The COP9 signalosome: A versatile regulatory hub of Cullin-RING ligases

The E3 ubiquitin ligase SlATL2 suppresses tomato immunity by promoting SlCSN5a degradation during Pseudomonas syringae pv. tomato DC3000 infection

Plant immunity involves complex regulatory mechanisms that mediate the activation of defense responses against pathogens. Protein degradation via ubiquitination plays a crucial role in modulating these defenses, with E3 ubiquitin ligases functioning as central regulators. This study investigates the role of SlATL2, an ARABIDOPSIS TÓXICOS EN LEVADURA (ATL)-type E3 ubiquitin ligase localized in the plasma membrane, in the immune response of tomato plants against Pseudomonas syringae pv. tomato (Pst) DC3000. Our findings demonstrate that SlATL2 expression is induced upon Pst DC3000 infection and treatment with defense hormones salicylic acid and jasmonic acid. Functionally, SlATL2 negatively regulates immune responses, impairing resistance to Pst DC3000 and suppressing flg22-triggered immunity. In addition, SlATL2 limits pathogen-induced reactive oxygen species and callose accumulation by targeting the COP9 signalosome subunit 5a (SlCSN5a), a key positive regulator of tomato defense responses against Pst DC3000. This interaction, which occurs via the N-terminal residue of SlATL2, results in the ubiquitination and 26S proteasomal degradation of SlCSN5a, thereby suppressing SA-dependent expression of defense response genes associated and limiting reactive oxygen species production. This work sheds light on the molecular mechanism through which the E3 ubiquitin ligase SlATL2 attenuates tomato immune responses by targeting a COP9 signalosome subunit for degradation. These discoveries deepen our insights into the post-translational mechanisms governing plant immune responses and provide fresh opportunities to bolster crop resistance against bacterial pathogens.

Discovery of New Azaindole Metallo-Deubiquitinase CSN5 Inhibitors

CSN5 is responsible for the deneddylation of cullin-RING E3 ubiquitin ligases and is closely linked to the development of various cancers. We previously developed a noncatalytic activity assay platform using novel fluorescent probes derived from azaindole inhibitors, which also highlighted the potential for further structural optimization of azaindoles. Herein, we report a series of new 4-NH-substituted azaindole derivatives, some of which showed nanomolar activity against the CSN5 subunit. Cellular assays revealed that the new azaindoles increase the cullin 1 neddylation in cancer cells. Importantly, they exhibit synergistic anticancer effects in combination with poly(ADP-ribose) polymerase inhibitors through increasing DNA damage. This work presents a new lead compound and a potential combination strategy for drug discovery targeting CSN5.

Proteome changes during the germination and early seedling development of carnauba palm (Copernicia prunifera) under skotomorphogenic conditions

We analyze the proteome changes during the development of the carnauba palm (Copernicia prunifera) seedlings under skotomorphogenic conditions, by separating the embryo into its two components: haustorium (HA) and cotyledonary petiole (CP) and established the descriptive and quantitative proteomes of these tissues across four developmental stages. 5205 proteins were identified in HA and 6028 in CP. These proteomes are rich in proteins known to maintain the skotomorphogenic state, and in a complete set of proteins involved in cellular respiration and biosynthesis of secondary metabolites. The quantitative analysis employing a label-free approach revealed that 583 proteins in HA and 383 in CP were differentially abundant, with 251 proteins shared between the datasets. The results showed that HA participates in the digestion of food reserves present in HA itself and in the endosperm, acting as a conduit of nitrogen and carbon sources for the growing embryo axis. Among the differentially abundant proteins in the CP, we identified the presence of proteins from the cellular metabolism and proteins involved in the hydrolysis of food reserves such as starch and proteins. This indicates that the CP, in addition to the endosperm and HA, serves as a source of food reserves for the embryo axis. SIGNIFICANCE: Our results also reveal the differential regulation of specific proteins involved in reactive oxygen species scavenging, cell wall remodeling, respiratory metabolism, and protein repair in seeds and seedlings of C. prunifera. These findings have broad implications for understanding the energy metabolism that drives the transition from seed to seedling. For this study, we employed state-of-the-art proteomic techniques, including quantitative mass spectrometry and bioinformatic analysis, that allowed us to create a large dataset that will be a valuable resource for future research on the physiological and biochemical aspects of skotomorphogenesis, photomorphogenesis, and the transition between these states.

