Hemagglutinin (H9 subtype) (HA)

The H9N2 influenza virus hemagglutinin (HA) is a critical surface glycoprotein that mediates the initial stages of viral infection by facilitating attachment and entry into host cells [1, 3]. It is a homotrimeric Class I fusion protein composed of two subunits: the HA1 globular head, which contains the receptor-binding site (RBS), and the HA2 stalk region, which governs pH-dependent membrane fusion [4, 7]. While H9N2 HA typically binds to avian-type α2,3-linked sialic acid receptors, specific mutations like Q226L have been shown to increase affinity for human-type α2,6-linked receptors, significantly enhancing the virus's zoonotic and pandemic potential [12, 14, 18]. As the primary target for neutralizing antibodies and seasonal vaccines, HA undergoes constant antigenic drift to evade host immune responses [5, 20]. Current therapeutic strategies include the use of small-molecule fusion inhibitors like Umifenovir (Arbidol) and the development of broadly neutralizing monoclonal antibodies (e.g., CR9114, FI6) that target the more conserved stalk domain to provide heterosubtypic protection [1, 11, 21]. Monitoring the genetic evolution of the HA protein is essential for vaccine strain selection and the identification of emerging strains with increased human infectivity [6, 10].

Other names

H9 HAH9N2 HAHemagglutinin proteinInfluenza A virus H9 hemagglutininViral surface glycoproteinHA1/HA2

Binding inhibition, Membrane fusion inhibition, Viral entry blockade, Virus neutralization