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

An important tool for our analysis was the use of streptolysin O in a complementary ultrastructural and biochemical approach (see also Wilson et al., 1993) . For the structural studies this pore forming toxin was useful in extracting the cytoplasm, thus allowing a clearer ultrastructural definition of the budding compartment. In the study by Tooze et al. (1988) the Epon section images suggested that the budding structures, which were close to or part of, the first, cis-Golgi cisterna were also continuous with the rough ER, However, the complexity of the membrane structures where the virus buds precluded an unequivocal conclusion. It is important to note that, whereas the fine structure of the MHV budding compartment may be altered by the infection, the same structure can be seen in uninfected cells. Thus, Tooze et al. (1988) carried out an extensive serial section analysis of uninfected sac(-) cells and described irregular tubular-cisternal elements reminiscent of the transitional elements shown earlier in the exocrine pancreas by Jamieson and Palade (1967) . The images shown by Tooze et al. (1988) also suggested that these structures were continuous with the rough ER. The only difference between these structures in uninfected and infected cells which was noted by Tooze et al. (1988) was that the organeUes were more dilated after infection.