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

In our Epon sections of MHV infected L cells in the absence of SLO we could find evidence for direct membrane continuities between the budding compartment and the rough ER. In SLO-treated cells, however, membrane continuities became more clear. Collectively, these data convinced us that the budding compartment is part of a continuum extending from the rough ER and from parts of the nuclear envelope down to a cisternal element on one side of the Golgi complex. These observations were further supported by the distribution of glucose 6 phosphatase reaction product. Furthermore, the budding compartment, as well as a cisternal element on one side of the Golgi stack labels for PDI, a marker that from immunofluoresence data is often considered to be exclusively present in the rough ER. We recently obtained a similar result in ts045 VSV-infected cells at 15°C; the structures where the VSV-G protein accumulated, including the first Golgi cisterna, also labeled for PDI (see below; Grifliths, G., R. Pepperkok, and T. Kreis, manuscript in preparation) . That the localization of PDI extends beyond the rough ER is also documented by Hobman et ai. (1992) , who showed that the post rough ER membranes where the E1 protein of rubella virus accumulates are also labeled with anti-PDI. Similarly, Oprins et al. (1993) found that in rat exocrine pancreatic cells the amount of labeling for PDI in the transitional elements was similar to that found over the rough ER.