Phase separation: from phenomenon to function

Liquid–liquid phase separation is a recently proposed model for how biochemical reactions can be compartmentalized in a cell without the use of membranes. Specifically, proteins, nucleic acids, and lipids can self-organize to create fluid-like compartments, known as “biomolecular condensates.” Many examples of biomolecular condensates are popping up in the literature, but these studies are largely phenomenological. The Minisymposium “Phase separation: from phenomenon to function” included talks that addressed how biomolecular condensates create new functions and revealed new techniques to study meso-scale organization within cells.

One of the first proposed functions for biomolecular condensates was improving reactions by concentrating enzymes and their substrates. Indeed, Saumya Saurabh (Stanford University) showed that protein condensates concentrated DivJ kinase, thereby enhancing its activity within the Caulobacter crescentus pole. By using synthetic condensates encapsulated into minimal cells, Celina Love (Max Planck Institute, Dresden) suggested that condensates might also increase enzyme efficiency in addition to concentrating reactants.

Several speakers revealed unexpected roles for biomolecular condensates in intracellular organization. Feng Yuan (University of Texas at Austin) showed that the protein FUS–-which can self-assemble into droplets–-was sufficient to bind to and deform vesicles and create internal tubulation. He speculated that phase separation of FUS creates compressive forces that would cause membrane invagination. Marina Feric (NIGMS) revealed that the protein TFAM condenses into gel-like structures called “nucleoids,” which she predicted could help organize the mitochondrial genome and play a role in aging. Using a hypotonic blebbing assay, Christopher King (Janelia Research Campus) showed that phase separation of lipids into ordered and disordered domains dictates the contacts made between the endoplasmic reticulum and other organelles.

Biomolecular condensates also affect cells at the level of cell fate decisions and ability to live in different environs. For example, Dan Jarosz (Stanford) demonstrated that condensation of SMAUG protein traps mRNAs and prevents budding yeasts from entering meiosis, even under conditions that normally drive this transition, such as nutrient starvation. Benjamin Stormo (University of North Carolina Chapel Hill) showed that the fungus Ashbya gossypii had modified the condensation temperature of its Whi3 protein to match the climate in which it lived. He revealed that differences in polyQ repeat length and RNA annealing temperatures of Whi3 underlie this behavior.

For the field of biological phase separation to move forward, new techniques are needed to characterize and quantify the functional properties of biomolecular condensates. Martin Wühr (Princeton) created a 3D printed centrifugal filter to fractionate Xenopus laevis egg extracts and determine how much of the proteome is organized into biomolecular condensates. Aaron Foote (University of Texas at Austin) used label-free microscopy to measure the liquidity of fission yeast cytoplasm. Finally, Jeffrey Woodruff (UT Southwestern Medical Center) demonstrated how noninvasive, light-induced flow perturbations can be used to deform biomolecular condensates and measure their functional material properties in living Caenorhabditis elegans embryos.