Towards Decoding the Sequence-Based Grammar Governing the Functions of Intrinsically Disordered Protein Regions

Long non-coding RNAs: roles in cellular stress responses and epigenetic mechanisms regulating chromatin

Most of the genome is transcribed into RNA but only 2% of the sequence codes for proteins. Non-coding RNA transcripts include a very large number of long noncoding RNAs (lncRNAs). A growing number of identified lncRNAs operate in cellular stress responses, for example in response to hypoxia, genotoxic stress, and oxidative stress. Additionally, lncRNA plays important roles in epigenetic mechanisms operating at chromatin and in maintaining chromatin architecture. Here, we address three lncRNA topics that have had significant recent advances. The first is an emerging role for many lncRNAs in cellular stress responses. The second is the development of high throughput screening assays to develop causal relationships between lncRNAs across the genome with cellular functions. Finally, we turn to recent advances in understanding the role of lncRNAs in regulating chromatin architecture and epigenetics, advances that build on some of the earliest work linking RNA to chromatin architecture.

Making the cut: Multiscale simulation of membrane remodeling

Biological membranes are dynamic heterogeneous materials, and their shape and organization are tightly coupled to the properties of the proteins in and around them. However, the length scales of lipid and protein dynamics are far below the size of membrane-bound organelles, much less an entire cell. Therefore, multiscale modeling approaches are often necessary to build a comprehensive picture of the interplay of these factors, and have provided critical insights into our understanding of membrane dynamics. Here, we review computational methods for studying membrane remodeling, as well as passive and active examples of protein-driven membrane remodeling. As the field advances towards the modeling of key aspects of organelles and whole cells - an increasingly accessible regime of study - we summarize here recent successes and offer comments on future trends.

Enhancer selectivity in space and time: from enhancer-promoter interactions to promoter activation

The primary regulators of metazoan gene expression are enhancers, originally functionally defined as DNA sequences that can activate transcription at promoters in an orientation-independent and distance-independent manner. Despite being crucial for gene regulation in animals, what mechanisms underlie enhancer selectivity for promoters, and more fundamentally, how enhancers interact with promoters and activate transcription, remain poorly understood. In this Review, we first discuss current models of enhancer-promoter interactions in space and time and how enhancers affect transcription activation. Next, we discuss different mechanisms that mediate enhancer selectivity, including repression, biochemical compatibility and regulation of 3D genome structure. Through 3D polymer simulations, we illustrate how the ability of 3D genome folding mechanisms to mediate enhancer selectivity strongly varies for different enhancer-promoter interaction mechanisms. Finally, we discuss how recent technical advances may provide new insights into mechanisms of enhancer-promoter interactions and how technical biases in methods such as Hi-C and Micro-C and imaging techniques may affect their interpretation.

A fine kinetic balance of interactions directs transcription factor hubs to genes

Eukaryotic gene regulation relies on the binding of sequence-specific transcription factors (TFs). TFs bind chromatin transiently yet occupy their target sites by forming high-local concentration microenvironments (hubs and condensates) that increase the frequency of binding events. Despite their ubiquity, such microenvironments have been difficult to study in endogenous contexts due to technical limitations. Here, we overcome these limitations and investigate how hubs drive TF occupancy at their targets. Using a DNA binding perturbation to a hub-forming TF, Zelda, in Drosophila embryos, we find that hub properties, including the stability and frequencies of associations to targets, are key determinants of TF occupancy. Our data suggest that the targeting of these hubs is driven not just by specific DNA motif recognition, but also by a fine-tuned kinetic balance of interactions between TFs and their co-binding partners.

