The function of Hemagglutinin (HA), often referred to as H protein, is primarily to mediate the entry of the virus into host cells by binding to sialic acid receptors on the cell surface.
Detailed Explanation of Hemagglutinin (H Protein) Function
Hemagglutinin is a crucial surface glycoprotein found on influenza viruses. Its role is fundamental to the viral infection process, enabling the virus to attach to and enter host cells.
Binding to Host Cells
- Receptor Binding: The primary function of HA is to bind to sialic acid receptors present on the surface of host cells, typically epithelial cells in the respiratory tract. This binding is a crucial first step in initiating infection. The specificity of this binding helps determine which species a particular influenza virus can infect.
- Agglutination: The name "hemagglutinin" stems from its ability to cause agglutination (clumping) of red blood cells (erythrocytes) in vitro. This property is often used in laboratory assays to detect and quantify the presence of the virus.
Viral Entry
- Membrane Fusion: Following receptor binding, HA undergoes a conformational change triggered by the acidic environment of endosomes (internal vesicles within the host cell). This change exposes a fusion peptide that inserts into the host cell membrane.
- Fusion Mediation: The conformational change and fusion peptide insertion mediate the fusion of the viral membrane with the host cell membrane. This fusion releases the viral genome into the host cell cytoplasm, allowing the virus to replicate.
Analogy
Think of Hemagglutinin as a key that unlocks the door to a cell. The "key" (HA) specifically fits into the "lock" (sialic acid receptors) on the cell surface, allowing the virus to enter and replicate within the cell.
Summary
In summary, the main function of Hemagglutinin (H protein) is to facilitate the attachment and entry of influenza viruses into host cells, making it an essential factor for viral infection.
