The 1-127 HA2 construct of influenza virus hemagglutinin induces cell-cellhemifusion

Conformational changes in the HA2 subunit of influenza hemagglutinin (HA) a re coupled to membrane fusion. We investigated the fusogenic activity of th e polypeptide FHA2 representing 127 aminoterminal residues of the ectodomai n of HA2. While the conformation of FHA2 both at neutral and at low pH is n early identical to the final low-pH conformation of HA2, FHA2 still induces lipid mixing between liposomes in a low-pH-dependent manner. Here, we foun d that FHA2 induces lipid mixing between bound cells, indicating that the " spring-loaded" energy is not required for FHA2-mediated membrane merger. Al though, unlike HA, FHA2 did not form an expanding fusion pore, both acidic pH and membrane concentrations of FHA2, required for lipid mixing, have bee n close to those required for HA-mediated fusion. Similar to what is observ ed for HA, FHA2-induced lipid mixing was reversibly blocked by lysophosphat idylcholine and low temperature, 4 degreesC. The same genetic modification of the fusion peptide inhibits both HA- and FHA2-fusogenic activities. The kink region of FHA2, critical for FHA2-mediated lipid mixing, was exposed i n the low-pH conformation of the whole HA prior to fusion. The ability of F HA2 to mediate lipid mixing very similar to HA-mediated lipid mixing is con sistent with the hypothesis that hemifusion requires just a portion of the energy released in the conformational change of HA at acidic pH.