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Glycopeptide export from the endoplasmic reticulum into cytosol is mediated by a mechanism distinct from that for export of misfolded glycoprotein.
When glycoproteins formed in the endoplasmic reticulum (ER) are misfolded, they are generally translocated into the cytosol for ubiquitination and are subsequently degraded by the proteasome. This system, the so-called ER-associated glycoprotein degradation, is important for eukaryotes to maintain the quality of glycoproteins generated in the ER. It has been established in yeast that several distinct proteins are involved in this translocation and degradation processes. Small glycopeptides formed in the ER are exported to the cytosol in a similar manner. This glycopeptide export system is conserved from yeast to mammalian cells, suggesting its basic biological significance for eukaryotic cells. These two export systems (for misfolded glycoproteins and glycopeptides) share some properties, such as a requirement for ATP and involvement of Sec61p, a central membrane protein presumably forming a dislocon channel for export of proteins. However, the machinery of glycopeptide export is poorly understood. In this study, various mutants known to have an effect on export/degradation of misfolded glycoproteins were examined for glycopeptide export activity with a newly established assay method. Surprisingly, most of the mutants were found not to exhibit a defect in glycopeptide export. The only gene that was found to be required on efficient export of both types of substrates was PMR1, the gene encoding the medial-Golgi Ca(2+)/Mn(2+)-ion pump. These results provide evidence that although the systems involved in export of misfolded glycoproteins and glycopeptides share some properties, they have exhibited distinct differences.
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