Encapsidates the negative strand viral RNA, protecting it from nucleases (Probable). The encapsidated genomic RNA is termed the ribonucleoprotein (RNP) and serves as template for transcription and replication (PubMed:15522447). Homomultimerizes to form the nucleocapsid. 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 (By similarity).