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 previous studies (Roth, 1984; Pavelka and Ellinger, 1985; Deschuyteneer et al., 1988; Ihida et al., 1991) HPA was found in a variety of cells to localize predominantly to the cis-Golgi region. Our studies, both with and without SLO treatment, confirm these findings and importantly, showed that the rough ER was essentially devoid of labeling in infected L cells (as well as in other cells, not shown). Since M is the major protein made upon infection this observation suggests that, at steady state no significant amount of GalNAc-containing M protein has recycled back to the rough ER. The tubular membranes containing the budding and budded virus were significantly labeled with HPA (Fig. 7, A-D). There was also some labeling of the Golgi stack, the plasma membrane as well as structures we believe to be endocytic organeUes (not shown). In uninfected L cells the labeling was qualitatively similar but the extent of labeling was small when compared with infected cells (not shown).