Biosynthesis and processing of ribophorins in the endoplasmic reticulum

It is not at all clear what mechanisms could effect the permanent segregation of certain polypeptides in ER membranes, since no common features have been identified among the ER membrane proteins so far studied. It has been noted that several ER membrane proteins have substantial portions of their polypeptide chains exposed on the cytoplasmic aspect of the membrane, but this feature is not one shared by the ribophorins (28, 29) or by the El glycoprotein of coronavirus, which is thought to trigger budding of the virion into the ER (48). Obviously, glycosylation is not required for retention in the ER since, in contrast to the ribophorins and the recently characterized HMG-CoA reductase (6, 7, 3 l), all other ER proteins investigated, such as cytochrome P-450 (3) and its reductase, cytochrome b5 and its reductase, epoxide hydrolase (29, 41), and Ca ++ ATPase (8, 37) are not glycoproteins. Moreover, retention of the signal for co-translational insertion, which is characteristic of all ER proteins studied except the ribophorins, is also known to take place for several plasma membrane proteins (39, 42, 50, 51). It is perhaps important to note that ribophorins are thought to form an intramembranous meshwork in the rough ER and that the tendency of these proteins to interact with each other may be sufficient to ensure that they are not transported from this organelle to the Golgi apparatus. Thus, oligomeric interaction of at least some other proteins with the ribophorins could also ensure their retention in the ER.