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Title: The influenza A virus NS1 protein and viral mRNA nuclear export
Author: Fernandes Pereira, Carina
ISNI:       0000 0004 7229 4239
Awarding Body: University of Cambridge
Current Institution: University of Cambridge
Date of Award: 2018
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Influenza A virus (IAV) replication and transcription occur in the host cell nucleus; a feature which means both the viral genome (vRNA) and mRNA must be exported from the nucleus to the cytoplasm. The mechanism by which vRNA nuclear export is achieved has been well characterised, but how viral mRNAs are exported is poorly understood. The cellular NXF1-dependent mRNA export pathway has been shown to be involved in the export of some viral mRNAs, but how they are recruited to this pathway is unknown. Prior work from our laboratory showed that segment 7 mRNA was inefficiently exported to the cytoplasm in a sub-viral ‘minireplicon’ system, providing the first indication that there were viral requirements for IAV mRNA nuclear export. Further addition of individual viral polypeptides was tested and the effect on segment 7 mRNA export was analysed by fluorescent in situ hybridization (FISH) and confocal microscopy. This identified the NS1 protein as the viral factor required for efficient segment 7 nuclear export. Mutational studies on NS1 were carried out to unveil the mechanistic role of this protein in viral mRNA nuclear export, by plasmid transfection as well as in the context of recombinant viruses. These approaches indicated that both functional domains of NS1 were necessary to preserve the mRNA export function. Furthermore, these mutant proteins were used to examine the association between NS1 and the NXF1-dependent pathway in the context of mRNA nuclear export. Protein-protein and protein-RNA binding assays indicated that interactions between NXF1 and NS1, and NXF1 and segment 7 mRNA were necessary, but not sufficient to promote segment 7 viral mRNA export. Lastly, the role of NS1 protein in the nuclear export of viral mRNAs from other genome segments was studied. The intracellular localisation of most viral mRNAs was not affected by the absence of NS1 or the presence of an export-incompetent NS1 mutant protein. However, segment 4 mRNA exhibited a similar phenotype to segment 7 mRNA in showing a dependence on NS1 for efficient nuclear export. Overall, the results presented in this dissertation suggest that NS1 acts as an adaptor protein between the viral RNA synthesis machinery and cellular export pathway. This provides deeper insights for the characterization of a recently identified function of the IAV NS1 protein, of being required for the efficient nuclear export of mRNA from “late” kinetic class viral genes.
Supervisor: Digard, Paul ; Crump, Colin Sponsor: Fundação para a Ciência e a Tecnologia (FCT)
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
Keywords: NS1 ; NXF1 ; influenza A virus mRNA export ; nuclear import/export