The trans-Golgi network (TGN) of MDCK cells is exquisitely sensitive to the fungal metabolite
brefeldin A (BFA), in contrast to the refractory Golgi stack of these cells. At a concentration of 1 microgram/ml, BFA promoted extensive tubulation of the TGN while the medical Golgi marker
alpha-mannosidase II was not affected. Tubules emerging minutes after addition of the
drug contained both the apical marker
influenza hemagglutinin (HA), previously accumulated at 20 degrees C, and the fusion
protein interleukin receptor/TGN38 (TGG), a TGN marker that recycles basolaterally, indicating that, in contrast to TGN vesicles, TGN-derived tubules cannot sort apical and basolateral
proteins. After 60 minutes treatment with BFA, HA and TGG tubules formed extensive networks widely spread throughout the cell, different from the focused centrosomal localization previously described in non-polarized cells. The TGG network partially codistributed with an early endosomal tubular network loaded with
transferrin, suggesting that the TGG and endosomal networks had fused or that TGG had entered the endosomal network via surface recycling and endocytosis. The extensive structural alterations of the TGN were accompanied by functional disruptions, such as the extensive mis-sorting of
influenza HA, and by the release of the TGN marker
gamma-adaptin. Our results suggest the involvement of BFA-sensitive adaptor
proteins in TGN-->surface transport.