Effects of CSN1/CSN2 Mutants in Flavonoid Metabolism on Rice (Oryza sativa L.)

The key flavonoid biosynthesis-related genes and their molecular features in rice have not been comprehensively and systematically characterized. In this study, we investigated the glumes of OsCSN1 mutants and OsCSN2 mutants and found the changes in the total flavonoid contents of the OsCSN2 mutants to be more pronounced than those of the OsCSN1 mutants and the changes in the anthocyanin contents of the OsCSN1 mutants to be more pronounced than those of the OsCSN2 mutants. In addition, key genes related to flavonoid synthesis, OsCHI, showed a more pronounced up-regulation trend, and the OsDFR gene, which encodes a precursor enzyme for anthocyanin synthesis, showed a clear down-regulation trend. And yeast two-hybrid experiments showed that OsCSN1 and OsCSN2 had the ability to interact with OsCUL4. In summary, OsCSN1 and OsCSN2 may regulate the metabolism of flavonoids in rice through CUL4-based E3 ligase, and the two subunits play different roles, laying a foundation for the study of the mechanism of flavonoid metabolism in monocotyledonous plants.

CSN-CRL Complexes: New Regulators of Adipogenesis

Recent discoveries revealed mechanistic insights into the control of adipogenesis by the Constitutive Photomorphogenesis 9 Signalosome (CSN) and its variants, CSNCSN7A and CSNCSN7B, which differ in the paralog subunits, CSN7A and CSN7B. CSNCSN7A and CSNCSN7B variants form permanent complexes with cullin-RING-ubiquitin ligases 3 and 4A (CRL3 and CRL4A), respectively. These complexes can be found in most eukaryotic cells and represent a critical reservoir for cellular functions. In an early stage of adipogenesis, mitotic clonal expansion (MCE), CSN-CRL1, and CSNCSN7B-CRL4A are blocked to ubiquitinate the cell cycle inhibitor p27KIP, leading to cell cycle arrest. In addition, in MCE CSN-CRL complexes rearrange the cytoskeleton for adipogenic differentiation and CRL3KEAP1 ubiquitylates the inhibitor of adipogenesis C/EBP homologous protein (CHOP) for degradation by the 26S proteasome, an adipogenesis-specific proteolysis. During terminal adipocyte differentiation, the CSNCSN7A-CRL3 complex is recruited to a lipid droplet (LD) membrane by RAB18. Currently, the configuration of the substrate receptors of CSNCSN7A-CRL3 on LDs is unclear. CSNCSN7A-CRL3 is activated by neddylation on the LD membrane, an essential adipogenic step. Damage to CSN/CUL3/CUL4A genes is associated with diverse diseases, including obesity. Due to the tremendous impact of CSN-CRLs on adipogenesis, we need strategies for adequate treatment in the event of malfunctions.

Molecular mechanisms of CAND2 in regulating SCF ubiquitin ligases

Protein degradation orchestrated by SKP1·CUL1·F-box protein (SCF) ubiquitin ligases is a fundamental process essential for cellular and organismal function. The dynamic assembly of SCFs, facilitated by CAND1, ensures timely ubiquitination of diverse SCF target proteins. As a homolog of CAND1, CAND2 alone has been implicated in various human diseases, yet its functional mechanisms remain elusive. Here, we investigate the role of CAND2 in human cells and its distinct mode of action compared to CAND1. Using an array of quantitative assays, we demonstrate that CAND2 promotes SCF-mediated protein degradation as an F-box protein exchange factor. While CAND2 binds CUL1 with structure and affinity comparable to CAND1, it exhibits lower efficiency in exchanging F-box proteins. Kinetic measurements reveal a significantly higher KM for CAND2-catalyzed SCF disassembly than CAND1, which explains the lower exchange efficiency of CAND2 and is likely due to conformations of the CAND2·SCF exchange intermediate complex being less favorable for F-box protein dissociation. Our study provides mechanistic insights into the biochemical and structural properties of CAND2, as well as its role in regulating cellular dynamics of SCFs, laying a foundation for understanding contributions of CAND2 to healthy and diseased human cells.

