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 we have characterized the organization of the budding compartment in more detail in streptolysin O (SLO) permeabilized cells. The use of SLO, by extracting cytoplasmic components, greatly facilitated the visualization of the fine tubules that appear to make up a significant fraction of the intermediate compartment (see also Lindsey and Ellisman, 1985a,b) . Continuities between the rough ER and the MHV budding compartment then became easier to recognize. The use of thawed cryosections enabled us to localize both the binding of a GalNAc-specific lectin as well as two markers of the intermediate compartment, p58 and the small GTPase tab2. In a complementary biochemical approach, using intact cells as well as SLO permeabilization, we investigated the conditions required for the M protein to acquire, first, GalNAc, and second, the Golgi modifications. Collectively, our data are consistent with a model whereby a single vesicular transport step would mediate traffic between the ER and the Golgi complex.