'ER degradation' of a mutant yeast plasma membrane protein by the ubiquitin-proteasome pathway

The yeast plasma membrane, uracil permease, undergoes ubiquitin-dependent endocytosis and subsequent degradation in the vacuole via a process 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 the intracellular fate of a mutant form of uracil permease carrying a three amino acid insertion in a cytoplasmic loop, Most of this protein is not deployed beyond the ER, and is degraded by the 26S proteasome, Mutant permease degradation is almost unaffected in cells with impaired Npilp/Rsp5p, but is dependent on the Ubc6p and Ubc7p ubiquitin-conjugating enzymes, suggesting that proteolysis of the protein requires its prior ubiquitination, Overproduction of a derivative of ubiquitin with a modified Lys(48) strongly impairs mutant permease degradation, This suggests that, Like other proteasome substrates, mutant permease might be polyubiquitinated with Lys(48)-linked ubiquitin chains, These findings provide an example of a yeast plasma membrane protein that is routed to the 'ER degradation' pathway, and highlight the versatility of the ubiquitin system.