Hemagglutinin protein (H9) on Avian Influenza Virus type A subtype H9N2

Hemagglutinin (HA) is a major surface glycoprotein of the influenza A virus, including subtype H9N2. It plays a critical role in viral infectivity by mediating both the attachment of the virus to host cells and subsequent membrane fusion required for viral entry. The HA protein is also a primary determinant of host specificity and antigenicity, making it central to immune recognition and vaccine design. HA exists as a homotrimeric integral membrane glycoprotein, with each trimer composed of three identical monomers. Each monomer is initially synthesized as an inactive precursor (HA0), which must be cleaved by host proteases into two subunits: HA1 and HA2, linked by disulfide bonds. The globular head domain (mainly HA1) contains the receptor-binding site responsible for recognizing sialic acid residues on host cell surfaces. The stem region (mainly HA2) contains elements necessary for membrane fusion; at neutral pH, its N-terminal "fusion peptide" is hidden but becomes exposed under acidic conditions in endosomes to trigger fusion. In H9 hemagglutinin specifically, structural differences compared to other subtypes are observed in the interhelical loop near residue 75 of HA2. Unlike H3 subtype which has glycine at this position allowing a sharp turn into an α-helix, H9 has alternative residues resulting in a ‘taller’ turn stabilized by hydrogen bonds—this may affect overall conformation and function.