N-glycosylation affects endoplasmic reticulum degradation of a mutated derivative of carboxypeptidase yscY in yeast

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Abstract
The endoplasmic reticulum (ER) of eukaryotic cells contains a quality control system, that is required for the proteolytic removal of aberrantly folded proteins that accumulate in this organelle. We used genetic and biochemical methods to analyse the involvement of N‐glycosylation in the degradation of a mutant derivative of carboxypeptidase yscY in the ER of the yeast Saccharomyces cerevisiae. Our results demonstrate that N‐glycosylation of this protein is required for its degradation since an unglycosylated species is retained stably in the ER. Cells that were devoid of the ER‐processing α1,2‐mannosidase showed reduced degradation of the glycosylated substrate protein. Disruption of CNE1, a gene encoding a putative yeast homologue for calnexin, did not exhibit any effects on the degradation of this substrate protein in vivo. Also, the α1,2‐mannosidase‐dependent reduction in the degradation rate did not show any correlation with the function of the CNE1 gene product. Our results suggest that the ER of yeast contains a glycosylation‐dependent quality control system, as has been shown for higher eukaryotic cells.