Biology subjects

Stachowiak, J.

Publications and source records attributed to Stachowiak, J..

At least 19 recordsLinked to original sources

bioRxiv2026

Transmembrane coupling of protein condensates via membrane-mediated interactions: A simulation study

Recent experiments show that protein condensates sitting on opposite surfaces of a flat lipid membrane move together and prefer to overlap, even though they cannot touch each other. This points to an indirect, membrane-mediated interaction. Two mechanisms could be responsible: a curvature-induced interaction, which is energetic in origin, and a fluctuation-induced interaction, which is entropic. Here we study both with coarse-grained molecular dynamics simulations, using Cookes implicit-solvent lipid model together with a generic bead-spring polymer model for the condensate. We compute the potential of mean force between two condensates across the membrane. For condensates of the same size, full overlap is unfavorable, and the pair instead settles into a partially overlapping state that bends the membrane into an S-like shape. When the two condensates differ strongly in size, full overlap becomes favorable. We explain this with a simple geometric picture. The condensate wets the membrane as a thin film and imposes curvature only along its rim, while membrane tension flattens the membrane under its interior. The resulting ring of curvature can trap a smaller condensate on the opposite side. We also compare the bending undulations and the effective bending modulus of a bare membrane, a membrane with one condensate, and a membrane with condensates on both sides. A wetting condensate suppresses the undulation modes and stiffens the membrane, but whether this makes overlap entropically favorable remains inconclusive. Our results indicate that the coupling is driven mainly by curvature, and that it depends on the wetting mechanism and on the membrane tension.

biophysics↗

bioRxiv2026

Simulations show increased lipid interdigitation in transmembrane coupling of protein condensates

Liquid-liquid phase separation of proteins has been observed to occur on biological membranes, where it is thought to play a role in diverse cellular behaviors. Recent work has demonstrated colocalization between protein condensates on opposing leaflets of the bilayer, suggesting that protein phase separation may be coupled across the bilayer. However, the mechanism behind this coupling phenomenon remains poorly understood. Here we seek to understand the protein-protein and protein-membrane interactions that give rise to transbilayer coupling of protein condensates. We perform coarse-grained molecular dynamics simulations of a bilayer with a disordered protein condensate tethered to each leaflet surface. In this system, we observe stable, coupled diffusion of the condensates across the membrane. We find that increasing the protein-protein interaction strength leads to decoupling, driven by competing membrane curvatures induced by each condensate. However, by applying membrane tension we suppress curvature and restore coupling even at higher protein interaction strengths. Under coupling conditions, we find that lipid entropy is reduced upon direct contact with proteins, but this effect is not transferred to the opposing leaflet. Interestingly, further analysis reveals increased transverse lipid packing (interdigitation) beneath the condensates relative to protein-free regions. Based on these observations, we propose that enhanced lipid interdigitation mediates interleaflet communication and serves as the primary mechanism driving transbilayer coupling of condensates in this system. This work provides insight into a potential physical mechanism for transmembrane communication in cellular contexts and suggests directions for future investigation. Significance StatementLiquid-like condensates are active participants at cellular membranes, where they act as organizers and catalysts for various cellular processes. Recent work has demonstrated that protein condensates can couple across the bilayer; however, the molecular mechanism of this transbilayer coupling remained unknown. Here, we investigate the molecular basis of transmembrane condensate coupling through detailed analysis and propose a mechanism for the phenomenon. This work advances our understanding of how information is transmitted across the bilayer, with implications in cellular requiring coordination across the membrane, such as signaling, and more broadly in the field of membrane biophysics.

biophysics↗

bioRxiv2026

Endocytic accessory proteins assemble clathrin while simultaneously destabilizing protein condensates

