Small molecules in regulating protein phase separation

Computer-Aided Drug Discovery for Undruggable Targets

Undruggable targets are those of therapeutical significance but challenging for conventional drug design approaches. Such targets often exhibit unique features, including highly dynamic structures, a lack of well-defined ligand-binding pockets, the presence of highly conserved active sites, and functional modulation by protein-protein interactions. Recent advances in computational simulations and artificial intelligence have revolutionized the drug design landscape, giving rise to innovative strategies for overcoming these obstacles. In this review, we highlight the latest progress in computational approaches for drug design against undruggable targets, present several successful case studies, and discuss remaining challenges and future directions. Special emphasis is placed on four primary target categories: intrinsically disordered proteins, protein allosteric regulation, protein-protein interactions, and protein degradation, along with discussion of emerging target types. We also examine how AI-driven methodologies have transformed the field, from applications in protein-ligand complex structure prediction and virtual screening to de novo ligand generation for undruggable targets. Integration of computational methods with experimental techniques is expected to bring further breakthroughs to overcome the hurdles of undruggable targets. As the field continues to evolve, these advancements hold great promise to expand the druggable space, offering new therapeutic opportunities for previously untreatable diseases.

Ponceau S as a Targeted Modulator for Protein Liquid-Liquid Phase Separation

Liquid-liquid phase separation (LLPS) in proteins is essential for cellular organization and biomolecular condensation. However, current methods to induce phase separation often lack precise spatiotemporal control. This study introduces Ponceau S as a selective modulator and monitors phase separation in bovine serum albumin and lysozyme. We demonstrate that Ponceau S effectively promotes the protein complex into liquid droplets by binding to hydrophobic regions and driving intermolecular interactions. Notably, Ponceau S fluorescence increases within protein-rich phases, reflecting the restricted molecular motion in these environments. Furthermore, the phase separation induced by Ponceau S is finely tunable through salt and 1,6-hexanediol adjustments, which influence droplet fusion and dissolution dynamics. This work highlights the potential of small molecules like Ponceau S to precisely regulate and monitor protein phase separation, providing a new dimension of control for applications in biomolecular engineering, drug delivery, and synthetic biology.

Current perspectives in drug targeting intrinsically disordered proteins and biomolecular condensates

Intrinsically disordered proteins (IDPs) and biomolecular condensates are critical for cellular processes and physiological functions. Abnormal biomolecular condensates can cause diseases such as cancer and neurodegenerative disorders. IDPs, including intrinsically disordered regions (IDRs), were previously considered undruggable due to their lack of stable binding pockets. However, recent evidence indicates that targeting them can influence cellular processes. This review explores current strategies to target IDPs and biomolecular condensates, potential improvements, and the challenges and opportunities in this evolving field.

Phase separation in DNA repair: orchestrating the cellular response to genomic stability

DNA repair is a hierarchically organized, spatially and temporally regulated process involving numerous repair factors that respond to various types of damage. Despite decades of research, the mechanisms by which these factors are recruited to and depart from repair sites have been a subject of intrigue. Recent advancements in the field have increasingly highlighted the role of phase separation as a critical facilitator of the efficiency of DNA repair. This review emphasizes how phase separation enhances the concentration and coordination of repair factors at damage sites, optimizing repair efficiency. Understanding how dysregulation of phase separation can impair DNA repair and alter nuclear organization, potentially leading to diseases such as cancer and neurodegenerative disorders, is crucial. This manuscript provides a comprehensive understanding of the pivotal role of phase separation in DNA repair, sheds light on the current research, and suggests potential future directions for research and therapeutic interventions.

Design and development of drug delivery nanocarriers based on liquid-liquid phase separation, improved stability, cell-penetration and anti-cancer effect

