Coronavirus (CoV) nucleocapsid (N) proteins have 3 highly conserved domains. The N-terminal domain (NTD) (N1b), the C-terminal domain (CTD)(N2b) and the N3 region. The N1b and N2b domains from SARS CoV, infectious bronchitis virus (IBV), human CoV 229E and mouse hepatic virus (MHV) display similar topological organisations. N proteins form dimers, which are asymmetrically arranged into octamers via their N2b domains. Domains N1b and N2b are linked by another domain N2a that contains an SR-rich region in which phosphorylation of specific serine residues allows the N protein to associate with the RNA helicase DDX1 permitting template read-through, and enabling the transition from discontinuous transcription of subgenomic mRNAs (sgmRNAs) to continuous synthesis of longer sgmRNAs and genomic RNA (gRNA)[1], [2], [3]. It has been shown that N proteins interact with nonstructural protein 3 (NSP3) and thus are recruited to the replication-transcription complexes (RTCs) [1].

1: Nucleocapsid Protein Recruitment to Replication-Transcription Complexes Plays a Crucial Role in Coronaviral Life Cycle. Cong Y, Ulasli M, Schepers H, Mauthe M, V'kovski P, Kriegenburg F, Thiel V, de Haan CAM, Reggiori F; J Virol. 2020; [Epub ahead of print] PMID:31776274

2: Coronavirus nucleocapsid proteins assemble constitutively in high molecular oligomers. Cong Y, Kriegenburg F, de Haan CAM, Reggiori F; Sci Rep. 2017;7:5740. PMID:28720894

3: Nucleocapsid phosphorylation and RNA helicase DDX1 recruitment enables coronavirus transition from discontinuous to continuous transcription. Wu CH, Chen PJ, Yeh SH; Cell Host Microbe. 2014;16:462-472. PMID:25299332