During endocytosis a dynamic protein network is responsible for assembly of the clathrin coat. Early in the process the Eps15 protein is thought to undergo liquid-liquid phase-separation to form biomolecular condensates. These condensates facilitate clathrin assembly, producing clathrin-coated vesicles that are ultimately excluded from the condensate as the vesicles depart from the plasma membrane. An array of distinct clathrin accessory proteins drive this process, yet their interactions with protein condensates have not been explored. Here we show that clathrin accessory proteins promote the assembly of the clathrin lattice by simultaneously destabilizing protein condensates. By adding diverse accessory proteins to Eps15 condensates, we observed that accessory proteins which cross-linked Eps15 proteins stabilized condensates, while accessory proteins that compete with Eps15-Eps15 interactions destabilized condensates. Interestingly, accessory proteins that destabilized condensates also promoted the assembly of clathrin and its exclusion from condensates. In contrast, accessory proteins that stabilized condensates opposed exclusion of clathrin. Finally, we demonstrated that decreasing condensate stability enhanced clathrin assembly and exclusion, even when no additional clathrin assembly motifs were introduced. Taken together, these results help to explain how condensates can catalyze the assembly of coated vesicles by providing both a substrate for their initiation and a driving force for their exclusion and ultimate departure. Statement of SignificanceThis work is significant because it reveals how biomolecular condensates could actively drive, rather than merely host, clathrin-mediated endocytosis. By showing that clathrin accessory proteins can tune condensate stability to simultaneously promote clathrin lattice assembly and exclusion, this study uncovers a physical mechanism that could link phase separation to membrane trafficking. The finding that condensate destabilization alone enhances clathrin exclusion reframes condensates as dynamic regulators that could provide both an initiation platform and a force for vesicle maturation and release. More broadly, this work advances our understanding of how cells coordinate assembly and disassembly of complex macromolecular structures through regulated changes in condensate material properties.

biophysics↗

bioRxiv2026

PROTAC internalization and target degradation require clathrin-mediatedendocytosis

Proteolysis-targeting chimeras (PROTACs) are emerging as potent tools for targeted protein degradation that overcome many of the limitations of traditional small molecule inhibitors. Yet how these hetero-bifunctional therapeutics enter cells remains a mystery. While passive diffusion is conventionally assumed, the bulky structure of PROTACs suggests that active transport may be required. Recently, the fatty acid transporter CD36 was identified as a key receptor for PROTACs. However, because the uptake mechanism of CD36 is itself unknown, how PROTACs enter cells remains a mystery. Here we show that PROTAC uptake and function require clathrin-mediated endocytosis. We uncover previously unrecognized clathrin adaptor-binding motifs in the CD36 C-terminus and use live-cell imaging to visualize the recruitment of both CD36 and PROTACs to sites of clathrin-mediated endocytosis on the cellular plasma membrane. Strikingly, disruption of clathrin assembly through either genetic or pharmacological means abolishes all detectable PROTAC-induced protein degradation, demonstrating that the clathrin pathway is required for the function of PROTACs that utilize diverse E3 enzymes against multiple targets. These results elucidate the molecular mechanism of PROTAC entry into cells, providing critical information for optimizing cellular uptake and response to targeted degraders.

pharmacology and toxicology↗

bioRxiv2026

Intrinsically disordered ligands for the control of receptor uptake by endocytosis

Endocytosis plays a crucial role in signaling, recycling, and degradation of receptors. Controlling endocytosis of specific receptors is therefore a major goal for both basic science and medicine. While antibody-induced dimerization can drive signaling-induced uptake of some receptors, the steric bulk of antibodies generally inhibits endocytosis, such that control over receptor uptake remains an unmet need. Recent work has demonstrated that attractive interactions between intrinsically disordered proteins drive inward membrane curvature, a key step in endocytosis. To harness this phenomenon for the control of receptor uptake, we designed chimeras that consisted of disordered domains fused to ligands for specific receptors. These chimeras condensed on membrane surfaces, driving receptor clustering and uptake via clathrin-mediated endocytosis. In contrast, chimeras that repelled one another resisted condensation, helping receptors escape endocytosis and remain on the cell surface. Taken together, these results suggest that by modulating the amino acid sequence of intrinsically disordered ligands, we can promote or hinder the internalization of specific receptors by endocytic pathways. More broadly, these findings suggest a generalizable strategy for controlling the plasma membrane lifetime of diverse receptors, opening up new pathways for modulating cellular behavior and delivering therapeutics. Significance StatementCells regulate signaling by continuously internalizing receptors from their surfaces using endocytosis. Controlling receptor internalization would provide new tools for addressing diverse diseases. While antibodies can cluster specific receptors on the cell surface, their steric bulk and rigidity inhibit endocytosis. In contrast, here we demonstrate that engineered ligands containing intrinsically disordered domains form flexible complexes that precisely control receptor internalization. Disordered ligands that attract one another condense receptors at sites of endocytosis, driving uptake, while repulsive disordered ligands prevent condensation such that receptors remain on the cell surface. By tuning the amino acid sequence of the disordered domain, a ligand can be switched from promoting to inhibiting receptor internalization, offering the opportunity to control cell signaling and therapeutic delivery.

