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Influenza A H3N2 subtype virus NS1 protein targets into the nucleus and binds primarily via its C-terminal NLS2/NoLS to nucleolin and fibrillarin

Krister Melén1*, Janne Tynell1, Riku Fagerlund2, Pascal Roussel3, Danièle Hernandez-Verdun4 and Ilkka Julkunen1

Author Affiliations

1 Virology Unit, Department of Infectious Disease Surveillance and Control, National Institute for Health and Welfare (THL), Mannerheimintie 166, FIN-00300, Helsinki, Finland

2 Signaling Systems Laboratory, Department of Chemistry and Biochemistry, University of California, San Diego, La Jolla, CA, 92093, USA

3 Functional Organization of the Nucleolus, RNA Biology-FRE 3402 CNRS, Université Pierre et Marie Curie, 75252, Paris cedex 5, France

4 Nuclei and Cell Cycle, Institut Jacques Monod-UMR 7592 CNRS, Université Paris Diderot, 75205, Paris cedex 13, France

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Virology Journal 2012, 9:167  doi:10.1186/1743-422X-9-167

Published: 21 August 2012



Influenza A virus non-structural protein 1 (NS1) is a virulence factor, which is targeted into the cell cytoplasm, nucleus and nucleolus. NS1 is a multi-functional protein that inhibits host cell pre-mRNA processing and counteracts host cell antiviral responses. Previously, we have shown that the NS1 protein of the H3N2 subtype influenza viruses possesses a C-terminal nuclear localization signal (NLS) that also functions as a nucleolar localization signal (NoLS) and targets the protein into the nucleolus.


Here, we show that the NS1 protein of the human H3N2 virus subtype interacts in vitro primarily via its C-terminal NLS2/NoLS and to a minor extent via its N-terminal NLS1 with the nucleolar proteins, nucleolin and fibrillarin. Using chimeric green fluorescence protein (GFP)-NS1 fusion constructs, we show that the nucleolar retention of the NS1 protein is determined by its C-terminal NLS2/NoLS in vivo. Confocal laser microscopy analysis shows that the NS1 protein colocalizes with nucleolin in nucleoplasm and nucleolus and with B23 and fibrillarin in the nucleolus of influenza A/Udorn/72 virus-infected A549 cells. Since some viral proteins contain NoLSs, it is likely that viruses have evolved specific nucleolar functions.


NS1 protein of the human H3N2 virus interacts primarily via the C-terminal NLS2/NoLS and to a minor extent via the N-terminal NLS1 with the main nucleolar proteins, nucleolin, B23 and fibrillarin.

Influenza A virus; NS1 protein; NoLS; Nucleolus; Nucleolin; B23; Fibrillarin