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

Currently a good deal of interest is focused on characterizing these intermediate structures involved in ER to Golgi transport, and the overexpression of viral membrane glycoproteins has been invaluable in these studies. To further address aspects of ER to Golgi transport, we have studied the expression and intracellular transport of the structural proteins of rubella virus (RV). RV, a Togavirus that assembles 1. Abbreviations usedin thispaper: BFA, Brefeldin A; BiP, binding protein; DFBS, dialyzed fetal bovine serum; dhfr, dihydrofolate reductase; DSP, dithiobis succinimidylpropionate; PDI, protein disulfide isomerase; RER, rough ER; RV, rubella virus. on intracellular membranes (2, 68) , contains two type I membrane envelope glycoproteins E2 and E1 (12, 40, 67) which are thought to form a heterodimeric complex within the ER. Transient expression of RV E1 and E2 together or E1 alone in COS cells resulted in accumulation of the proteins in a post-RER, juxtanuclear compartment believed to be part of the Golgi complex (20) . In the present study we have further characterized the site of arrest of E1 by analysis of stably transfected CHO cells expressing El. We found that when RV E1 and E2 are expressed together in CHO cells they are targeted to the Golgi; however, when E1 is expressed alone it is arrested in a novel, post-RER/pre-Golgi compartment composed of tubular smooth membrane elements that appear to be located near the site of exit from the ER. In this paper, we have characterized this compartment and found that it has distinctive properties from either the RER or the Golgi complex.