[Active site for fusion activity of influenza virus hemagglutinin]

A sequence of hydrophobic amino acids at the N-terminus of HA2 subunit of hemagglutinin (HA) is thought to be the active site for fusion activity, and called "fusion peptide". At neutral pH, fusion peptides are located inside the stem of HA spike. At pH 5, the heads of HA spike are dissociated and thereby fusion peptides are exposed and relocated probably at the top of newly formed long alpha-helix trimer. Fusogenic HA2 bridges two adjacent membranes by plunging fusion peptide into the target membrane. HA molecule is metastable at neutral pH. Oligosaccharides in the stem region, which are strictly conserved among various strains, maintain HA in the metastable form required for fusion activity. Cytoplasmic tail of HA also participates in fusion process. Addition of 5 amino acids at the end of cytoplasmic tail abolishes the fusion activity without affecting other biological properties of HA.