Shallow Boomerang-Shaped Influenza Hemagglutinin G13A Mutant Structure Promotes Leaky Membrane Fusion

Abstract

Our previous studies showed that an angled boomerang-shaped structure of the influenza hemagglutinin (HA) fusion domain is critical for virus entry into host cells by membrane fusion. Since the acute angle of ∼105 degree of the wild-type fusion domain promotes efficient non-leaky membrane fusion, we asked whether different angles would still support fusion and thus facilitate virus entry. Here, we show that the G13A fusion domain mutant produces a new leaky fusion phenotype. The mutant fusion domain structure was solved by NMR spectroscopy in a lipid environment at fusion pH. The mutant adopts a similar boomerang structure as wild-type, but with a shallower kink angle of ∼150 degree. G13A perturbs the structure of model membranes to a lesser degree than wild-type, but to a greater degree than non-fusogenic fusion domain mutants. The strength of G13A binding to lipid bilayers is also intermediate between that of wild-type and non-fusogenic mutants. These membrane interactions provide a clear link between structure and function of influenza fusion domains: An acute angle is required to promote clean non-leaky fusion suitable for virus entry, presumably by interaction of the fusion domain with the transmembrane domain deep in the lipid bilayer. A shallower angle perturbs the bilayer of the target membrane so that it becomes leaky and unable to form a clean fusion pore. Mutants with no fixed boomerang angle interact with bilayers weakly and do not promote any fusion or membrane perturbation.