In Vitro Generation from the trans-Golgi Network of Coatomer-Coated Vesicles Containing Sialylated Vesicular Stomatitis Virus-G Protein

Abstract We describe an in vitro system in which post-Golgi vesicles containing metabolically labeled, sialylated, vesicular stomatitis virus (VSV) G protein molecules (VSV-G) are produced from the trans -Golgi network (TGN) of an isolated Golgi membrane fraction. This fraction is prepared from VSV-infected Madin–Darby canine kidney (MDCK) cells in which the 35 S-labeled viral envelope glycoprotein was allowed to accumulate in the trans -Golgi network during a prolonged incubation at 20°C. The vesicles produced in this system are separated from the remnant Golgi membranes by differential centrifugation or by velocity sedimentation in a sucrose gradient. Vesicle production, quantified as the percentage of labeled VSV-G released from the Golgi membranes, is optimal at 37°C and does not occur below 20°C. It requires GTP and the small GTP-binding protein Arf (ADP-ribosylation factor), as well as coat protein type I (COPI) coat components (coatomer) and vesicle scission factors—one of which corresponds to the phosphatidylinositol transfer protein (PITP). Formation of the vesicles does not require GTP hydrolysis which, however, is necessary for their uncoating. Thus, vesicles generated in the presence of the nonhydrolyzable GTP analogs, GTPγS or GMP–PNP, retain a coatomer coat visible in the electron microscope, sediment more rapidly in sucrose density gradients than those generated with ATP or GTP, and can be captured with anticoatomerantibodies. The process of coatomer-coated vesicle formation from the TGN can be dissected into two distinct sequential phases, corresponding to coat assembly/bud formation and vesicle scission. The first phase is completed when Golgi fractions are incubated with cytosolic proteins and nonhydrolyzable GTP analogs at 20°C. The scission phase, which leads to vesicle release, takes place when coated Golgi membranes, recovered after phase I, are incubated at higher temperatures in the presence of cytosolic proteins. The scission phase does not take place if protein kinase C inhibitors are added during the first phase, even though these inhibitors do not prevent membrane coating and bud formation. The phosphorylating activity of a protein kinase C, however, plays no role in vesicle formation, since this process does not require ATP.