Several protein regions contribute to determine the nuclear and cytoplasmic localization of the influenza A virus nucleoprotein

A systematic analysis was carried out to identify the amino acid signals th at regulate the nucleocytoplasmic transport of the influenza A virus nucleo protein (NP). The analysis involved determining the intracellular localizat ion of eight deleted recombinant NF proteins and 14 chimeric proteins conta ining the green fluorescent protein fused to different NF fragments. In add ition, the subcellular distribution of NP derivatives that contained specif ic substitutions at serine-3, which is the major phosphorylation site of th e A/Victoria/3/75 NP, were analysed. From the results obtained, it is concl uded that the NP contains three signals involved in nuclear accumulation an d two regions that cause cytoplasmic accumulation of the fusion proteins. O ne of the karyophilic signals was located at the N terminus of the protein, and the data obtained suggest that the functionality of this signal can be modified by phosphorylation at serine-3. These findings are discussed in t he context of the transport of influenza virus ribonucleoprotein complexes into and out of the nucleus.