Mouse Heterochromatin Adopts Digital Compaction States without Showing Hallmarks of HP1-Driven Liquid-Liquid Phase Separation

Mol Cell. 2020 Apr 16;78(2):236-249.e7. doi: 10.1016/j.molcel.2020.02.005. Epub 2020 Feb 25.


The formation of silenced and condensed heterochromatin foci involves enrichment of heterochromatin protein 1 (HP1). HP1 can bridge chromatin segments and form liquid droplets, but the biophysical principles underlying heterochromatin compartmentalization in the cell nucleus are elusive. Here, we assess mechanistically relevant features of pericentric heterochromatin compaction in mouse fibroblasts. We find that (1) HP1 has only a weak capacity to form liquid droplets in living cells; (2) the size, global accessibility, and compaction of heterochromatin foci are independent of HP1; (3) heterochromatin foci lack a separated liquid HP1 pool; and (4) heterochromatin compaction can toggle between two "digital" states depending on the presence of a strong transcriptional activator. These findings indicate that heterochromatin foci resemble collapsed polymer globules that are percolated with the same nucleoplasmic liquid as the surrounding euchromatin, which has implications for our understanding of chromatin compartmentalization and its functional consequences.

Keywords: Heterochromatin protein 1; chromatin accessibility; chromatin compartmentalization; epigenetic editing; intracellular viscosity; liquid- liquid phase separation; nuclear organization; optodroplets; polarization-dependent fluorescence correlation spectroscopy; polymer collapse.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Animals
  • Chromatin / genetics*
  • Chromobox Protein Homolog 5
  • Chromosomal Proteins, Non-Histone / genetics*
  • Euchromatin / genetics*
  • Fibroblasts
  • Heterochromatin / genetics*
  • Mice


  • Chromatin
  • Chromosomal Proteins, Non-Histone
  • Euchromatin
  • Heterochromatin
  • Chromobox Protein Homolog 5