THE structure of a key protein of the 1918 flu virus, which killed 40 million people worldwide, has been worked out.
The main surface protein on influenza viruses is called haemagglutinin (HA). For the virus to enter cells, it is thought to have to bind to a sugar called sialic acid on the surface of lung cells. Birds and humans have different forms of sialic acid, which is why bird flu viruses do not usually infect people.
Samples from 1918 flu victims have allowed parts of the 1918 virus, including HA, to be sequenced. Now John Skehel鈥檚 team at the National Institute for Medical Research in London has used X-ray crystallography to work out the three-dimensional structure of the HA protein. The team compared it with the HA proteins from two viruses of the same class from the 1930s, one of which binds both human and bird sialic acids, and another that binds only bird sialic acid.
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The 1918 HA looked like the bird variety, they report in two papers published online by Science, supporting the idea that the 1918 flu was related to bird flu. The work shows that multiple changes in amino acids were needed to allow it to bind to human sialic acid. By contrast, Skehel told New 杏吧原创, it鈥檚 known that a change in a single amino acid could allow the HA in the H5N1 bird flu now circulating in Asia to bind readily to human sialic acid.
Intriguingly, the 1918 HA also had a structure that had never been seen before. It is possible that this allowed the 1918 virus to bypass the sialic acid binding site altogether when entering lung cells.