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

If the ribophorins rapidly became resistant to the action of the Golgi glycosyltransferases as a consequence of their incorporation into an oligomeric assembly, one would expect that overexpression of one of these proteins, so that its levels far exceeded those of other components of the assembly, would lead to much more efficient O-glycosylation. Surprisingly, when the full-length ribophorin I was transiently overexpressed in HeLa cells no O-glycosylation was observed when BFA treatment was initiated after completion of ribophorin I synthesis (Fig. 7 B, lanes d-f) . Furthermore, the level of O-glycosylation of ribophorin I observed when synthesis occurred in the presence of BFA did not markedly change (Fig. 7 B, lanes a-c) when compared with that seen in control cells ~ig. 7 A, lanes a-c) . This result suggests that ribophorin I, before being integrated into a heterooligomeric complex, undergoes a conformational change that renders it resistant to O-glycosylation in BFA-treated cells.