Cullin-RING ligase BioE3 reveals molecular-glue-induced neosubstrates and rewiring of the endogenous Cereblon ubiquitome

BACKGROUND

The specificity of the ubiquitination process is mediated by the E3 ligases. Discriminating genuine substrates of E3s from mere interacting proteins is one of the major challenges in the field. We previously developed BioE3, a biotin-based approach that uses BirA-E3 fusions together with ubiquitin fused to a low-affinity AviTag to obtain a site-specific and proximity-dependent biotinylation of the substrates. We proved the suitability of BioE3 to identify targets of RING and HECT-type E3 ligases.

METHODS

BioE3 experiments were performed in HEK293FT and U2OS stable cell lines expressing TRIPZ-bioGEFUb transiently transfected with BirA-cereblon (CRBN). Cells were seeded using biotin-free media, followed later by a short-biotin pulse. We evaluated the applicability of the BioE3 system to CRBN and molecular glues by Western blot and confocal microscopy, blocking the proteasome with bortezomib, inhibiting NEDDylation with MLN4924 and treating the cells with pomalidomide. For the identification of endogenous substrates and neosubstrates we analyzed the eluates of streptavidin pull-downs of BioE3 experiments by LC-MS/MS. Analysis of targets for which ubiquitination changes significantly upon treatment was done using two-sided Student's t-test. Orthogonal validations were performed by histidine pull-down, GFP-trap and computational modelling.

RESULTS

Here we demonstrate that BioE3 is suitable for the multi-protein complex Cullin-RING E3s ligases (CRLs), the most utilized E3-type for targeted protein degradation (TPD) strategies. Using CRBN as proof of concept, one of the substrate receptors of CRL4 E3 ligase, we identified both endogenous substrates and novel neosubstrates upon pomalidomide treatment, including CSDE1 which contains a G-loop motif potentially involved in the binding to CRBN in presence of pomalidomide. Importantly, we observed a major rearrangement of the endogenous ubiquitination landscape upon treatment with this molecular glue.

CONCLUSIONS

The ability of BioE3 to detect and compare both substrates and neosubstrates, as well as how substrates change in response to treatments, will facilitate both on-target and off-target identifications and offer a broader characterization and validation of TPD compounds, like molecular glues and PROTACs.

Ubiquitin-mediated degradation of the inhibitor FvMYB1 and the activator FvBBX20 by FvCSN5 balances anthocyanin biosynthesis in strawberry fruit

Plant CSN5 is widely recognized as the subunit of the COP9 signalosome and CSN5 is mainly involved in plant growth and development, and tolerance to biotic and abiotic stresses. However, the molecular mechanism of CSN5 regulating anthocyanin biosynthesis in plants is still largely unknown. Here, we identified FvCSN5 from the woodland strawberry yeast two-hybrid library using the anthocyanin pathway inhibitor MYB1 as bait. We demonstrated the interaction of FvCSN5 and FvMYB1 by H2Y, Pull-down, LCI, and BiFC assays. FvCSN5 was expressed in all test tissues and localized in the nucleus and cytosol with self-activation activity. Stable overexpression of FvCSN5 in woodland strawberries reduced anthocyanin accumulation in fruits. The protein level of FvMYB1 greatly decreased in overexpressing FvCSN5 plants compared with wild-type plants. Protein degradation assay and MG-132 treatment (a proteasome inhibitor blocking 26S proteasome activity) revealed FvCSN5 degraded FvMYB1 through the ubiquitination pathway. In addition, FvCSN5 also interacted with the anthocyanin activator FvBBX20 and FvBBX20 could be degraded by FvCSN5. Moreover, transient expression analysis showed the expression of anthocyanin biosynthetic genes FvCHS and FvF3H was greatly increased and decreased when FvCSN5 was co-expressed with FvMYB1 and FvBBX20, respectively. These results indicate that FvMYB1-FvCSN5-FvBBX20 is a novel ternary complex that regulates anthocyanin biosynthesis by the ubiquitination pathway.

