COPI- and COPII-coated vesicles bud directly from the endoplasmic reticulum in yeast

The cytosolic yeast proteins Sec13p-Sec31p, Sec23p-Sec24p, and the small GTP-binding protein Sar1p generate protein transport vesicles by forming the membrane coat termed COPII. We demonstrate by thin section and immunoelectron microscopy that purified COPII components form transport vesicles directly from the outer membrane of isolated yeast nuclei. Another set of yeast cytosolic proteins, coatomer and Arf1p (COPI), also form coated buds and vesicles from the nuclear envelope. Formation of COPI-coated, but not COPII-coated, buds and vesicles on the nuclear envelope is inhibited by the fungal metabolite brefeldin A. The two vesicle populations are distinct. However, both vesicle types are devoid of endoplasmic reticulum (ER) resident proteins, and each contains targeting proteins necessary for docking at the Golgi complex. Our data suggest that COPI and COPII mediate separate vesicular transport pathways from the ER.