GOLGI TUBULE TRAFFIC AND THE EFFECTS OF BREFELDIN-A VISUALIZED IN LIVING CELLS

The Golgi complex is a dynamic organelle engaged in both secretory and retrograde membrane traffic. Here, we use green fluorescent protein-G olgi protein chimeras to study Golgi morphology in vivo. In untreated cells, membrane tubules were a ubiquitous, prominent feature of the Go lgi complex, serving both to interconnect adjacent Golgi elements and to carry membrane outward along microtubules after detaching from stab le Golgi structures. Brefeldin A treatment, which reversibly disassemb les the Golgi complex, accentuated tubule formation without tubule det achment. A tubule network extending throughout the cytoplasm was quick ly generated and persisted for 5-10 min until rapidly emptying Golgi c ontents into the ER within 15-30 s. Both lipid and protein emptied fro m the Golgi at similar rapid rates, leaving no Golgi structure behind, indicating that Golgi membranes do not simply mix but are absorbed in to the ER in BFA-treated cells. The directionality of redistribution i mplied Golgi membranes are at a higher free energy state than ER membr anes. Analysis of its kinetics suggested a mechanism that is analogous to wetting or adsorptive phenomena in which a tension-driven membrane flow supplements diffusive transfer of Golgi membrane into the ER. Su ch nonselective, flow-assisted transport of Golgi membranes into ER su ggests that mechanisms that regulate retrograde tubule formation and d etachment from the Golgi complex are integral to the existence and mai ntenance of this organelle.