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

A striking observation in this study was that, in BFAtreated cells, newly synthesized, but not preexisting intact ribophorin I molecules, were susceptible to O-glycosylation by the relocated Golgi enzymes. As a consequence, only a very small fraction of the total cell complement of endogenous ribophorins acquired the modification. The intact ribophorin molecules that received the modification (<50 % of those that were labeled during a 5-min pulse) did so during a brief period after their synthesis. In addition, overexpressed full-length ribophorin I molecules, synthesized in excess of the stoichiometric amounts present in untransfected cells, were also susceptible to O-glycosylation for a limited period after their synthesis. On the other hand, the truncated ribophorin molecules were suspectible to O-glycosylation throughout their lifetime and were quantitatively modified during the first 30-rain period after the addition of BFA. It seems likely that this difference between the intact and the truncated ribophorins results from a conformational change that only the intact polypeptide molecules undergo soon after their synthesis, and possibly also from their sequestration in the supramolecular assembly of the ER membrane that is responsible for their retention within the organelle. The relatively rapid degradation of full-length ribophorin molecules expressed at levels above the stoichiometric amounts of other components of the supramolecular assembly in the RER membrane is likely to be a consequence of their failure to be assembled normally. This would be analogous to the rapid turnover of other monomeric components of multimeric proteins that is observed when individual subunits are synthesized in excess (Bonifacino and Lippincott-Schwartz, 1991) .