Influenza Hemagglutinin (HA) Protein
Detailed Structure of Hemagglutinin (HA) Protein
The HA protein is a trimeric glycoprotein, meaning it is composed of three identical subunits. Each subunit has two major components:
The Globular Head Domain:
- Composed of chains A, C, and E.
- This region is responsible for receptor binding and is the main target of neutralizing antibodies.
The Stem Domain:
- Composed of chains B, D, and F.
- It anchors the protein to the viral membrane and plays a role in the fusion of the viral and host cell membranes.
The cylindrical shape of the HA trimer facilitates its function in binding and fusion. Advanced imaging techniques, like cryo-electron microscopy, have provided insights into its dynamic structural changes during viral entry.
Biological Function of HA Protein
The HA protein is crucial for influenza virus infection, performing two main functions:
1. Receptor Binding
- The HA protein binds to sialic acid-containing receptors on the surface of host cells. This specificity determines the host range of the virus.
2. Membrane Fusion
- After binding, HA undergoes a conformational change triggered by the acidic environment of the endosome, facilitating the fusion of the viral envelope with the host membrane.
Interestingly, the term "hemagglutinin" originates from its ability to cause erythrocyte agglutination in vitro, a property widely used in diagnostic assays.