Biosynthesis and processing of ribophorins in the endoplasmic reticulum

Golgi apparatus, since proteins of these different classes may be processed by the same Golgi enzymes (12, 33) , and some have been found in the same Golgi cisternae (44, 53) . On the other hand, it has not yet been determined if the biosynthesis of membrane polypeptides characteristic of the ER requires their transfer to the Golgi apparatus and if this organelle plays a role in ensuring the segregation of certain proteins within the ER. Significant amounts of several ER proteins have been detected in fractions of Golgi membranes (5, 17-19, 20, 22, 23) , and it has been proposed (47) that an important function of the cis portion of the Golgi apparatus is the refinement of a crude export from the ER, from which membrane proteins to be segregated in the ER must be continuously removed and returned to this organelle. If this is the case, it may be expected that some ER membrane polypeptides are subjected to posttranslational modifications carried out by Golgi enzymes, such as those that affect oligosaccharide chains in glycoproteins. Of the many ER proteins characterized to date, however, only the ribophorins, which are restricted to the rough ER and appear to play a role in ribosome binding (26) (27) (28) , and HMG-CoA reductase, an enzyme of cholesterol biosynthesis (31) , have been found to bear covalently bound oligosaccharides, and therefore may serve as natural probes to study this question.