Compartmentation of the Golgi complex: brefeldin-A distinguishes trans- Golgi cisternae from the trans-Golgi network

Ulmer and Palade have recently shown that the protein, glycophorin, acquired O-linked sialic acid when present in the ER of BFA-treated erythroleukemia cells (45) . However, these workers noted that sialylation was not complete, even after 6 h. The O-linked oligosaccharides of glycophorin contain sialic acid both in ~-2,3 linkage to galactose, as well as in ot-2,6 linkage to GalNAc (43) . If the GalNAc-specific sialyltransferase is located in the trans-Golgi along with galactosyltransferase, whereas the Gal-specific sialyltransferase is located primarily in the "I'GN, only the GalNAcspecific enzyme would redistribute to the ER in BFA. Under these conditions, O-linked oligosaccharides would become partially sialylated, while N-linked sugars would not. If this is correct, the findings of Ulmer and Palade (45) would be entirely consistent with those reported here and elsewhere (9, 28) .