biophysics↗

bioRxiv2025

Mechanochemical feedback between confinement and actin crosslinking drives the shape dynamics of liquid-like droplets

Several actin-binding proteins can form liquid-liquid phase-separated condensates that promote actin filament assembly and bundling, which is crucial for local actin network organization. Previous studies have established that phase-separated condensates composed of actin-binding proteins, such as vasodilator-stimulated phosphoprotein (VASP) and Lamellipodin (Lpd), restrict the organization of actin filaments to structures such as rings, shells, discs, and rods through kinetic trapping. However, the mechanism by which crosslinker multivalency, actin growth, and condensate properties tune actin organization and droplet shape is not well understood. Using a combination of agent-based simulations and experiments, we find that the deformability of the droplet interface allows for the emergence of not just tightly-bundled actin rings but also weakly-bundled actin discs. We find two major quantitative relationships between actin bundling and droplet deformation. The first relationship shows that the crosslinked bundle thickness and droplet diameter followed a power law, consistent with experiments. The second one is that the kinetics of droplet deformation follows a dynamic snapping behavior that depends on the droplet surface tension and the multivalent VASP-actin binding kinetics. We predicted that these two relationships were generalizable to dynamic multimers and to weak actin crosslinkers. Our predictions were experimentally tested using two additional condensate-forming proteins, lamellipodin and RGG. Taken together, we show that mechanochemical feedback between the droplet interface properties and crosslinker multivalency tune actin organization and control the dynamics of droplet deformation by actin networks.

biophysics↗

bioRxiv2025

Stressed Actin Binding by the Prickle2 LIM Domains and itsRegulation in the Full-Length Protein

Cells are capable of sensing mechanical changes in their cytoskeletal network via stress-sensitive actin-binding proteins. Recently, a novel stress-sensing mechanism was described whereby LIM domains from diverse protein families bind directly to stressed actin filaments. It remains unclear, however, how the interaction of these domains with actin is regulated in the context of full-length proteins. Here, we show that the LIM domain containing region (LCR) of the planar cell polarity protein Prickle2 (Pk2) associated with stressed actin filaments in Xenopus mesoderm alongside known stress-sensitive LIM domains. By contrast, the full-length Pk2 did not exhibit similar recruitment along actin filaments. Structure function analysis revealed that both the structured PET domain and unstructured C-terminal region of Pk2 suppress recruitment of Pk2s LCR to stressed actin and promote recruitment to Pk2-rich nodes. Finally, we show that two human patient-derived variants associated with epilepsy result in a loss of Pk2-LCR recruitment to actin filaments. These data provide new insights into the regulation of stress-sensitive LIM domains and may inform our understanding of planar cell polarity.

cell biology↗

bioRxiv2025

Of condensates and coats - reciprocal regulation of clathrin assembly and the growth of protein networks

