Trafficking of mRNAs from the nucleus to the cytoplasm is essential for gene expression and is highly regulated by pathogens and signaling pathways. To exit the nucleus, most mRNAs interact with the receptors NXF1-p15 (TAP-p15) via adaptor proteins, such as E1B-AP5. Another mRNA export factor is Rae1, which aids in docking mRNAs onto nuclear pore complex proteins, such as the nucleoporin Nup98. The mRNA export complex is then translocated through the nuclear pore complex to the cytoplasm. We have reported the interaction of viral proteins with constituents of the mRNA export pathway, which induced inhibition of Mrna export and provided insights on regulatory mechanisms of mRNA export. We showed that the vesicular stomatitis virus (VSV) matrix (M) protein binds Rae1 and that the nonstructural protein 1 (NS1) of influenza virus interacts with NXF1-p15, Rae1, and E1B-1AP5. NS1 is a major virulence factor of influenza A virus that is essential for pathogenesis. These viral-host interactions inhibit expression of antiviral proteins. However, Nup98 and Rae1 are up-regulated by interferons (IFN), which constitute a mechanism of antiviral response that can revert the mRNA export block mediated by these viral proteins. Another nucleoporin involved in antiviral response is Nup96, which is regulated by IFN and in turn preferentially facilitates expression of IFN-regulated mRNAs. Using viral proteins as tools, we propose to uncover novel molecular mechanisms of mRNA export.
Our specific aims are: 1. To investigate the mechanisms through which viral proteins disrupt the mRNA export machinery. We have evidence that NS1 and VSV M proteins interact with different forms of the mRNA export complex. We have also identified novel constituents of the complex. Biochemical approaches will be used to assemble these mRNA export complexes in vitro. Functional studies will be carried out using knockdown, overexpression, and mutagenesis strategies in combination with mRNA export assays. 2. To study regulation of mRNA export by antagonists of viral-mediated mRNA export block. We have identified novel chemical inhibitors of the NS1 protein of influenza virus, some of which target constituents of the mRNA export machinery. Biochemical analyses of interactions between these inhibitors and the mRNA export machinery will be performed to investigate key regulatory mechanisms. Functional assays to assess the effect of these inhibitors on mRNA export will also be performed in vitro and in vivo. 3. To determine the role of mRNA export in antiviral response. We have reported that Nup96 is up-regulated by IFN and is involved in antiviral response to facilitate mRNA export of IFN-regulated mRNAs. Nup96 interacts with Sec13 and Seh1. To study the relationship between these Nups in nuclear transport and Nup96-mediated regulation of mRNA export and IFN response, we have generated novel hypomorphic mice and cells that allow gradual down-regulation of Sec13 and Seh1 alone or in combination with Nup96. Altogether, these studies will generate new information on basic mechanisms of mRNA export and on how viruses and host regulate this machinery to their own advantage.
Trafficking of molecules between the two major compartments in the cell, the nucleus and the cytoplasm, involve processes that are targeted by viruses, as they are important for antiviral defense. Our laboratory studies the mechanisms and key players of these trafficking processes. By understanding these mechanisms, we were able to design inhibitors or drugs that work against viral toxicity.
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