A window into intracellular events in myositis through subcellular proteomics

OBJECTIVE AND DESIGN

Idiopathic inflammatory myopathies (IIM) are a heterogeneous group of inflammatory muscle disorders of unknown etiology. It is postulated that mitochondrial dysfunction and protein aggregation in skeletal muscle contribute to myofiber degeneration. However, molecular pathways that lead to protein aggregation in skeletal muscle are not well defined.

SUBJECTS

Here we have isolated membrane-bound organelles (e.g., nuclei, mitochondria, sarcoplasmic/endoplasmic reticulum, Golgi apparatus, and plasma membrane) from muscle biopsies of normal (n = 3) and muscle disease patients (n = 11). Of the myopathy group, 10 patients displayed mitochondrial abnormalities (IIM (n = 9); mitochondrial myopathy (n = 1)), and one IIM patient did not show mitochondrial abnormalities (polymyositis).

METHODS

Global proteomic analysis was performed using an Orbitrap Fusion mass spectrometer. Upon unsupervised clustering, normal and mitochondrial myopathy muscle samples clustered separately from IIM samples.

RESULTS

We have confirmed previously known protein alterations in IIM and identified several new ones. For example, we found differential expression of (i) nuclear proteins that control cell division, transcription, RNA regulation, and stability, (ii) ER and Golgi proteins involved in protein folding, degradation, and protein trafficking in the cytosol, and (iii) mitochondrial proteins involved in energy production/metabolism and alterations in cytoskeletal and contractile machinery of the muscle.

CONCLUSIONS

Our data demonstrates that molecular alterations are not limited to protein aggregations in the cytosol (inclusions) and occur in nuclear, mitochondrial, and membrane compartments of IIM skeletal muscle.

Dysregulation of deubiquitinylases: a linchpin of gastrointestinal diseases

Ubiquitinylation of proteins regulates manifold processes and is reversed by deubiquitinylating enzymes (DUBs), which are therefore implicated in a plethora of cellular processes. DUBs are frequently upregulated in many diseases, while in a few cases downregulation of DUBs is associated with disease progression. This review focuses on the involvement of DUBs in the development and progression of gastrointestinal diseases with a particular emphasis on hepatic steatosis and hepatocellular, cholangio-, esophageal, gastric, colorectal, and pancreatic ductal carcinomas. In addition, pathogens that trigger the activity of several DUBs and thus suppress the immune response and cell survival are discussed. Finally, we highlight recent approaches made towards the therapeutic treatment of gastrointestinal diseases using DUB inhibitors.

CUL4-Based Ubiquitin Ligases in Chromatin Regulation: An Evolutionary Perspective

Ubiquitylation is a post-translational modification that modulates protein function and stability. It is orchestrated by the concerted action of three types of enzymes, with substrate specificity governed by ubiquitin ligases (E3s), which may exist as single proteins or as part of multi-protein complexes. Although Cullin (CUL) proteins lack intrinsic enzymatic activity, they participate in the formation of active ubiquitin ligase complexes, known as Cullin-Ring ubiquitin Ligases (CRLs), through their association with ROC1 or ROC2, along with substrate adaptor and receptor proteins. Mammalian genomes encode several CUL proteins (CUL1-9), each contributing to distinct CRLs. Among these CUL proteins, CUL1, CUL3, and CUL4 are believed to be the most ancient and evolutionarily conserved from yeast to mammals, with CUL4 uniquely duplicated in vertebrates. Genetic evidence strongly implicates CUL4-based ubiquitin ligases (CRL4s) in chromatin regulation across various species and suggests that, in vertebrates, CRL4s have also acquired a cytosolic role, which is facilitated by a cytosol-localizing paralog of CUL4. Substrates identified through biochemical studies have elucidated the molecular mechanisms by which CRL4s regulate chromatin and cytosolic processes. The substantial body of knowledge on CUL4 biology amassed over the past two decades provides a unique opportunity to explore the functional evolution of CRL4. In this review, we synthesize the available structural, genetic, and biochemical data on CRL4 from various model organisms and discuss the conserved and novel functions of CRL4s.