Phase separation of multicomponent peptide mixtures into dehydrated clusters with hydrophilic cores

Phase separation of biomolecules underlies the formation and regulation of various membraneless condensates in cells. How condensates function reliably while surrounded by heterogeneous and dynamic mixtures of biomolecular components with specific and nonspecific interactions is yet to be understood. Studying multicomponent biomolecular mixtures with designer peptides has recently become an attractive avenue for learning about physicochemical principles governing cellular condensates. In this work, we employed long-timescale atomistic simulations of multicomponent tripeptide mixtures with all residue substitutions to illuminate the nature of direct and water-mediated interactions in a prototypical cellular condensate environment. We find that peptide mixtures form clusters with inverse hydrophobic order. Most multivalent and charged residues are localized in the cluster's core, with a large fraction of nonaromatic hydrophobic residues remaining on the surface. This inverse hydrophobic order in peptide clusters is partly driven by the expulsion of nonspecifically bound water molecules following peptide cluster growth. The growth of clusters is also accompanied by the formation of increasing numbers of specific water-mediated interactions between polar and charged residues. While the present study focused on the condensation of short peptide motifs, the general findings and analysis techniques should be helpful for future studies on larger peptides and protein condensation.

Phase Separation as a Driver of Stem Cell Organization and Function during Development

A properly organized subcellular composition is essential to cell function. The canonical organizing principle within eukaryotic cells involves membrane-bound organelles; yet, such structures do not fully explain cellular complexity. Furthermore, discrete non-membrane-bound structures have been known for over a century. Liquid-liquid phase separation (LLPS) has emerged as a ubiquitous mode of cellular organization without the need for formal lipid membranes, with an ever-expanding and diverse list of cellular functions that appear to be regulated by this process. In comparison to traditional organelles, LLPS can occur across wider spatial and temporal scales and involves more distinct protein and RNA complexes. In this review, we discuss the impacts of LLPS on the organization of stem cells and their function during development. Specifically, the roles of LLPS in developmental signaling pathways, chromatin organization, and gene expression will be detailed, as well as its impacts on essential processes of asymmetric cell division. We will also discuss how the dynamic and regulated nature of LLPS may afford stem cells an adaptable mode of organization throughout the developmental time to control cell fate. Finally, we will discuss how aberrant LLPS in these processes may contribute to developmental defects and disease.

Phase separation modulates the functional amyloid assembly of human CPEB3

How functional amyloids are regulated to restrict their activity is poorly understood. The cytoplasmic polyadenylation element-binding protein 3 (CPEB3) is an RNA-binding protein that adopts an amyloid state key for memory persistence. Its monomer represses the translation of synaptic target mRNAs while phase separated, whereas its aggregated state acts as a translational activator. Here, we have explored the sequence-driven molecular determinants behind the functional aggregation of human CPEB3 (hCPEB3). We found that the intrinsically disordered region (IDR) of hCPEB3 encodes both an amyloidogenic and a phase separation domain, separated by a poly-A-rich region. The hCPEB3 amyloid core is composed by a hydrophobic region instead of the Q-rich stretch found in the Drosophila orthologue. The hCPEB3 phase separation domain relies on hydrophobic interactions with ionic strength dependence, and its droplet ageing process leads to a liquid-to-solid transition with the formation of a non-fibril-based hydrogel surrounded by starburst droplets. Furthermore, we demonstrate the differential behavior of the protein depending on its environment. Under physiological-like conditions, hCPEB3 can establish additional electrostatic interactions with ions, increasing the stability of its liquid droplets and driving a condensation-based amyloid pathway.

Intrinsically disordered regions and RNA binding domains contribute to protein enrichment in biomolecular condensates in Xenopus oocytes

Proteins containing both intrinsically disordered regions (IDRs) and RNA binding domains (RBDs) can phase separate in vitro, forming bodies similar to cellular biomolecular condensates. However, how IDR and RBD domains contribute to in vivo recruitment of proteins to biomolecular condensates remains poorly understood. Here, we analyzed the roles of IDRs and RBDs in L-bodies, biomolecular condensates present in Xenopus oocytes. We show that a cytoplasmic isoform of hnRNPAB, which contains two RBDs and an IDR, is highly enriched in L-bodies. While both of these domains contribute to hnRNPAB self-association and phase separation in vitro and mediate enrichment into L-bodies in oocytes, neither the RBDs nor the IDR replicate the localization of full-length hnRNPAB. Our results suggest a model where the additive effects of the IDR and RBDs regulate hnRNPAB partitioning into L-bodies. This model likely has widespread applications as proteins containing RBD and IDR domains are common biomolecular condensate residents.

