Packages the positive strand viral genome RNA into a helical ribonucleocapsid (RNP) and plays a fundamental role during virion assembly through its interactions with the viral genome and membrane protein M. Plays an important role in enhancing the efficiency of subgenomic viral RNA transcription as well as viral replication. Homooligomer. Both monomeric and oligomeric forms interact with RNA. Interacts with protein M. Interacts with NSP3; this interaction serves to tether the genome to the newly translated replicase-transcriptase complex at a very early stage of infection. Located inside the virion, complexed with the viral RNA. Probably associates with ER-derived membranes where it participates in viral RNA synthesis and virus budding. ADP-ribosylated. The ADP-ribosylation is retained in the virion during infection. Phosphorylated on serine and threonine residues. Belongs to the gammacoronavirus nucleocapsid protein family.