The mature ternary influenza viral polymerase complex consists of PB1, PB2, and PA. The N-terminus of PB1 (residues 1-48) interacts with PB2, and amino acids 506-659 in PB1 interact with the PA subunit (Gonzalez, 1996; Perez, 2001). Although monomeric PB1, PB2 and PA, as well as PB1-PB2 and PB1-PA dimers are likely to exist in infected cells, it is believed that most of the polymerase proteins are assembled into the trimeric PB1-PB2-PA complex (Detjen, 1987). Newly synthesized subunits of the polymerase are imported into the nucleus through nuclear localization signals (NLS), which interact with cellular importin family proteins (Jones, 1986; Buolo, 2006). Importin beta-3 (Ran binding protein 5) facilitates nuclear import of PB1 and a PB1-PA dimer (Deng, 2006); coexpression of PA with PB1 was shown to enhance the import of PB1 (Fodor, 2004). A PB1-PB2 dimer has been found to interact with the molecular chaperone heat shock protein 90 (HSP90) to facilitate import (Naito, 2007). The three subunits assembled in the nucleus form a mature ternary polymerase complex that binds viral vRNA or cRNA (Jones, 1986; Buolo, 2006).