The SARS-CoV-2 nucleocapsid phosphoprotein forms mutually exclusive condensates with RNA and the membrane-associated M protein

The multifunctional nucleocapsid (N) protein in SARS-CoV-2 binds the ~30 kb viral RNA genome to aid its packaging into the 80-90nm membrane-enveloped virion. The N protein is composed of N-terminal RNA-binding and C-terminal dimerization domains that are flanked by three intrinsically disordered regions. Here we demonstrate that a centrally located 40 amino acid intrinsically disordered domain drives phase separation of N protein when bound to RNA, with the morphology of the resulting condensates affected by inclusion in the RNA of the putative SARS-CoV-2 packaging signal. The SARS-CoV-2 M protein, normally embedded in the virion membrane with its C-terminus extending into the virion core, independently induces N protein phase separation that is dependent on the N protein’s C-terminal dimerization domain and disordered region. Three-component mixtures of N+M+RNA form condensates with mutually exclusive compartments containing N+M or N+RNA, including spherical annular structures in which the M protein coats the outside of an N+RNA condensate. These findings support a model in which phase separation of the N protein with both the viral genomic RNA and the SARS-CoV-2 M protein facilitates RNA packaging and virion assembly.