Effect of the viral proteins on the fluidity of the membrane lipids in Sindbis virus

The fluidity of the lipids in the membrane of Sindbis virus was studied by electron paramagnetic resonance spectroscopy. The lipids in the viral membrane are noticeably less fluid than the lipids in the membrane of the cells in which the virus was grown. This difference in fluidity is not due simply to differences in lipid composition but instead appears to be the result of the interaction of the viral proteins with the membrane lipids. This conclusion seems clear because (i) the viral lipids are more fluid after extraction with chloroform/methanol than in the lipid bilayer of the virion and because (ii) the viral membrane is made more fluid by proteolytic digestion of the viral glycoproteins.The possible role of this viral protein-mediated alteration of the physical state of membrane lipids in the maturation of Sindbis virions is discussed.