Hierarchical interactions between nucleolar and heterochromatin condensates are mediated by a dual-affinity protein

Abstract Nucleoli are surrounded by pericentromeric heterochromatin (PCH), reflecting a conserved spatial association between the two largest biomolecular condensates in eukaryotic nuclei. Nucleoli are the sites of ribosome synthesis, whereas the repeat-rich PCH is essential for chromosome segregation, genome stability and transcriptional silencing, yet the mechanisms for their co-assembly are unclear. Here we use high-resolution live imaging during Drosophila embryogenesis and reveal that de novo establishment of PCH–nucleolar associations is highly dynamic, as PCH transitions from extending along the nuclear edge to surrounding the nucleolus. Elimination of the nucleolus by removing the ribosomal RNA genes disrupted this process causing increased PCH compaction, followed by its reorganization into a toroidal structure. Furthermore, in embryos lacking ribosomal RNA genes, nucleolar proteins were redistributed into new bodies or ‘neocondensates’, including enrichment in the PCH toroidal hole. Combining these in vivo observations with molecular dynamics simulations based on multiphase wetting theory revealed that nucleolar–PCH associations can be mediated by a hierarchy of interaction strengths between PCH, nucleoli and proteins with dual affinities for both compartments. We validate this model by identifying such a protein, a DEAD-box RNA helicase called Pitchoune, and show that modulation of its affinity for either nucleolar or PCH components alters nucleolar–PCH organization. Together, this study unveils a dynamic programme for establishing nucleolar–PCH associations during animal development and demonstrates how interaction hierarchies and dual-affinity molecular linkers co-organize compositionally distinct condensates.