Encapsidates the genome, protecting it from nucleases. The encapsidated genomic RNA is termed the NC and serves as template for transcription and replication. Plays a critical role in the nucleocytoplasmic transport of viral RNP by interaction with other viral proteins. The nuclear export signal is masked by the interaction with the phosphoprotein P. Delays the host cell cycle progression by interacting with the host CDK1-cyclin B1 phase in late G2. Inhibits host NF-kappaB activation. Homomultimerizes to form the nucleocapsid (PubMed:19945724). Binds to viral genomic RNA (PubMed:19945724). Interacts with phosphoprotein P (PubMed:9880009, PubMed:19945724). Interacts with host CDK1 (PubMed:14512566); this interaction delays host cell cycle in late G2. Interacts with host NFKB1 (PubMed:25733193). Shuttles between the host nucleus and cytoplasm. Isoform P38 does not have the nuclear localization signal, nonetheless it is localized in the nucleus.