ER DEGRADATION OF A MUTANT YEAST PLASMA-MEMBRANE PROTEIN BY THE UBIQUITIN-PROTEASOME PATHWAY

The yeast plasma membrane, uracil permease, undergoes ubiquitin-depend ent endocytosis and subsequent degradation in the vacuole via a proces s that does not involve the proteasome, Cell-surface ubiquitination of this protein is mediated by the ubiquitin-protein ligase Npi1p/Rsp5p and involves Lys(63)-linked ubiquitin chains, This report describes th e intracellular fate of a mutant form of uracil permease carrying a th ree amino acid insertion in a cytoplasmic loop, Most of this protein i s not deployed beyond the ER, and is degraded by the 26S proteasome, M utant permease degradation is almost unaffected in cells with impaired Npilp/Rsp5p, but is dependent on the Ubc6p and Ubc7p ubiquitin-conjug ating enzymes, suggesting that proteolysis of the protein requires its prior ubiquitination, Overproduction of a derivative of ubiquitin wit h a modified Lys(48) strongly impairs mutant permease degradation, Thi s suggests that, Like other proteasome substrates, mutant permease mig ht be polyubiquitinated with Lys(48)-linked ubiquitin chains, These fi ndings provide an example of a yeast plasma membrane protein that is r outed to the 'ER degradation' pathway, and highlight the versatility o f the ubiquitin system.