‘ER degradation’ of a mutant yeast plasma membrane protein by the ubiquitin‐proteasome pathway

Abstract

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 Npi1p/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.—Galan, J.‐M., Cantegrit, B., Garnier, C., Namy, O., Haguenauer‐Tsapis, R. ‘ER degradation’ of a mutant yeast plasma membrane protein by the ubiquitin‐proteasome pathway. FASEB J. 12, 315–323 (1998)