Stepwise assembly of functionally active transport vesicles.

Budding of COP-coated vesicles (the likely carriers of newly synthesized proteins from the endoplasmic reticulum through the Golgi stack) from Golgi cisternae requires ADP-ribosylation factor (ARF), coatomer proteins from the cytosol, GTP, and fatty acyl-coenzyme A (CoA). The assembly of coated buds on the membranes requires coatomer, ARF, and GTP. When palmitoyl-CoA is added, membrane fission occurs at the coated bud, releasing coated vesicles. We show that COP-coated vesicles can be generated stepwise in vitro and isolated in a functionally active form, demonstrating that the minimal set of cytosolic components required for their formation as well as principal steps in their assembly have been identified.