The metastable state of nucleocapsids of enveloped viruses as probed by high hydrostatic pressure

Enveloped viruses fuse their membranes with cellular membranes to transfer their genomes into cells at the beginning of infection. What is not clear, however, is the role of the envelope (lipid bilayer and glycoproteins) in t he stability of the viral particle. To address this question, we compared t he stability between enveloped and nucleocapsid particles of the alphavirus Mayaro using hydrostatic pressure and urea. The effects were monitored by intrinsic fluorescence, light scattering, and binding of fluorescent dyes, including bis(8-anilinonaphthalene-1-sulfonate) and ethidium bromide. Press ure caused a drastic dissociation of the nucleocapsids as determined by try ptophan fluorescence, light scattering, and gel filtration chromatography, Pressure-induced dissociation of the nucleocapsids was poorly reversible, I n contrast, when the envelope was present, pressure effects were much less marked and were highly reversible. Binding of ethidium bromide occurred whe n nucleocapsids were dissociated under pressure, indicating exposure of the nucleic acid, whereas enveloped particles underwent no changes, Overall, o ur results demonstrate that removal of the envelope with the glycoproteins leads the particle to a metastable state and, during infection, may serve a s the trigger for disassembly and delivery of the genome. The envelope acts as a "Trojan horse," gaining entry into the host cell to allow release of a metastable nucleocapsid prone to disassembly.