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

The idea that the intermediate compartment represents a distinct functional domain(s) of the ER that extends close to, and probably includes a part of the first, morphologically distinct, (cis) Golgi cisterna agrees with the earlier ultrastructural studies of Lindsey and Ellisman (1985a,b) . It is also supported by extensive unpublished studies we have done on many cell types using a variety of markers including PDI, p58, p53, and rab2. The same pattern has been consistently seen in uninfected cells, in cells infected with vaccinia virus (Sodeik et al., 1993) and in cells infected with VSV (Pepperkok et ai., 1993) . Continuity between these structures also provides a simple explanation for the observed budding of a fraction of the virus population into the rough ER itself (Tooze et al., 1984 , and this study).