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 the present study the labeling of MHV infected cells with anti H-cop antibodies was too weak to make a definitive conclusion about its localization. Low amounts of label were found on the Golgi stack as well as in association with the membranes of the MHV budding structure (not shown). However, after treatment of SLO-permeabilized cells for 30 rain at 37°C with 50 I~M GTP'yS a dramatic increase in the number of ~-cop containing vesicles was seen close to, and around the Golgi stack (Figs. I0, I I B, and 12 C). As shown in Figs. 10 and 12 C, the compartment into which MHV budded contained a high concentration of/3-cop buds and vesicles. Some of the membrane structures containing the H-cop-reactive buds were contiguous, and possibly continuous, with the rough ER (Figs. I0 B and II B); these ~-cop structures were also reactive for the lectin HPA (Figs. 10 B and 12 C). These data argue strongly that/~-cop vesicles are formed from the MHV budding compartment. This idea is also supported by the Epon section analysis (see Fig. 2 ).