Probing the structure of influenza B hemagglutinin using site-directed mutagenesis

The crystal structure of the hemagglutinin (HA) of influenza virus A/Aichi/68 (H3N2) from the X-31 reassortant virus was reported in 1981, but as yet there are no X-ray diffraction structures for hemagglutinins of other types or even subtypes of influenza virus. We have used site-directed mutagenesis to probe the structure of the hemagglutinin of influenza B/Hong Kong/8/73. We investigated a region in the globular head domain that is helical in the influenza A HA structure, targeting sidechains that in the H3 HA point toward solvent (Thr196) or into the receptor-binding pocket (Gln197). None of the mutations affected hemagglutination activity, but mutations T196P or Q1971 eliminated binding of a monoclonal antibody. The data suggest that this region of the influenza B HA forms a surface structure different from the alpha-helix of the influenza A HA structure and that it accounts for much of the antigenic activity of influenza B HA.