Nuclear import of influenza B virus nucleoprotein: involvement of an N-terminal nuclear localization signal and a cleavage-protection motif.

The nucleoprotein of influenza B virus (BNP) shares several characteristics with its influenza A virus counterpart (ANP), including localization in the host's nucleus. However, while the nuclear localization signal(s) (NLS) of ANP are well characterized, little is known about those of BNP. In this study, we showed that the fusion protein bearing the BNP N-terminus fused with GFP (N70-GFP) is exclusively nuclear, and identified a highly conserved KRXR motif spanning residues 44-47 as a putative NLS. In addition, we demonstrated that residues 3-15 of BNP, though not an NLS, are also crucial for nuclear import. Results from mutational analyses of N70-GFP and the full-length BNP suggest that this region may be required for protection of the N-terminus from proteolytic cleavage. Altogether, we propose that the N-terminal region of BNP contains the NLS and cleavage-protection motif, which together drive its nuclear localization.