Liposome Flotation Assay for Studying Interactions Between Rubella Virus Particles and Lipid Membranes

Rubella virus (RuV) is an enveloped, positive-sense single-stranded RNA virus that is pathogenic to humans. RuV binds to the target cell via the viral envelope protein E1, but the specific receptor molecules on the target cell are yet to be fully elucidated. Here, we describe a protocol for liposome flotation assay to study direct interactions between RuV particles and lipid membranes in a qualitative manner. Interactions are examined by a Nycodenz density gradient fractionation using UV-inactivated RuV particles and fluorescent-labeled liposomes consisting of pure lipids. Fractionated RuV particles are detected using standard sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) followed by Western blot analysis for viral proteins. On the Nycodenz gradient, RuV particles bound to liposomes shift to lower density fractions than unbound RuV particles. Using this protocol, we provide compelling evidence that, at neutral pH in a calcium-dependent manner, RuV particles bind to lipid membranes containing both sphingomyelin (SM) and cholesterol in certain cell types.