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

Our data also suggest that the first morphological cisterna of the Golgi stack may also be a part of the intermediate compartment rather than representing a functional part of the bona fide Golgi complex. If this view is correct it would argue that the beginning of the functional Golgi complex (which we define as the compartment containing mannosi-dase I) would comprise the second cisterna on the cis side of the stack. In other words, we suggest that the first vesicular transport step would occur from the first cisterna into the second cisterna of the stack. This idea does not rule out the possibility that vesicles may also bud from more proximal regions of the intermediate compartment and still be targeted to the cisterna containing mannosidase I. We emphasize, however, that the three-dimensional organization of this ER/Golgi boundary is exceedingly complex. Clearly, more extensive structural analyses combined with double-labeling experiments using defined markers of both the intermediate compartment and the bona fide Golgi complex, and in particular mannosidase I, will be required in order to clarify this point.