Clathrin-mediated endocytosis is essential for membrane traffic, impacting a diverse range of cellular processes including cell signaling homeostasis, cell adhesion, and receptor recycling. During endocytosis, invagination of the plasma membrane is coordinated by a network of proteins that recruit and assemble the clathrin coat. Recent work demonstrated that clathrin accessory proteins which arrive early at endocytic sites, such as Eps15 and Fcho2, form phase-separated condensates that recruit downstream machinery, promoting assembly and maturation of clathrin-coated vesicles. However, the mechanisms by which protein condensates regulate - and are regulated by - clathrin assembly remain unclear. Using in vitro reconstitution and nuclear magnetic resonance spectroscopy, we demonstrate that protein condensates provide a platform for recruitment and assembly of clathrin triskelia. This condensate driven assembly is enhanced in the presence of the accessory protein, AP2, which is readily incorporated within condensates. In turn, clathrin assembly restricted the growth of condensates, exhibiting surfactant-like behavior that stabilized protein-protein interactions while imposing the preferred curvature of the clathrin lattice. This mutual regulation promotes efficient assembly of clathrin-coated vesicles while preventing uncontrolled expansion of protein condensates. More broadly, reciprocal regulation of protein condensates and clathrin coats may provide a framework for understanding how intrinsically disordered and structured protein assemblies can work together to build complex cellular architectures.

biophysics↗

bioRxiv2025

Epsin1 enforces a condensation-dependent checkpoint for ubiquitylated cargo during clathrin-mediated endocytosis

Clathrin-mediated endocytosis internalizes proteins and lipids from the cell surface, supporting nutrient uptake, signaling, and membrane trafficking. Recent work has demonstrated that a flexible, liquid-like network of initiator proteins is responsible for catalyzing assembly of clathrin-coated vesicles in diverse organisms including yeast, mammals, and plants. How do cells regulate the assembly of this dynamic network to produce cargo-loaded vesicles? Here we reveal the ability of an endocytic adaptor protein, Epsin1, to conditionally stabilize the initiator protein network, creating a cargo-dependent checkpoint during clathrin-mediated endocytosis. Epsin1 is known to recruit ubiquitylated transmembrane proteins to endocytic sites. Using in vitro assays, we demonstrate that Epsin1 uses competitive binding and steric repulsion to destabilize condensation of initiator proteins in the absence of ubiquitin. However, when polyubiquitin is present, Epsin1 binds to both ubiquitin and initiator proteins, creating attractive interactions that stabilize condensation. Similarly, in mammalian cells, endocytic dynamics and ligand uptake are disrupted by removal of either ubiquitin or Epsin1. Surprisingly, when Epsin1 and ubiquitin are removed simultaneously, endocytic defects are rescued to near wildtype levels, although endocytic sites lose the ability to distinguish between ubiquitylated and non-ubiquitylated cargos. Taken together, these results suggest that Epsin1 tunes protein condensation to ensure the presence of ubiquitylated cargo during assembly of clathrin-coated vesicles. More broadly, these findings illustrate how a balance of attractive and repulsive molecular interactions controls the stability of liquid-like protein networks, providing dynamic control over key cellular events.

cell biology↗

bioRxiv2024

Poly-ubiquitylated transmembrane proteins outcompete other cargo for limited space inside clathrin-coated vesicles

Endocytic recycling of transmembrane proteins is essential to cell signaling, ligand uptake, protein traffic and degradation. The intracellular domains of many transmembrane proteins are ubiquitylated, which promotes their internalization by clathrin-mediated endocytosis. How might this enhanced internalization impact endocytic uptake of transmembrane proteins that lack ubiquitylation? Recent work demonstrates that diverse transmembrane proteins compete for space within highly crowded endocytic structures, suggesting that enhanced internalization of one group of transmembrane proteins may come at the expense of other groups. Here we show that preferential internalization of poly-ubiquitylated transmembrane proteins results in reduced endocytosis of mono-ubiquitylated and non-ubiquitylated proteins. Using a combination of live-cell imaging and ligand uptake assays, we confirmed that increased ubiquitylation correlates with increased internalization by clathrin-coated vesicles. Further, poly-ubiquitylated receptors significantly outcompeted their mono-ubiquitylated and non-ubiquitylated counterparts for localization to endocytic sites and uptake of extracellular ligands. These findings demonstrate the inherent interdependence of transmembrane protein recycling, suggesting that clathrin-coated vesicles act as selective filters, prioritizing highly ubiquitylated transmembrane proteins for uptake while leaving proteins with little or no ubiquitylation behind. Given that poly-ubiquitylation is thought to signal protein aging and damage, our findings suggest a mechanism for selective internalization of high priority cargo proteins, with simultaneously exclusion and protection of functional proteins that lack poly-ubiquitylation.

