RING tetramerization is required for nuclear body biogenesis and PML sumoylation

Mutation in the Unrearranged PML Allele Confers Resistance to Arsenic Trioxide in Acute Promyelocytic Leukemia

Arsenic trioxide (ATO) is able to selectively target and degrade the disease-causing PML::RARα (P/R) oncoprotein in acute promyelocytic leukemia (APL) for curing the disease. However, some relapsed patients develop resistance to ATO due to mutations in the promyelocytic leukemia (PML) part of the PML::RARα fusion gene. A relapsed APL patient had shown resistance to ATO and chemotherapy and was identified to harbor a point mutation (A216V) in the unrearranged PML allele rather than the PML::RARα fusion gene. Here, we report that mutations in the unrearranged PML allele impede the ATO-induced destabilization and degradation of the wild-type P/R oncoprotein. Deletion of the coiled-coil domain in a PML mutant completely reversed wild-type P/R protein resistance to ATO by abolishing the interaction between PML and P/R proteins. Collectively, our findings reveal that a point mutation in the unrearranged PML allele can confer ATO resistance through a protein-protein interaction. Therefore, the unrearranged PML allele should also be screened for drug-resistant mutations in relapsed APL patients.

Induced proximity to PML protects TDP-43 from aggregation via SUMO-ubiquitin networks

The established role of cytosolic and nuclear inclusions of TDP-43 in the pathogenesis of neurodegenerative disorders has multiplied efforts to understand mechanisms that control TDP-43 aggregation and has spurred searches for approaches limiting this process. Formation and clearance of TDP-43 aggregates are controlled by an intricate interplay of cellular proteostasis systems that involve post-translational modifications and frequently rely on spatial control. We demonstrate that attachment of the ubiquitin-like SUMO2 modifier compartmentalizes TDP-43 in promyelocytic leukemia protein (PML) nuclear bodies and limits the aggregation of TDP-43 in response to proteotoxic stress. Exploiting this pathway through proximity-inducing recruitment of TDP-43 to PML triggers a SUMOylation-ubiquitylation cascade protecting TDP-43 from stress-induced insolubility. The protective function of PML is mediated by ubiquitylation in conjunction with the p97 disaggregase. Altogether, we demonstrate that SUMO-ubiquitin networks protect cells from insoluble TDP-43 inclusions and propose the functionalization of PML as a potential future therapeutic avenue countering aggregation.

Identification of RING E3 pseudoligases in the TRIM protein family

TRIpartite Motif (TRIM) family proteins have diverse roles across a broad variety of cellular functions, which are largely presumed to depend on their ubiquitin E3 ligase activity, conferred by a RING domain. However, recent reports have shown that some TRIMs lack detectable ubiquitination activity in isolation, despite containing a RING domain. Here, we present parallel in cellulo, in vitro, and in silico structure-function analyses of the ubiquitin E3 ligase activity and RING domain structural characteristics of the whole TRIM protein family. In-depth follow-up studies of this comprehensive dataset reveals a number of 'pseudoligases', whose RING domains have structurally diverged at either the homodimerisation or E2~ubiquitin interfaces, thereby disrupting their ability to catalyse ubiquitin transfer. Together, these data raise intriguing open questions regarding the unknown TRIM functions in physiology and disease.

PML Nuclear bodies: the cancer connection and beyond

Promyelocytic leukemia (PML) nuclear bodies, membrane-less organelles in the nucleus, play a crucial role in cellular homeostasis. These dynamic structures result from the assembly of scaffolding PML proteins and various partners. Recent crystal structure analyses revealed essential self-interacting domains, while liquid-liquid phase separation contributes to their formation. PML bodies orchestrate post-translational modifications, particularly stress-induced SUMOylation, impacting target protein functions. Serving as hubs in multiple signaling pathways, they influence cellular processes like senescence. Dysregulation of PML expression contributes to diseases, including cancer, highlighting their significance. Therapeutically, PML bodies are promising targets, exemplified by successful acute promyelocytic leukemia treatment with arsenic trioxide and retinoic acid restoring PML bodies. Understanding their functions illuminates both normal and pathological cellular physiology, guiding potential therapies. This review explores recent advancements in PML body biogenesis, biochemical activity, and their evolving biological roles.

Cryo-EM structure of PML RBCC dimer reveals CC-mediated octopus-like nuclear body assembly mechanism

Promyelocytic leukemia protein (PML) nuclear bodies (NBs) are essential in regulating tumor suppression, antiviral response, inflammation, metabolism, aging, and other important life processes. The re-assembly of PML NBs might lead to an ~100% cure of acute promyelocytic leukemia. However, until now, the molecular mechanism underpinning PML NB biogenesis remains elusive due to the lack of structural information. In this study, we present the cryo-electron microscopy (cryo-EM) structure of the PML dimer at an overall resolution of 5.3 Å, encompassing the RING, B-box1/2 and part of the coiled-coil (RBCC) domains. The integrated approach, combining crosslinking and mass spectrometry (XL-MS) and functional analyses, enabled us to observe a unique folding event within the RBCC domains. The RING and B-box1/2 domains fold around the α3 helix, and the α6 helix serves as a pivotal interface for PML dimerization. More importantly, further characterizations of the cryo-EM structure in conjugation with AlphaFold2 prediction, XL-MS, and NB formation assays, help unveil an unprecedented octopus-like mechanism in NB assembly, wherein each CC helix of a PML dimer (PML dimer A) interacts with a CC helix from a neighboring PML dimer (PML dimer B) in an anti-parallel configuration, ultimately leading to the formation of a 2 µm membrane-less subcellular organelle.

The landscape of lncRNAs in cell granules: Insights into their significance in cancer

Cellular compartmentalization, achieved through membrane-based compartments, is a fundamental aspect of cell biology that contributes to the evolutionary success of cells. While organelles have traditionally been the focus of research, membrane-less organelles (MLOs) are emerging as critical players, exhibiting distinct morphological features and unique molecular compositions. Recent research highlights the pivotal role of long noncoding RNAs (lncRNAs) in MLOs and their involvement in various cellular processes across different organisms. In the context of cancer, dysregulation of MLO formation, influenced by altered lncRNA expression, impacts chromatin organization, oncogenic transcription, signaling pathways, and telomere lengthening. This review synthesizes the current understanding of lncRNA composition within MLOs, delineating their functions and exploring how their dysregulation contributes to human cancers. Environmental challenges in tumorigenesis, such as nutrient deprivation and hypoxia, induce stress granules, promoting cancer cell survival and progression. Advancements in biochemical techniques, particularly single RNA imaging methods, offer valuable tools for studying RNA functions within live cells. However, detecting low-abundance lncRNAs remains challenging due to their limited expression levels. The correlation between lncRNA expression and pathological conditions, particularly cancer, should be explored, emphasizing the importance of single-cell studies for precise biomarker identification and the development of personalized therapeutic strategies. This article is categorized under: RNA Export and Localization > RNA Localization RNA in Disease and Development > RNA in Disease RNA Interactions with Proteins and Other Molecules > RNA-Protein Complexes.

