Association of Transmembrane Helices in Viral Fusion Peptides Suggests a Protein-Centric Mechanism of Membrane Fusion

A broad range of biological functions, from neurotransmitter release to infection by enveloped viruses, is achieved by fusogenic proteins, which increase the intrinsically slow rate of membrane fusion by using their energetically downhill conformational changes. Among viral fusogenic proteins, many are identical trimers that feature a fusion mechanism reminiscent of the SNARE proteins responsible for vesicle fusion. In this mechanism, the N-terminal hydrophobic fusion peptide domains (FP) of the three protein chains insert into the host cell's membrane, and then zipper with the viral-membrane attached TM domains of the same protein, driving the two membranes together in the process. So far, only high resolution structures of the soluble portions exist, and the minimal number of trimeric proteins is also undetermined. By analytical ultra-centrifugation and polarized infrared spectroscopy, we proved that the FP of the parainfluenza virus (PIV) fusogenic protein forms hexameric helical bundles that lie transverse to lipid bilayers. We modeled the FP hexamer's structure and refined it by molecular dynamics simulations, observing an association mode mediated by water and a glutamine amino acid, frequently occurring in diverse fusion peptide sequences. This structure suggests that as few as two trimeric proteins may form a hexameric bundle of their FP domains, and induce curvature on the cellular membrane through a mechanism analogous to mechanosensitive channels. The FP hexamer is at the geometric center of such fusion mechanism at all times. By simulations at atomic detail of the fusion process, we identified the amino acids that control the associated hydration and dehydration events, and suggest new strategies to inhibit viral infections.