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

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, while 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. Eliminating the nucleolus by removing the ribosomal RNA genes
(rDNA) disrupted this process, causing increased PCH compaction followed
by its reorganization into a toroidal structure. Further, in embryos
lacking rDNA, 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 could
be mediated by a hierarchy of interaction strengths between PCH, nucleoli,
and proteins with dual affinities for both compartments. We validated this
model by identifying such a protein, a DEAD-Box RNA Helicase called
Pitchoune, and showed that modulating its affinity for either nucleolar or
PCH components alters nucleolar-PCH organization. Together, this study
unveils a dynamic program for establishing nucleolar-PCH associations
during animal development and demonstrates how interaction hierarchies and
dual-affinity molecular linkers co-organize compositionally distinct
condensates.