The nucleocapsid (N) protein of coronaviruses serves two major functions: compaction of the RNA genome in the virion and regulation of viral gene transcription. It is not clear how the N protein mediates such distinct functions. The N protein contains two RNA-binding domains surrounded by regions of intrinsic disorder. Phosphorylation of the central disordered region promotes the protein’s transcriptional function, but the underlying mechanism is not known. Here, we show that the N protein of SARS-CoV-2, together with viral RNA, forms biomolecular condensates. Unmodified N protein forms partially ordered gel-like condensates and discrete 15-nm particles based on multivalent RNA-protein and protein-protein interactions. Phosphorylation reduces these interactions, generating a more liquid-like droplet. We propose 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.

冠状病毒的核衣壳 (N) 蛋白有两个主要功能：压缩病毒颗粒中的 RNA 基因组和调节病毒基因转录。目前尚不清楚 N 蛋白如何介导如此独特的功能。N 蛋白包含两个 RNA 结合域，周围环绕着内在无序区域。中央无序区域的磷酸化促进了蛋白质的转录功能，但其潜在机制尚不清楚。在这里，我们证明 SARS-CoV-2 的 N 蛋白与病毒 RNA 一起形成生物分子缩合物。未修饰的 N 蛋白基于多价 RNA-蛋白和蛋白-蛋白相互作用形成部分有序的凝胶状凝聚物和离散的 15 nm 颗粒。磷酸化减少了这些相互作用，产生了更像液体的液滴。我们认为不同的寡聚状态支持 N 蛋白的两种功能：未修饰的蛋白质形成适合核衣壳组装的结构化寡聚物，磷酸化的蛋白质形成用于病毒基因组加工的液体样区室。