Sequence variations of phase-separating proteins and resources for studying biomolecular condensates

Phase separation (PS) is an important mechanism underlying the formation of biomolecular condensates. Physiological condensates are associated with numerous biological processes, such as transcription, immunity, signaling, and synaptic transmission. Changes in particular amino acids or segments can disturb the protein's phase behavior and interactions with other biomolecules in condensates. It is thus presumed that variations in the phase-separating-prone domains can significantly impact the properties and functions of condensates. The dysfunction of condensates contributes to a number of pathological processes. Pharmacological perturbation of these condensates is proposed as a promising way to restore physiological states. In this review, we characterize the variations observed in PS proteins that lead to aberrant biomolecular compartmentalization. We also showcase recent advancements in bioinformatics of membraneless organelles (MLOs), focusing on available databases useful for screening PS proteins and describing endogenous condensates, guiding researchers to seek the underlying pathogenic mechanisms of biomolecular condensates.

The Role of ATP-Binding Cassette Proteins in Stem Cell Pluripotency

Pluripotent stem cells (PSCs) are highly proliferative cells that can self-renew indefinitely in vitro. Upon receiving appropriate signals, PSCs undergo differentiation and can generate every cell type in the body. These unique properties of PSCs require specific gene expression patterns that define stem cell identity and dynamic regulation of intracellular metabolism to support cell growth and cell fate transitions. PSCs are prone to DNA damage due to elevated replicative and transcriptional stress. Therefore, mechanisms to prevent deleterious mutations in PSCs that compromise stem cell function or increase the risk of tumor formation from becoming amplified and propagated to progenitor cells are essential for embryonic development and for using PSCs including induced PSCs (iPSCs) as a cell source for regenerative medicine. In this review, we discuss the role of the ATP-binding cassette (ABC) superfamily in maintaining PSC homeostasis, and propose how their activities can influence cellular signaling and stem cell fate decisions. Finally, we highlight recent discoveries that not all ABC family members perform only canonical metabolite and peptide transport functions in PSCs; rather, they can participate in diverse cellular processes from genome surveillance to gene transcription and mRNA translation, which are likely to maintain the pristine state of PSCs.

The molecular grammar of protein disorder guiding genome-binding locations

Intrinsically disordered regions (IDRs) direct transcription factors (TFs) towards selected genomic occurrences of their binding motif, as exemplified by budding yeast's Msn2. However, the sequence basis of IDR-directed TF binding selectivity remains unknown. To reveal this sequence grammar, we analyze the genomic localizations of >100 designed IDR mutants, each carrying up to 122 mutations within this 567-AA region. Our data points at multivalent interactions, carried by hydrophobic-mostly aliphatic-residues dispersed within a disordered environment and independent of linear sequence motifs, as the key determinants of Msn2 genomic localization. The implications of our results for the mechanistic basis of IDR-based TF binding preferences are discussed.

A viral biomolecular condensate coordinates assembly of progeny particles

Biomolecular condensates formed by phase separation can compartmentalize and regulate cellular processes1,2. Emerging evidence has suggested that membraneless subcellular compartments in virus-infected cells form by phase separation3-8. Although linked to several viral processes3-5,9,10, evidence that phase separation contributes functionally to the assembly of progeny particles in infected cells is lacking. Here we show that phase separation of the human adenovirus 52-kDa protein has a critical role in the coordinated assembly of infectious progeny particles. We demonstrate that the 52-kDa protein is essential for the organization of viral structural proteins into biomolecular condensates. This organization regulates viral assembly such that capsid assembly is coordinated with the provision of viral genomes needed to produce complete packaged particles. We show that this function is governed by the molecular grammar of an intrinsically disordered region of the 52-kDa protein, and that failure to form condensates or to recruit viral factors that are critical for assembly results in failed packaging and assembly of only non-infectious particles. Our findings identify essential requirements for coordinated assembly of progeny particles and demonstrate that phase separation of a viral protein is critical for production of infectious progeny during adenovirus infection.