History of Developing Acute Promyelocytic Leukemia Treatment and Role of Promyelocytic Leukemia Bodies

The story of acute promyelocytic leukemia (APL) discovery, physiopathology, and treatment is a unique journey, transforming the most aggressive form of leukemia to the most curable. It followed an empirical route fueled by clinical breakthroughs driving major advances in biochemistry and cell biology, including the discovery of PML nuclear bodies (PML NBs) and their central role in APL physiopathology. Beyond APL, PML NBs have emerged as key players in a wide variety of biological functions, including tumor-suppression and SUMO-initiated protein degradation, underscoring their broad importance. The APL story is an example of how clinical observations led to the incremental development of the first targeted leukemia therapy. The understanding of APL pathogenesis and the basis for cure now opens new insights in the treatment of other diseases, especially other acute myeloid leukemias.

Function of PML-RARA in Acute Promyelocytic Leukemia

The transformation of acute promyelocytic leukemia (APL) from the most fatal to the most curable subtype of acute myeloid leukemia (AML), with long-term survival exceeding 90%, has represented one of the most exciting successes in hematology and in oncology. APL is a paradigm for oncoprotein-targeted cure.APL is caused by a 15/17 chromosomal translocation which generates the PML-RARA fusion protein and can be cured by the chemotherapy-free approach based on the combination of two therapies targeting PML-RARA: retinoic acid (RA) and arsenic. PML-RARA is the key driver of APL and acts by deregulating transcriptional control, particularly RAR targets involved in self-renewal or myeloid differentiation, also disrupting PML nuclear bodies. PML-RARA mainly acts as a modulator of the expression of specific target genes: genes whose regulatory elements recruit PML-RARA are not uniformly repressed but also may be upregulated or remain unchanged. RA and arsenic trioxide directly target PML-RARA-mediated transcriptional deregulation and protein stability, removing the differentiation block at promyelocytic stage and inducing clinical remission of APL patients.

On the Prevalence and Roles of Proteins Undergoing Liquid-Liquid Phase Separation in the Biogenesis of PML-Bodies

The formation and function of membrane-less organelles (MLOs) is one of the main driving forces in the molecular life of the cell. These processes are based on the separation of biopolymers into phases regulated by multiple specific and nonspecific inter- and intramolecular interactions. Among the realm of MLOs, a special place is taken by the promyelocytic leukemia nuclear bodies (PML-NBs or PML bodies), which are the intranuclear compartments involved in the regulation of cellular metabolism, transcription, the maintenance of genome stability, responses to viral infection, apoptosis, and tumor suppression. According to the accepted models, specific interactions, such as SUMO/SIM, the formation of disulfide bonds, etc., play a decisive role in the biogenesis of PML bodies. In this work, a number of bioinformatics approaches were used to study proteins found in the proteome of PML bodies for their tendency for spontaneous liquid-liquid phase separation (LLPS), which is usually caused by weak nonspecific interactions. A total of 205 proteins found in PML bodies have been identified. It has been suggested that UBC9, P53, HIPK2, and SUMO1 can be considered as the scaffold proteins of PML bodies. It was shown that more than half of the proteins in the analyzed proteome are capable of spontaneous LLPS, with 85% of the analyzed proteins being intrinsically disordered proteins (IDPs) and the remaining 15% being proteins with intrinsically disordered protein regions (IDPRs). About 44% of all proteins analyzed in this study contain SUMO binding sites and can potentially be SUMOylated. These data suggest that weak nonspecific interactions play a significantly larger role in the formation and biogenesis of PML bodies than previously expected.

Viruses and Cajal Bodies: A Critical Cellular Target in Virus Infection?

Nuclear bodies (NBs) are dynamic structures present in eukaryotic cell nuclei. They are not bounded by membranes and are often considered biomolecular condensates, defined structurally and functionally by the localisation of core components. Nuclear architecture can be reorganised during normal cellular processes such as the cell cycle as well as in response to cellular stress. Many plant and animal viruses target their proteins to NBs, in some cases triggering their structural disruption and redistribution. Although not all such interactions have been well characterised, subversion of NBs and their functions may form a key part of the life cycle of eukaryotic viruses that require the nucleus for their replication. This review will focus on Cajal bodies (CBs) and the viruses that target them. Since CBs are dynamic structures, other NBs (principally nucleoli and promyelocytic leukaemia, PML and bodies), whose components interact with CBs, will also be considered. As well as providing important insights into key virus-host cell interactions, studies on Cajal and associated NBs may identify novel cellular targets for development of antiviral compounds.

PML Body Biogenesis: A Delicate Balance of Interactions

PML bodies are subnuclear protein complexes that play a crucial role in various physiological and pathological cellular processes. One of the general structural proteins of PML bodies is a member of the tripartite motif (TRIM) family-promyelocytic leukemia protein (PML). It is known that PML interacts with over a hundred partners, and the protein itself is represented by several major isoforms, differing in their variable and disordered C-terminal end due to alternative splicing. Despite nearly 30 years of research, the mechanisms underlying PML body formation and the role of PML proteins in this process remain largely unclear. In this review, we examine the literature and highlight recent progress in this field, with a particular focus on understanding the role of individual domains of the PML protein, its post-translational modifications, and polyvalent nonspecific interactions in the formation of PML bodies. Additionally, based on the available literature, we propose a new hypothetical model of PML body formation.

Structure and activation of the RING E3 ubiquitin ligase TRIM72 on the membrane

Defects in plasma membrane repair can lead to muscle and heart diseases in humans. Tripartite motif-containing protein (TRIM)72 (mitsugumin 53; MG53) has been determined to rapidly nucleate vesicles at the site of membrane damage, but the underlying molecular mechanisms remain poorly understood. Here we present the structure of Mus musculus TRIM72, a complete model of a TRIM E3 ubiquitin ligase. We demonstrated that the interaction between TRIM72 and phosphatidylserine-enriched membranes is necessary for its oligomeric assembly and ubiquitination activity. Using cryogenic electron tomography and subtomogram averaging, we elucidated a higher-order model of TRIM72 assembly on the phospholipid bilayer. Combining structural and biochemical techniques, we developed a working molecular model of TRIM72, providing insights into the regulation of RING-type E3 ligases through the cooperation of multiple domains in higher-order assemblies. Our findings establish a fundamental basis for the study of TRIM E3 ligases and have therapeutic implications for diseases associated with membrane repair.