Increased CSN5 expression enhances the sensitivity to lenalidomide in multiple myeloma cells

Lenalidomide (LEN) is commonly used as an effective therapeutic agent for multiple myeloma (MM). However, in some patients, primary resistance to LEN is observed, the mechanisms of which remain poorly understood. In this study, we combined a LEN sensitivity assay with proteomics data from 15 MM cell lines to identify protein expression profiles associated with primary LEN resistance. Our findings revealed that CSN5 expression is lower in LEN-resistant cell lines than in LEN-sensitive lines. Moreover, we established that CSN5 is degraded via the cullin-RING ubiquitin ligase (CRL)-mediated ubiquitin-proteasome pathway through ubiquitination at lysine 194. Our data suggest that reduced CSN5 expression leads to abnormalities in the ubiquitination cycle of CRL4A, resulting in the inhibition of LEN-mediated degradation of IKZF1 and IKZF3. These findings delineate an additional mechanism of LEN resistance in MM cells and may contribute to the development of alternative therapeutic strategies to overcome LEN resistance.

Dysregulation of deubiquitinases in gastric cancer progression

Gastric cancer (GC), characterized by a high incidence rate, poses significant clinical challenges owing to its poor prognosis despite advancements in diagnostic and therapeutic approaches. Therefore, a comprehensive understanding of the molecular mechanisms driving GC progression is crucial for identifying predictive markers and defining treatment targets. Deubiquitinating enzymes (DUBs), also called deubiquitinases, function as reverse transcriptases within the ubiquitin-proteasome system to counteract protein degradation. Recent findings suggest that DUB dysregulation could be a crucial factor in GC pathogenesis. In this review, we examined recent research findings on DUBs in the context of GC, elucidating their molecular characteristics, categorizations, and roles while also exploring the potential mechanisms underlying their dysregulation in GC. Furthermore, we assessed the therapeutic efficacy of DUB inhibitors in treating malignancies and evaluated the prevalence of aberrant DUB expression in GC.

Discovering New Metallo-Deubiquitinase CSN5 Inhibitors by a Non-Catalytic Activity Assay Platform

COP9 signalosome catalytic subunit CSN5 plays a key role in tumorigenesis and tumor immunity, showing potential as an anticancer target. Currently, only a few CSN5 inhibitors have been reported, at least partially, due to the challenges in establishing assays for CSN5 deubiquitinase activity. Here, we present the establishment and validation of a simple and reliable non-catalytic activity assay platform for identifying CSN5 inhibitors utilizing a new fluorescent probe, CFP-1, that exhibits enhanced fluorescence and fluorescence polarization features upon binding to CSN5. By using this platform, we identified 2-aminothiazole-4-carboxylic acids as new CSN5 inhibitors, which inhibited CSN5 but slightly downregulated PD-L1 in cancer cells. Furthermore, through the integration of deep learning-enabled virtual screening, we discovered that shikonins are nanomolar CSN5 inhibitors, which can upregulate PD-L1 in HCT116 cells. The binding modes of these structurally distinct inhibitors with CSN5 were explored by using microsecond-scale molecular dynamics simulations and tryptophan quenching assays.