Prediction of protein-protein interactions using sequences of intrinsically disordered regions

Protein-protein interactions (PPIs) play a crucial role in numerous molecular processes. Despite many efforts, mechanisms governing molecular recognition between interacting proteins remain poorly understood and it is particularly challenging to predict from sequence whether two proteins can interact. Here we present a new method to tackle this challenge using intrinsically disordered regions (IDRs). IDRs are protein segments that are functional despite lacking a single invariant three-dimensional structure. The prevalence of IDRs in eukaryotic proteins suggests that IDRs are critical for interactions. To test this hypothesis, we predicted PPIs using IDR sequences in candidate proteins in humans. Moreover, we divide the PPI prediction problem into two specific subproblems and adapt appropriate training and test strategies based on problem type. Our findings underline the importance of defining clearly the problem type and show that sequences encoding IDRs can aid in predicting specific features of the protein interaction network of intrinsically disordered proteins. Our findings further suggest that accounting for IDRs in future analyses should accelerate efforts to elucidate the eukaryotic PPI network.

Hotspot mutations in the structured ENL YEATS domain link aberrant transcriptional condensates and cancer

Growing evidence suggests prevalence of transcriptional condensates on chromatin, yet their mechanisms of formation and functional significance remain largely unclear. In human cancer, a series of mutations in the histone acetylation reader ENL create gain-of-function mutants with increased transcriptional activation ability. Here, we show that these mutations, clustered in ENL's structured acetyl-reading YEATS domain, trigger aberrant condensates at native genomic targets through multivalent homotypic and heterotypic interactions. Mechanistically, mutation-induced structural changes in the YEATS domain, ENL's two disordered regions of opposing charges, and the incorporation of extrinsic elongation factors are all required for ENL condensate formation. Extensive mutagenesis establishes condensate formation as a driver of oncogenic gene activation. Furthermore, expression of ENL mutants beyond the endogenous level leads to non-functional condensates. Our findings provide new mechanistic and functional insights into cancer-associated condensates and support condensate dysregulation as an oncogenic mechanism.

Mechanisms governing target search and binding dynamics of hypoxia-inducible factors

Transcription factors (TFs) are classically attributed a modular construction, containing well-structured sequence-specific DNA-binding domains (DBDs) paired with disordered activation domains (ADs) responsible for protein-protein interactions targeting co-factors or the core transcription initiation machinery. However, this simple division of labor model struggles to explain why TFs with identical DNA-binding sequence specificity determined in vitro exhibit distinct binding profiles in vivo. The family of hypoxia-inducible factors (HIFs) offer a stark example: aberrantly expressed in several cancer types, HIF-1α and HIF-2α subunit isoforms recognize the same DNA motif in vitro - the hypoxia response element (HRE) - but only share a subset of their target genes in vivo, while eliciting contrasting effects on cancer development and progression under certain circumstances. To probe the mechanisms mediating isoform-specific gene regulation, we used live-cell single particle tracking (SPT) to investigate HIF nuclear dynamics and how they change upon genetic perturbation or drug treatment. We found that HIF-α subunits and their dimerization partner HIF-1β exhibit distinct diffusion and binding characteristics that are exquisitely sensitive to concentration and subunit stoichiometry. Using domain-swap variants, mutations, and a HIF-2α specific inhibitor, we found that although the DBD and dimerization domains are important, another main determinant of chromatin binding and diffusion behavior is the AD-containing intrinsically disordered region (IDR). Using Cut&Run and RNA-seq as orthogonal genomic approaches, we also confirmed IDR-dependent binding and activation of a specific subset of HIF target genes. These findings reveal a previously unappreciated role of IDRs in regulating the TF search and binding process that contribute to functional target site selectivity on chromatin.

