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

IOCHEMICAL fractionation experiments and the immunocytochemical localization of specific glycosyltransferases and their products have led to the notion that the Golgi complex is divided into at least three distinct subcomparmaents (11, 21, 23, 27) . The first compartment, termed the cis-Golgi, is thought to house N-acetylglucosamine (Glc-NAc)t-l-phosphodiester ot-N-acetylglucosaminidase (18) , an enzyme involved in the construction of mannose 6-phosphate (man6P) residues on lysosomal enzyme oligosaccharide side chains. The medial Golgi houses GlcNAc transferase I (12) , and galactosyltransferase is located predominantly in trans-Golgi cisternae (37) . Since proteins pass through the Golgi by a series of vesicular transfers (39) from the cisto the medial to the trans-Golgi (3, 42) , the assembly of N-linked oligosaccharide chains is regulated by the order in which proteins gain access to particular mannosidases and glycosyltransferases (27) . Beyond oligosaccharide assembly, the significance of Golgi compartmentation is not fully understood. phate receptors, when pulse-labeled in the ER of BFAtreated cells, readily acquired N-linked galactose, yet only rarely acquired sialic acid residues. We show here that the failure of proteins to acquire sialic acid is most likely due to a failure of sialyltransferase to relocate to the ER, rather than an inability of the enzyme to act within the ER. Our results indicate that unlike the cis-, medial, and trans-Golgi cisternae, the TGN is insensitive to BFA action.