The nascent vRNP complexes, one for each gene segment, contain the negative-sense viral RNA and polymerase proteins (PB1, PB2, PA, and NP). In a model using negative-sense viral RNP reconstituted from transfected cells, there are multiple NP complexes and one polymerase complex arranged along a closed vRNA loop (Area et al., 2004). The three-dimensional structure of NP has revealed that three NP molecules form a stable trimer, interacting through beta-sheets b5, b6, and b7 in the C-terminal domain of the protein (Ye, 2006), with the viral RNA wrapping around the outside of the complex. Viral RNA from purified virions is present in an RNase-sensitive complex with NP and PB1, PB2, and PA, consistent with this structural model (Baudin et al, 1994; Ruigrok et al., 1995; Klumpp et al., 1997). It is not clear what controls the fate of vRNP, whether it is destined to become a template for transcription, for replication, or for export into the cytoplasm for packaging into virions at the plasma membrane, nor how distinct sub-nuclear localization and NP distribution at the nuclear matrix might mark, or polarize, a vRNP for export (Elton, 2005; Takizawa et al., 2006).