cell biology↗

bioRxiv2024

Biophysical modeling of membrane curvature generation and curvature sensing by the glycocalyx

Generation of membrane curvature is fundamental to cellular function. Recent studies have established that the glycocalyx, a sugar-rich polymer layer at the cell surface, can generate membrane curvature. While there have been some theoretical efforts to understand the interplay between the glycocalyx and membrane bending, there remain open questions about how the properties of the glycocalyx affect membrane bending. For example, the relationship between membrane curvature and the density of glycosylated proteins on its surface remains unclear. In this work, we use polymer brush theory to develop a detailed biophysical model of the energetic interactions of the glycocalyx with the membrane. Using this model, we identify the conditions under which the glycocalyx can both generate and sense curvature. Our model predicts that the extent of membrane curvature generated depends on the grafting density of the glycocalyx and the length of the polymers constituting the glycocalyx. Furthermore, when coupled with the intrinsic membrane properties such as spontaneous curvature and a line tension along the membrane, the curvature generation properties of the glycocalyx are enhanced. These predictions were tested experimentally by examining the propensity of glycosylated transmembrane proteins to drive the assembly of highly-curved filopodial protrusions at the plasma membrane of adherent mammalian cells. Our model also predicts that the glycocalyx has curvature sensing capabilities, in agreement with the results of our experiments. Thus, our study develops a quantitative framework for mapping the properties of the glycocalyx to the curvature generation capability of the membrane. Significance StatementThe glycocalyx is a dense layer of glycosylated transmembrane proteins and lipids distributed on the extracellular surface of eukaryotic cells. It is known to mediate cell-cell interactions and protect cells from invasion by pathogens. However, recently it has been found to play a role in generating membrane curvature, which is essential to diverse cellular functions spanning from endocytosis to cell division. Experiments have revealed that different membrane shapes: spherical vesicles, pearls, and tubes are regulated by the glycocalyx. However, currently, we lack a quantitative physical explanation of how glycocalyx properties determine membrane geometries. Here we develop a polymer brush theory-based model, which suggests that the interplay between glycocalyx polymers and membrane bending captures the wide variety of membrane shapes from spherical buds to elongated pearl-like shapes found in previously published experiments. We predict that the physical properties of glycocalyx polymers, line tension, and membrane elastic parameters play significant roles in regulating membrane morphologies. We find that the glycocalyx prefers higher local membrane curvature indicating that the glycocalyx can sense local curvature.

biophysics↗

bioRxiv2024

Liquid-like condensates that bind actin can polymerize and bundle actin filaments

Liquid-like protein condensates perform diverse physiological functions. Previous work showed that VASP, a processive actin polymerase, forms condensates that polymerize and bundle actin. To minimize their curvature, filaments accumulated at the inner condensate surface, ultimately deforming the condensate into a rod-like shape, filled with a bundle of parallel filaments. Here we show that this behavior does not require proteins with specific polymerase activity. Specifically, we found that condensates composed of Lamellipodin, a protein that binds actin but is not an actin polymerase, were also capable of polymerizing and bundling actin filaments. To probe the minimum requirements for condensate-mediated actin bundling, we developed an agent-based computational model. Guided by its predictions, we hypothesized that any condensate-forming protein that binds actin could bundle filaments through multivalent crosslinking. To test this idea, we added an actin-binding motif to Eps15, a condensate-forming protein that does not normally bind actin. The resulting chimera formed condensates that drove efficient actin polymerization and bundling. Collectively, these findings broaden the family of proteins that could organize cytoskeletal filaments to include any actin-binding protein that participates in protein condensation.

biophysics↗

bioRxiv2024

A live cell imaging-based assay for tracking particle uptake by clathrin-mediated endocytosis