Conformational exchange at a C2H2 zinc-binding site facilitates redox sensing by the PML protein

The promyelocytic leukemia protein, PML, plays a vital role in the cellular response to oxidative stress; however, the molecular mechanism of its action remains poorly understood. Here, we identify redox-sensitive sites of PML. A molecule of PML is cysteine-rich and contains three zinc-binding domains including RING, B-box1, and B-box2. Using in vitro assays, we have compared the sensitivity of the isolated RING and B-box1 domains and shown that B-box1 is more sensitive to oxidation. NMR studies of PML dynamics showed that one of the Zn-coordination sites within the B-box1 undergoes significant conformational exchange, revealing a hotspot for exposure of reactive cysteines. In agreement with the in vitro data, enhancement of the B-box1 Zn-coordination dynamics led to more efficient recruitment of PML into PML nuclear bodies in cells. Overall, our results suggest that the increased sensitivity of B-box1 to oxidative stress makes this domain an important redox-sensing component of PML.

Tripartite Motif Protein Family in Central Nervous System Diseases

Tripartite motif (TRIM) protein superfamily is a group of E3 ubiquitin ligases characterized by the conserved RING domain, the B-box domain, and the coiled-coil domain (RBCC). It is widely involved in various physiological and pathological processes, such as intracellular signal transduction, cell cycle regulation, oncogenesis, and innate immune response. Central nervous system (CNS) diseases are composed of encephalopathy and spinal cord diseases, which have a high disability and mortality rate. Patients are often unable to take care of themselves and their life quality can be seriously declined. Initially, the function research of TRIM proteins mainly focused on cancer. However, in recent years, accumulating attention is paid to the roles they play in CNS diseases. In this review, we integrate the reported roles of TRIM proteins in the pathological process of CNS diseases and related signaling pathways, hoping to provide theoretical bases for further research in treating CNS diseases targeting TRIM proteins. TRIM proteins participated in CNS diseases. TRIM protein family is characterized by a highly conserved RBCC domain, referring to the RING domain, the B-box domain, and the coiled-coil domain. Recent research has discovered the relations between TRIM proteins and various CNS diseases, especially Alzheimer's disease, Parkinson's disease, and ischemic stroke.

The interactions between PML nuclear bodies and small and medium size DNA viruses

Promyelocytic leukemia nuclear bodies (PM NBs), often referred to as membraneless organelles, are dynamic macromolecular protein complexes composed of a PML protein core and other transient or permanent components. PML NBs have been shown to play a role in a wide variety of cellular processes. This review describes in detail the diverse and complex interactions between small and medium size DNA viruses and PML NBs that have been described to date. The PML NB components that interact with small and medium size DNA viruses include PML protein isoforms, ATRX/Daxx, Sp100, Sp110, HP1, and p53, among others. Interaction between viruses and components of these NBs can result in different outcomes, such as influencing viral genome expression and/or replication or impacting IFN-mediated or apoptotic cell responses to viral infection. We discuss how PML NB components abrogate the ability of adenoviruses or Hepatitis B virus to transcribe and/or replicate their genomes and how papillomaviruses use PML NBs and their components to promote their propagation. Interactions between polyomaviruses and PML NBs that are poorly understood but nevertheless suggest that the NBs can serve as scaffolds for viral replication or assembly are also presented. Furthermore, complex interactions between the HBx protein of hepadnaviruses and several PML NBs-associated proteins are also described. Finally, current but scarce information regarding the interactions of VP3/apoptin of the avian anellovirus with PML NBs is provided. Despite the considerable number of studies that have investigated the functions of the PML NBs in the context of viral infection, gaps in our understanding of the fine interactions between viruses and the very dynamic PML NBs remain. The complexity of the bodies is undoubtedly a great challenge that needs to be further addressed.

A Novel Recognition by the E3 Ubiquitin Ligase of HSV-1 ICP0 Enhances the Degradation of PML Isoform I to Prevent ND10 Reformation in Late Infection

Upon viral entry, components of ND10 nuclear bodies converge with incoming DNA to repress viral expression. The infected cell protein 0 (ICP0) of herpes simplex virus 1 (HSV-1) contains a RING-type E3 ubiquitin ligase that targets the ND10 organizer, PML, for proteasomal degradation. Consequently, ND10 components are dispersed and viral genes are activated. Previously, we reported that ICP0 E3 differentiates two similar substrates, PML isoforms I and II, and demonstrated that SUMO-interaction has profound regulatory effects on PML II degradation. In the present study, we investigated elements that regulate the PML I degradation and found that: (i) two regions of ICP0 flanking the RING redundantly facilitate the degradation of PML I; (ii) downstream of the RING, the SUMO-interaction motif located at residues 362-364 (SIM_362-364) targets the SUMOylated PML I in the same manner as that of PML II; (iii) upstream of the RING, the N-terminal residues 1-83 mediate PML I degradation regardless of its SUMOylation status or subcellular localization; (iv) the reposition of residues 1-83 to downstream of the RING does not affect its function in PML I degradation; and (v) the deletion of 1-83 allows the resurgence of PML I and reformation of ND10-like structures late in HSV-1 infection. Taken together, we identified a novel substrate recognition specific for PML I, by which ICP0 E3 enforces a continuous PML I degradation throughout the infection to prevent the ND10 reformation.

Structural basis for TRIM72 oligomerization during membrane damage repair

Tripartite Motif Protein 72 (TRIM72, also named MG53) mediates membrane damage repair through membrane fusion and exocytosis. During injury, TRIM72 molecules form intermolecular disulfide bonds in response to the oxidative environment and TRIM72 oligomers are proposed to connect vesicles to the plasma membrane and promote membrane fusion in conjunction with other partners like dysferlin and caveolin. However, the detailed mechanism of TRIM72 oligomerization and action remains unclear. Here we present the crystal structure of TRIM72 B-box-coiled-coil-SPRY domains (BCC-SPRY), revealing the molecular basis of TRIM72 oligomerization, which is closely linked to disulfide bond formation. Through structure-guided mutagenesis, we have identified and characterized key residues that are important for the membrane repair function of TRIM72. Our results also demonstrate that TRIM72 interacts with several kinds of negatively charged lipids in addition to phosphatidylserine. Our work provides a structural foundation for further mechanistic studies as well as the clinical application of TRIM72.

