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

Our observations that in BFA-treated ceils galactose residues are incorporated into the truncated ribophorin I variant and that neuraminidase treatment increases the electrophoretic mobility of this protein, provide biochemical evidence that oligosaccharide-modifying enzymes of the trans-region of the Golgi apparatus become relocated to the ER after BFA treatment. The presence of a galactosyltransferase in the ER of BFA-treated cells was previously demonstrated immunocytochemically (Lippincott-Schwartz et al., 1990) , as well as by the finding that mannose-6-phosphate receptor molecules synthesized in the presence of the drug bind to RCA-I lectin columns (Chege and Pfeffer, 1990) . The sialic acid residue(s) that are present in the truncated ribophorin I and are removed by neuraminidase must be part of O-linked oligosaccharide chain(s), since the protein remained endo H sensitive and, therefore, its single N-linked oligosaccharide did not contain sialic acid.