A popular strategy for therapeutic delivery to cells and tissues is to encapsulate therapeutics inside particles that cells internalize via endocytosis. The efficacy of particle uptake by endocytosis is often studied in bulk using flow cytometry and Western blot analysis and confirmed using confocal microscopy. However, these techniques do not reveal the detailed dynamics of particle internalization and how the inherent heterogeneity of many types of particles may impact their endocytic uptake. Toward addressing these gaps, here we present a live-cell imaging-based method that utilizes total internal reflection fluorescence (TIRF) microscopy to track the uptake of a large ensemble of individual particles in parallel, as they interact with the cellular endocytic machinery. To analyze the resulting data, we employ an open-source tracking algorithm in combination with custom data filters. This analysis reveals the dynamic interactions between particles and endocytic structures, which determine the probability of particle uptake. In particular, our approach can be used to examine how variations in the physical properties of particles (size, targeting, rigidity), as well as heterogeneity within the particle population, impact endocytic uptake. These data impact the design of particles toward more selective and efficient delivery of therapeutics to cells.

biophysics↗

bioRxiv2023

Ubiquitin-driven protein condensation promotes clathrin-mediated endocytosis

Clathrin-mediated endocytosis is an essential cellular pathway that enables signaling and recycling of transmembrane proteins and lipids. During endocytosis, dozens of cytosolic proteins come together at the plasma membrane, assembling into a highly interconnected network that drives endocytic vesicle biogenesis. Recently, multiple groups have reported that early endocytic proteins form flexible condensates, which provide a platform for efficient assembly of endocytic vesicles. Given the importance of this network in the dynamics of endocytosis, how might cells regulate its stability? Many receptors and endocytic proteins are ubiquitylated, while early endocytic proteins such as Eps15 contain ubiquitin-interacting motifs. Therefore, we examined the influence of ubiquitin on the stability of the early endocytic protein network. In vitro, we found that recruitment of small amounts of polyubiquitin dramatically increased the stability of Eps15 condensates, suggesting that ubiquitylation could nucleate endocytic assemblies. In live cell imaging experiments, a version of Eps15 that lacked the ubiquitin-interacting motif failed to rescue defects in endocytic initiation created by Eps15 knockout. Furthermore, fusion of Eps15 to a deubiquitylase enzyme destabilized nascent endocytic sites within minutes. In both in vitro and live cell settings, dynamic exchange of Eps15 proteins, a hallmark of liquid-like systems, was modulated by Eps15-Ub interactions. These results collectively suggest that ubiquitylation drives assembly of the flexible protein network responsible for catalyzing endocytic events. More broadly, this work illustrates a biophysical mechanism by which ubiquitylated transmembrane proteins at the plasma membrane could regulate the efficiency of endocytic recycling. Significance StatementThe assembly of proteins into dynamic, liquid-like condensates is an emerging principle of cellular organization. During clathrin-mediated endocytosis, a liquid-like protein network catalyzes vesicle assembly. How do cells regulate these assemblies? Here we show that ubiquitin and endocytic proteins form a dynamic, mutually-reinforcing protein network in vitro and in live cells. To probe the impact of ubiquitylation on the dynamics of endocytosis, we engineered opto-genetic control over recruitment of proteins to nascent endocytic sites. While recruitment of wildtype proteins promoted endocytosis, recruitment of deubiquitylases, enzymes capable of removing ubiquitin, resulted in disassembly of endocytic sites within minutes. These results illustrate that ubiquitylation can regulate the fate of endocytic structures, elucidating a functional connection between protein condensates, endocytosis, and ubiquitin signaling.

cell biology↗

bioRxiv2023

Selective endocytic uptake of targeted liposomes occurs within a narrow range of liposome diameter

Cell surface receptors facilitate signaling and nutrient uptake. These processes are dynamic, requiring receptors to be actively recycled by endocytosis. Due to their differential expression in disease states, receptors are often the target of drug-carrier particles, which are adorned with ligands that bind specifically to receptors. These targeted particles are taken into the cell by multiple routes of internalization, where the best-characterized pathway is clathrin-mediated endocytosis. Most studies of particle uptake have utilized bulk assays, rather than observing individual endocytic events. As a result, the detailed mechanisms of particle uptake remain obscure. To address this gap, we have employed a live-cell imaging approach to study the uptake of individual liposomes as they interact with clathrin-coated structures. By tracking individual internalization events, we find that the size of liposomes, rather than the density of the ligands on their surfaces, primarily determines their probability of uptake. Interestingly, targeting has the greatest impact on endocytosis of liposomes of intermediate diameters, with the smallest and largest liposomes being internalized or excluded, respectively, regardless of whether they are targeted. These findings, which highlight a previously unexplored limitation of targeted delivery, can be used to design more effective drug carriers.