Recruitment of TRIM33 to cell-context specific PML nuclear bodies regulates nodal signaling in mESCs

TRIM33 is a chromatin reader required for mammalian mesendoderm differentiation after activation of Nodal signaling, while its role in mESCs is still elusive. Here, we report that TRIM33 co-localizes with promyelocytic leukemia nuclear bodies (PML-NBs) specifically in mESCs, to mediate Nodal signaling-directed transcription of Lefty1/2. We show that TRIM33 puncta formation in mESCs depends on PML and on specific assembly of PML-NBs. Moreover, TRIM33 and PML co-regulate Lefty1/2 expression in mESCs, with both PML protein and formation of mESCs-specific PML-NBs being required for TRIM33 recruitment to these loci, and PML-NBs directly associating with the Lefty1/2 loci. Finally, a TurboID proximity-labeling experiment confirmed that TRIM33 is highly enriched only in mESCs-specific PML-NBs. Thus, our study supports a model in which TRIM33 condensates regulate Nodal signaling-directed transcription in mESCs and shows that PML-NBs can recruit distinct sets of client proteins in a cell-context-dependent manner.

Regulating the p53 Tumor Suppressor Network at PML Biomolecular Condensates

By forming specific functional entities, nuclear biomolecular condensates play an important function in guiding biological processes. PML biomolecular condensates, also known as PML nuclear bodies (NBs), are macro-molecular sub-nuclear organelles involved in central biological processes, including anti-viral response and cell fate control upon genotoxic stress. PML condensate formation is stimulated upon cellular stress, and relies on protein-protein interactions establishing a PML protein meshwork capable of recruiting the tumor suppressor p53, along with numerous modifiers of p53, thus balancing p53 posttranslational modifications and activity. This stress-regulated process appears to be controlled by liquid-liquid phase separation (LLPS), which may facilitate regulated protein-unmixing of p53 and its regulators into PML nuclear condensates. In this review, we summarize and discuss the molecular mechanisms underlying PML nuclear condensate formation, and how these impact the biological function of p53 in driving the cell death and senescence responses. In addition, by using an in silico approach, we identify 299 proteins which share PML and p53 as binding partners, thus representing novel candidate proteins controlling p53 function and cell fate decision-making at the level of PML nuclear biocondensates.

TRIM family proteins: roles in proteostasis and neurodegenerative diseases

Neurodegenerative diseases (NDs) are a diverse group of disorders characterized by the progressive degeneration of the structure and function of the central or peripheral nervous systems. One of the major features of NDs, such as Alzheimer's disease (AD), Parkinson's disease (PD) and Huntington's disease (HD), is the aggregation of specific misfolded proteins, which induces cellular dysfunction, neuronal death, loss of synaptic connections and eventually brain damage. By far, a great amount of evidence has suggested that TRIM family proteins play crucial roles in the turnover of normal regulatory and misfolded proteins. To maintain cellular protein quality control, cells rely on two major classes of proteostasis: molecular chaperones and the degradative systems, the latter includes the ubiquitin-proteasome system (UPS) and autophagy; and their dysfunction has been established to result in various physiological disorders including NDs. Emerging evidence has shown that TRIM proteins are key players in facilitating the clearance of misfolded protein aggregates associated with neurodegenerative disorders. Understanding the different pathways these TRIM proteins employ during episodes of neurodegenerative disorder represents a promising therapeutic target. In this review, we elucidated and summarized the diverse roles with underlying mechanisms of members of the TRIM family proteins in NDs.

Effects of arsenic on the topology and solubility of promyelocytic leukemia (PML)-nuclear bodies

Promyelocytic leukemia (PML) proteins are involved in the pathogenesis of acute promyelocytic leukemia (APL). Trivalent arsenic (As3+) is known to cure APL by binding to cysteine residues of PML and enhance the degradation of PML-retinoic acid receptor α (RARα), a t(15;17) gene translocation product in APL cells, and restore PML-nuclear bodies (NBs). The size, number, and shape of PML-NBs vary among cell types and during cell division. However, topological changes of PML-NBs in As3+-exposed cells have not been well-documented. We report that As3+-induced solubility shift underlies rapid SUMOylation of PML and late agglomeration of PML-NBs. Most PML-NBs were toroidal and granular dot-like in GFPPML-transduced CHO-K1 and HEK293 cells, respectively. Exposure to As3+ and antimony (Sb3+) greatly reduced the solubility of PML and enhanced SUMOylation within 2 h in the absence of changes in the number and size of PML-NBs. However, the prolonged exposure to As3+ and Sb3+ resulted in agglomeration of PML-NBs. Exposure to bismuth (Bi3+), another Group 15 element, did not induce any of these changes. ML792, a SUMO activation inhibitor, reduced the number of PML-NBs and increased the size of the NBs, but had little effect on the As3+-induced solubility change of PML. These results warrant the importance of As3+- or Sb3+-induced solubility shift of PML for the regulation intranuclear dynamics of PML-NBs.

Liquid-liquid phase separation as an organizing principle of intracellular space: overview of the evolution of the cell compartmentalization concept

At the turn of the twenty-first century, fundamental changes took place in the understanding of the structure and function of proteins and then in the appreciation of the intracellular space organization. A rather mechanistic model of the organization of living matter, where the function of proteins is determined by their rigid globular structure, and the intracellular processes occur in rigidly determined compartments, was replaced by an idea that highly dynamic and multifunctional "soft matter" lies at the heart of all living things. According this "new view", the most important role in the spatio-temporal organization of the intracellular space is played by liquid-liquid phase transitions of biopolymers. These self-organizing cellular compartments are open dynamic systems existing at the edge of chaos. They are characterized by the exceptional structural and compositional dynamics, and their multicomponent nature and polyfunctionality provide means for the finely tuned regulation of various intracellular processes. Changes in the external conditions can cause a disruption of the biogenesis of these cellular bodies leading to the irreversible aggregation of their constituent proteins, followed by the transition to a gel-like state and the emergence of amyloid fibrils. This work represents a historical overview of changes in our understanding of the intracellular space compartmentalization. It also reflects methodological breakthroughs that led to a change in paradigms in this area of science and discusses modern ideas about the organization of the intracellular space. It is emphasized here that the membrane-less organelles have to combine a certain resistance to the changes in their environment and, at the same time, show high sensitivity to the external signals, which ensures the normal functioning of the cell.

Sumoylation in Physiology, Pathology and Therapy

Sumoylation is an essential post-translational modification that has evolved to regulate intricate networks within emerging complexities of eukaryotic cells. Thousands of target substrates are modified by SUMO peptides, leading to changes in protein function, stability or localization, often by modulating interactions. At the cellular level, sumoylation functions as a key regulator of transcription, nuclear integrity, proliferation, senescence, lineage commitment and stemness. A growing number of prokaryotic and viral proteins are also emerging as prime sumoylation targets, highlighting the role of this modification during infection and in immune processes. Sumoylation also oversees epigenetic processes. Accordingly, at the physiological level, it acts as a crucial regulator of development. Yet, perhaps the most prominent function of sumoylation, from mammals to plants, is its role in orchestrating organismal responses to environmental stresses ranging from hypoxia to nutrient stress. Consequently, a growing list of pathological conditions, including cancer and neurodegeneration, have now been unambiguously associated with either aberrant sumoylation of specific proteins and/or dysregulated global cellular sumoylation. Therapeutic enforcement of sumoylation can also accomplish remarkable clinical responses in various diseases, notably acute promyelocytic leukemia (APL). In this review, we will discuss how this modification is emerging as a novel drug target, highlighting from the perspective of translational medicine, its potential and limitations.