The small CRL4CSA ubiquitin ligase component DDA1 regulates transcription-coupled repair dynamics

Transcription-blocking DNA lesions are specifically targeted by transcription-coupled nucleotide excision repair (TC-NER), which removes a broad spectrum of DNA lesions to preserve transcriptional output and thereby cellular homeostasis to counteract aging. TC-NER is initiated by the stalling of RNA polymerase II at DNA lesions, which triggers the assembly of the TC-NER-specific proteins CSA, CSB and UVSSA. CSA, a WD40-repeat containing protein, is the substrate receptor subunit of a cullin-RING ubiquitin ligase complex composed of DDB1, CUL4A/B and RBX1 (CRL4CSA). Although ubiquitination of several TC-NER proteins by CRL4CSA has been reported, it is still unknown how this complex is regulated. To unravel the dynamic molecular interactions and the regulation of this complex, we apply a single-step protein-complex isolation coupled to mass spectrometry analysis and identified DDA1 as a CSA interacting protein. Cryo-EM analysis shows that DDA1 is an integral component of the CRL4CSA complex. Functional analysis reveals that DDA1 coordinates ubiquitination dynamics during TC-NER and is required for efficient turnover and progression of this process.

Microsatellite break-induced replication generates highly mutagenized extrachromosomal circular DNAs

Extrachromosomal circular DNAs (eccDNAs) are produced from all regions of the eucaryotic genome. We used inverse PCR of non-B microsatellites capable of forming hairpin, triplex, quadruplex and AT-rich structures integrated at a common ectopic chromosomal site to show that these non-B DNAs generate highly mutagenized eccDNAs by replication-dependent mechanisms. Mutagenesis occurs within the non-B DNAs and extends several kilobases bidirectionally into flanking and nonallelic DNA. Each non-B DNA exhibits a different pattern of mutagenesis, while sister clones containing the same non-B DNA also display distinct patterns of recombination, microhomology-mediated template switching and base substitutions. Mutations include mismatches, short duplications, long nontemplated insertions, large deletions and template switches to sister chromatids and nonallelic chromosomes. Drug-induced replication stress or the depletion of DNA repair factors Rad51, the COPS2 signalosome subunit or POLη change the pattern of template switching and alter the eccDNA mutagenic profiles. We propose an asynchronous capture model based on break-induced replication from microsatellite-induced DNA double strand breaks to account for the generation and circularization of mutagenized eccDNAs and the appearance of genomic homologous recombination deficiency (HRD) scars. These results may help to explain the appearance of tumor eccDNAS and their roles in neoantigen production, oncogenesis and resistance to chemotherapy.

The Toxoplasma gondii F-Box Protein L2 Functions as a Repressor of Stage Specific Gene Expression

Toxoplasma gondii is a foodborne pathogen that can cause severe and life-threatening infections in fetuses and immunocompromised patients. Felids are its only definitive hosts, and a wide range of animals, including humans, serve as intermediate hosts. When the transmissible bradyzoite stage is orally ingested by felids, they transform into merozoites that expand asexually, ultimately generating millions of gametes for the parasite sexual cycle. However, bradyzoites in intermediate hosts differentiate exclusively to disease-causing tachyzoites, which rapidly disseminate throughout the host. Though tachyzoites are well-studied, the molecular mechanisms governing transitioning between developmental stages are poorly understood. Each parasite stage can be distinguished by a characteristic transcriptional signature, with one signature being repressed during the other stages. Switching between stages require substantial changes in the proteome, which is achieved in part by ubiquitination. F-box proteins mediate protein poly-ubiquitination by recruiting substrates to SKP1, Cullin-1, F-Box protein E3 ubiquitin ligase (SCF-E3) complexes. We have identified an F-box protein named Toxoplasma gondii F-Box Protein L2 (TgFBXL2), which localizes to distinct perinucleolar sites. TgFBXL2 is stably engaged in an SCF-E3 complex that is surprisingly also associated with a COP9 signalosome complex that negatively regulates SCF-E3 function. At the cellular level, TgFBXL2-depleted parasites are severely defective in centrosome replication and daughter cell development. Most remarkable, RNAseq data show that TgFBXL2 conditional depletion induces the expression of stage-specific genes including a large cohort of genes necessary for sexual commitment. Together, these data suggest that TgFBXL2 is a latent guardian of stage specific gene expression in Toxoplasma and poised to remove conflicting proteins in response to an unknown trigger of development.

