Encapsidates the viral RNA genome by forming a left-handed helical nucleocapsid that protects the RNA from nucleases (PubMed:23677789, PubMed:19965480, PubMed:31229488). RNA replication depends on the availability of soluble nucleoprotein (PubMed:9299631, PubMed:22623798). The encapsidated genomic RNA is termed the NC and serves as template for transcription and replication (PubMed:22623798). Together with the phosphoprotein, sequesters host NF-kappa-B in inclusion bodies (IBs) thereby inhibiting this host defense pathway (PubMed:32878896). May also act as a modulator of the innate immune response by sequestration of host IFIH1/MDA5 and MAVS into IBs (PubMed:22623778). Homomultimerizes to form the nucleocapsid (PubMed:23677789, PubMed:19965480). Binds to viral genomic RNA. Interacts (via N-terminus) with the phosphoprotein P (via C-terminus); the phosphorylated phosphoprotein P binds to N-RNA complex (PubMed:26474524, PubMed:17170452, PubMed:22623798, PubMed:25407889, PubMed:11861854, PubMed:12368320, PubMed:28031463). When in a monomeric RNA-free form, interacts with the phosphoprotein (via N-terminus) (PubMed:25568210, PubMed:30626736). Interacts with protein M2-1; this interaction allows the association of nucleocapsid with the matrix protein (PubMed:10846068). Interacts with host EIF2AK2/PKR; this interaction inhibits EIF2AK2 phosphorylation of EIF2S1 and blocks EIF2AK2-mediated translation shutoff (PubMed:20519500). Interacts with host EIF1AX; this interaction recruits EIF1AX to the viral replication complex to facilitate viral genomic RNA synthesis and virus production (PubMed:25479059). Interacts with host NF-kappa-B; this interaction sequesters NF-kappa-B in inclusion bodies (By similarity). Localizes in cytoplasmic inclusion bodies. Tyrosine phosphorylation modulates viral transcription and replication. Belongs to the paramyxoviruses nucleocapsid family.