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

Except for PDI and BiP which only partially overlap with El, no other marker proteins have been found that colocalize to the site of E1 arrest. HMG CoA reductase remains a potential marker; however, we were unable to detect this protein by immunofluorescence in our CHOE1 cells, presumably because expression levels of this enzyme are low except in these cells containing amplified copies of the gene. The GTPbinding protein rablb, which colocalizes with tab2 to the intermediate compartment (10) and is found predominantly in smooth ER fractions of tissue homogenates (45) also represents a potential marker, but we were not able to ascertain by immunofluorescence whether or not it was present at the site of E1 arrest. Similarly, our attempts to label CHOE1 cells using antibodies to the 72-kD KDEL receptor (65) or gp58 (53) were unsuccessful. We did not try p53 (54) The results presented here as well as those reported by others (11, 61, 63, 66) demonstrate that the smooth membrane pre-Golgi compartments that lie between the RER and cis-Golgi are diversified in organization and function and have the capacity to undergo amplification. They also raise the possibility that the smooth membrane pre-Golgi compartments may consist of two or more distinct subcompartments with different specialized functions. We consider that the site of E1 arrest may represent a new compartment or alternatively a differentiated proximal moiety of the intermediate compartment (54, 55) . Without appropriate markers it is difficult to know where to draw the line between the ER and intermediate compartments. The high level of expression of E1 in the CHOE1 cells should facilitate the isolation of the El-containing tubular elements by cell fractionation and immunoisolation, and make it possible to characterize the resident proteins of this novel compartment.