Encapsidates the negative strand viral RNA, protecting it from nucleases. The encapsidated genomic RNA is termed the ribonucleoprotein (RNP) and serves as template for transcription and replication. The RNP needs to be localized in the host nucleus to start an infectious cycle, but is too large to diffuse through the nuclear pore complex. NP comprises at least 2 nuclear localization signals that are responsible for the active RNP import into the nucleus through cellular importin alpha/beta pathway. Later in the infection, nclear export of RNPs are mediated through viral proteins NEP interacting with M1 which binds nucleoproteins. It is possible that nucleoprotein binds directly host exportin-1/XPO1 and plays an active role in RNPs nuclear export. M1 interaction with RNP seems to hide nucleoprotein's nuclear localization signals. Soon after a virion infects a new cell, M1 dissociates from the RNP under acidification of the virion driven by M2 protein. Dissociation of M1 from RNP unmasks nucleoprotein's nuclear localization signals, targeting the RNP to the nucleus. Homomultimerizes to form the nucleocapsid. May bind host exportin-1/XPO1. Binds to viral genomic RNA. Protein-RNA contacts are mediated by a combination of electrostatic interactions between positively charged residues and the phosphate backbone and planar interactions between aromatic side chains and bases. Late in virus-infected cells, may be cleaved from a 56-kDa protein to a 53-kDa protein by a cellular caspase. This cleavage might be a marker for the onset of apoptosis in infected cells or have a specific function in virus host interaction. South Carolina isolate has been sequenced from formalid fixed-lung tissues of a 21-year-old male which died in 1918 at Ft. Jackson, SC. Brevig Mission isolate has been sequenced from lung tissues of an Inuit woman buried in the permafrost in a gravesite near Brevig Mission, Alaska. This sample was recovered by John Hultin, retired pathologist. Belongs to the influenza viruses nucleoprotein family.