Alleles A and B of non-structural protein 1 of avian influenza A viruses differentially inhibit beta interferon production in human and mink lung cells

Non-structural protein 1 (NS1) counteracts the production of host type I interferons (IFN-alpha/beta) for the efficient replication and pathogenicity of influenza A viruses. Here, we reveal another dimension of the NS1 protein of avian influenza A viruses in suppressing IFN-beta production in cultured cell lines. We found that allele A NS1 proteins of H6N8 and H4N6 have a strong capacity to inhibit the activation of IFN-beta production, compared with allele B from corresponding subtypes, as measured by IFN stimulatory response element (ISRE) promoter activation, IFN-beta mRNA transcription and IFN-beta protein expression. Furthermore, the ability to suppress IFN-beta promoter activation was mapped to the C-terminal effector domain (ED), while the RNA-binding domain (RBD) alone was unable to suppress IFN-beta promoter activation. Chimeric studies indicated that when the RBD of allele A was fused to the ED of allele B, it was a strong inhibitor of IFN-beta promoter activity. This shows that well-matched ED and RBD are crucial for the function of the NS1 protein and that the RBD could be one possible cause for this differential IFN-beta inhibition. Notably, mutagenesis studies indicated that the F103Y and Y103F substitutions in alleles A and B, respectively, do not influence the ISRE promoter activation. Apart from dsRNA signalling, differences were observed in the expression pattern of NS1 in transfected human and mink lung cells. This study therefore expands the versatile nature of the NS1 protein in inhibiting IFN responses at multiple levels, by demonstrating for the first time that it occurs in a manner dependent on allele type.