Uncoupling of brefeldin a-mediated coatomer protein complex-I dissociation from Golgi redistribution.

The Golgi complex functions in transport of molecules from the endoplasmic reticulum (ER) to the plasma membrane and other distal organelles as well as in retrograde transport to the ER. The fungal metabolite brefeldin A (BFA) promotes dissociation of ADP-ribosylation-factor-1 (ARF1) and the coatomer protein complex-I (COP-I) from Golgi membranes, followed by Golgi tubulation and fusion with the ER. Here we demonstrate that the cationic ionophore monensin inhibited the BFA-mediated Golgi redistribution to the ER without interfering with ARF1 and COP-I dissociation. Preservation of a perinuclear Golgi despite COP-I and ARF1 dissociation enables addressing the involvement of these proteins in anterograde ER to Golgi transport. The thermo-reversible folding mutant of vesicular stomatitis virus G protein (VSVGtsO45) was retained in the ER in the presence of both monensin and BFA, thus supporting ARF1/COP-I participation in ER-exit processes. Live-cell imaging revealed that BFA-induced Golgi tubulation persisted longer in the presence of monensin, suggesting that monensin inhibits tubule fusion with the ER. Moreover, monensin also augmented Golgi-derived tubules that contained the ER-Golgi-intermediate compartment marker, p58, in the absence of BFA, signifying the generality of this effect. Taken together, we propose that monensin inhibits membrane fusion processes in the presence or absence of BFA.