Retention of glucose units added by the UDP-GLC:glycoprotein glucosyltransferase delays exit of glycoproteins from the endoplasmic reticulum.

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Abstract
It has been proposed that the UDP-Glc:glycoprotein glucosyltransferase, an endoplasmic reticulum enzyme that only glucosylates improperly folded glycoproteins forming protein-linked Glc1Man7-9-GlcNAc2 from the corresponding unglucosylated species, participates together with lectin-like chaperones that recognize monoglucosylated oligosaccharides in the control mechanism by which cells only allow passage of properly folded glycoproteins to the Golgi apparatus. Trypanosoma cruzi cells were used to test this model as in trypanosomatids addition of glucosidase inhibitors leads to the accumulation of only monoglucosylated oligosaccharides, their formation being catalyzed by the UDP-Glc:glycoprotein glucosyltransferase. In all other eukaryotic cells the inhibitors produce underglycosylation of proteins and/or accumulation of oliogosaccharides containing two or three glucose units. Cruzipain, a lysosomal proteinase having three potential N-glycosylation sites, two at the catalytic domain and one at the COOH-terminal domain, was isolated in a glucosylated form from cells grown in the presence of the glucosidase II inhibitor 1-deoxynojirimycin. The oligosaccharides present at the single glycosylation site of the COOH-terminal domain were glucosylated in some cruzipain molecules but not in others, this result being consistent with an asynchronous folding of glycoproteins in the endoplasmic reticulum. In spite of not affecting cell growth rate or the cellular general metabolism in short and long term incubations, 1-deoxynojirimycin caused a marked delay in the arrival of cruzipain to lysosomes. These results are compatible with the model proposed by which monoglucosylated glycoproteins may be transiently retained in the endoplasmic reticulum by lectin-like anchors recognizing monoglucosylated oligosaccharides.

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