New Evidence of the Importance of Weak Interactions in the Formation of PML-Bodies

In this work, we performed a comparative study of the formation of PML bodies by full-length PML isoforms and their C-terminal domains in the presence and absence of endogenous PML. Based on the analysis of the distribution of intrinsic disorder predisposition in the amino acid sequences of PML isoforms, regions starting from the amino acid residue 395 (i.e., sequences encoded by exons 4–6) were assigned as the C-terminal domains of these proteins. We demonstrate that each of the full-sized nuclear isoforms of PML is capable of forming nuclear liquid-droplet compartments in the absence of other PML isoforms. These droplets possess dynamic characteristics of the exchange with the nucleoplasm close to those observed in the wild-type cells. Only the C-terminal domains of the PML-II and PML-V isoforms are able to be included in the composition of the endogenous PML bodies, while being partially distributed in the nucleoplasm. The bodies formed by the C-terminal domain of the PML-II isoform are dynamic liquid droplet compartments, regardless of the presence or absence of endogenous PML. The C-terminal domain of PML-V forms dynamic liquid droplet compartments in the knockout cells (PML−/−), but when the C-terminus of the PML-V isoform is inserted into the existing endogenous PML bodies, the molecules of this protein cease to exchange with the nucleoplasm. It was demonstrated that the K490R substitution, which disrupts the PML sumoylation, promotes diffuse distribution of the C-terminal domains of PML-II and PML-V isoforms in endogenous PML knockout HeLa cells, but not in the wild-type cells. These data indicate the ability of the C-terminal domains of the PML-II and PML-V isoforms to form dynamic liquid droplet-like compartments, regardless of the ordered N-terminal RBCC motifs of the PML. This indicates a significant role of the non-specific interactions between the mostly disordered C-terminal domains of PML isoforms for the initiation of liquid–liquid phase separation (LLPS) leading to the formation of PML bodies.

SUMOylation regulates the number and size of promyelocytic leukemia-nuclear bodies (PML-NBs) and arsenic perturbs SUMO dynamics on PML by insolubilizing PML in THP-1 cells

The functional roles of protein modification by small ubiquitin-like modifier (SUMO) proteins are not well understood compared to ubiquitination. Promyelocytic leukemia (PML) proteins are good substrates for SUMOylation, and PML-nuclear bodies (PML-NBs) may function as a platform for the PML SUMOylation. PML proteins are rapidly modified both with SUMO2/3 and SUMO1 after exposure to arsenite (As³⁺) and SUMOylated PML are further ubiquitinated and degraded by proteasomes. However, effects of As³⁺ on SUMO dynamics on PML-NBs are not well investigated. In the present study, we report that (1) the number and size of PML-NBs were regulated by SUMO E1-activating enzyme, (2) SUMO2/3 co-localized with PML irrespective of As³⁺ exposure and was restricted to PML-nuclear bodies (PML-NBs) via covalent binding in response to As³⁺, and (3) As³⁺-induced biochemical changes in PML were not modulated by ubiquitin-proteasome system (UPS) in THP-1 cells. Undifferentiated and differentiated THP-1 cells responded to As³⁺ similarly and PML proteins were changed from the detergent soluble to the insoluble form and further SUMOylated with SUMO2/3 and SUMO1. ML792, a SUMO E1 inhibitor, decreased the number of PML-NBs and reciprocally increased the size irrespective of exposure to As³⁺, which itself slightly decrease both the number and size of PML-NBs. TAK243, a ubiquitin E1 inhibitor, did not change the PML-NBs, while SUMOylated proteins accumulated in the TAK243-exposed cells. Proteasome inhibitors did not change the As³⁺-induced SUMOylation levels of PML. Co-localization and further restriction of SUMO2/3 to PML-NBs were confirmed by PML-transfected CHO-K1 cells. Collectively, SUMOylation regulates PML-NBs and As³⁺ restricts SUMO dynamics on PML by changing its solubility.

Differential Interactome Based Drug Repositioning Unraveled Abacavir, Exemestane, Nortriptyline Hydrochloride, and Tolcapone as Potential Therapeutics for Colorectal Cancers

There is a critical requirement for alternative strategies to provide the better treatment in colorectal cancer (CRC). Hence, our goal was to propose novel biomarkers as well as drug candidates for its treatment through differential interactome based drug repositioning. Differentially interacting proteins and their modules were identified, and their prognostic power were estimated through survival analyses. Drug repositioning was carried out for significant target proteins, and candidate drugs were analyzed via in silico molecular docking prior to in vitro cell viability assays in CRC cell lines. Six modules (mAPEX1, mCCT7, mHSD17B10, mMYC, mPSMB5, mRAN) were highlighted considering their prognostic performance. Drug repositioning resulted in eight drugs (abacavir, ribociclib, exemestane, voriconazole, nortriptyline hydrochloride, theophylline, bromocriptine mesylate, and tolcapone). Moreover, significant in vitro inhibition profiles were obtained in abacavir, nortriptyline hydrochloride, exemestane, tolcapone, and theophylline (positive control). Our findings may provide new and complementary strategies for the treatment of CRC.

TRIM21/Ro52 - Roles in Innate Immunity and Autoimmune Disease

TRIM21 (Ro52/SSA1) is an E3 ubiquitin ligase with key roles in immune host defence, signal transduction, and possibly cell cycle regulation. It is also an autoantibody target in Sjögren's syndrome, systemic lupus erythematosus, and other rheumatic autoimmune diseases. Here, we summarise the structure and function of this enzyme, its roles in innate immunity, adaptive immunity and cellular homeostasis, the pathogenesis of autoimmunity against TRIM21, and the potential impacts of autoantibodies to this intracellular protein.

Does it take two to tango? RING domain self-association and activity in TRIM E3 ubiquitin ligases

TRIM proteins form a protein family that is characterized by a conserved tripartite motif domain comprising a RING domain, one or two B-box domains and a coiled-coil region. Members of this large protein family are important regulators of numerous cellular functions including innate immune responses, transcriptional regulation and apoptosis. Key to their cellular role is their E3 ligase activity which is conferred by the RING domain. Self-association is an important characteristic of TRIM protein activity and is mediated by homodimerization via the coiled-coil region, and in some cases higher order association via additional domains of the tripartite motif. In many of the TRIM family proteins studied thus far, RING dimerization is an important prerequisite for E3 ligase enzymatic activity though the propensity of RING domains to dimerize differs significantly between different TRIMs and can be influenced by other regions of the protein.

