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

When cells were harvested immediately after surface labeling, essentially no radioactivity was found in excluded column fractions (Fig. 4, open circles) , as would be expected, since at this time, man6P receptor glycopeptides should not contain sialic acid. In contrast, sialic acid addition was readily observed when cells were recultured for 6 h in either the presence or the absence of BFA, as monitored by the presence of 3H-glycopeptides in the void volume fraction (Fig. 4, solid symbols) . Although the extent of sialylation was reproducibly lower ('o65 %) in the presence of BFA than in its absence, significant addition of sialic acid was detected under both conditions. The lower level of sialylation observed in BFA may be explained, in part, by a 20 % decrease in the rate of endocytosis observed in the presence of 10/zg/ml BFA (32) . In summary, this experiment demonstrates that at least a large proportion of sialyltransferase activity was present in a compartment reached by cell surface man6P receptors in BFA-treated cells. This compartment cannot represent the ER, since man6P receptors pulse-labeled there were not modified by sialyltransferase (Fig. 3) .