Fatty acids on the A/USSR/77 influenza virus hemagglutinin facilitate the transition from hemifusion to fusion pore formation

Influenza virus hemagglutinin (HA) has three highly conserved acylation sites close to the carboxyl terminus of the HA2 subunit, one in the transmembrane domain and two in the cytoplasmic domain. Each site is modified by palmitic acid through a thioester linkage to cysteine. To elucidate the biological significance of HA acylation, the acylation sites of HA of influenza virus strain A/USSR/77 (H1N1) were changed by site-directed mutagenesis, and the membrane fusion activity of mutant HAs lacking the acylation site(s) was examined quantitatively using transfer assays of lipid (R18) and aqueous (calcein) dyes. Lipid mixing, so-called hemifusion, activity was not affected by deacylation, whereas transfer of aqueous dye, so-called fusion pore formation, was dramatically restricted. When the fusion reaction was induced by a lower pH than the optimal one, calcein transfer with the mutant HAs was improved, but simultaneously a considerable calcein leakage into the medium was observed. From these results, we conclude that the palmitic acids on the H1 subtype HA facilitate the transition from hemifusion to fusion pore formation.