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Influenza A virus NS1 protein is a multifunctional virulence factor. Here, we report a crystal structure for the NS1 effector domain of avian influenza virus A/Duck/Albany/76. Comparison of this structure with that reported for a human strain shows both proteins share a common monomer conformation, albeit with subtle differences. Strikingly, our data reveal a novel helix–helix dimeric interface between monomers of the avian NS1 protein, which is also found in the human NS1 crystal lattice. We re-evaluate the current model of NS1 dimeric assembly, and provide biochemical evidence to show tryptophan-187 (a residue located at the helix–helix interface) is essential for dimerization of this effector domain.

Whilst screening various cell lines for their ability to respond to interferon (IFN), we noted that in comparison to other tissue culture cells AGS tumour cells, which are widely used in biomedical research, had very low levels of STAT1. Subsequent analysis showed that the reason for this is that AGS cells are persistently infected with parainfluenza virus type 5 (PIV5; formally known as SV5), a virus that blocks the interferon (IFN) response by targeting STAT1 for proteasome-mediated degradation. Virus protein expression in AGS is altered in comparison to the normal pattern of virus protein synthesis observed in acutely infected cells, suggesting that the AGS virus is defective. We discuss the relevance of these results in terms of the need to screen cell lines for persistent virus infections that can alter cellular functions.