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

marker. This is a classical ER marker which extends to structures very close to, if not part of, the cis-Golgi region (Griffiths et al., 1983) . In their study Tooze et al. (1984) found that the compartment where MHV buds was partially reactive for this marker, but the three-dimensional complexity of the budding compartment precluded an unequivocal interpretation as to whether or not this compartment was continuous with the rough ER. Again, after SLO permeabilization the structures containing the product of the glucose 6 phosphatase reaction were better visualized, especially the fine tubules in the perinuclear region. In general, the reaction product was equally patchy in the rough ER, nuclear en-velope and around the budding and budded virions (Fig. 6 ). The lumen of the compartment surrounding some virions reacted strongly while others were devoid of reaction product. This probably reflects a technical limitation of the method. The fine tubules in continuity with cisternal elements (presumably on the cis side of the Golgi) were also partially reactive (Fig. 6 , A and C). As shown in Fig. 6 , A and C the reaction product around the virions could often be seen to be continuous with the reaction in some Golgi cisternae. These tubules, as before, appeared to contact the nuclear envelope: some spotty reaction product occasionally extended into these tubules, at least their peripheral parts (Fig. 6 B) . cell. Note that the labeling (arrows) is associated with tubulo-cisternal structures that are predominantly on one side of the Golgi stack (G). These structures include budding virions (/arge arrowheads) or budded virions (small arrowheads). In B a virion is also shown in the rough ER that is not labeled for p58. N, nucleus. Bars, 100 nm.