Protein conformation and biomolecular condensates

Protein conformation and cell compartmentalization are fundamental concepts and subjects of vast scientific endeavors. In the last two decades, we have witnessed exciting advances that unveiled the conjunction of these concepts. An avalanche of studies highlighted the central role of biomolecular condensates in membraneless subcellular compartmentalization that permits the spatiotemporal organization and regulation of myriads of simultaneous biochemical reactions and macromolecular interactions. These studies have also shown that biomolecular condensation, driven by multivalent intermolecular interactions, is mediated by order-disorder transitions of protein conformation and by protein domain architecture. Conceptually, protein condensation is a distinct level in protein conformational landscape in which collective folding of large collections of molecules takes place. Biomolecular condensates arise by the physical process of phase separation and comprise a variety of bodies ranging from membraneless organelles to liquid condensates to solid-like conglomerates, spanning lengths from mesoscopic clusters (nanometers) to micrometer-sized objects. In this review, we summarize and discuss recent work on the assembly, composition, conformation, material properties, thermodynamics, regulation, and functions of these bodies. We also review the conceptual framework for future studies on the conformational dynamics of condensed proteins in the regulation of cellular processes.

The ribonucleoprotein network of the nucleus: a historical perspective

There is a long experimental history supporting the principle that RNA is essential for normal nuclear and chromatin architecture. Most of the genome is transcribed into RNA but only 2% of the sequence codes for proteins. In the nucleus, most non-coding RNA, packaged in proteins, is bound into structures including chromatin and a non-chromatin scaffolding, the nuclear matrix, which was first observed by electron microscopy. Removing nuclear RNA or inhibiting its transcription causes the condensation of chromatin, showing the importance of RNA in spatially and functionally organizing the genome. Today, powerful techniques for the molecular characterization of RNA and for mapping its spatial organization in the nucleus have provided molecular detail to these principles.

Tuning levels of low-complexity domain interactions to modulate endogenous oncogenic transcription

Gene activation by mammalian transcription factors (TFs) requires multivalent interactions of their low-complexity domains (LCDs), but how such interactions regulate transcription remains unclear. It has been proposed that extensive LCD-LCD interactions culminating in liquid-liquid phase separation (LLPS) of TFs is the dominant mechanism underlying transactivation. Here, we investigated how tuning the amount and localization of LCD-LCD interactions in vivo affects transcription of endogenous human genes. Quantitative single-cell and single-molecule imaging reveals that the oncogenic TF EWS::FLI1 requires a narrow optimum of LCD-LCD interactions to activate its target genes associated with GGAA microsatellites. Increasing LCD-LCD interactions toward putative LLPS represses transcription of these genes in patient-derived cells. Likewise, ectopically creating LCD-LCD interactions to sequester EWS::FLI1 into a well-documented LLPS compartment, the nucleolus, inhibits EWS::FLI1-driven transcription and oncogenic transformation. Our findings show how altering the balance of LCD-LCD interactions can influence transcriptional regulation and suggest a potential therapeutic strategy for targeting disease-causing TFs.

RNA chain length and stoichiometry govern surface tension and stability of protein-RNA condensates

Proteomic studies have shown that cellular condensates are frequently enriched in diverse RNA molecules, which is suggestive of mechanistic links between phase separation and transcriptional activities. Here, we report a systematic experimental and computational study of thermodynamic landscapes and interfacial properties of protein-RNA condensates. We have studied the affinity of protein-RNA condensation as a function of variable RNA sequence length and RNA-protein stoichiometry under different ionic environments and external crowding. We have chosen the PolyU sequences for RNA and arginine/glycine-rich intrinsically disordered peptide (RGG) for proteins as a model system of RNA-protein condensates, which we then investigate through in vitro microscopy measurements and coarse-grained molecular dynamics simulations. We find that crowding and RNA chain length can have a major stabilizing effect on the condensation. We also find that the RNA-protein charge ratio is a crucial variable controlling stability, interfacial properties, and the reentrant phase behavior of RGG-RNA mixtures.

