, and -5 was found to inhibit m altose permease expression at the post-transcriptional level.