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

The El-containing tubular elements are morphologically reminiscent of tubular smooth ER seen in phenobarbitaltreated hepatocytes where the volume of the system increases greatly in order to accommodate the increased concentration of detoxifying enzymes (41, 46) . Unlike the structures described in this paper which form relatively homogeneous tubular networks, smooth ER of this type is not arranged in tubules and is often interspersed with other organelles such as mitochondria (41) . Several other ER-associated specialized pre-Golgi smooth membrane compartments have been described previously including, dilated smooth membrane cisternae that are the intracellular site of mouse hepatitis virus (61), spleen focus forming virus, and murine leukemia virus (63) budding in certain cell types; a smooth ER subcompartment consisting of anastamosing tubules where concentration and assembly of chondroitin sulfate proteoglycan precursors is believed to take place in chondrocytes (66); "crystalloid smooth ER" found in mutant CHO cells synthesizing massive amounts of the resident ER membrane protein HMG CoA reductase, (11) , which ts045 G protein can also enter and exit (4); flattened smooth cisternae postulated to be amplified transitional elements in continuity with the PER in transfected COS cells expressing a rat growth hormone-influenza hemagglutinin chimeric protein (48) . None of these structures resembles morphologically the Elcontaining compartment described here; however, the latter compartment may be functionally equivalent since the growth hormone-influenza hemagglutinin protein remained endo H-sensitive and was modified with palmitate (47, 48) . The only other structures described to date that morphologically resemble the El-containing tubules are those that develop after endocytosis of SV-40 in CV-I cells; however, these tubules do not appear to be involved in exocytic processes, because newly synthesized virus particles do not enter them (23) .