Presence of Golgi remnant membranes in the cytoplasm of brefeldin A-treated cells

Brefeldin A (BFA) rapidly blocks secretion, induces disassembly of the Golgi complex and causes a redistribution of Golgi components into the endoplasmic reticulum (ER). In addition to these effects on the exocytotic pathway, BFA has been shown to induce fusion of endosomal membranes with the trans-Golgi network in some cell types. To better understand the mechanism through which BFA disrupts the exocytotic traffic, we have examined its effects on the ultrastructural organization of the Golgi complex. Within minutes of exposure to BFA, the Golgi cisternae were fragmented into a number of small tubules and vesicles, many of which had a non-clathrin coat on their cytosolic surface. In addition, a complex structure consisting of anastomosing tubules and associated vesicles appeared in the cytoplasm of cells incubated with BFA for 10 min or longer. These tubular networks were permanent, distinct structures separated from the ER cisternae. They contained cis, middle, and trans Golgi proteins as well as the lipid analogue C5-DMB-ceramide. Furthermore, secretory proteoglycans en route through the Golgi were retained in the lumen of the tubular networks. As judged by the endocytosis of cationized ferritin, endosomes do not contribute to the formation of these tubular networks. Reassembly of the Golgi complex after BFA incubation involved fragmentation and reorganization of the tubular networks as well as fusion with vesicles budded from the ER. We conclude that although in the presence of BFA the bulk of Golgi membranes are induced to fuse with the ER, as indicated by the detection of Golgi markers in this organelle, a fraction of these membranes remain in the cytoplasm organized as Golgi remnants.