Two modes of fusogenic action for influenza virus fusion peptide

Abstract

The entry of influenza virus into the host cell requires fusion of its lipid envelope with the host membrane. It is catalysed by viral hemagglutinin protein, whose fragments called fusion peptides become inserted into the target bilayer and initiate its merging with the viral membrane. Isolated fusion peptides are already capable of inducing lipid mixing between liposomes. Years of studies indicate that upon membrane binding they form bend helical structure whose degree of opening fluctuates between tightly closed hairpin and an extended boomerang. The actual way in which they initiate fusion remains elusive. In this work we employ atomistic simulations of wild type and fusion inactive W14A mutant of influenza fusion peptides confined between two closely apposed lipid bilayers. We characterise peptide induced membrane perturbation and determine the potential of mean force for the formation of the first fusion intermediate, an interbilayer lipid bridge called stalk. Our results demonstrate two routes through which the peptides can lower free energy barrier towards fusion. The first one assumes peptides capability to adopt transmembrane configuration which subsequently promotes the creation of a stalk-hole complex. The second involves surface bound peptide configuration and proceeds owing to its ability to stabilise stalk by fitting into the region of extreme negative membrane curvature resulting from its formation. In both cases, the active peptide conformation corresponds to tight helical hairpin, whereas extended boomerang geometry appears to be unable to provide favourable thermodynamic effect. The latter observation offers plausible explanation for long known inactivity of boomerang-stabilising W14A mutation. In order to deliver genetic material into host cells enveloped viruses, such as Influenza, HIV or Corona, need to fuse their lipid envelope with the cell membrane. This process is mediated by viral fusion proteins, whose specialised elements, called fusion peptides, are inserted into target membranes and initiate their merging with the viral lipid bilayer. Being dynamic and dependent on individual molecules, this process is inherently difficult to study, and hence, its details remain elusive. Here, we report results of fully atomistic simulations aimed at revealing the mode of action of influenza virus fusion peptides. Aside from analysing structural aspects of peptide-induced lipids perturbation, we calculate free energy changes during the formation of the first fusion intermediate between two closely apposed lipid bilayers, that is a hydrophobic bridge called stalk. We demonstrate that the peptides are indeed capable of lowering the associated free energy barrier in comparison to membrane only system. They appear to do so via two possible routes. They can either remain at the membrane-water interface and stabilise stalk by shielding its core from aqueous environment, or they can adopt membrane-spanning configurations and lower the cost of membrane deformation by forming so called stalk-hole complexes.