Phosphorylation modulates liquid-liquid phase separation of the SARS-CoV-2 N protein

ABSTRACT

The nucleocapsid (N) protein of coronaviruses serves two major functions compaction of the RNA genome in the virion and regulation of viral gene transcription in the infected cell (1-3) . The N protein contains two globular RNA-binding domains surrounded by regions of intrinsic disorder (4) . Phosphorylation of the central disordered region is required for normal viral genome transcription (5,6) , which occurs in a cytoplasmic structure called the replication transcription complex (RTC) (7-11) . It is not known how phosphorylation controls N protein function. Here we show that the N protein of SARS-CoV-2, together with viral RNA, forms biomolecular condensates (12-15) . Unmodified N protein forms partially ordered gel-like structures that depend on multivalent RNA-protein and protein-protein interactions. Phosphorylation reduces a subset of these interactions, generating a more liquid-like droplet. We speculate that distinct oligomeric states support the two functions of the N protein unmodified protein forms a structured oligomer that is suited for nucleocapsid assembly, and phosphorylated protein forms a liquid-like compartment for viral genome processing. Inhibitors of N protein phosphorylation could therefore serve as antiviral therapy.