Influenza A virus hemagglutinin H9 (HA H9)

Influenza A virus hemagglutinin H9 (HA H9) is a trimeric glycoprotein on the viral envelope responsible for host cell attachment and entry during infection. It binds sialic acid receptors on respiratory epithelial cells via its globular head domain, determining host tropism, particularly in avian species with potential zoonotic spillover to humans. Following endocytosis, the stem domain mediates membrane fusion in the acidic endosome through conformational changes that expose and insert a fusion peptide into the host membrane, enabling viral genome release. HA H9 belongs to one of 16 HA subtypes (H1-H16) in influenza A viruses, often paired with neuraminidase subtypes like N2, and is associated with low-pathogenic avian influenza strains such as A/swine/Hong Kong/9/98(H9N2). As a key viral antigen, it emits neutralizing antibodies primarily against the variable head but also conserved stem epitopes targeted by broadly protective responses. Therapeutic strategies focus on HA inhibitors like TBHQ, which bind a conserved pocket in group 2 HA subtypes including H9, blocking fusion without broad host toxicity observed in host-targeted antivirals. Its role in antigenic drift necessitates annual surveillance for vaccine matching, as mutations alter receptor binding and immune evasion.

Other names

H9 hemagglutininInfluenza hemagglutinin subtype H9HA H9

Inhibits membrane fusion by binding to a hydrophobic pocket in the HA trimer, stabilizing the neutral pH conformation and preventing low-pH-induced rearrangements required for fusion