Enhanced human-type receptor binding by ferret transmissible H5N1 with a K193T mutation

All human influenza pandemics have originated from avian influenza viruses. Although multiple changes are needed for an avian virus to be able to transmit between humans, binding to human-type receptors is essential. Several research groups have reported mutations in H5N1 viruses that exhibit specificity to human-type receptors and promote respiratory droplet transmission between ferrets. Upon detailed analysis we have found that these mutants exhibit significant differences in fine receptor specificity compared to human H1N1 and H3N2 and retain avian-type receptor binding. We have recently shown that human influenza viruses preferentially bind to α2-6 sialylated branched N-linked glycans, where the sialic acids on each branch can bind to receptor sites on two protomers of the same HA trimer. In this binding mode the glycan projects over the 190-helix at the top of the receptor-binding pocket, which in H5N1 would create stearic clash with lysine at 193. Thus we hypothesized that a K193T mutation, would improve binding to branched N-linked receptors. Indeed, adding the K193T mutation to the H5 HA of a respiratory droplet transmissible virus dramatically improves both binding to human trachea epithelial cells and specificity for extended α2-6 sialylated N-linked glycans recognized by human influenza viruses.