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

Yung Shyeng Tsao's present address is Schering-Plough Research, Union, NJ. Akira Takatsuki's present address is RIKEN, the Institute of Physical and Chemical Research, Wako, Saitama, Japan. appear to be part of the apparatus that effects the transit)cation of polypeptides synthesized on membrane-bound ribosomes across the ER membrane (Yu et al., 1989 (Yu et al., , 1990 . Ribophorins and other components of the translocation apparatus, such as the signal peptidase and the receptors for the signal recognition particle and for the signal peptide itself, form a suprarnolecular complex, or proteinaceous network, within the ER membrane that, after treatment of rough microsomes with neutral detergents, can be recovered together with associated ribosomes in a rapidly sedimenting fraction (Kreibich et al., 1983; Marcantonio et al., 1984; Amar-Costesec et al., 1984; Wiedmann et al., 1987) . The incorpo-ration of ribophorins and other components of the translocation apparatus into this network may be responsible for the characteristic cisternal morphology of the rough portions of the ER, that is quite distinct from the tubular arrangement of membranes in the smooth portions of the organelle (Kreibich et al., 1978a,b) . Moreover, retention of the components of the translocation apparatus in the ER may simply result from the fact that, when assembled into the intramembranous network, they cannot gain access to the transport vesicles that normally flow from the ER to the Golgi apparatus.