Paramagnetic spin label interactions with the envelope of a group A arbovirus. Lipid organization

Electron paramagnetic resonance observations were made on nitroxide spin- labeled molecules which were bound to the TC-83 vaccine strain of Venezuelan equine-encephalomyelitis virus. Paramagnetic resonance parameters derived from the observations and their dependence on sample temperature were similar but not identical to those which have been reported for these labels dissolved in lipid bilayer membranes of mammalian and bacterial origin. The data has a mechanical rigidity substantially greater than that of bilayers in cellular membranes. A model is presented which assumes the location of the lipid bilayer outside the nucleoprotein capsid and inside a spherical layer of envelope proteins. The model is in accord with Harrison's X-ray diffraction results for Sindbis virus. The model is discussed in terms of its implications with respects to the role played by lipid in viral maturation and infectivity.