Involvement of GTP-binding “G” proteins in transport through the Golgi stack

GTPγS irreversibly inhibits protein transport between successive compartments of the Golgi stack in a cell-free system. Fluoride, potentiated by the addition of aluminum ion, also causes a strong inhibition. These are hallmarks of the involvement of a guanine nucleotide-binding or regulatory “G” protein. Inhibition by GTPγS requires a cytosolic inhibitory factor that binds to Golgi membranes during inhibition. Preincubation experiments reveal that GTPγS blocks the function of acceptor Golgi but not donor Golgi membranes. More specifically, a processing step in between vesicle attachment and the actual fusion event seems to be affected. Electron microscopy demonstrates a corresponding 5-fold accumulation of non-clathrin-coated buds and vesicles associated with the Golgi cisternae during inhibition by GTPγS.