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

If our observation that transport from the rough ER into the intermediate compartment does not depend on vesicular transport is shown to be a general phenomenon, we also have to explain the data suggesting that this transport step is not a simple constitutive process, but may be stringently regulated (Lodish et al., 1987; Beckers and Balch, 1989) . The nature of this regulation is at present unclear. We suggest that one important aspect in this process that has until now been neglected is a consideration of the structure that defines the boundary between the rough ER and the intermediate compartment or between the rough ER and the smooth ER in cells such as rat liver parenchyma. It is conceivable that this structure may act as a gating mechanism that regulates the exit of both membrane and soluble components from the rough ER.