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. Homomultimerizes to form the nucleocapsid. 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. Localizes in cytoplasmic inclusion bodies. Belongs to the paramyxoviruses nucleocapsid family.