Characterization of the budding compartment of mouse hepatitis virus: evidence that transport from the RER to the Golgi complex requires only one vesicular transport step

After GTP-yS treatment/~-cop vesicles and buds accumulate in structures that contain budded and budding virions, which are enriched in GalNAc and also contain PDI. Since our data show that vesicular transport is not required for Gal-NAc addition, the logical consequence is that the budding compartment (or intermediate compartment) is the first compartment along the biosynthetic pathway from which /~-cop vesicles appear to bud. In agreement with this notion /3-cop was found in a recent study to localize mainly to cis-Golgi structures (Oprins et al., 1993) and a yeast homologue of E-cop, SEC21, has been found to be required for ER to Golgi transport in yeast (Hosobuchi et al., 1992) . In addition, when the transport of the newly synthesized G protein of VSV into the Golgi stack is blocked at 15°C much of the G protein is localized in/3-cop-containing buds found in close proximity to the Golgi stack (Griftiths, G., R. Pepperkok, and T. Kreis, manuscript in preparation). Further, in a parallel study, microinjection of anti-E-cop antibodies was shown to block the transport of the G protein into the Golgi stack; the compartment where the G protein accumulated under these conditions overlapped significantly at the immunofluorescence level with the intermediate compartment marker p53 (Pepperkok et al., 1993) .