Liquid-liquid phase separation (LLPS) of nuclear pore complex (NPC) with nuclear transport proteins (NTPs) via intrinsically disordered regions (IDRs) plays a crucial role in the nucleocytoplasmic transport. The development of efficient targeted delivery systems based on LLPS has attracted widespread attention. Here, we developed nanocarriers of casein peptides, a natural intrinsically disordered proteins (IDPs), modified with fatty acids of different alkyl chains (C10-C18) and decorated by shellac for highly effective drug delivery and cancer therapy. The curcumin (Cur)-loading nanocarriers (CSLNCs) showed excellent stability and dispersity in the natural environment over 30 days, with Cur encapsulation efficiency and loading capacity of ~90 % and ~57 %. Electron microscope (EM) indicated an aggregated homogeneous elliptical shape of CSLNCs(C10) and the morphology of CSLNCs(C18) transited to a distributed cubic shape. CSLNCs(C10, C12, C14 and C18) exhibited cytotoxicity against human lung adenocarcinoma NCI-H1975 cells with an IC50 value of 17.5 μM, 17.3 μM, 10.2 μM and 19.3 μM after 24 h of incubation, respectively. CSLNCs were also found to inhibit the cell wound healing with a migration rate of 12.72 %, 10.93 %, 4.28 % and 13.62 %, respectively. CSLNCs especially increased the percentage of late apoptotic cells. As indications of confocal microscopy, the fluorescence intensities of NCI-H1975 cells were enhanced with a cytosolic distribution and noticeably florescence in the nucleus after 0.5 h of incubation CSLNCs. CSLNCs treated cells adopted a rounded morphology with a dramatic reduction in fluorescence intensity after 1 h of incubation. Among CSLNCs, CSLNCs(C14) improved considerably the cytotoxicity activity and intercellular localization in the nucleus. The cell-penetration ability was also confirmed by the binding of CSLNCs in a model bicelles membrane system composed of DMPC and DHPC investigated by 1H NMR. It was proposed that CSLNCs with cell-penetrating and nuclear targeting performance may regulate the LLPS of nuclear pore complex and thus improved its nuclear penetration and cytotoxic activity.

A globular protein exhibits rare phase behavior and forms chemically regulated orthogonal condensates in cells

Proteins with chemically regulatable phase separation are of great interest in the fields of biomolecular condensates and synthetic biology. Intrinsically disordered proteins (IDPs) are the dominating building blocks of biomolecular condensates which often lack orthogonality and small-molecule regulation desired to create synthetic biomolecular condensates or membraneless organelles (MLOs). Here, we discover a well-folded globular protein, lipoate-protein ligase A (LplA) from E. coli involved in lipoylation of enzymes essential for one-carbon and energy metabolisms, that exhibits structural homomeric oligomerization and a rare LCST-type reversible phase separation in vitro. In both E. coli and human U2OS cells, LplA can form orthogonal condensates, which can be specifically dissolved by its natural substrate, the small molecule lipoic acid and its analogue lipoamide. The study of LplA phase behavior and its regulatability expands our understanding and toolkit of small-molecule regulatable protein phase behavior with impacts on biomedicine and synthetic biology.

Liquid-liquid phase separation in microorganisms: Insights into existence, functions, and applications

Liquid-liquid phase separation (LLPS) is a universal mechanism essential for maintaining cellular integrity and function in microorganisms, facilitating the organization of biomolecules into dynamic compartments. Although extensively studied in mammalian cells, research on LLPS formation and regulation in microorganisms remains limited. This review integrates insights from diverse studies exploring LLPS across microorganisms. We discuss the role of intrinsic disorders in microbial proteins and their relationship with environmental adaptation. Additionally, we examine how microorganisms utilize LLPS to sense changes in environmental parameters, such as temperature, pH, and nutrient levels, enabling them to respond to stresses and regulate cellular processes, such as cell division, protein synthesis, and metabolic flux. We highlight that LLPS is a promising target for synthetic biology and therapeutic intervention against pathogenic microorganisms. We also explore the research landscape of LLPS in microorganisms and address challenges associated with the techniques used in LLPS research. Further research is needed to focus on the detailed molecular regulatory mechanisms of condensates, biotechnological and synthetic biology applications, facilitating improved manipulation of microorganisms, and the identification of novel therapeutic targets.

Ensemble docking for intrinsically disordered proteins

Intrinsically disordered proteins (IDPs) are implicated in many human diseases and are increasingly being pursued as drug targets. Conventional structure-based drug design methods that rely on well-defined binding sites are however, largely unsuitable for IDPs. Here, we present computationally efficient ensemble docking approaches to predict the relative affinities of small molecules to IDPs and characterize their dynamic, heterogenous binding mechanisms at atomic resolution. We demonstrate that these ensemble docking protocols accurately predict the relative binding affinities of small molecule α-synuclein ligands measured by NMR spectroscopy and generate conformational ensembles of ligand binding modes in remarkable agreement with experimentally validated long-timescale molecular dynamics simulations. Our results display the potential of ensemble docking approaches for predicting small molecule binding to IDPs and suggest that these methods may be valuable tools for IDP drug discovery campaigns.

