Hemagglutinin

In molecular biology, hemagglutinins (or haemagglutinin in British English) (from the Greek haima, 'blood' + Latin gluten, 'glue') are receptor-binding membrane fusion glycoproteins produced by viruses in the Paramyxoviridae family. Hemagglutinins are responsible for binding to receptors on red blood cells to initiate viral attachment and infection. The agglutination of red cells occurs when antibodies on one cell bind to those on others, causing amorphous aggregates of clumped cells.Hemagglutinins recognize cell-surface glycoconjugates containing sialic acid on the surface of host red blood cells with a low affinity, and use them to enter the endosome of host cells. In the endosome, hemagglutinins are activated at a pH of 5 - 6.5 to undergo conformational changes that enable viral attachment through a fusion peptide. Agglutination and hemagglutinins were discovered by virologist George K. Hirst in 1941. Alfred Gottschalk proved in 1957 that hemagglutinins bind a virus to a host cell by attaching to sialic acids on carbohydrate side chains of cell-membrane glycoproteins and glycolipids. Influenza hemagglutinin or haemagglutinin: a homotrimeric glycoprotein that is found on the surface of influenza viruses which is responsible for their infectivity. Influenza strains are named for the specific hemagglutinin variant they produce, along with the specific variant of another surface protein, neuraminidase. Measles hemagglutinin: a hemagglutinin produced by the measles virus which encodes six structural proteins, of which, hemagglutinin and fusion are surface glycoproteins involved in attachment and entry. Parainfluenza hemagglutinin-neuraminidase: a type of hemagglutinin-neuraminidase produced by parainfluenza which is closely associated with both human and veterinary disease. Mumps hemagglutinin-neuraminidase: a kind of hemagglutinin that the mumps virus (MuV) produces, which is the virus that causes mumps. The PH-E form of phytohaemagglutinin. Hemagglutinins are small proteins that project from the virus membrane surface as 135 Å long spikes with a diameter of 30-50 Å.