Encapsidates the viral RNA genome by forming a left-handed helical nucleocapsid that protects the RNA from nucleases. RNA replication depends on the availability of soluble nucleoprotein. The encapsidated genomic RNA is termed the NC and serves as template for transcription and replication. Together with the phosphoprotein, sequesters host NF-kappa-B in inclusion bodies (IBs) thereby inhibiting this host defense pathway. May also act as a modulator of the innate immune response by sequestration of host IFIH1/MDA5 and MAVS into IBs. Homomultimerizes to form the nucleocapsid. Binds to viral genomic RNA. Interacts with the phosphoprotein P. When in a monomeric RNA-free form, interacts with the phosphoprotein (via N-terminus). Interacts with protein M2-1; this interaction allows the association of nucleocapsid with the matrix protein. Interacts with host EIF2AK2/PKR; this interaction inhibits EIF2AK2 phosphorylation of EIF2S1 and blocks EIF2AK2-mediated translation shutoff. Interacts with host EIF1AX; this interaction recruits EIF1AX to the viral replication complex to facilitate viral genomic RNA synthesis and virus production (By similarity). 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.