Liquid-liquid phase separation, often abbreviated LLPS, is the process by which proteins and nucleic acids inside a cell spontaneously demix from the surrounding fluid and concentrate into dense, liquid droplets. These droplets act as compartments that lack any surrounding membrane yet still concentrate specific molecules and speed up particular biochemical reactions. Over the past decade, LLPS has gone from a curiosity of polymer physics to one of the most actively studied organizing principles in cell biology, with implications stretching from how genes get switched on to why certain neurodegenerative diseases form toxic protein clumps.
Membraneless Organelles and Cellular Organization
Cells have long been understood to contain membrane-bound compartments like the nucleus, mitochondria, and lysosomes. But a large number of cellular structures have no membrane at all. Nucleoli, Cajal bodies, P-bodies, and stress granules are all examples of compartments that exist as liquid droplets, concentrating proteins and nucleic acids without a lipid barrier separating them from the rest of the cell.1PubMed Central. Biomolecular condensates: organizers of cellular biochemistry These structures, now commonly called biomolecular condensates, form through LLPS and can assemble, dissolve, and reform within seconds to minutes depending on cellular conditions.2Journal of Biological Chemistry. Liquid–Liquid Phase Separation in Biology: Key Implications
The formation of these condensates is, at its core, a thermodynamic process. Certain proteins contain stretches that lack a fixed three-dimensional shape, known as intrinsically disordered regions. These floppy stretches can make numerous weak, transient contacts with one another, and when enough of these interactions accumulate, the proteins collectively separate from the surrounding solution into a concentrated phase.3PubMed. Liquid-liquid phase separation of intrinsically disordered proteins: Effect of osmolytes and crowders Think of it like oil droplets forming in salad dressing: the oil doesn’t vanish, it just collects into its own domain. The biological version is more nuanced, but the physical intuition is similar.
What makes condensates more than just organizational tools is that concentrating the right molecules in one place can dramatically change the rate of biochemical reactions. A signaling enzyme that would rarely encounter its target in the dilute cytoplasm might find that target readily inside a condensate, turning a sluggish reaction into a rapid one. Cells also use condensates to improve their fitness during stress, as when stress granules form to stall translation and protect messenger RNAs until conditions improve.2Journal of Biological Chemistry. Liquid–Liquid Phase Separation in Biology: Key Implications
Driving Gene Expression at Super-Enhancers
One of the most consequential discoveries in the LLPS field involves how cells control gene expression. Genes critical to a cell’s identity are often regulated by clusters of enhancer elements called super-enhancers. At these sites, transcriptional coactivators like BRD4 and MED1 form visible puncta in the nucleus that behave like liquid droplets. Researchers found that these coactivator condensates can compartmentalize and concentrate the transcription machinery, pulling in RNA polymerase and associated factors to drive robust gene activation.4PubMed Central. Coactivator condensation at super-enhancers links phase separation and gene control Chemicals that disrupt condensate properties also disrupt these puncta, suggesting the liquid-like state is functionally important rather than incidental.
Complementary work showed that the Mediator complex and RNA polymerase II colocalize in stable clusters associated with chromatin, and that these clusters have hallmarks of phase-separated condensates.5PubMed Central. Mediator and RNA polymerase II clusters associate in transcription-dependent condensates The picture that emerges is that phase separation acts as a kind of amplifier: by bringing transcription factors, coactivators, and polymerase together into a dense droplet at the right genomic location, cells can crank up expression of key genes far beyond what individual factor-binding events could achieve. This has direct relevance to cancer biology, where dysregulated super-enhancers can hijack this condensation machinery to drive oncogene expression.6PubMed Central. Super-Enhancers, Phase-Separated Condensates, and 3D Genome Organization in Cancer
Phase Separation in Genome Architecture
Beyond individual gene regulation, LLPS appears to shape how the genome itself is physically packaged. Heterochromatin, the tightly packed, largely silent portion of the genome, was traditionally thought to be compacted through a scaffolding mechanism. Research in fruit fly embryos showed that HP1a, a protein essential for heterochromatin, undergoes liquid-liquid demixing in the test tube and forms foci with liquid-like properties during the earliest stages of heterochromatin domain formation.7PubMed Central. Phase separation drives heterochromatin domain formation These domains are sensitive to disruption of weak hydrophobic interactions and show reduced diffusion at their boundaries, consistent with a phase-separated state. The implication is that large-scale genome organization, not just gene-by-gene regulation, can be driven by condensation.