Protein neddylation and its role in health and diseases

NEDD8 (Neural precursor cell expressed developmentally downregulated protein 8) is an ubiquitin-like protein that is covalently attached to a lysine residue of a protein substrate through a process known as neddylation, catalyzed by the enzyme cascade, namely NEDD8 activating enzyme (E1), NEDD8 conjugating enzyme (E2), and NEDD8 ligase (E3). The substrates of neddylation are categorized into cullins and non-cullin proteins. Neddylation of cullins activates CRLs (cullin RING ligases), the largest family of E3 ligases, whereas neddylation of non-cullin substrates alters their stability and activity, as well as subcellular localization. Significantly, the neddylation pathway and/or many neddylation substrates are abnormally activated or over-expressed in various human diseases, such as metabolic disorders, liver dysfunction, neurodegenerative disorders, and cancers, among others. Thus, targeting neddylation becomes an attractive strategy for the treatment of these diseases. In this review, we first provide a general introduction on the neddylation cascade, its biochemical process and regulation, and the crystal structures of neddylation enzymes in complex with cullin substrates; then discuss how neddylation governs various key biological processes via the modification of cullins and non-cullin substrates. We further review the literature data on dysregulated neddylation in several human diseases, particularly cancer, followed by an outline of current efforts in the discovery of small molecule inhibitors of neddylation as a promising therapeutic approach. Finally, few perspectives were proposed for extensive future investigations.

Unveiling the Biological Function of Phyllostachys edulis FBA6 (PeFBA6) through the Identification of the Fructose-1,6-Bisphosphate Aldolase Gene

Fructose-1,6-bisphosphate aldolase (FBA) is a pivotal enzyme in various metabolic pathways, including glycolysis, gluconeogenesis, and the Calvin cycle. It plays a critical role in CO2 fixation. Building on previous studies on the FBA gene family in Moso bamboo, our study revealed the biological function of PeFBA6. To identify CSN5 candidate genes, this study conducted a yeast two-hybrid library screening experiment. Subsequently, the interaction between CSN5 and PeFBA6 was verified using yeast two-hybrid and LCI experiments. This investigation uncovered evidence that FBA may undergo deubiquitination to maintain glycolytic stability. To further assess the function of PeFBA6, it was overexpressed in rice. Various parameters were determined, including the light response curve, CO2 response curve, and the levels of glucose, fructose, sucrose, and starch in the leaves of overexpressing rice. The results demonstrated that overexpressed rice exhibited a higher saturation light intensity, net photosynthetic rate, maximum carboxylation rate, respiration rate, and increased levels of glucose, fructose, and starch than wild-type rice. These findings indicated that PeFBA6 not only enhanced the photoprotection ability of rice but also improved the photosynthetic carbon metabolism. Overall, this study enhanced our understanding of the function of FBA and revealed the biological function of PeFBA6, thereby providing a foundation for the development of excellent carbon fixation bamboo varieties through breeding.

CAND1 inhibits Cullin-2-RING ubiquitin ligases for enhanced substrate specificity

Through targeting essential cellular regulators for ubiquitination and serving as a major platform for discovering proteolysis-targeting chimera (PROTAC) drugs, Cullin-2 (CUL2)-RING ubiquitin ligases (CRL2s) comprise an important family of CRLs. The founding members of CRLs, the CUL1-based CRL1s, are known to be activated by CAND1, which exchanges the variable substrate receptors associated with the common CUL1 core and promotes the dynamic assembly of CRL1s. Here we find that CAND1 inhibits CRL2-mediated protein degradation in human cells. This effect arises due to altered binding kinetics, involving CAND1 and CRL2VHL, as we illustrate that CAND1 dramatically increases the dissociation rate of CRL2s but barely accelerates the assembly of stable CRL2s. Using PROTACs that differently recruit neo-substrates to CRL2VHL, we demonstrate that the inhibitory effect of CAND1 helps distinguish target proteins with different affinities for CRL2s, presenting a mechanism for selective protein degradation with proper pacing in the changing cellular environment.

