Journal Article

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

Burlingame, Alma L., Narlikar, Geeta J., Keenen, Madeline M., Elnatan, Daniel, Redding, Sy, Johnston, Jonathan B., Larson, Adam G., Trnka, Michael J., Agard, David A.

2017

Overview

Phosphorylation or DNA binding promotes the physical partitioning of HP1α out of a soluble aqueous phase into droplets, suggesting that the repressive action of heterochromatin may in part be mediated by the phase separation of HP1. HP1α forms reversible droplets The gene-silencing action of heterochromatin is thought to arise from the spread of proteins such as HP1 that compact the underlying chromatin and recruit repressors. Two papers in this issue demonstrate that HP1α has the ability to form phase-separated droplets. Gary Karpen and colleagues show that HP1α can nucleate into foci that display liquid properties during the early stages of heterochromatin domain formation in Drosophila embryos. Geeta Narlikar and colleagues demonstrate that human HP1α protein also forms phase-separated droplets. Phosphorylation or DNA binding promotes the physical partitioning of HP1α out of the soluble aqueous phase into droplets. These related findings suggest that the repressive action of heterochromatin may be in part mediated by the phase separation of HP1, with the droplets being initiated or dissolved by various ligands depending on nuclear context. Gene silencing by heterochromatin is proposed to occur in part as a result of the ability of heterochromatin protein 1 (HP1) proteins to spread across large regions of the genome, compact the underlying chromatin and recruit diverse ligands 1 , 2 , 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 molecules such as the transcription factor TFIIB show no preference. Using a single-molecule DNA curtain assay, we find that both unmodified and phosphorylated HP1α induce rapid compaction of DNA strands into puncta, although with different characteristics 4 . 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 on the basis of nuclear context.

• Publisher: Nature Publishing Group UK
• Subject: 631/337/100 / 631/45/56 / 631/80/386 / 82/80 / Animals / Chromobox Protein Homolog 5 / Chromosomal Proteins, Non-Histone - chemistry / Chromosomal Proteins, Non-Histone - genetics / Chromosomal Proteins, Non-Histone - metabolism / DNA - metabolism / Gene Silencing / Heterochromatin - chemistry / Heterochromatin - genetics / Heterochromatin - metabolism / Humanities and Social Sciences / Humans / letter / Ligands / Mice / multidisciplinary / NIH 3T3 Cells / Nucleosomes - chemistry / Nucleosomes - genetics / Nucleosomes - metabolism / Phosphorylation / Science / Solubility / Transcription Factor TFIIB - metabolism