Molecular basis of heterochromatin formation and regulation

Heterochromatin protein 1α (HP1α) plays a central role in the organization of nuclear content and in the regulation of gene expression and chromatin compaction. HP1α binds to chromatin regions enriched in histone H3 K9 trimethylation and coordinates a complex network of events that leads to the formation of dense heterochromatin neighborhoods and gene silencing. It is believed that this process involves the phase separation of HP1α proteins into liquid droplets and gel-like compartments. Capturing the molecular interactions that drive heterochromatin formation has been difficult due to the heterogeneous nature of chromatin and HP1α, both of which contain rigid and dynamic components. To understand this system, we have used a biophysical toolbox that includes solution and solid-state NMR spectroscopy, fluorescence imaging, and coarse-grained molecular dynamics simulations. These approaches have allowed us to 1) capture the molecular interactions that drive the phase separation of HP1α, 2) describe how HP1α interacts with chromatin to bring about gene silencing, and 3) understand how these processes can be regulated by chromatin and HP1α binding partners. Our strategy can also be applied to other protein systems that undergo phase separation to provide atomic resolution details of this elusive biological process.