Liquid-liquid Phase Separation

In the classic textbook picture, biomolecules are spatially organized within the cell using membranes . However, it has recently been appreciated that a wide variety of biomolecules, like nucleic acids and proteins, can also self-organize without the need of membranes into intracellar structures called biomolecular condensates, which can be used as hubs of assembly, molecular bioreactors, or transport systems. In our lab, we are interested in understanding the mechanism of condensate formation.

We use multivalent DNA complexes, called ‘nanostars’, as a model system to understand biomolecular phase separation rather than using RNA and proteins, which form the majority of condensates found within cells. The tunability of DNA nanostars makes them a great model system to learn about how intermolecular interactions and molecular structure control phase separation.

DNA nanostars are flexible DNA structures comprised of a double-stranded ‘arm’ region and a single-stranded ‘hand’ region. The self-complementary hand sequences allow the DNA nanostars to bind to each other, thus forming a transient network of DNA. The following schematics show a tetravalent DNA nanostar with palindromic handles and its phase diagram. Below the upper critical solution temperature (UCST), the DNA nanostar solution phase separates into dense liquid droplets and a dilute phase. Above the binodal curve, the interactions between nanostars are negligibly weak and the system enters a one-phase region. In our lab, we use a microfluidics-based approach for constructing the phase diagram of DNA nanostars. Using our approach, we quantify the equilibrium densities of both the liquid and gas phases as a function of temperature for different star architectures, thereby constructing the phase diagrams in the density-temperature plane. Our research sheds light on the mechanisms that drive phase separation or macromolecular condensation in other material platforms, including proteins and other nucleic acids.