Hemagglutinin
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Hemagglutinin (HA) is an antigenic glycoprotein found on the surface of the Influenza virus and is responsible for binding the virus to the cell that is being infected.
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Subtypes
Fifteen hemagglutinin subtypes are known.
Function
Hemagglutinin binds to a sialic acid residue on the cell to be infected. The name derives from the protein's ability to adhere to red blood cells and causes them to agglutinate, or clump together.
Structure
The HA monomer is synthesized as a precursor and it is glycosylated then cleaved into two smaller proteins: The HA1 and HA2 subunits (it is cleaved by a trypsin enzyme, known as clara tryptase in human cells). Each HA monomer consists of a long, helical chain anchored in the membrane by HA2 and topped by a large HA1 globule.