Chimeras containing influenza NS1 and HIV-1 Rev protein sequences: mechanism of their inhibition of nuclear export of Rev protein-RNA complexes

Influenza virus non-structural protein NS1: interferon antagonism and beyond

Most viruses express one or several proteins that counter the antiviral defences of the host cell. This is the task of non-structural protein NS1 in influenza viruses. Absent in the viral particle, but highly expressed in the infected cell, NS1 dramatically inhibits cellular gene expression and prevents the activation of key players in the IFN system. In addition, NS1 selectively enhances the translation of viral mRNAs and may regulate the synthesis of viral RNAs. Our knowledge of the virus and of NS1 has increased dramatically during the last 15 years. The atomic structure of NS1 has been determined, many cellular partners have been identified and its multiple activities have been studied in depth. This review presents our current knowledge, and attempts to establish relationships between the RNA sequence, the structure of the protein, its ligands, its activities and the pathogenicity of the virus. A better understanding of NS1 could help in elaborating novel antiviral strategies, based on either live vaccines with altered NS1 or on small-compound inhibitors of NS1.

TNF-alpha and IFN-alpha enhance influenza-A-virus-induced chemokine gene expression in human A549 lung epithelial cells

Lung epithelial cells are the primary cellular targets for respiratory virus pathogens such as influenza and parainfluenza viruses. Here, we have analyzed influenza A, influenza B and Sendai virus-induced chemokine response in human A549 lung epithelial cells. Influenza virus infection resulted in low CCL2/MCP-1, CCL5/RANTES, CXCL8/IL-8 and CXCL10/IP-10 production at late times of infection. However, when cells were pretreated with TNF-alpha or IFN-alpha, influenza-A-virus-induced chemokine production was greatly enhanced. Cytokine pretreatment resulted in enhanced expression of RIG-I, IKKepsilon, interferon regulatory factor (IRF)1, IRF7 and p50 proteins. Most importantly, influenza-A-virus-induced DNA binding of IRF1, IRF3, IRF7 and NF-kappaB onto CXCL10 ISRE and NF-kappaB elements, respectively, was markedly enhanced in cytokine-pretreated cells. Our results suggest that IFN-alpha and TNF-alpha have a significant role in priming epithelial cells for higher cytokine and chemokine production in influenza A virus infection.

Influenza A virus NS1 protein targets poly(A)-binding protein II of the cellular 3'-end processing machinery

Influenza A virus NS1 protein (NS1A protein) via its effector domain targets the poly(A)-binding protein II (PABII) of the cellular 3'-end processing machinery. In vitro the NS1A protein binds the PABII protein, and in vivo causes PABII protein molecules to relocalize from nuclear speckles to a uniform distribution throughout the nucleoplasm. In vitro the NS1A protein inhibits the ability of PABII to stimulate the processive synthesis of long poly(A) tails catalyzed by poly(A) polymerase (PAP). Such inhibition also occurs in vivo in influenza virus-infected cells, where the NS1A protein via its effector domain causes the nuclear accumulation of cellular pre-mRNAs which contain short ( approximately 12 nucleotide) poly(A) tails. Consequently, although the NS1A protein also binds the 30 kDa subunit of the cleavage and polyadenylation specificity factor (CPSF), 3' cleavage of some cellular pre-mRNAs still occurs in virus-infected cells, followed by the PAP-catalyzed addition of short poly(A) tails. Subsequent elongation of these short poly(A) tails is blocked because the NS1A protein inhibits PABII function. Nuclear-cytoplasmic shuttling of PABII, an activity implicating this protein in the nuclear export of cellular mRNAs, is also inhibited by the NS1A protein. In vitro assays suggest that the 30 kDa CPSF and PABII proteins bind to non-overlapping regions of the NS1A protein effector domain and indicate that these two 3' processing proteins also directly bind to each other.