Structural biology of the influenza virus fusion peptide

The release of influenza RNA inside the host cell occurs through the fusion of two membranes, the viral envelope and that of the cellular endosome. The fusion is mediated by the influenza hemagglutinin protein (HA), in particular by the fusion peptide (HAfp) located in the N-terminal fragment of HA2 subunit. This protein fragment anchors in the internal endosomal membrane, whereas the C-terminal HA2 part comprises a transmembrane domain (TMD) embedded in the viral envelope. A drop of pH in the endosome acts as the main trigger for HA2 large conformational change that leads to anchoring of the fusion peptide, close contact of the membranes and the subsequent fusion. Throughout the years the major research effort was focused on a 20-aminoacid fragment (HAfp1-20), shown by NMR to adopt a 'boomerang'-like structure. However, recent studies showed that extending HAfp1-20 by three highly conserved residues W21-Y22-G23 leads to formation of a unique, tight helical hairpin structure. This review summarizes recently discovered structural aspects of influenza fusion peptides and their relations with the membrane fusion mechanism.