COATED VESICLE ASSEMBLY IN THE GOLGI REQUIRES ONLY COATOMER AND ARF PROTEINS FROM THE CYTOSOL

TRANSPORT vesicles derived from the Golgi apparatus are thought to med iate biosynthetic transport across the Golgi stack1-3. These vesicles are surrounded by a protein coat4 whose principal constituents are coa tomer (a complex of seven distinct subunits or COPs)5 and ADP-ribosyla tion factor (ARF, an N-myristylated small GTP-binding protein). The co at proteins of the COP-coated vesicles were originally defined by ultr astructural criteria4-9, however, and it is possible that important bu t minor coat proteins or cytoplasmic proteins needed for coat assembly may have been overlooked. Here we show that coatomer and ARF are the only cytoplasmic proteins needed for the assembly and budding of COP-c oated vesicles. COP-coated buds may therefore form essentially by self -assembly from Golgi cisternae after an initial step in which GTP is u sed to allow ARF binding.