O-glycosylation of intact and truncated ribophorins in brefeldin A- treated cells: newly synthesized intact ribophorins are only transiently accessible to the relocated glycosyltransferases

That the BFA-induced modification of ribophorin I does not involve, or even require, the presence of the N-linked oligosaccharide chain was definitively established by the finding (Fig. 1 B) that BFA treatment led to a decrease in the electrophoretic mobility of both the normal full length and the truncated (RI332) molecules, even when these were synthesized in the presence of tunicamycin, a drug that prevents the addition of N-linked oligosaccharide chains to newly synthesized polypeptides (Takatsuki et al., 1975) . Moreover, in this experiment it is apparent that the endogenous intact ribophorin I molecules also undergo a similar decrease in electrophoretic mobility although only a fraction (<50%) of the population of the newly synthesized native ribophorin I molecules was eventually modified ( Fig. 1 B , R/*m). The possibility that in BFA-treated cells the ribophorin molecules were modified by the addition of phosphate or sulfate groups was eliminated by the finding that prolonged incubation with medium containing radioactive phosphate (Capasso et al., 1989) or sulfate (Hutmer, 1988) did not result in detectable labeling of either the intact or truncated ribophorin I molecules (results not shown). We therefore examined the possibility that the modified ribophorins had acquired O-linked oligosaccharides, which are normally not present in these molecules.