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

Although the tubular networks found in CHOE1 ceils are not seen in non-transfected CHO cells, it is quite possible that this compartment has a less-developed counterpart in wild type cells. It is the part rough/part smooth transitional elements of the ER that are presumed to represent the exit site for vesicles shuttling between the ER and Golgi complex or the ER and the intermediate compartment; however, the morphology of this region varies considerably from one cell type to another as detailed below. One possibility is that the El-containing tubular networks may arise as a consequence of extensive proliferation of smooth regions of the ER which correspond to transitional elements. The fact that the Elcontaining compartment is not destabilized by Ca 2+ removal is in keeping with the model proposed by Sambrook (51) which predicts that transport vesicles bud from regions of the ER that are not sensitive to fluctuations in calcium ions. Our findings are compatible with the assumption that proteins in transit to these specialized extensions of the ER may still be associated with molecular chaperones and enzymes required for attainment of correct tertiary structure such as BiP and PDI, respectively. Upon arrival at this juncture, properly assembled transport-competent protein complexes would presumably then enter transport vesicles, and the chaperone proteins could be returned to the PER by diffusion, or they may enter the vesicles, leave the ER system, and be returned via receptor-mediated retrieval from a salvage compartment (39) . By contrast unassembled subunits that are unable to enter the transport vesicles are retained by some unknown mechanism and are ultimately degraded. Alternatively, these membranes may develop from other as yet uncharacterized subregions of the ER. The situation will be clarified only when antibodies to resident proteins of the El-containing compartment are generated which should allow identification of these membranes in nontransfected cells.