Rubella Virus Pseudotypes and a Cell-cell Fusion Assay As Tools for Functional Analysis of the Rubella Virus E2 and E1 Envelope Glycoproteins

The rubivirus Rubella virus contains the two envelope glycoproteins E2 and E1 as a heterodimeric spike complex embedded in its lipid envelope. The functions of both proteins, especially of E2, in the process of viral entry are still not entirely understood. In order to dissect E2 and E1 entry functions from post-entry steps, pseudotypes of lentiviral vectors based on Simian immunodeficiency virus were used. C-terminally modified E2 and E1 variants successfully pseudotyped lentiviral vector particles. This is the first report to show that not only E1, but also E2, is able to mediate infectious viral entry. Furthermore, a cell-cell fusion assay was used to further clarify membrane-fusion activities of E2 and E1 as one of the early steps of infection. It was demonstrated that the capsid protein, when coexpressed in cis, enhances the degree of E2- and E1-mediated cell-cell fusion.