X-ray structure of NS1 from a highly pathogenic H5N1 influenza virus

Recent emergence of highly pathogenic avian (H5N1) influenza viruses, their epizootic and panzootic nature, and their association with lethal human infections have raised significant global health concerns1,2. Several studies have underscored the importance of non-structural protein NS1 in the increased pathogenicity and virulence of these strains3,4. NS1, which consists of two domains, a dsRNA binding domain (RBD)5,6 and the effector domain (ED)7 separated through a linker, is an antagonist of antiviral type-I interferon (IFN) response in the host8,9. Here we report the x-ray structure of the full length NS1 from a H5N1 (A/Vietnam/1203/2004) strain that was associated with 60% of human deaths in an outbreak in Vietnam1,2. Compared to the individually determined structures of RBD and ED from non-H5N1 strains, the RBD within H5N1 NS1 exhibits modest structural changes, while the H5N1 ED shows significant alteration particularly in the dimeric interface. Although both domains in the full-length NS1 individually participate in dimeric interactions, an unexpected finding is that these interactions result in the formation of a chain of NS1 molecules instead of distinct dimeric units. Three such chains in the crystal interact with one another extensively to form a tubular organization of similar dimensions observed in the cryo-EM images of NS1 in the presence of dsRNA. The tubular oligomeric organization of NS1 in which residues implicated in dsRNA binding face a 20 Å wide central tunnel provides a plausible mechanism for how NS1 sequesters varying lengths of dsRNA, to counter cellular antiviral dsRNA response pathways, while simultaneously interacting with other cellular ligands during an infection.