Interaction of influenza virus NS1 protein and the human homologue of Staufen in vivo and in vitro

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Abstract
A screening for human proteins capable of interacting with influenza virus NS1 has been carried out using the two-hybrid genetic trap in yeast. A cDNA corresponding to the human homologue of Drosophila melanogaster Staufen protein (hStaufen) was isolated that fulfilled all genetic controls of the two-hybrid protocol. Using a hStaufen cDNA isolated from a λ human library, the interaction of hStaufen and NS1 proteins was characterised in vivo and in vitro. Co-transfection of NS1 cDNA and a partial cDNA of hStaufen led to the relocalisation of recombinant hStaufen protein from its normal accumulation site in the cytoplasm to the nuclear location of NS1 protein. NS1 and hStaufen proteins could be co-immunoprecipitated from extracts of co-transfected cells and from mixtures of extracts containing either protein, as well as from extracts of influenza virus-infected cells. Furthermore, both proteins co-localised in the ribosomal and polysomal fractions of influenza virus-infected cells. The interaction was also detected in pull-down experiments using a resin containing purified hStaufen and NS1 protein translated in vitro. Deletion mapping of the NS1 gene indicated that a mutant protein containing the N-terminal 81 amino acids is unable to interact with hStaufen, in spite of retaining full RNA-binding capacity. These results are discussed in relation to the possible mechanisms of action of hStaufen and its relevance for influenza virus infection.