Immune Signaling Condensates
When a T cell recognizes a foreign antigen, its receptor triggers a cascade of signaling events that ultimately leads to immune activation. Researchers discovered that upon T cell receptor phosphorylation, downstream signaling proteins spontaneously separate into liquid-like clusters that boost signaling outputs both in reconstituted systems and in living human T cells.8PubMed Central. Phase separation of signaling molecules promotes T cell receptor signal transduction Follow-up work identified specific components of this mechanism, showing that the CD3ε chain can condense with Lck kinase through phase separation to form structures called TCR signalosomes.9PubMed Central. Self-programmed dynamics of T cell receptor condensation
T cell receptors are only the beginning. Phase separation has been implicated in signaling downstream of B cell receptors and innate immune sensors like the cGAS-STING and RIG-I pathways, which detect viral DNA and RNA inside cells.10PubMed Central. Phase separation in immune signalling The common thread is that condensation creates a distinct physical and biochemical compartment, concentrating signaling molecules and excluding inhibitory ones, to sharpen the cell’s decision about whether to mount an immune response.
When Condensates Go Wrong
The liquid state of protein condensates is not always stable. Over time, or under pathological conditions, liquid droplets can mature into gels and eventually into solid, fibrillar aggregates rich in a specific structural motif called beta sheets.11PubMed Central. Functional Biomaterials Derived from Protein Liquid-Liquid Phase Separation and Liquid-to-Solid Transition This liquid-to-solid transition is now understood to be central to several neurodegenerative diseases.
Two proteins in particular, TDP-43 and FUS, have received intense scrutiny. Both normally participate in RNA processing and form liquid-like condensates as part of their function. But mutations or prolonged stress can cause these droplets to age into less dynamic assemblies, including hydrogels, inclusions, and amyloid fibrils. These insoluble aggregates are pathological hallmarks of amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD).12PubMed Central. Liquid-Liquid Phase Separation of TDP-43 and FUS in Physiology and Pathology of Neurodegenerative Diseases 13PubMed Central. Molecular Mechanisms of Phase Separation and Amyloidosis of ALS/FTD-linked FUS and TDP-43 The insight here is that the disease-causing aggregates may not form from scratch in the cytoplasm. Instead, they appear to begin as normal, functional condensates that then undergo a catastrophic material transition. Dysfunctional LLPS has emerged as a mechanism linking ALS-related proteins to the disease process itself.14Journal of Molecular Cell Biology. Role and therapeutic potential of liquid–liquid phase separation in amyotrophic lateral sclerosis
Prion diseases offer another example. Regulated cleavage of the prion protein within phase-separated condensates can trigger an aberrant phase transition from liquid to aggregate, and the chaperone Clusterin was found to interfere with this process and even reduce the amplification of infectious human prions isolated from patients with Creutzfeldt-Jakob disease.15PubMed. Regulated Proteolysis Induces Aberrant Phase Transition of Biomolecular Condensates into Aggregates: A Protective Role for the Chaperone Clusterin
How Cells Keep Condensates Under Control
Given that runaway condensation can be toxic, cells have multiple layers of quality control. Molecular chaperones, long known for helping proteins fold correctly, are increasingly recognized as regulators of condensate formation, material state, and dispersal.16PubMed Central. Chaperone regulation of biomolecular condensates In yeast, for instance, condensation of proteins into stress-induced assemblies after a brief heat shock is completely reversible without requiring autophagy. Instead, a chaperone pathway involving Hsp70, its co-chaperones, and the ring-shaped unfoldase Hsp104 uses energy from ATP to thread misfolded proteins through a central pore and resolubilize them.17Frontiers in Biophysics. Chaperone regulation of biomolecular condensates – Section: Chaperones inhibit and dissolve condensates
Post-translational modifications offer another dial cells can turn. Phosphorylation, acetylation, ubiquitination, methylation, and poly(ADP-ribosyl)ation can all alter a protein’s charge, hydrophobicity, or interaction surfaces, shifting whether it condenses or stays dispersed.18PubMed Central. Post-translational modifications in liquid-liquid phase separation: a comprehensive review 19PubMed. Regulation of liquid-liquid phase separation with focus on post-translational modifications A kinase that phosphorylates a disordered region can dissolve a condensate in minutes by adding negative charges that repel other molecules, while removing those phosphates can trigger reassembly. This gives cells a rapid, reversible toolkit for controlling when and where condensates exist.