Short Linear Motifs (SLiMs) in “Core” RxLR Effectors of Phytophthora parasitica var. nicotianae : a Case of PpRxLR1 Effector

Oomycetes of the genus Phytophthora encompass several of the most successful plant pathogens described to date. The success of infection by Phytophthora species is attributed to the pathogens’ ability to secrete effector proteins that alter the host’s physiological processes. Structural analyses of effector proteins mainly from bacterial and viral pathogens have revealed the presence of intrinsically disordered regions that host short linear motifs (SLiMs). These motifs play important biological roles by facilitating protein-protein interactions as well as protein translocation. Nonetheless, SLiMs in Phytophthora species RxLR effectors have not been investigated previously and their roles remain unknown. Using a bioinformatics pipeline, we identified 333 candidate RxLR effectors in the strain INRA 310 of Phytophthora parasitica. Of these, 71 (21%) were also found to be present in 10 other genomes of P. parasitica, and hence, these were designated core RxLR effectors (CREs). Within the CRE sequences, the N terminus exhibited enrichment in intrinsically disordered regions compared to the C terminus, suggesting a potential role of disorder in effector translocation. Although the disorder content was reduced in the C-terminal regions, it is important to mention that most SLiMs were in this terminus. PpRxLR1 is one of the 71 CREs identified in this study, and its genes encode a 6-amino acid (aa)-long SLiM at the C terminus. We showed that PpRxLR1 interacts with several host proteins that are implicated in defense. Structural analysis of this effector using homology modeling revealed the presence of potential ligand-binding sites. Among key residues that were predicted to be crucial for ligand binding, L¹⁰² and Y¹⁰⁶ were of interest since they form part of the 6-aa-long PpRxLR1 SLiM. In silico substitution of these two residues to alanine was predicted to have a significant effect on both the function and the structure of PpRxLR1 effector. Molecular docking simulations revealed possible interactions between PpRxLR1 effector and ubiquitin-associated proteins. The ubiquitin-like SLiM carried in this effector was shown to be a potential mediator of these interactions. Further studies are required to validate and elucidate the underlying molecular mechanism of action.

IMPORTANCE The continuous gain and loss of RxLR effectors makes the control of Phytophthora spp. difficult. Therefore, in this study, we endeavored to identify RxLR effectors that are highly conserved among species, also known as “core” RxLR effectors (CREs). We reason that these highly conserved effectors target conserved proteins or processes; thus, they can be harnessed in breeding for durable resistance in plants. To further understand the mechanisms of action of CREs, structural dissection of these proteins is crucial. Intrinsically disordered regions (IDRs) that do not adopt a fixed, three-dimensional fold carry short linear motifs (SLiMs) that mediate biological functions of proteins. The presence and potential role of these SLiMs in CREs of Phytophthora spp. have been overlooked. To our knowledge, we have effectively identified CREs as well as SLiMs with the potential of promoting effector virulence. Together, this work has advanced our comprehension of Phytophthora RxLR effector function and may facilitate the development of innovative and effective control strategies.