bioengineering↗

bioRxiv2023

Lipid droplets as substrates for protein phase separation

Membrane-associated protein phase separation plays critical roles in cell biology, driving essential cellular phenomena from immune signaling to membrane traffic. Importantly, by restricting diffusion to a two-dimensional surface, lipid bilayers can nucleate phase separation at far lower concentrations compared to those required for phase separation in solution. How might other intracellular lipid substrates, such as lipid droplets, contribute to nucleation of phase separation? Distinct from bilayer membranes, lipid droplets consist of a phospholipid monolayer surrounding a core of neutral lipids, and they are energy storage organelles that protect cells from lipotoxicity and oxidative stress. Here, we show that intrinsically disordered proteins can undergo phase separation on the surface of synthetic and cell-derived lipid droplets. Specifically, we find that model disordered domains, FUS LC and LAF1-RGG, separate into protein-rich and protein-depleted phases on the surfaces of lipid droplets. Owing to the hydrophobic nature of interactions between FUS LC proteins, increasing ionic strength drives an increase in its phase separation on droplet surfaces. The opposite is true for LAF1-RGG, owing to the electrostatic nature of its interprotein interactions. In both cases, protein-rich phases on the surfaces of synthetic and cell-derived lipid droplets demonstrate molecular mobility indicative of a liquid-like state. Our results show that lipid droplets can nucleate protein condensates, suggesting that protein phase separation could be key in organizing biological processes involving lipid droplets.

biophysics↗

bioRxiv2023

Kinetic trapping organizes actin filaments within liquid-like protein droplets

AbstractActin is essential for various cellular functions such as growth, migration, and endocytosis. Recent evidence suggests that several actin-binding proteins phase separate to form condensates and that actin networks have different architectures in these droplets. In this study, we use computational modeling to investigate the conditions under which actin forms different network organizations in VASP droplets. Our simulations reveal that the binding and unbinding rates of actin and VASP determine the probability of formation of shells and rings, with shells being more probable than rings. The different actin networks are highly dependent on the kinetics of VASP-actin interactions, suggesting that they arise from kinetic trapping. Specifically, we showed that reducing the residence time of VASP on actin filaments promotes assembly of shells rather than rings, where rings require a greater degree of actin bundling. These predictions were tested experimentally using a mutant of VASP, which has decreased bundling capability. Experiments reveal an increase in the abundance of shells in VASP droplets, consistent with our predictions. Finally, we investigated the arrangements of filaments within deformed droplets and found that the filament length largely determines whether a droplet will straighten into a bundle or remain kinetically trapped in a ring-like architecture. The sphere-to-ellipsoid transition is favored under a wide range of conditions while the ellipse-to-rod transition is only permitted when filaments have a specific range of lengths. Our findings have implications for understanding how the interactions between phase-separated actin binding proteins and actin filaments can give rise to different actin network architectures.

biophysics↗

bioRxiv2022

The ins and outs of membrane bending by intrinsically disordered proteins

Membrane curvature is essential to diverse cellular functions. While classically attributed to structured domains, recent work illustrates that intrinsically disordered proteins are also potent drivers of membrane bending. Specifically, repulsive interactions among disordered domains drive convex bending, while attractive interactions, which lead to liquid-like condensates, drive concave bending. How might disordered domains that contain both repulsive and attractive domains impact curvature? Here we examine chimeras that combine attractive and repulsive interactions. When the attractive domain was closer to the membrane, its condensation amplified steric pressure among repulsive domains, leading to convex curvature. In contrast, when the repulsive domain was closer to the membrane, attractive interactions dominated, resulting in concave curvature. Further, a transition from convex to concave curvature occurred with increasing ionic strength, which reduced repulsion while enhancing condensation. In agreement with a simple mechanical model, these results illustrate a set of design rules for membrane bending by disordered proteins.

biophysics↗