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

The PDI labeling extends from the rough ER down to the structures containing the budding virions (large arrowheads) that label for the M protein. Note that one budding virion is very closely situated to the nuclear envelope (N, nucleus). The double arrowheads show an apparent continuity between the rough ER and the budding compartment. In B, labeled as in A, the small amount of PDI labeling over the virion containing structures is evident. In C, treated with GTP~,S and double labeled for/3-cop (10 nm gold) and HPA (6 nm gold) the membranes of the structures enriched in virions (arrowheads) and budding virions (double arrowheads) is directly continuous with a/~-copcontaining bud (small arrowhead). On the right of this image a ~-cop reactive bud/ vesicle (small arrow) is also labeled with the lectin. Bars, 100 nm. rough ER and the budding compartment (Figs. 11 and 12, A and B). No obvious difference was observed with or without SLO. But as before, the identification of the structure was much easier in SLO-treated cells and this facilitated a quantitative analysis (Table I) . These data confirmed that both the rough ER/nuclear envelope and the structures where MHV buds are enriched in PDI. The amount of label over the ribosome-containing membranes was about twice as high as over the budding compartment (Table I) . This difference should be interpreted with caution however, since the labeling efficiency may be different over the different structures (Griffiths, 1993) . Since we used GTPTS in many experiments in this study we also investigated whether this drug had any quantitative effect on the labeling of PDI. As shown in Table I , both the amount and the distribution of the label was essentially unchanged in the presence of GTPTS.