Nuclear localization of influenza B polymerase proteins and their binary complexes.

The viral RNA-dependent RNA polymerases of influenza A and B are trimeric complexes of PA, PB1, and PB2 subunits that are crucial for both transcription and replication of the viral genome. Unlike the significant progress made recently in understanding nuclear transport and molecular assembly of influenza A polymerase, little is known about the influenza B polymerase, although influenza B viruses cause severe upper respiratory disease in humans. The aim of this study was to characterize nuclear localization of the influenza B RNA polymerase proteins and binary complexes. We demonstrated that each polymerase protein has a nuclear localization function, and among them, the PB2 protein exclusively locates to the nucleus while PA and PB1 proteins are associated with the cytoplasm and the nucleus. Furthermore, we show that pairwise binary complexes are formed among the influenza B subunits (PA-PB1, PA-PB2, and PB1-PB2) and both PB1-PB2 and PA-PB2 complexes are predominantly associated with the nucleus while the PA-PB1 complex exhibits both nuclear and cytoplasmic fluorescence signals. Results of our studies represent the first step toward the understanding of nuclear transport and molecular assembly within the influenza B polymerase complex.