Liquid droplet formation by HP1α suggests a role for phase separation in heterochromatin

Abstract

Gene silencing by heterochromatin is proposed to occur in part from the ability of HP1 proteins to spread across large regions of the genome, compact the underlying chromatin and recruit repressive activities1–3. Here we identify a new property of the human HP1α protein: the ability to form phase-separated droplets. While unmodified HP1α is soluble, either phosphorylation of its N-terminal extension or DNA binding promotes the formation of phase-separated droplets. Phosphorylation driven phase-separation can be promoted or reversed by specific HP1α ligands. Known components of heterochromatin such as nucleosomes and DNA preferentially partition into the HP1α droplets but other molecules such as the transcription factor TFIIB show no preference. Using single-molecule DNA curtains we find that unmodified and phosphorylated HP1α induce rapid compaction of DNA strands into puncta, though with different characteristics. We show by direct protein delivery into mammalian cells that an HP1α mutant incapable of phase separation in vitro forms smaller and fewer nuclear puncta than phosphorylated HP1α. These findings suggest that heterochromatin mediated gene silencing may occur in part through sequestration of compacted chromatin in phase-separated HP1 droplets, which are dissolved or formed by specific ligands based on nuclear context.

Disruption of an RNA-binding hinge region abolishes LHP1-mediated epigenetic repression

“…Such regions can undergo phase transitions to form non-membranebound subcellular compartments, frequently containing RNA (Lin et al 2015). It is interesting to speculate that the subnuclear foci that we observed may therefore represent a phase-separated compartment, as observed recently for mammalian HP1α (Larson et al 2017) and Drosophila HP1a (Strom et al 2017). RNA-binding to the LHP1 hinge region could precipitate formation of nuclear LHP1 bodies to maintain a locally high concentration of LHP1 at Polycomb target genes.…”

Transcription organizes euchromatin similar to an active microemulsion

“…This raises the question how such packing is achieved. In the case of heterochromatin, which is transcriptionally repressed, organization can be explained by phase separation (Larson et al, 2017;Strom et al, 2017). For euchromatin, the transcriptionally permissive part of chromatin, spatial organization is required in development and health (Lupiáñez et al, 2015;Flavahan et al, 2016;Hnisz et al, 2016;Symmons et al, 2016), but a mechanism of euchromatin organization has not yet been described.…”

What lava lamps and vinaigrette can teach us about cell biology

“…Last year, biochemist Geeta Narlikar and her colleagues at the University of California, San Francisco, reported 15 that phase separation helps to mothball parts of the human genome that are perpetually inactive and serve mainly a structural function. A team led by structural biologist Mingjie Zhang at the Hong Kong University of Science and Technology found 16 that a piece of cellular machinery that helps brain cells receive signals is built using phase separation.…”