Membraneless organelles, though less famous than their lipid encapsulated counterparts are important and ubiquitous cellular compartments. A number of these bodies have been observed to have characteristics of liquid droplets, and that their formation can be understood as a polymeric phase separation. The protein Ddx4 is associated with nuage, membraneless organelles in germ cells. We have quantitatively studied the formation of membraneless organelles by this protein both inside cells and in vitro. Through rigorous thermodynamic analysis, we determine that the mechanism of formation arises through its disordered tails, and that specific cation-pi interactions between blocks of arginine and phenylalanine residues provide sufficiently strong interactions to cause the protein to phase separate. The behaviour is well described by Flory-Huggin’s theory, a model that considers the phase separation of disordered polymers. We determine that the formation of the bodies is regulated by, for example, physiologically relevant methylation of the arginine residues. Notably, the dielectric of the interior of the droplets is more similar to organic solvent such as DMSO rather than bulk water. The bodies are effectively concentrated solutions of polymer, and that the interior dielectric is very different to that of water: these membraneless organelles are regions of organic solvent, inside cells. In a manner akin to an organic chemist in a synthesis lab, biology has harnessed the properties of organic solvents for biochemistry. We demonstrate this by showing the bodies have novel biochemical properties. Proteins are taken up, or excluded by the bodies in a manner that reflects their amino acid sequence, revealing they have predictable solubilities in these blobs of organic solvent. Moreover, we demonstrate that double stranded DNA is effectively melted when it moves inside these bodies. Thus the interior of the Ddx4 organelles behave as a passive helicase. These results indicate that cells have developed membraneless organelles as a means for altering the environment for biochemical reactions.

Alternative solvent for biology

253rd National Meeting (2017)

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