Targeting TRIM Proteins: A Quest towards Drugging an Emerging Protein Class

The ubiquitylation machinery regulates several fundamental biological processes from protein homeostasis to a wide variety of cellular signaling pathways. As a consequence, its dysregulation is linked to diseases including cancer, neurodegeneration, and autoimmunity. With this review, we aim to highlight the therapeutic potential of targeting E3 ligases, with a special focus on an emerging class of RING ligases, named tri-partite motif (TRIM) proteins, whose role as targets for drug development is currently gaining pharmaceutical attention. TRIM proteins exert their catalytic activity as scaffolds involved in many protein-protein interactions, whose multidomains and adapter-like nature make their druggability very challenging. Herein, we give an overview of the current understanding of this class of single polypeptide RING E3 ligases and discuss potential targeting options.

The Role of Non-Specific Interactions in Canonical and ALT-Associated PML-Bodies Formation and Dynamics

In this work, we put forward a hypothesis about the decisive role of multivalent nonspecific interactions in the early stages of PML body formation. Our analysis of the PML isoform sequences showed that some of the PML isoforms, primarily PML-II, are prone to phase separation due to their polyampholytic properties and the disordered structure of their C-terminal domains. The similarity of the charge properties of the C-terminal domains of PML-II and PML-VI isoforms made it possible for the first time to detect migration of PML-VI from PML bodies to the periphery of the cell nucleus, similar to the migration of PML-II isoforms. We found a population of “small” (area less than 1 µm²) spherical PML bodies with high dynamics of PML isoforms exchange with nucleoplasm and a low fraction of immobilized proteins, which indicates their liquid state properties. Such structures can act as “seeds” of functionally active PML bodies, providing the necessary concentration of PML isoforms for the formation of intermolecular disulfide bonds between PML monomers. FRAP analysis of larger bodies of toroidal topology showed the existence of an insoluble scaffold in their structure. The hypothesis about the role of nonspecific multiple weak interactions in the formation of PML bodies is further supported by the change in the composition of the scaffold proteins of PML bodies, but not their solidification, under conditions of induction of dimerization of PML isoforms under oxidative stress. Using the colocalization of ALT-associated PML bodies (APBs) with TRF1, we identified APBs and showed the difference in the dynamic properties of APBs and canonical PML bodies.

SUMO-Targeted Ubiquitin Ligases and Their Functions in Maintaining Genome Stability

Small ubiquitin-like modifier (SUMO)-targeted E3 ubiquitin ligases (STUbLs) are specialized enzymes that recognize SUMOylated proteins and attach ubiquitin to them. They therefore connect the cellular SUMOylation and ubiquitination circuits. STUbLs participate in diverse molecular processes that span cell cycle regulated events, including DNA repair, replication, mitosis, and transcription. They operate during unperturbed conditions and in response to challenges, such as genotoxic stress. These E3 ubiquitin ligases modify their target substrates by catalyzing ubiquitin chains that form different linkages, resulting in proteolytic or non-proteolytic outcomes. Often, STUbLs function in compartmentalized environments, such as the nuclear envelope or kinetochore, and actively aid in nuclear relocalization of damaged DNA and stalled replication forks to promote DNA repair or fork restart. Furthermore, STUbLs reside in the same vicinity as SUMO proteases and deubiquitinases (DUBs), providing spatiotemporal control of their targets. In this review, we focus on the molecular mechanisms by which STUbLs help to maintain genome stability across different species.

To Ubiquitinate or Not to Ubiquitinate: TRIM17 in Cell Life and Death

TRIM17 is a member of the TRIM family, a large class of RING-containing E3 ubiquitin-ligases. It is expressed at low levels in adult tissues, except in testis and in some brain regions. However, it can be highly induced in stress conditions which makes it a putative stress sensor required for the triggering of key cellular responses. As most TRIM members, TRIM17 can act as an E3 ubiquitin-ligase and promote the degradation by the proteasome of substrates such as the antiapoptotic protein MCL1. Intriguingly, TRIM17 can also prevent the ubiquitination of other proteins and stabilize them, by binding to other TRIM proteins and inhibiting their E3 ubiquitin-ligase activity. This duality of action confers several pivotal roles to TRIM17 in crucial cellular processes such as apoptosis, autophagy or cell division, but also in pathological conditions as diverse as Parkinson's disease or cancer. Here, in addition to recent data that endorse this duality, we review what is currently known from public databases and the literature about TRIM17 gene regulation and expression, TRIM17 protein structure and interactions, as well as its involvement in cell physiology and human disorders.

Interplay between RNA Viruses and Promyelocytic Leukemia Nuclear Bodies

Promyelocytic leukemia nuclear bodies (PML NBs) are nuclear membrane-less sub structures that play a critical role in diverse cellular pathways including cell proliferation, DNA damage, apoptosis, transcriptional regulation, stem cell renewal, alternative lengthening of telomeres, chromatin organization, epigenetic regulation, protein turnover, autophagy, intrinsic and innate antiviral immunity. While intrinsic and innate immune functions of PML NBs or PML NB core proteins are well defined in the context of nuclear replicating DNA viruses, several studies also confirm their substantial roles in the context of RNA viruses. In the present review, antiviral activities of PML NBs or its core proteins on diverse RNA viruses that replicate in cytoplasm or the nucleus were discussed. In addition, viral counter mechanisms that reorganize PML NBs, and specifically how viruses usurp PML NB functions in order to create a cellular environment favorable for replication and pathogenesis, are also discussed.

A PML/RARα direct target atlas redefines transcriptional deregulation in acute promyelocytic leukemia

Transcriptional deregulation initiated by oncogenic fusion proteins plays a vital role in leukemia. The prevailing view is that the oncogenic fusion protein promyelocytic leukemia/retinoic acid receptor-α (PML/RARα), generated by the chromosome translocation t(15;17), functions as a transcriptional repressor in acute promyelocytic leukemia (APL). Here, we provide rich evidence of how PML/RARα drives oncogenesis through both repressive and activating functions, particularly the importance of the newly identified activation role for the leukemogenesis of APL. The activating function of PML/RARα is achieved by recruiting both abundant P300 and HDAC1 and by the formation of super-enhancers. All-trans retinoic acid and arsenic trioxide, 2 widely used drugs in APL therapy, exert synergistic effects on controlling super-enhancer-associated PML/RARα-regulated targets in APL cells. We use a series of in vitro and in vivo experiments to demonstrate that PML/RARα-activated target gene GFI1 is necessary for the maintenance of APL cells and that PML/RARα, likely oligomerized, transactivates GFI1 through chromatin conformation at the super-enhancer region. Finally, we profile GFI1 targets and reveal the interplay between GFI1 and PML/RARα on chromatin in coregulating target genes. Our study provides genomic insight into the dual role of fusion transcription factors in transcriptional deregulation to drive leukemia development, highlighting the importance of globally dissecting regulatory circuits.

