The N-terminus of the influenza B virus nucleoprotein is essential for virus viability, nuclear localization and is required for optimal transcription and replication of the viral genome

The nucleoprotein (NP) of influenza viruses is a multifunctional protein with essential roles throughout viral replication. Despite influenza A and B viruses belonging to separate genera of the Orthomyxoviridae family, their NP proteins share a relatively high level of sequence conservation. However NP of influenza B viruses (BNP) contains an evolutionarily conserved N-terminal 50 amino acid extension that is absent from NP of influenza A viruses. There is conflicting evidence as to the functions of the BNP N-terminal extension, however this has never been assessed in the context of viral infection. We have used reverse genetics to assess the significance of this region on the functions of BNP and virus viability. Truncation of more than three amino acids prevented virus recovery suggesting that the N-terminal extension is essential for virus viability. Mutational analysis indicated that multiple regions of the protein are involved in nuclear localization of BNP with the entire N-terminal extension required for this to function efficiently. Viruses containing mutations in the first ten residues of BNP demonstrated little differences in nuclear localization, however the viruses exhibited significant reductions in viral mRNA transcription and genome replication resulting in significantly attenuated phenotypes. Mutations introduced to ablate a previously reported nuclear localization signal also resulted in a significant decrease in mRNA production during early stages of viral replication. Overall our results demonstrate that the N-terminal extension of BNP is essential to virus viability not only for directing nuclear localization of BNP, but also for regulating viral mRNA transcription and genome replication.