Microsatellite break-induced replication generates highly mutagenized extrachromosomal circular DNAs

Extrachromosomal circular DNAs (eccDNAs) are produced from all regions of the eucaryotic genome. In tumors, highly transcribed eccDNAs have been implicated in oncogenesis, neoantigen production and resistance to chemotherapy. Here we show that unstable microsatellites capable of forming hairpin, triplex, quadruplex and AT-rich structures generate eccDNAs when integrated at a common ectopic site in human cells. These non-B DNA prone microsatellites form eccDNAs by replication-dependent mechanisms. The microsatellite-based eccDNAs are highly mutagenized and display template switches to sister chromatids and to nonallelic chromosomal sites. High frequency mutagenesis occurs within the eccDNA microsatellites and extends bidirectionally for several kilobases into flanking DNA and nonallelic DNA. Mutations include mismatches, short duplications, longer nontemplated insertions and large deletions. Template switching leads to recurrent deletions and recombination domains within the eccDNAs. Template switching events are microhomology-mediated, but do not occur at all potential sites of complementarity. Each microsatellite exhibits a distinct pattern of recombination, microhomology choice and base substitution signature. Depletion of Rad51, the COPS2 signalosome subunit or POLη alter the eccDNA mutagenic profiles. We propose an asynchronous capture model based on break-induced replication from microsatellite-induced DNA breaks for the generation and circularization of mutagenized eccDNAs and genomic homologous recombination deficiency (HRD) scars.

Toxoplasma gondii F-Box Protein L2 Silences Feline-Restricted Genes Necessary for Sexual Commitment

Toxoplasma gondii is a foodborne pathogen that can cause severe and life-threatening infections in fetuses and immunocompromised patients. Felids are its only definitive hosts, and a wide range of animals, including humans, serve as intermediate hosts. When the transmissible bradyzoite stage is orally ingested by felids, they transform into merozoites that expand asexually, ultimately generating millions of gametes for the parasite sexual cycle. However, bradyzoites in intermediate hosts differentiate exclusively to disease-causing tachyzoites, which rapidly disseminate throughout the host. Though tachyzoites are well-studied, the molecular mechanisms governing transitioning between developmental stages are poorly understood. Each parasite stage can be distinguished by a characteristic transcriptional signature, with one signature being repressed during the other stages. Switching between stages requires substantial changes in the proteome, which is achieved in part by ubiquitination. F-box proteins mediate protein poly-ubiquitination by recruiting substrates to SKP1, Cullin-1, F-Box protein E3 ubiquitin ligase (SCF-E3) complexes. We have identified an F-box protein named Toxoplasma gondii F-Box Protein L2 (TgFBXL2), which localizes to distinct nuclear sites. TgFBXL2 is stably engaged in an SCF-E3 complex that is surprisingly also associated with a COP9 signalosome complex that negatively regulates SCF-E3 function. At the cellular level, TgFBXL2-depleted parasites are severely defective in centrosome replication and daughter cell development. Most remarkable, RNA seq data show that TgFBXL2 conditional depletion induces the expression of genes necessary for sexual commitment. We suggest that TgFBXL2 is a latent guardian of sexual stage development in Toxoplasma and poised to remove conflicting proteins in response to an unknown trigger of sexual development.

Regulatory mechanisms and therapeutic potential of JAB1 in neurological development and disorders

c-Jun activation domain binding protein-1 (JAB1) is a multifunctional regulator that plays vital roles in diverse cellular processes. It regulates AP-1 transcriptional activity and also acts as the fifth component of the COP9 signalosome complex. While JAB1 is considered an oncoprotein that triggers tumor development, recent studies have shown that it also functions in neurological development and disorders. In this review, we summarize the general features of the JAB1 gene and protein, and present recent updates on the regulation of JAB1 expression. Moreover, we also highlight the functional roles and regulatory mechanisms of JAB1 in neurodevelopmental processes such as neuronal differentiation, synaptic morphogenesis, myelination, and hair cell development and in the pathogenesis of some neurological disorders such as Alzheimer's disease, multiple sclerosis, neuropathic pain, and peripheral nerve injury. Furthermore, current challenges and prospects are discussed, including updates on drug development targeting JAB1.