The Role of ND10 Nuclear Bodies in Herpesvirus Infection: A Frenemy for the Virus?

Nuclear domains 10 (ND10), a.k.a. promyelocytic leukemia nuclear bodies (PML-NBs), are membraneless subnuclear domains that are highly dynamic in their protein composition in response to cellular cues. They are known to be involved in many key cellular processes including DNA damage response, transcription regulation, apoptosis, oncogenesis, and antiviral defenses. The diversity and dynamics of ND10 residents enable them to play seemingly opposite roles under different physiological conditions. Although the molecular mechanisms are not completely clear, the pro- and anti-cancer effects of ND10 have been well established in tumorigenesis. However, in herpesvirus research, until the recently emerged evidence of pro-viral contributions, ND10 nuclear bodies have been generally recognized as part of the intrinsic antiviral defenses that converge to the incoming viral DNA to inhibit the viral gene expression. In this review, we evaluate the newly discovered pro-infection influences of ND10 in various human herpesviruses and analyze their molecular foundation along with the traditional antiviral functions of ND10. We hope to shed light on the explicit role of ND10 in both the lytic and latent cycles of herpesvirus infection, which is imperative to the delineation of herpes pathogenesis and the development of prophylactic/therapeutic treatments for herpetic diseases.

Towards understanding the multifaceted role of SUMOylation in plant growth and development

Post-translational modifications (PTMs) play a critical role in regulating plant growth and development through the modulation of protein functionality and its interaction with its partners. Analysis of the functional implication of PTMs on plant cellular signalling presents grand challenges in understanding their significance. Proteins decorated or modified with another chemical group or polypeptide play a significant role in regulating physiological processes as compared with non-decorated or non-modified proteins. In the past decade, SUMOylation has been emerging as a potent PTM influencing the adaptability of plants to growth, in response to various environmental cues. Deciphering the SUMO-mediated regulation of plant stress responses and its consequences is required to understand the mechanism underneath. Here, we will discuss the recent advances in the role and significance of SUMOylation in plant growth, development and stress response.

PML nuclear bodies and chromatin dynamics: catch me if you can!

Eukaryotic cells compartmentalize their internal milieu in order to achieve specific reactions in time and space. This organization in distinct compartments is essential to allow subcellular processing of regulatory signals and generate specific cellular responses. In the nucleus, genetic information is packaged in the form of chromatin, an organized and repeated nucleoprotein structure that is a source of epigenetic information. In addition, cells organize the distribution of macromolecules via various membrane-less nuclear organelles, which have gathered considerable attention in the last few years. The macromolecular multiprotein complexes known as Promyelocytic Leukemia Nuclear Bodies (PML NBs) are an archetype for nuclear membrane-less organelles. Chromatin interactions with nuclear bodies are important to regulate genome function. In this review, we will focus on the dynamic interplay between PML NBs and chromatin. We report how the structure and formation of PML NBs, which may involve phase separation mechanisms, might impact their functions in the regulation of chromatin dynamics. In particular, we will discuss how PML NBs participate in the chromatinization of viral genomes, as well as in the control of specific cellular chromatin assembly pathways which govern physiological mechanisms such as senescence or telomere maintenance.

The RING domain of TRIM69 promotes higher-order assembly

Members of the TRIM protein family have been shown to inhibit a range of viral infections. Recently, TRIM69 was identified as a potent inhibitor of Vesicular stomatitis Indiana virus infection, with its inhibition being dependent upon multimerization. Using SEC-MALLS analysis, it is demonstrated that the assembly of TRIM69 is mediated through the RING domain and not the Bbox domain as has been shown for other TRIM proteins. Using X-ray crystallography, the structure of the TRIM69 RING domain has been determined to a resolution of 2.1 Å, the oligomerization interface has been identified and regions outside the four-helix bundle have been observed to form interactions that are likely to support assembly.

Emerging Roles of Tripartite Motif-Containing Family Proteins (TRIMs) in Eliminating Misfolded Proteins

Protein quality control (PQC) is pivotal for eukaryotic cells to eliminate misfolded proteins and maintain cellular homeostasis. A decreased or increased capacity of PQC is associated with various diseases, e.g., neurodegenerative diseases and cancers. Recently, increasing evidences have suggested that tripartite motif-containing family proteins (TRIMs) are the key players in PQC regulation. Most TRIMs are E3 ubiquitin ligases, such as TRIM11/19/25, which, through the ubiquitination modifications, can contribute to effectively remove the cellular misfolded proteins or protein aggregates via the UPS pathway. In this review, we summarized the participation of TRIM members in misfolded protein elimination through distinct pathways, including the ubiquitin-proteasome system (UPS), autophagy system, and ER-associated degradation (ERAD).

The Promyelocytic Leukemia Protein facilitates human herpesvirus 6B chromosomal integration, immediate-early 1 protein multiSUMOylation and its localization at telomeres

Human herpesvirus 6B (HHV-6B) is a betaherpesvirus capable of integrating its genome into the telomeres of host chromosomes. Until now, the cellular and/or viral proteins facilitating HHV-6B integration have remained elusive. Here we show that a cellular protein, the promyelocytic leukemia protein (PML) that forms nuclear bodies (PML-NBs), associates with the HHV-6B immediate early 1 (IE1) protein at telomeres. We report enhanced levels of SUMOylated IE1 in the presence of PML and have identified a putative SUMO Interacting Motif (SIM) within IE1, essential for its nuclear distribution, overall SUMOylation and association with PML to nuclear bodies. Furthermore, using PML knockout cell lines we made the original observation that PML is required for efficient HHV-6B integration into host chromosomes. Taken together, we could demonstrate that PML-NBs are important for IE1 multiSUMOylation and that PML plays an important role in HHV-6B integration into chromosomes, a strategy developed by this virus to maintain its genome in its host over long periods of time.

Human herpesvirus 6B (HHV-6B) is a ubiquitous virus that can be life threatening in immunocompromised patients. HHV-6B is among a few other herpesviruses that integrate their genome in host chromosomes as a mean to establish dormancy. Integration of HHV-6B occurs in host telomeres, a region that protects our genome from deterioration and controls the cellular lifespan. To date, the mechanisms leading to HHV-6B integration remain elusive. Our laboratory has identified that the IE1 protein of HHV-6B associates with PML, a cellular protein that is responsible for the regulation of important cellular mechanisms including DNA recombination and repair. With the objective of understanding how IE1 is brought to PML, we discovered that PML aids the SUMOylation of IE1. This finding led us to identify a putative SUMO interaction motif on IE1 that is essentials for both its SUMOylation and IE1 oligomerization with PML-NBs. We next studied the role of PML on HHV-6B integration and identified that cells that are deficient for PML were less susceptible to HHV-6B integration. These results correlate with the fact that PML influences IE1 localization at telomeres, the site of HHV-6B integration. Our study further contributes to our understanding of the mechanisms leading to HHV-6B chromosomal integration.

