Atypical characteristics of nucleoprotein of pandemic influenza virus H1N1 and their roles in reassortment restriction

Sequence analysis of the nucleoprotein (NP) of swine-origin influenza virus H1N1 (S-OIV) reveals a number of atypical characteristics including an early start codon and a highly conserved, non-aromatic residue at position 313. Using an in vitro viral polymerase reconstitution assay, we found that the polymerase complex containing the NP of S-OIV (NPS-OIV) yielded substantially lower activity than those assayed with NP derived from other influenza virus strains. Moreover, alteration of the early start codon or introduction of an aromatic residue at position 313 (V313Y) did not increase but instead exacerbated the poor polymerase activity. Interestingly, when NPS-OIV was allowed to compete with that of a mouse-adapted influenza virus (A/PR/8/34) to form progeny virions, only progeny bearing NPS-OIV were produced, despite the low polymerase activity associated with NPS-OIV. Our results indicated that NPS-OIV requires both the early start codon and the V313 residue for its optimal function. These characteristics are required for a strong compatibility between the S-OIV polymerase subunits and its indigenous NP over that of other strains, which might explain why productive reassortment between S-OIV and seasonal influenza viruses has yet to occur in nature.