N-GLYCOSYLATION AFFECTS ENDOPLASMIC-RETICULUM DEGRADATION OF A MUTATED DERIVATIVE OF CARBOXYPEPTIDASE YSCY IN YEAST

The endoplasmic reticulum (ER) of eukaryotic cells contains a quality control system, that is required for the proteolytic removal of aberra ntly folded proteins that accumulate in this organelle. We used geneti c and biochemical methods to analyse the involvement of N-glycosylatio n in the degradation of a mutant derivative of carboxypeptidase yscY i n the ER of the yeast Saccharomyces cerevisiae. Our results demonstrat e that N-glycosylation of this protein is required for its degradation since an unglycosylated species is retained stably in the ER. Cells t hat were devoid of the ER-processing alpha 1,2-mannosidase showed redu ced degradation of the glycosylated substrate protein. Disruption of C NE1, a gene encoding a putative yeast homologue for calnexin, did not exhibit any effects on the degradation of this substrate protein in vi vo. Also, the alpha 1,2-mannosidase-dependent reduction in the degrada tion rate did not show any correlation with the function of the CNE1 g ene product. Our results suggest that the ER of yeast contains a glyco sylation-dependent quality control system, as has been shown for highe r eukaryotic cells.