Retention of Glucose Units Added by the UDP-GLC:Glycoprotein Glucosyltransferase Delays Exit of Glycoproteins from the Endoplasmic Reticulum

CARLOS LABRIOLA, JUAN J. CAZZULO, AND ARMANDO J. PARODI

JOURNAL OF CELL BIOLOGY

ROCKEFELLER UNIV PRESS

Lugar: New York; Año: 1995 vol. 130 p. 771 - 779

0021-9525

Abstract. It has been proposed that the UDPGlc:glycoprotein glucosyltransferase, an endoplasmicreticulum enzyme that only glucosylates improperlyfolded glycoproteins forming protein-linkedGlclMan7.9- GlcNAc2 from the corresponding unglucosylatedspecies, participates together with lectin-likechaperones that recognize monoglucosylated oligosaccharidesin the control mechanism by which cells onlyallow passage of properly folded glycoproteins to theGolgi apparatus. Trypanosoma cruzi cells were used totest this model as in trypanosomatids addition of glucosidaseinhibitors leads to the accumulation of onlymonoglucosylated oligosaccharides, their formation beingcatalyzed by the UDP-Glc:glycoprotein glucosyltransferase.In all other eukaryotic cells the inhibitorsproduce underglycosylation of proteins and/or accumulationof oliogosaccharides containing two or three glucoseunits. Cruzipain, a lysosomal proteinase havingthree potential N-glycosylation sites, two at the catalyticdomain and one at the COOH-terminal domain,was isolated in a glucosylated form from cells grown inthe presence of the glucosidase II inhibitor 1-deoxynojirimycin.The oligosaccharides present at the singleglycosylation site of the COOH-terminal domain wereglucosylated in some cruzipain molecules but not inothers, this result being consistent with an asynchronousfolding of glycoproteins in the endoplasmic reticulum.In spite of not affecting cell growth rate or thecellular general metabolism in short and long term incubations,1-deoxynojirimycin caused a marked delayin the arrival of cruzipain to lysosomes. These resultsare compatible with the model proposed by whichmonoglucosylated glycoproteins may be transiently retainedin the endoplasmic reticulum by lectin-like anchorsrecognizing monoglucosylated oligosaccharides.