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Fusion of influenza virus particles with liposomes: requirement for cholesterol and virus receptors to allow fusion with and lysis of neutral but not of negatively charged liposomes

Ofer Nussbaum^1,

¹ Department of Biological Chemistry, Institute of Life Sciences, The Hebrew University of Jerusalem, Jerusalem 91904, Israel
and² Institute of Virology, Frankfurterstrasse 107, D-6300 Giessen, Germany

Influenza virus particles are able to fuse with liposomes composed of negatively charged or neutral phospholipids, as shown by using fluorochrome-labelled virions and fluorescence dequenching methods. Fusion with liposomes composed of only phosphatidylcholine (PC) was dependent on the presence of cholesterol (Chol), whereas fusion with liposomes containing negatively charged phospholipids, such as phosphatidylserine (PS), or of PC and phosphatidylethanolamine (PE) occurred in the absence of Chol. Fusion of influenza virions with PC:Chol liposomes was observed at pH 5.0, but not at pH 7.4, whereas a low degree of fusion with negatively charged liposomes or those containing PE was observed at pH 7.4. In addition, non-fusogenic influenza virions or HA₀ influenza virions fused with PS- or PE-containing liposomes, especially at pH 5.0. Influenza virus particles were also able to induce the release of the fluorochrome calcein from negatively charged calcein-loaded liposomes at pH 5.0, as well as at pH 7.4, but failed to do so with PC:Chol liposomes. Lysis of PC:Chol by influenza virions was dependent on the presence of virus receptors, namely gangliosides (sialoglycolipids), and was observed only at pH 5.0. The results show that fusion of influenza virions with negatively charged or PE-containing liposomes does not reflect the biological activity of the virus needed for penetration and infection of living cells. On the other hand, fusion with PC:Chol liposomes is probably due to the activity of the viral fusion protein, the haemagglutinin glycoprotein.

Present address: National Institutes of Health, NIAID-LVD, 9000 Rockville Pike, Building 4, Room 210, Bethesda, Maryland 20892, U.S.A.

Received 9 June 1992; accepted 5 August 1992.