Sequential intermediates in the pathway of intercompartmental transport in a cell-free system.

Two-stage incubations and the selective inhibitory effects of N-ethylmaleimide have revealed three steps in the transport of the vesicular stomatitis viral glycoprotein (G protein) between compartments of the Golgi. These are "priming" of the donor membrane, making G protein available for transfer to the acceptor Golgi stack; "transfer" of G protein to the acceptor stack to form a prefusion complex in which G protein is still separate from the GlcNAc transferase; and "fusion," the steps that result in the delivery of G protein to the same cisternal membranes that contain the GlcNAc transferase. Electron microscopy shows that priming of the donor membrane is accompanied by the formation of a uniform population of small (60-80 nm diameter) vesicles that bud from the rims of the cisternae of the Golgi stacks. This suggests the working hypothesis that the above steps correspond to stages in the budding and fusion of transport vesicles.