PML Nuclear Body Biogenesis, Carcinogenesis, and Targeted Therapy

Targeted therapy has become increasingly important in cancer therapy. For example, targeting the promyelocytic leukemia PML protein in leukemia has proved to be an effective treatment. PML is the core component of super-assembled structures called PML nuclear bodies (NBs). Although this nuclear megaDalton complex was first observed in the 1960s, the mechanism of its assembly remains poorly understood. We review recent breakthroughs in the PML field ranging from a revised assembly mechanism to PML-driven genome organization and carcinogenesis. In addition, we highlight that oncogenic oligomerization might also represent a promising target in the treatment of leukemias and solid tumors.

Effect of SUMO-SIM Interaction on the ICP0-Mediated Degradation of PML Isoform II and Its Associated Proteins in Herpes Simplex Virus 1 Infection

ND10 nuclear bodies, as part of the intrinsic defenses, impose repression on incoming DNA. Infected cell protein 0 (ICP0), an E3 ubiquitin ligase of herpes simplex virus 1 (HSV-1), can derepress viral genes by degrading ND10 organizers to disrupt ND10. These events are part of the initial tug of war between HSV-1 and host, which determines the ultimate outcome of infection. Previously, we reported that ICP0 differentially recognizes promyelocytic leukemia (PML) isoforms. ICP0 depends on a SUMO-interaction motif located at residues 362 to 364 (SIM_362-364) to trigger the degradation of PML isoforms II, IV, and VI, while using a bipartite sequence flanking the RING domain to degrade PML I. In this study, we investigated how the SUMO-SIM interaction regulates the degradation of PML II and PML II-associated proteins in ND10. We found that (i) the same regulatory mechanism for PML II degradation was detected in cells permissive or nonpermissive to the ICP0-null virus; (ii) the loss of a single SIM_362-364 motif was restored by the presence of four consecutive SIMs from RNF4, but was not rescued by only two of the RNF4 SIMs; (iii) the loss of three C-terminal SIMs of ICP0 was fully restored by four RNF4 SIMs and also partially rescued by two RNF4 SIMs; and (iv) a PML II mutant lacking both lysine SUMOylation and SIM was not recognized by ICP0 for degradation, but was localized to ND10 and mitigated the degradation of other ND10 components, leading to delayed viral production. Taken together, SUMO regulates ICP0 substrate recognition via multiple fine-tuned mechanisms in HSV-1 infection.IMPORTANCE HSV-1 ICP0 is a multifunctional immediate early protein key to effective replication in the HSV-1 lytic cycle and reactivation in the latent cycle. ICP0 transactivates gene expression by orchestrating an overall mitigation in host intrinsic/innate restrictions. How ICP0 coordinates its multiple active domains and its diverse protein-protein interactions is a key question in understanding the HSV-1 life cycle and pathogenesis. The present study focuses on delineating the regulatory effects of the SUMO-SIM interaction on ICP0 E3 ubiquitin ligase activity regarding PML II degradation. For the first time, we discovered the importance of multivalency in the PML II-ICP0 interaction network and report the involvement of different regulatory mechanisms in PML II recognition by ICP0 in HSV-1 infection.

Emerging RNA-binding roles in the TRIM family of ubiquitin ligases

TRIM proteins constitute a large, diverse and ancient protein family which play a key role in processes including cellular differentiation, autophagy, apoptosis, DNA repair, and tumour suppression. Mostly known and studied through the lens of their ubiquitination activity as E3 ligases, it has recently emerged that many of these proteins are involved in direct RNA binding through their NHL or PRY/SPRY domains. We summarise the current knowledge concerning the mechanism of RNA binding by TRIM proteins and its biological role. We discuss how RNA-binding relates to their previously described functions such as E3 ubiquitin ligase activity, and we will consider the potential role of enrichment in membrane-less organelles.

Role of PML SUMOylation in arsenic trioxide-induced fibrosis in HSCs

BACKGROUND

Arsenic trioxide (ATO) can bind directly to the human promyelocytic leukemia (PML) protein, leading to modification of PML by SUMOs. UBC9 is the only known E2-conjugating enzyme involved in SUMOylation. PML degradation via RNF4, an E3 ubiquitin ligases family member. PML is key organizer of nuclear bodies (NBs) that regulate many biological processes such as senescence, and DNA damage. ATO can activate the TGFβ/Smad signaling pathway, causing liver fibrosis. However, the roles of PML Sumoylation in ATO-induced liver fibrosis remain unclear.

OBJECTIVE

This study aimed to investigate the role of PML Sumoylation in the ATO-induced HSCs activation and to improve the mechanism of ATO-induced liver fibrosis.

METHODS

Hepatic stellate cells (HSCs) were treated with 2 μmol/L ATO. Cell viability was detected by CCK-8 analysis. Immunoblot analysis and real-time quantitative PCR were used to detect the expression of IL-1β, TNF-α, TGF-β1, p-Smad2/3, α-SMA, Collagen I and PML SUMOylation after silencing PML, UBC9, and RNF4, respectively. The formation of PML-NBs was observed by immunofluorescence staining.

RESULTS

2 and 5 μmol/L ATO intervention increased HSCs cell viability. ATO was able to significantly trigger PML SUMOylation and the formation of PML-NBs. Inhibition of SUMOylated PML by silencing UBC9, subsequently preventing the downregulation of HSCs activation indicators induced by ATO (P < 0.05). Conversely, enhancing SUMOylated PML accumulation by silencing RNF4, activating TGFβ/Smad signaling pathway, eventually promoting the induction of liver fibrosis.

CONCLUSION

These results indicated that PML SUMOylation plays a critical role in the development of liver fibrosis induced by ATO.

PML nuclear body biogenesis and oligomerization-driven leukemogenesis

PML nuclear bodies (NBs), which are increasingly recognized as the central hub of many cellular signaling events, are superassembled spherical complexes with diameters of 0.1–2 μm. Recent studies reveal that RING tetramerization and B1-box polymerization are key factors to the overall PML NBs assembly. The productive RBCC oligomerization allows subsequent PML biogenesis steps, including the PML auto-sumoylation and partners recruitment via SUMO–SIM interactions. In promyelocytic leukemia, the oncoprotein PML/RARα (P/R) inhibits PML NBs assembly and leads to a full-fledged leukemogenesis. In this review, we review the recent progress in PML and acute promyelocytic leukemia fields, highlighting the protein oligomerization as an important direction of future targeted therapy.