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

We have shown that the 300-kD man6P receptor, when pulse-labeled within the ER of BFA-treated cells, acquires numerous N-linked galactose residues and many fewer sialic acid residues. These data confirm the findings of Lippincott-Schwartz et al. (29) who have used immunofluoresccnce to show that the trans-Golgi enzyme, galactosyltransferase, returns to the ER in the presence of BFA. In addition, our experiments suggest that sialyltransferase has a different fate in BFA than galactosyltransferase, since the half time for sialic acid addition to proteins within the ER (,~60 h) was 30 times greater than that measured for galactose addition. The simplest explanation for these findings would be that sialyltransferase, a component of the TGN, is not redistributed to the ER in BFA, while the trans-Golgi enzyme, galactosyltransferase, is. If true, this supports the proposal that the trans-Golgi and TGN are distinct compartments (21) .