We next sought to determine the effect of MA NS1 mutations on NS1 localization in mouse and human infected cells, and whether altered NS1 localization correlated with IAV adaptation to a novel host

We next sought to determine the effect of MA NS1 mutations on NS1 localization in mouse and human infected cells, and whether altered NS1 localization correlated with IAV adaptation to a novel host. adaptive mutations increased the proportion of NS1 in the cytoplasm of mouse cells with the greatest effects for mutations M106I Vatalanib free base and D125G. The host gene expression profile of the adaptive mutants was determined by microarray analysis of infected mouse cells to show either high or low extents of host-gene regulation (HGR or LGR) phenotypes. While host genes were predominantly down regulated for the HGR group of mutants (D2N, V23A, F103L, M106I+L98S, L98S, M106V, and M106V+M124I), the LGR phenotype mutants (D125G, M106I, V180A, V226I, and R227K) were characterized by a predominant up regulation of host genes. CPSF30 binding affinity of NS1 mutants did not predict effects on host gene expression. To our knowledge this is the first report of functions of adaptive NS1 mutations that impact intracellular localization and regulation of host gene expression. == Introduction == The influenza A computer virus (IAV) NS1 protein possesses multiple functions that support computer virus replication. NS1 can engage in many functions due to its ability to translocate to both the nucleus and cytoplasm of infected cells and interact with numerous cellular and viral factors including RNA. Cytoplasmic activities include blocking viral RNA detection by RIG-I signalling of type I interferon (IFN) induction as well as inhibition of IFN-stimulated antiviral proteins, suppression of the host cell apoptotic response, and enhancement of viral protein synthesis[1]. In the nucleus, NS1 binds Vatalanib free base cellular post-transcriptional processing factors including the cellular post-transcriptional host factor cleavage and polyadenylation specificity factor 30 (CPSF30), which has been reported to result in a blockade of host gene expression, including type I IFN[2],[3]. NS1 localization is usually governed by two known nuclear localization signals (NLS) (NLS1: aa 3438, NLS2: aa 203237), which interact with the cellular protein importin [4]and induce rapid nuclear localization following translation[5]. Later in infection, the protein is usually detected in both the nucleus and the cytoplasm[6], which is usually attributed to conversation of its NES element (137147)[6]with the nuclear pore complex. Cytoplasmic NS1 levels are also influenced by an inhibitory sequence (148161) adjacent to the NES[7]. NS1 also localizes to the nucleolus and contains a NoLS involving key basic residues Arg-224 and Arg-229[4],[8], however the role of NS1 nucleolar localization and its contribution to viral replication is usually unknown. The ability of the NS1 protein to translocate within discrete cellular compartments and engage in pro-viral functions including blockade of host gene expression and type I interferon (IFN) induction has been shown to be essential for optimum computer virus replication. Alanine substitutions at NLS residues 38 and 41 were Vatalanib free base observed to reduce NS1 nuclear localization and the ability to bind RNA, resulting in increased IFN/ production and attenuation of computer virus replication[9],[10]. Liet Vatalanib free base al.(2011) showed alanine substitutions at Leu-69 and Leu-77 within the highly conserved NS1 linker region inducedde novolocalization to the nucleolus, and dramatically reduced the ability of NS1 to limit IFN inductionin vitro[11]. In addition, NS1 binding affinity for nuclear and nucleolar factors, including CPSF30, nucleolin, and fibrillarin, also influence proteins sequestration in the nucleus and nucleolus, respectively[8],[12]. In the case of CPSF30, amino acid mutations F103S and M106I disrupted CPSF30 binding while shifting NS1 localization to the cytoplasm[12]. Although intracellular localization of the NS1 protein is usually well documented, the effect of virus adaptation to a novel host on NS1 protein localization remains unknown. Cellular distribution of the NS1 protein is dependent upon virus-host interactions; several reports have shown strain-specific and cell-line specific localization patterns of the NS1 protein[4],[12],[13]. Moreover, the nuclear import signals of NOTCH1 influenza RNP (polymerase subunits (PB1, PB2, and PA) and NP) have been shown to be determinants of adaptive evolution in mice[14][17]and for highly pathogenic avian influenza in mammalian cells (reviewed in[18]). We hypothesize that subcellular trafficking of NS1 is usually a determinant of host adaptation. The Vatalanib free base influenza A computer virus host range is usually extensive; while aquatic waterfowl species define the primary viral reservoir, many avian and mammalian species are also hosts. However, the genetic determinants of IAV host range and virulence remain largely unknown. To this end, we previously reported the experimental evolution of the human influenza A computer virus isolate A/Hong Kong/1/1968 (H3N2) (HK) to high virulence in the mouse[15],[17]and identified multiple mouse-adapted (MA) mutations in the PB2[16],[17], HA[16],[17],[19]as well as the NS1 protein[20],[21]. These mutations, including 11 in the NS1 gene, were confirmed to mediate host adaptation, assessed by enhanced computer virus replication in mouse lungs and/or mouse cells[21]. These mutants possessed additional adaptive phenotypes, including enhanced viral gene expression.