14-3-3 Proteins are Potential Regulators of Liquid-Liquid Phase Separation

The 14-3-3 family proteins are vital scaffold proteins that ubiquitously expressed in various tissues. They interact with numerous protein targets and mediate many cellular signaling pathways. The 14-3-3 binding motifs are often embedded in intrinsically disordered regions which are closely associated with liquid-liquid phase separation (LLPS). In the past ten years, LLPS has been observed for a variety of proteins and biological processes, indicating that LLPS plays a fundamental role in the formation of membraneless organelles and cellular condensates. While extensive investigations have been performed on 14-3-3 proteins, its involvement in LLPS is overlooked. To date, 14-3-3 proteins have not been reported to undergo LLPS alone or regulate LLPS of their binding partners. To reveal the potential involvement of 14-3-3 proteins in LLPS, in this review, we summarized the LLPS propensity of 14-3-3 binding partners and found that about one half of them may undergo LLPS spontaneously. We further analyzed the phase separation behavior of representative 14-3-3 binders and discussed how 14-3-3 proteins may be involved. By modulating the conformation and valence of interactions and recruiting other molecules, we speculate that 14-3-3 proteins can efficiently regulate the functions of their targets in the context of LLPS. Considering the critical roles of 14-3-3 proteins, there is an urgent need for investigating the involvement of 14-3-3 proteins in the phase separation process of their targets and the underling mechanisms.

Uses and Abuses of the Atomic Displacement Parameters in Structural Biology

B-factors determined with X-ray crystallographic analyses are commonly used to estimate the flexibility degree of atoms, residues, and molecular moieties in biological macromolecules. In this chapter, the most recent studies and applications of B-factors in protein engineering and structural biology are briefly summarized. Particular emphasis is given to the limitations in using B-factors, in order to prevent inappropriate applications. It is eventually predicted that future applications will involve anisotropically refined B-factors, deep learning, and data produced by cryo-EM.

Regulation of spatially restricted gene expression: linking RNA localization and phase separation

Subcellular restriction of gene expression is crucial to the functioning of a wide variety of cell types. The cellular machinery driving spatially restricted gene expression has been studied for many years, but recent advances have highlighted novel mechanisms by which cells can generate subcellular microenvironments with specialized gene expression profiles. Particularly intriguing are recent findings that phase separation plays a role in certain RNA localization pathways. The burgeoning field of phase separation has revolutionized how we view cellular compartmentalization, revealing that, in addition to membrane-bound organelles, phase-separated cytoplasmic microenvironments - termed biomolecular condensates - are compositionally and functionally distinct from the surrounding cytoplasm, without the need for a lipid membrane. The coupling of phase separation and RNA localization allows for precise subcellular targeting, robust translational repression and dynamic recruitment of accessory proteins. Despite the growing interest in the intersection between RNA localization and phase separation, it remains to be seen how exactly components of the localization machinery, particularly motor proteins, are able to associate with these biomolecular condensates. Further studies of the formation, function, and transport of biomolecular condensates promise to provide a new mechanistic understanding of how cells restrict gene expression at a subcellular level.

Disordered and dead, but in good company: How a catalytically inactive UTX retains its function

Shi et al. (2021) demonstrate that tumor-suppressive and developmental functions of UTX require an intrinsically disordered region (IDR) capable of condensate formation. These results provide further evidence for the functional role of IDR-mediated spatial organization in regulating gene expression in development and disease.

RNA-Binding Proteins at the Host-Pathogen Interface Targeting Viral Regulatory Elements

Viral RNAs contain the information needed to synthesize their own proteins, to replicate, and to spread to susceptible cells. However, due to their reduced coding capacity RNA viruses rely on host cells to complete their multiplication cycle. This is largely achieved by the concerted action of regulatory structural elements on viral RNAs and a subset of host proteins, whose dedicated function across all stages of the infection steps is critical to complete the viral cycle. Importantly, not only the RNA sequence but also the RNA architecture imposed by the presence of specific structural domains mediates the interaction with host RNA-binding proteins (RBPs), ultimately affecting virus multiplication and spreading. In marked difference with other biological systems, the genome of positive strand RNA viruses is also the mRNA. Here we focus on distinct types of positive strand RNA viruses that differ in the regulatory elements used to promote translation of the viral RNA, as well as in the mechanisms used to evade the series of events connected to antiviral response, including translation shutoff induced in infected cells, assembly of stress granules, and trafficking stress.

TopBP1 comes into focus

Frattini et al. (2021) demonstrate that TopBP1 forms phase-separated nuclear condensates to promote activation of ATR in cells experiencing genomic stress.