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

THOUGH the boundary region between the ER and the Golgi complex has been extensively investigated during the past few years, many questions remain about the organization of this part of the cell. Foremost among these is whether a separate intermediate compartment exists or whether this structure is functionally continuous with the ER. The available evidence strongly suggests that both in mammalian cells (Beckers et al., 1990) and in yeast (Rexach and Schekman, 1991) at least one vesicular transport step exists between the rough ER and the Golgi complex. A separate intermediate compartment, as has been postulated by several authors (Warren, 1987; Pelham, 1989; MeUman and Simons, 1992) would necessitate two distinct vesicular transport steps, one from the ER to the intermediate compartment and the other from this site to Golgi. In contrast, if the intermediate compartment is continuous with the ER, transport to the Golgi complex would require a single vesicular transport step. This region between the ER and the Golgi has been given a number of different names, including the salvage compartment (Warren, 1987) , the cis-Golgi network (CGN; Duden et al., 1991b; Mellman and Simons, 1992) , the ER-Golgi intermediate compartment (ERGIC; Hauri and Schweizer, 1992) and the "budding compartment" for coronaviruses (Tooze et al., 1988; Griffiths and Rottier, 1992) .