Trifluoroacetic Acid as a Molecular Probe for the Dense Phase in Liquid-Liquid Phase-Separating Peptide Systems

Although trifluoroacetic acid (TFA) is not typically considered a Hofmeister reagent, it has been demonstrated to modulate biocoacervation. We show that TFA can be employed to probe specific interactions in coacervating bioinspired peptide phenylalanine (Phe) 19F-labeled at a single site, altering its liquid-liquid phase separation (LLPS) behavior. Solid-state nuclear magnetic resonance (NMR) spectroscopy revealed two dynamically distinct binding modes of TFA with Phe, resulting in a structured, dipolar-ordered complex and a more dynamic complex, highlighting the proximity between TFA and Phe. Quantum chemistry modeling of 19F chemical shift differences indicates that the structured complex is formed by the intercalation of one TFA molecule between two stacked Phe aromatic rings, possibly contributing to the stabilization of the condensed dense phase. Thus, we propose that TFA can be used as a convenient molecular probe in 19F NMR-based studies of the structure and dynamics of the dense phase in LLPS peptide systems.

Nonspecific Yet Selective Interactions Contribute to Small Molecule Condensate Binding

Biomolecular condensates are essential in various cellular processes, and their misregulation has been demonstrated to underlie disease. Small molecules that modulate condensate stability and material properties offer promising therapeutic approaches, but mechanistic insights into their interactions with condensates remain largely lacking. We employ a multiscale approach to enable long-time, equilibrated all-atom simulations of various condensate-ligand systems. Systematic characterization of the ligand binding poses reveals that condensates can form diverse and heterogeneous chemical environments with one or multiple chains to bind small molecules. Unlike traditional protein-ligand interactions, these chemical environments are dominated by nonspecific hydrophobic interactions. Nevertheless, the chemical environments feature unique amino acid compositions and physicochemical properties that favor certain small molecules over others, resulting in varied ligand partitioning coefficients within condensates. Notably, different condensates share similar sets of chemical environments but at different populations. This population shift drives ligand selectivity toward specific condensates. Our approach can enhance the interpretation of experimental screening data and may assist in the rational design of small molecules targeting specific condensates.

Divalent and multivalent cations control liquid-like assembly of poly(ADP-ribosyl)ated PARP1 into multimolecular associates in vitro

The formation of nuclear biomolecular condensates is often associated with local accumulation of proteins at a site of DNA damage. The key role in the formation of DNA repair foci belongs to PARP1, which is a sensor of DNA damage and catalyzes the synthesis of poly(ADP-ribose) attracting repair factors. We show here that biogenic cations such as Mg2+, Ca2+, Mn2+, spermidine3+, or spermine4+ can induce liquid-like assembly of poly(ADP-ribosyl)ated [PARylated] PARP1 into multimolecular associates (hereafter: self-assembly). The self-assembly of PARylated PARP1 affects the level of its automodification and hydrolysis of poly(ADP-ribose) by poly(ADP-ribose) glycohydrolase (PARG). Furthermore, association of PARylated PARP1 with repair proteins strongly stimulates strand displacement DNA synthesis by DNA polymerase β (Pol β) but has no noticeable effect on DNA ligase III activity. Thus, liquid-like self-assembly of PARylated PARP1 may play a critical part in the regulation of i) its own activity, ii) PARG-dependent hydrolysis of poly(ADP-ribose), and iii) Pol β-mediated DNA synthesis. The latter can be considered an additional factor influencing the choice between long-patch and short-patch DNA synthesis during repair.

Non-specific yet selective interactions contribute to small molecule condensate partitioning behavior

Biomolecular condensates are essential in various cellular processes, and their misregulation has been demonstrated to be underly disease. Small molecules that modulate condensate stability and material properties offer promising therapeutic approaches, but mechanistic insights into their interactions with condensates remain largely lacking. We employ a multiscale approach to enable long-time, equilibrated all-atom simulations of various condensate-ligand systems. Systematic characterization of the ligand binding poses reveals that condensates can form diverse and heterogeneous chemical environments with one or multiple chains to bind small molecules. Unlike traditional protein-ligand interactions, these chemical environments are dominated by non-specific hydrophobic interactions. Nevertheless, the chemical environments feature unique amino acid compositions and physicochemical properties that favor certain small molecules over others, resulting in varied ligand partitioning coefficients within condensates. Notably, different condensates share similar sets of chemical environments but at different populations. This population shift drives ligand selectivity towards specific condensates. Our approach can enhance the interpretation of experimental screening data and may assist in the rational design of small molecules targeting specific condensates.

