BETA-COP IS ESSENTIAL FOR TRANSPORT OF PROTEIN FROM THE ENDOPLASMIC-RETICULUM TO THE GOLGI IN-VITRO

Using a novel in vitro assay which allows us to distinguish vesicle bu dding from subsequent targeting and fusion steps, we provide the first biological evidence that beta-COP, a component of non-clathrin-coated vesicles believed to mediate intraGolgi transport, is essential for t ransport of protein from the ER to the cis-Golgi compartment. Incubati on in the presence of beta-COP specific antibodies and F(ab) fragments prevents the exit of vesicular stomatitis virus glycoprotein (VSV-G) from the ER. These results demonstrate that beta-COP is required for t he assembly of coat complexes mediating vesicle budding. Fractionation of rat liver cytosol revealed that a major biologically active form o f beta-COP was found in a high molecular pool (>1,000 kD) distinct fro m coatomer and which promoted efficient vesicle budding from the ER. S urprisingly, rab1B could be quantitatively coprecipitated with this be ta-COP containing complex and was also essential for function. We sugg est that beta-COP functions in an early step during vesicle formation and that rab1B may be recruited as a component of a precoat complex wh ich participates in the export of protein from the ER via vesicular ca rriers.