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

It has previously been observed that ribophorin I is a longlived protein of the RER (Rosenfeld et al., 1984 ; see also Fig. 8 A) . A comparison of the degradation rate of ribophorin I in transfected cells to that in control cells revealed that the overexpressed molecules were much more unstable (Fig. 8 B) . Since after 24 h of chase approximately equal amounts of labeled ribophorin I remained in transfected and control cells, it would appear that the overexpressed molecules turnover with a half-life of 8 h or less. It should be noted that in these transient transfection experiments only ~5 % of the cells express the foreign protein, so that in the expressing cells, ribophorin I is being synthesized at up to 50 times the control levels. This finding that the overexpressed portion of the ribophorin I molecules is degraded significantly faster than the endogenously expressed polypeptide is consistent with the notion that the high stability of endogenous ribophorin I results from its integration into an oligomeric assembly.