Hemagglutinin-mediated Influenza Viral Entry: Identification of Molecular Determinants on Host Tropism

Influenza A virus causes seasonal flu in humans and occasionally pandemics in humans and poultry, resulting in high mortality diseases and huge economic loss. The Biggest threat comes from the highly pathogenic avian influenza which may break species barrier and become the source of a potential flu pandemic. Entry is the first step in viral infection which is mediated by viral glycoprotein hemagglutinin (HA) binding to host cell surface sialic acid (SA)-terminated gangliosides. Species specific SA plays an important role in the determination of influenza host specificity. The SA binding pocket of the HA has been well studied and the important residues for receptor binding have been identified. In this study, we have characterized the new role of the surface located, highly conserved residues within all 16 subtypes of HA1 subunit in influenza A viral entry and host tropism determination. Also, we have evaluated the specificity of H5 HA1 subunit in H5N1 viral entry by using computer modeling and biochemical methods. Our results revealed two major categories of residues which are critical in SA recognition and viral entry. One such residue is Y161 which is highly conserved in all 16 subtypes of HA1. Y161 plays a critical role in influenza host tropism by affecting viral recognition of different species of SAs from different hosts. The other category includes H117 and D241 which are H5 HA1 specific conserved residues, whose charge at a specific position plays an essential role in SA binding affecting viral entry, however, without affecting SA species specificity. Our study contributes to a better understanding of the role of different domains of HA in influenza viral entry. These new insights will help elucidate the mechanism of influenza A viral entry and provides a promising lead to the development of HA inhibitors.