Phosphorylation Toggles the SARS-CoV-2 Nucleocapsid Protein Between Two Membrane-Associated Condensate States.

The Nucleocapsid protein (N) of SARS-CoV-2 plays a critical role in the viral lifecycle by regulating RNA replication and packaging the viral genome. N and RNA phase separate to form condensates that may be important for these functions. Both functions occur at membrane surfaces, but how N toggles between these two membrane-associated functional states is unclear. Here, we reveal that phosphorylation switches how N condensates interact with membranes, partly by modulating condensate material properties. Phosphorylated N forms liquid condensates that wet membranes, reminiscent of N's role in RNA replication. In contrast, unmodified N forms viscoelastic condensates that can be engulfed by membranes, evoking viral genome packaging. These results suggest that phosphorylation serves as a regulatory switch within the viral replication cycle by modulating N's association with membranes. We gained mechanistic insight through structural analysis and molecular simulations, which suggest phosphorylation induces a conformational change that softens condensate material properties. Our studies also show that phosphorylation alters N's interaction with viral membrane proteins. Together, our findings uncover a novel aspect of SARS-CoV-2 biology by identifying membrane association as a key feature of N condensates and providing mechanistic insights into the regulatory role of phosphorylation. Understanding this mechanism suggests potential therapeutic targets for COVID infection.