RNA itself acts as a tuning agent. At low ratios relative to RNA-binding proteins, RNA molecules serve as scaffolds that enhance condensation. But when RNA is present in excess, the resulting negatively charged protein-RNA complexes actually inhibit phase separation, a behavior called a reentrant phase transition. This ratio-dependent effect helps explain why certain condensates form more readily in the cytoplasm, where free RNA is relatively scarce, than in the RNA-rich nucleus.20Molecular Cell. RNA-driven phase transitions in ribonucleoprotein condensate biology
Viruses Exploit Phase Separation
Viruses, with their compact genomes and limited protein repertoires, have evolved to co-opt host cellular machinery. It turns out LLPS is no exception. Viral proteins, particularly those with intrinsically disordered regions, can undergo phase separation to form subcellular microenvironments variously called viral factories, inclusion bodies, or viroplasms.21PubMed. Liquid-liquid Phase Separation in Viral Function These viral condensates concentrate replicase proteins, viral genomes, and host factors needed for replication while simultaneously shielding viral components from intracellular immune detection.21PubMed. Liquid-liquid Phase Separation in Viral Function
Both SARS-CoV-2 and HIV-1 nucleocapsid proteins, for example, have intrinsic abilities to condense through LLPS, and this condensation contributes to viral replication.22PubMed Central. Liquid-liquid phase separation of nucleocapsid proteins during SARS-CoV-2 and HIV-1 replication More broadly, intrinsically disordered protein regions of viruses have been shown to drive LLPS that supports the viral life cycle and modulates virus-host interactions.23PubMed Central. Liquid-Liquid Phase Separation by Intrinsically Disordered Protein Regions of Viruses: Roles in Viral Life Cycle and Control of Virus-Host Interactions Understanding how viruses use condensation opens up the possibility of antiviral strategies that target the phase behavior of viral proteins rather than their enzymatic activity.
Drug Development Targeting Condensates
The realization that condensates are functionally important and that their dysregulation drives disease has sparked an emerging area of drug discovery. A new class of compounds called condensate-modifying therapeutics, or c-mods, aims to selectively inhibit or promote condensation. These small molecules, typically under about 1,500 daltons in molecular weight, target the weak, multivalent interactions that drive LLPS and are being explored as candidates for neurodegenerative disorders, cancer, and viral infections.24PubMed. Small Molecules as Regulators of Liquid-Liquid Phase Separation: Mechanisms and Strategies for New Drug Discovery
The therapeutic strategy is not limited to dissolving pathological condensates. Researchers are also exploring ways to induce, redirect, or reprogram condensate dynamics, composition, and material state.25PubMed Central. Targeting biomolecular condensates: beyond dissolution For instance, in some cancers the goal might be to disrupt a condensate that concentrates oncogenic transcription factors, while in a neurodegenerative context the goal might be to keep a condensate in its healthy liquid state and prevent it from solidifying into toxic aggregates.
Drug design itself may need to adapt. Many pharmaceutical targets partition into biomolecular condensates, and the microenvironment inside a condensate can differ from the bulk cytoplasm in ways that affect how drugs distribute. Recent work found that condensates enriched in nonpolar residues create more hydrophobic interiors, and that a drug’s hydrophobicity strongly predicts how efficiently it reaches phase-separated targets. Both binding affinity and hydrophobicity contributed to inhibitor potency in cellular experiments.26Nature Chemical Biology. Navigating condensate micropolarity to enhance small-molecule drug targeting In other words, a drug designed to hit a target that lives inside a condensate may fail if the drug itself is too hydrophilic to concentrate there. This is a genuinely new design principle that traditional pharmacology did not account for.
