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.