Glycoprotein synthesis in lysolecithin-treated cells using sugar nucleotides as glycosyl donors

The 3T3 cells were treated with 50 mu g/ml lysolecithin (LL) followed by the addition of exogenously supplied radiolabelled sugar nucleotides to serve as direct glycosyl donors. These were found to be 1.5 to 3.0 times more active than untreated cells in their glycosyl transferase activities depending on the particular sugar nucleotide used. Mannosyl transferase activity was not inhibited by 2-deoxyglucose or mannose-1-phosphate, indicating that the sugar nucleotide remained intact throughout the assay period. Preincubation of the cells with tunicamycin caused an 85% decrease in mannosyl transfer, which suggested that the normal pathway of glycosylation via lipid intermediates was still operable in the treated cells. Fractionation of control and LL-treated cells after incubation with UDP[3H]galactose revealed that only microsomal and cytosolic proteins from the treated cells were radioactive. Thus, intracellular labelling of permeabilized cells was allowed. About 80% of the radiolabeled product was glycoprotein in nature, based upon its solubilization with pronase.