Abstract
Viral fusion peptides are short N-terminal regions of type-1 viral fusion proteins that are critical for virus entry. Although the importance of viral fusion peptides in virus-cell membrane fusion is established, little is known about how they function. We report the effects of wild-type (WT) hemagglutinin (HA) fusion peptide and its G1S, G1V, and W14A mutants on the kinetics of poly(ethylene glycol)(PEG)-mediated fusion of small unilamellar vesicles composed of dioleoylphosphatidylcholine, dioleoylphosphatidylethanolamine, sphingomyelin, and cholesterol (molar ratio of 35:30:15:20). Time courses of lipid mixing, content mixing, and content leakage were obtained using fluorescence assays at multiple temperatures and analyzed globally using either a two-step or three-step sequential ensemble model of the fusion process to obtain the rate constant and activation thermodynamics of each step. We also monitored the influence of peptides on bilayer interfacial order, acyl chain order, bilayer free volume, and water penetration. All these data were considered in terms of a recently published mechanistic model for the thermodynamic transition states for each step of the fusion process. We propose that WT peptide catalyzes Step 1 by occupying bilayer regions vacated by acyl chains that protrude into interbilayer space to form the Step 1 transition state. It also uniquely contributes a positive intrinsic curvature to hemi-fused leaflets to eliminate Step 2 and catalyzes Step 3 by destabilizing the highly stressed edges of the hemi-fused microstructures that dominate the ensemble of the intermediate state directly preceding fusion pore formation. Similar arguments explain the catalytic and inhibitory properties of the mutant peptides and support the hypothesis that the membrane-contacting fusion peptide of HA fusion protein is key to its catalytic activity.