The rubella virus E1 glycoprotein is arrested in a novel post-ER, pre- Golgi compartment

To obtain further information on the nature of the site of E1 accumulation, we examined CHOE1 cells by EM. In routine Epon sections, structures composed of masses of tubular, smooth membranes were evident that were not present in non-transfected controls or in CHOE2E1 cells. These masses were surrounded by RER (Fig. 2 , A and C) and were located adjacent to Golgi stacks (Fig. 2, A and D) . In favorable sections continuity with RER membranes was seen (Fig. 2 B) . The smooth membranes differ from RER cisternae in that they are more tubular, have a narrower lumen, and form complicated networks. The tubular networks were relatively homogeneous in that other organelles were excluded from these areas (Fig. 2 C) . After immunogold labeling of ultrathin cryosections of CHOE1 cells, the membranes of the tubular network were heavily labeled for El, whereas the RER had a very low level of labeling and the Golgi complex was not significantly labeled above background (Fig. 3, A and B) .