Plant Stress Responses and Phase Separation
LLPS is not exclusive to animal cells. Accumulating evidence points to phase separation as a major mechanism for environmental stress sensing in plants. When plants face dehydration, osmotic stress, or temperature extremes, specific proteins condense into droplets that appear to act as sensors or signaling hubs, helping the plant mount an appropriate response.27PubMed Central. Liquid-liquid phase separation as a major mechanism of plant abiotic stress sensing and responses The speed of condensation, which can happen on the timescale of seconds, makes it well suited for rapid sensing of environmental changes. This area of research is younger than its counterparts in animal and disease biology, but it suggests that condensate formation is an ancient and broadly conserved strategy for coping with a changing environment.
Engineering Synthetic Condensates
As understanding of biological condensates has grown, so has interest in building artificial ones. Synthetic biomolecular condensates, designed from the ground up, offer a way to create new cellular functions that nature never evolved. Researchers have begun engineering condensates that can sequester specific enzymes, rewire metabolic pathways, or serve as scaffolds for multi-step reactions inside living cells.28PubMed Central. Engineering synthetic biomolecular condensates Meanwhile, optogenetic tools now allow researchers to trigger condensate formation at specific genomic locations with light, enabling precise dissection of how condensation at a particular spot in the genome affects transcription and chromatin structure.29PubMed Central. Light-activated macromolecular phase separation modulates transcription by reconfiguring chromatin interactions
Phase separation on membrane surfaces adds yet another dimension. When proteins phase-separate on the outer surface of a lipid membrane, the resulting condensate creates compressive stress in the plane of the membrane, which can drive the membrane to bend inward and form tubular protrusions.30PubMed Central. Membrane bending by protein phase separation This discovery suggests that membrane remodeling, previously thought to require rigid scaffold proteins, can also be driven by liquid-like protein networks. For bioengineering, this opens up possibilities for designing synthetic compartments that physically reshape membranes.
The Evidence Problem
For all the excitement, there is a persistent methodological tension in the field. Demonstrating that something undergoes LLPS in a test tube is relatively straightforward: you can watch droplets form, measure their fusion, and check that they dissolve when conditions change. Proving that the same process happens inside a living cell is far harder. A critical assessment of the field noted that evidence for in vivo LLPS is often phenomenological and insufficient to distinguish phase separation from other possible mechanisms that could produce similar-looking puncta or clusters.31Genes & Development. Evaluating phase separation in live cells: diagnosis, caveats, and functional consequences
This does not mean the in vivo claims are wrong, but it does mean the field has sometimes gotten ahead of its evidence. A bright spot under a microscope is not automatically a phase-separated condensate. Researchers now increasingly demand multiple independent lines of evidence, such as sensitivity to specific perturbations, fusion and relaxation dynamics, and concentration-dependent thresholds, before labeling a structure as phase-separated. New tools like light-activated condensate formation are helping bridge the gap between controlled in vitro experiments and the messy reality of intact cells.29PubMed Central. Light-activated macromolecular phase separation modulates transcription by reconfiguring chromatin interactions The evidence bar is rising, which is a sign of a field maturing rather than one in trouble.
Germ Cells and Development
Some of the earliest hints that phase separation mattered in biology came from studies of germ cells. Structures like P granules in worm embryos, which segregate to germ-cell precursors during the first cell divisions, were among the first cellular bodies shown to behave as liquid droplets. Evidence now suggests that phase separation plays a multifaceted role in both germ cells and multicellular development more broadly, contributing to the spatial organization of maternal factors, the regulation of RNA during early embryogenesis, and the partitioning of cell-fate determinants during division.32PubMed Central. Phase Separation in Germ Cells and Development Because early embryonic development often happens before the zygote’s own genome is fully active, condensate-based organization of inherited proteins and RNAs may be especially important during those first critical hours of life.