Intracellular transport of rubella virus structural proteins expressed from cloned cDNA

The structural proteins of rubella virus consist of a nucleocapsid protein (C) and two membrane-embedded spike glycoproteins (E1 and E2). Since many reports have suggested that rubella virus buds intracellularly, we have examined the intracellular transport of the structural proteins in the absence of virion formation, particularly whether the membrane glycoproteins are retained inside the cell or are transported to the cell surface. We have expressed the structural proteins from cloned cDNA either alone or in different combinations, have examined the intracellular location of the proteins by immunofluorescence and using biochemical methods, and have looked for plasma membrane-localized E1 or E2 using a cell surface biotinylation assay. The C protein was found in the Golgi complex when expressed with E2 and E1; without the membrane glycoproteins, C appeared to remain in the endoplasmic reticulum (ER). When expressed alone, E1 was retained in a pre-Golgi compartment, and was not detected at the cell surface in any cell line. When E2 was expressed alone a small fraction could be detected at the cell surface, but the majority was retained intracellularly, apparently in the ER and the Golgi. Both proteins were transported to the surface when they were expressed together, albeit with low efficiencies in all cell lines. These data suggest that, although neither glycoprotein carries a dominant intracellular retention signal, E2 and E1 are largely retained in the Golgi even when present as a transport-competent heterodimer.