Glucosylation of glycoproteins in Crithidia fasciculata.

High mannose-type, N-linked oligosaccharides devoid of glucose units may be glucosylated directly from UDP-Glc in mammalian, plant, fungal and protozoan cells. The glucosylated compounds thus formed (protein-linked Glc1Man5-9GlcNAc2, depending on the organisms) are immediately deglucosylated by glucosidase II, an enzyme located, the same as the glucosylating activity, in the endoplasmic reticulum. In order to evaluate the molar proportion of N-linked oligosaccharides that are glucosylated in the trypanosomatid Crithidia fasciculata (a microorganism transferring Man7GlcNAc2 in protein N-glycosylation) cells of the parasite were grown in the presence of [14C]glucose and concentrations of the glucosidase II inhibitors deoxynojirimycin and/or castanospermine that were several hundred-fold higher than those required to inhibit 50% of the activity of the protozoan enzyme. The inhibitors did not affect the cell growth rate and, although glucose analogs, did not interfere with the entry of glucose into the cells. About 40-43% of total N-linked oligosaccharides appeared to be glucosylated. As on the average there are several N-linked oligosaccharides per glycoprotein, more than 40-43% (but probably not all of them) are transiently glucosylated in the endoplasmic reticulum.