In yeast the export of small glycopeptides from the endoplasmic reticulum into the cytosol is not affected by the structure of their oligosaccharide chains

A "quality control" system associated with the endoplasmic reticulum (ER) t hat discriminates between misfolded proteins and correctly folded proteins is present in a variety of eukaryotic cells, including yeast, Recently, it has been shown that misfolded proteins that are N-glycosylated in the lumen of the ER are transported out of the ER, de-N-glycosylated by a soluble pe ptide:N-glycanase (PNGase) and degraded by action of the proteasome, It als o has been shown that small N-glycosylatable peptides follow a fate similar to that of misfolded proteins, i.e., glycosylation in the lumen of the ER, transport out of the ER, and de-N-glycosylation in the cytosol, These proc esses of retrograde glycopeptide transport and de-N-glycosylation have been observed in mammalian cells, as well as in yeast cells, However, little is known about the mechanism involved in the movement of glycopeptides from t he ER to the cytosol, Here we report a simple method for assaying N-glycosy lation/de-N-glycosylation by simple paper chromatographic and electrophoret ic techniques using an N-glycosylatable H-3-labeled tripeptide as a substra te. With this method, me confirmed the cytosolic localization of the de-N-g lycosylated peptide, which supports the idea that de-N-glycosylation occurs after the export of the glycopeptide from the lumen of the ER to the cytos ol, Further, me found that the variations in the structure of the oligosacc haride chain on the glycopeptide did not cause differences in the export of the glycopeptide, This finding suggests that the mechanism for the export of small glycopeptides may differ from that of misfolded (glyco)proteins.