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

Ribophorins I and II, as well as other components of the translocation apparatus, appear to represent a class of resident ER membrane proteins, that, in contrast with other well studied ER polypeptides, such as the El9 protein of adenovirus (P~bo et al., 1987; Nilsson et al., 1989) and UDPglucuronosyltransferase (Jackson et al., 1990) , do not contain retention signals at their extreme COOH-termini (Jackson et al., 1990) . Ribophorins form large macromolecular assemblies or complexes within the ER membrane (Kreibich et al., 1978a,b; Yu et al., 1989) and it seems likely that their retention in the ER simply results from the fact that, once incorporated into these complexes, they cannot gain access to the vesicles that mediate the transport to the Golgi apparatus of other proteins synthesized in the ER. Thus, the segregation of proteins that form large macromolecular complexes, although resulting from a true retention mechanism, would not require a signal common to all of them. Instead, the retention of each protein might only involve its interaction with another specific component of the macromolecular assembly.