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

The striking relocation of Golgi components caused by BFA was also manifested by the appearance of binding sites for the lectins RCA-I and WGA in the ER. The former lectin binds to exposed galactose residues (Baenziger and Fiete, 1979) and the latter to exposed N-acetyl glucosamine and sialic acid residues (Bhavanandan and Katlic, 1979) . As expected from previous studies with myeloma cells (Tartakoff and Vassalli, 1983) , we found that in control HeLa cells intracellular binding sites for both lectins were almost exclusively present in the trans-region of the Golgi apparatus and not to any significant extent in the ER or nuclear envelope. After BFA treatment, however, even in cells in which protein synthesis was inhibited with cycloheximide, both lectins intensely labeled the ER and the nuclear envelope. These electron microscopic results are in marked contrast with immunofluorescence observations by Lippincott-Schwartz et al. (1989) of cultured normal rat kidney (NRK) cells, in which BFA treatment did not lead to a relocation of WGA binding sites. In later work these authors observed that BFA caused the complete relocation to the ER of the trans-Golgi enzyme galactosyltransferase and, therefore, suggested that WGA binding sites are not relocated because they are predominantly localized in the trans-Golgi network (Lippincott-Schwartz et al., 1990) . This may indeed be the case in NRK cells, but it is also possible that the lectin binding sites that appear in the ER after BFA treatment do not all correspond to redistributed previously existing Golgi glycoproteins. Rather, to an extent which may vary with the cell type, such sites may be created by a sialyltransferase relocated from the medial or trans-Golgi region, operating on ER resident proteins, as well as on relocated cisand medial Golgi proteins. In considering the location of lectin binding sites within the Golgi apparatus, one must note that an integral membrane protein confined to cisand medial Golgi cisternae has been identified that contains sialic acid residues on both its N-linked and O-linked oligosaccharide chains (Yuan et al., 1987) . This led to the alternative suggestions that this protein is either modified in the trans-Golgi network and then returned to the more proximal region of the Golgi apparatus, or that an as yet uncharacterized sialyltransferase is located in cisand medial cisternae.