Substrate-induced phase transition within liquid condensates reverses the catalytic activity of nanoparticles

Liquid-liquid phase separation is reported to enhance the catalytic reaction rates severalfold. Herein, we explored the interactions between a catalyst and a range of substrate concentrations to understand the impact on the droplet phase and catalytic reaction kinetics. We observed that the substrate above a critical concentration induces phase transitions within liquid condensates and restricts the free movement of both the substrate and products, resulting in an overall reduction of the reaction rate, an observation not reported earlier.

The liquid-liquid phase separation in programmed cell death

In recent years, the physical phenomenon of liquid-liquid phase separation has been widely introduced into biological research. Membrane-free organelles have been found to exist in cells that were driven by liquid-liquid phase separation. Intermolecular multivalent interactions can drive liquid-liquid phase separation to form condensates that are independent of other substances in the environment and thus can play an effective role in regulating multiple biological processes in the cell. The way of cell death has also long been a focus in multiple research. In the face of various stresses, cell death-related mechanisms are crucial for maintaining cellular homeostasis and regulating cell fate. With the in-depth study of cell death pathways, it has been found that the process of cell death was also accompanied by the regulation of liquid-liquid phase separation and played a key role. Therefore, this review summarized the roles of liquid-liquid phase separation in various cell death pathways, and explored the regulation of cell fate by liquid-liquid phase separation, with the expectation that the exploration of the mechanism of liquid-liquid phase separation would provide new insights into the treatment of diseases caused by regulated cell death.

Fundamental Aspects of Phase-Separated Biomolecular Condensates

Biomolecular condensates, formed through phase separation, are upending our understanding in much of molecular, cell, and developmental biology. There is an urgent need to elucidate the physicochemical foundations of the behaviors and properties of biomolecular condensates. Here we aim to fill this need by writing a comprehensive, critical, and accessible review on the fundamental aspects of phase-separated biomolecular condensates. We introduce the relevant theoretical background, present the theoretical basis for the computation and experimental measurement of condensate properties, and give mechanistic interpretations of condensate behaviors and properties in terms of interactions at the molecular and residue levels.

Interplay between posttranslational modifications and liquid‒liquid phase separation in tumors

Liquid‒liquid phase separation (LLPS) is a general phenomenon recently recognized to be critically involved in the regulation of a variety of cellular biological processes, such as transcriptional regulation, heterochromatin formation and signal transduction, through the compartmentalization of proteins or nucleic acids into droplet-like condensates. These processes are directly or indirectly related to tumor initiation and treatment. Posttranslational modifications (PTMs), which represent a rapid and reversible mechanism involved in the functional regulation of proteins, have emerged as key events in modulating LLPS under physiological or pathophysiological conditions, including tumorigenesis and antitumor therapy. In this review, we introduce the biological functions participated in cancer-associated LLPS, discuss the potential roles of LLPS during tumor onset or therapy, and emphasize the mechanistic characteristics of LLPS regulated by PTMs and its effects on tumor progression. We then provide a perspective on further studies on LLPS and its regulation by PTMs in cancer research. This review aims to broaden the understanding of the functions of LLPS and its regulation by PTMs under normal or aberrant cellular conditions.

Liquid-Liquid Phase Separation Sheds New Light upon Cardiovascular Diseases

Liquid-liquid phase separation (LLPS) is a biophysical process that mediates the precise and complex spatiotemporal coordination of cellular processes. Proteins and nucleic acids are compartmentalized into micron-scale membrane-less droplets via LLPS. These droplets, termed biomolecular condensates, are highly dynamic, have concentrated components, and perform specific functions. Biomolecular condensates have been observed to organize diverse key biological processes, including gene transcription, signal transduction, DNA damage repair, chromatin organization, and autophagy. The dysregulation of these biological activities owing to aberrant LLPS is important in cardiovascular diseases. This review provides a detailed overview of the regulation and functions of biomolecular condensates, provides a comprehensive depiction of LLPS in several common cardiovascular diseases, and discusses the revolutionary therapeutic perspective of modulating LLPS in cardiovascular diseases and new treatment strategies relevant to LLPS.

Biological Liquid-Liquid Phase Separation, Biomolecular Condensates, and Membraneless Organelles: Now You See Me, Now You Don't

Liquid-liquid phase separation (LLPS, also known as biomolecular condensation) and the related biogenesis of various membraneless organelles (MLOs) and biomolecular condensates (BMCs) are now considered fundamental molecular mechanisms governing the spatiotemporal organization of the intracellular space [...].