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

Under normal conditions, man6P receptors recycle from the cell surface to the TGN, where they come into contact with sialyltransferase (10, 17, 26, 35), but not galactosyltransferase (10) . We reasoned that if sialyltransferase failed to return to the ER in BFA, this enzyme should remain in the TGN, and thus be able to act upon man6P receptors arriving in that compartment. To test this, we used an experimental scheme devised by Duncan and Kornfeld (10) to measure the transport of man6P receptors from the cell surface back to the TGN. The approach uses CHO clone 13 cells, in which glycoproteins lack galactose residues and thus can be labeled at the plasma membrane using galactosyltransferase and UDP-[3H]galactose. The transport of man6P receptors from the surface to the TGN is then monitored by the addition of sialic acid residues to their 3H-labeled oligosaccharide side chains (10) .