Two mutations in the viral hemagglutinin surface protein independently enable H5N1 influenza A virus to bind to human receptors, researchers report in Nature this week. The sequence changes might serve as molecular markers to assess the pandemic potential of H5N1."Before avian influenza viruses can replicate efficiently in humans, they must switch recognition from avian to human receptors," senior author Yoshiro Kawaoka of the University of Wisconsin-Madison told The Scientist. Avian and human influenza viruses differ in the receptor they bind. Avian hemagglutinin recognizes sugars ending in sialic acid a2,3-galactose, whereas human hemagglutinin recognizes a slightly different ending: sialic acid a 2,6-galactose.To look for evidence of the switch in recognition, Kawaoka and his colleagues screened avian and human isolates from individuals infected by H5N1 viruses. They analyzed receptor specificity with an assay that measured direct binding to sialylglycopolymers possessing either the 2,3 or the 2,6 sugars. Whereas viruses from chickens...

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