We have studied the role of hydrophobic interactions in the fusion activity of two lipid enveloped viruses, influenza and Sendai. Using the fluorescent probe ANS (1-aminonaphtalene-8-sulfonate) we have shown that low-pH-dependent influenza virus activation involves a marked increase in the viral envelope hydrophobicity. The effect of dehydrating agents on the fusion activity of both viruses towards model lipid membranes was studied using a fluorescence dequenching assay. Dehydrating agents such as dimethylsulfoxide and dimethylsulfone greatly enhanced the initial rate of the fusion process, the effect of dimethylsulfone doubling that of dimethylsulfoxide. The effect of poly(ethylene glycol) on the fusion process was found to be dependent on the polymer concentration and molecular weight. In general, similar observations were made for both viruses. These results stress the importance of dehydration and hydrophobic interactions in the fusion activity of influenza and Sendai viruses, and show that these factors may be generally involved in membrane fusion events mediated by many other lipid enveloped viruses.

Abbreviations: ANS, 1-aminonaphtalene-8-sulfonate; a.u., arbitrary units; DMSO, dimethylsulfoxide; DMSO2, dimethylsulfone; HA, influenza virus hemagglutinin; LUVs, large unilamellar vesicles; PC, phosphatidylcholine; PE, phosphatidyl-ethanolamine; PEG, 1500-poly(ethylene glycol), 1500 average molecular weight; PEG, 3000- poly(ethylene glycol), 3000 average molecular weight; PEG, 6000-poly(ethylene glycol), 6000 average molecular; weight; PS, phosphatidylserine; R18, octadecylrhodamine B chloride

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