The idea that liquid droplets contribute to the structure of the cell interior has existed since the late 1800s. In the past decade, interest in this concept has been rejuvenated with the discovery that many proteins (some of them characteristic of well-studied membraneless structures in cells), with or without nucleic acids, can spontaneously form liquid droplets in vitro via liquid-liquid phase separation (LLPS). There is growing evidence that such phase transitions are important for both intracellular organization as well as function. Our work focuses on the role of LLPS of nuclear proteins with chromatin as a possible mechanism for silencing genes through heterochromatin formation.
The cover art of the February 4th issue of Biophysical Journal highlights LLPS of histone proteins and chromatin in the cell nucleus using a minimalist and abstract representation. This is inspired by our study on LLPS of the linker histone H1 associated with heterochromatin in cells. A key concept in the artwork is the multiscale nature of biological LLPS. The orange background represents the nucleus. Green spheres within the nucleus represent liquid-like condensates enriched in H1, HP1a, and heterochromatin. Across the cell nucleus, the liquid-like condensates organize spatially and temporally, interacting with one another (neighboring condensates may merge if brought into contact). On the length scale of a single condensate, questions arise as to how the cell regulates their size and how the internal dynamics are tuned to remain liquid-like vs irreversible aggregates.
On the molecular length scale, a combination of specific and non-specific interactions (electrostatic, hydrogen bonding, hydrophobic, cation-π, etc.) govern the association and assembly of proteins and nucleic acids. The artwork shows chromatin consisting of the nuclear core particle (comprised of two copies of the four core histones) with DNA wrapped around the particle. Associated with the nuclear core particle are the proteins H1 and HP1a that are found in higher concentrations in heterochromatin. The structured regions of the proteins are depicted as rods, while the disordered regions are depicted as chains. While most proteins that undergo LLPS contain large intrinsically disordered domains that are highly charged, a full understanding of the structure-function relationship is not yet available. Indeed, key questions remain regarding how molecular interactions dictate selective LLPS in a cellular milieu that contains thousands of components.
One of the main interests of our lab is the role of LLPS of chromatin and nuclear proteins in genome organization and function. However, there are also clear engineering applications of biological LLPS more generally. Membraneless liquid-like compartments have been shown to selectively partition molecules and can be formed or dissolved on demand. The unique ability to generate reversible, triggerable micron-sized compartments have applications ranging from molecular storage to enhancing chemical reactions.
Additional information regarding the work in our lab can be found at: https://sites.google.com/view/king-group/home?authuser=1
- Anisha Shakya, Seonyoung Park, Neha Rana, John T. King