PRC2.1 Coordinates Peri‐Nucleolar H3K27me3‐Enriched Heterochromatin Organization and NPM1 Pentamerization to Maintain Nucleolar Integrity

ABSTRACT The nucleolus, a membrane‐less organelle within the nucleus, plays a fundamental role in ribosome biogenesis with cellular functions across diverse physiological and pathological states. While its multilayered, liquid‐like architecture is formed by complex interactions between nucleolar proteins and nucleic acids, the regulatory mechanism of nucleoplasmic components on nucleolus remains poorly understood, particularly heterochromatin and its associated proteins. In this study, we revealed that Polycomb repressive complex 2.1 (composed of PRC2 core subunits plus Polycomb‐like (PCL) homolog), associating with H3K27me3‐enriched heterochromatin, accumulates surrounding the nucleolus to form a ring‐like structure. Notably, as a key PRC2.1 component, PCL2 undergoes phase separation, while the intrinsically disordered region (IDR) in PCL2 regulates both the fluidity of the condensates and its interaction with nucleolar protein nucleophosmin (NPM1). PCL2 is a key regulator to facilitate NPM1 pentamerization and interaction among nucleolar components, ensuring the efficient progression of rRNA and protein synthesis. Taken together, our findings reveal that PRC2.1(PCL2) plays a crucial role in maintaining nucleolar integrity and rRNA synthesis, highlighting that PRC2.1(PCL2) is required for cell proliferation.