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

As shown above, all the truncated ribophorin I molecules (RI332) synthesized during a brief (5 rnin) pulse eventually become posttranslationally modified (Fig. 1) during a chase period in the presence of BFA, even when the drug was added 30 rain after labeling (Fig. 5) . On the other hand, <50% of the intact ribophorin molecules synthesized during a brief pulse undergo the posttranslational modification ( Fig. 1 B) . Moreover, in cells incubated with labeled monosaccharides (Fig. 3) , O-glycosylation of previously synthesized intact ribophorin I molecules does not appear to take place to any significant extent. This suggests that ribophorin I molecules are susceptible to O-glycosylation only during a limited period after their synthesis. It seems likely that this could correspond to the time required for the newly synthesized polypeptides to undergo a conformational change and/or eventually to become incorporated into the proteinaceous network in the RER where the mature protein has been shown to be contained (Kreibich et al., 1978a,b; Yu et al., 1989) .