Interaction of U-box E3 ligase SNEV with PSMB4, the beta7 subunit of the 20 S proteasome

Allosteric regulation of the 20S proteasome by the Catalytic Core Regulators (CCRs) family

Controlled degradation of proteins is necessary for ensuring their abundance and sustaining a healthy and accurately functioning proteome. One of the degradation routes involves the uncapped 20S proteasome, which cleaves proteins with a partially unfolded region, including those that are damaged or contain intrinsically disordered regions. This degradation route is tightly controlled by a recently discovered family of proteins named Catalytic Core Regulators (CCRs). Here, we show that CCRs function through an allosteric mechanism, coupling the physical binding of the PSMB4 β-subunit with attenuation of the complex's three proteolytic activities. In addition, by dissecting the structural properties that are required for CCR-like function, we could recapitulate this activity using a designed protein that is half the size of natural CCRs. These data uncover an allosteric path that does not involve the proteasome's enzymatic subunits but rather propagates through the non-catalytic subunit PSMB4. This way of 20S proteasome-specific attenuation opens avenues for decoupling the 20S and 26S proteasome degradation pathways as well as for developing selective 20S proteasome inhibitors.

Cellular PSMB4 Protein Suppresses Influenza A Virus Replication through Targeting NS1 Protein

The nonstructural protein 1 (NS1) of influenza A virus (IAV) possesses multiple functions, such as the inhibition of the host antiviral immune responses, to facilitate viral infection. To search for cellular proteins interacting with the IAV NS1 protein, the yeast two-hybrid system was adopted. Proteasome family member PSMB4 (proteasome subunit beta type 4) was found to interact with the NS1 protein in this screening experiment. The binding domains of these two proteins were also determined using this system. The physical interactions between the NS1 and cellular PSMB4 proteins were further confirmed by co-immunoprecipitation assay and confocal microscopy in mammalian cells. Neither transiently nor stably expressed NS1 protein affected the PSMB4 expression in cells. In contrast, PSMB4 reduced the NS1 protein expression level, especially in the presence of MG132. As expected, the functions of the NS1 protein, such as inhibition of interferon activity and enhancement of transient gene expression, were suppressed by PSMB4. PSMB4 knockdown enhances IAV replication, while its overexpression attenuates IAV replication. Thus, the results of this study suggest that the cellular PSMB4 protein interacts with and possibly facilitates the degradation of the NS1 protein, which in turn suppresses IAV replication.

A U-Box Type E3 Ubiquitin Ligase Prp19-Like Protein Negatively Regulates Lipid Accumulation and Cell Size in Chlamydomonas reinhardtii

Microalgae lipid triacylglycerol is considered as a promising feedstock for national production of biofuels. A hotspot issue in the biodiesel study is to increase TAG content and productivity of microalgae. Precursor RNA processing protein (Prp19), which is the core component of eukaryotic RNA splice NTC (nineteen associated complex), plays important roles in the mRNA maturation process in eukaryotic cells, has a variety of functions in cell development, and is even directly involved in the biosynthesis of oil bodies in mouse. Nevertheless, its function in Chlamydomonas reinhardtii remains unknown. Here, transcriptional level of CrPrp19 under nutrition deprivation was analyzed, and both its RNA interference and overexpressed transformants were constructed. The expression level of CrPrp19 was suppressed by nitrogen or sulfur deficiency. Cell densities of CrPrp19 RNAi lines decreased, and their neutral lipid contents increased 1.33 and 1.34 times over those of controls. The cells of CrPrp19 RNAi lines were larger and more resistant to sodium acetate than control. Considerably none of the alterations in growth or neutral lipid contents was found in the CrPrp19 overexpression transformants than wild type. Fatty acids were also significantly increased in CrPrp19 RNAi transformants. Subcellular localization and yeast two-hybrid analysis showed that CrPrp19 was a nuclear protein, which might be involved in cell cycle regulation. In conclusion, CrPrp19 protein was necessary for negatively regulating lipid enrichment and cell size, but not stimulatory for lipid storage.

PRPF19 promotes tongue cancer growth and chemoradiotherapy resistance

Pre-mRNA processing factor 19 (PRPF19) is a multifaceted protein and participates in DNA damage response and pre-mRNA processing. The role of PRPF19 in cancer is unclear. Here, we report that the expression of PRPF19 in human tongue cancer is associated with unfavorable prognosis. Overexpression of PRPF19 promotes while knockdown of PRPF19 inhibits tongue cancer cell migration, proliferation, and tumor growth. Overexpression of PRPF19 increases the resistance of tongue cancer cells to radiation and cisplatin treatment. Furthermore, PRPF19 regulates the expression of solute carrier family 40 member 1 (SLC40A1) and mono-ADP ribosylhydrolase 2 (MACROD2), knockdown of SLC40A1 or MACROD2 decreases the sensitivity of tongue cancer cells to radiation and cisplatin treatment. Thus, our results establish a key role of PRPF19 in tongue cancer growth and chemoradiotherapy resistance, targeting PRPF19 would be an effective therapeutic strategy for tongue cancer, especially for those resistant to chemoradiotherapy.

CircRNA-SORE mediates sorafenib resistance in hepatocellular carcinoma by stabilizing YBX1

Sorafenib is the first-line chemotherapeutic therapy for advanced hepatocellular carcinoma (HCC). However, sorafenib resistance significantly limits its therapeutic efficacy, and the mechanisms underlying resistance have not been fully clarified. Here we report that a circular RNA, circRNA-SORE (a circular RNA upregulated in sorafenib-resistant HCC cells), plays a significant role in sorafenib resistance in HCC. We found that circRNA-SORE is upregulated in sorafenib-resistant HCC cells and depletion of circRNA-SORE substantially increases the cell-killing ability of sorafenib. Further studies revealed that circRNA-SORE binds the master oncogenic protein YBX1 in the cytoplasm, which prevents YBX1 nuclear interaction with the E3 ubiquitin ligase PRP19 and thus blocks PRP19-mediated YBX1 degradation. Moreover, our in vitro and in vivo results suggest that circRNA-SORE is transported by exosomes to spread sorafenib resistance among HCC cells. Using different HCC mouse models, we demonstrated that silencing circRNA-SORE by injection of siRNA could substantially overcome sorafenib resistance. Our study provides a proof-of-concept demonstration for a potential strategy to overcome sorafenib resistance in HCC patients by targeting circRNA-SORE or YBX1.

Silencing Core Spliceosome Sm Gene Expression Induces a Cytotoxic Splicing Switch in the Proteasome Subunit Beta 3 mRNA in Non-Small Cell Lung Cancer Cells

The core spliceosomal Sm proteins were recently proposed as cancer-selective lethal targets in non-small cell lung cancer (NSCLC). In contrast, the loss of the commonly mutated cancer target SF3B1 appeared to be toxic to non-malignant cells as well. In the current study, the transcriptomes of A549 NSCLC cells, in which SF3B1 or SNRPD3 was silenced, were compared using RNA sequencing. The skipping of exon 4 of the proteasomal subunit beta type-3 (PSMB3) mRNA, resulting in a shorter PSMB3-S variant, occurred only after silencing SNRPD3. This observation was extended to the other six Sm genes. Remarkably, the alternative splicing of PSMB3 mRNA upon Sm gene silencing was not observed in non-malignant IMR-90 lung fibroblasts. Furthermore, PSMB3 was found to be overexpressed in NSCLC clinical samples and PSMB3 expression correlated with Sm gene expression. Moreover, a high PSMB3 expression corresponds to worse survival in patients with lung adenocarcinomas. Finally, silencing the canonical full-length PSMB3-L, but not the shorter PSMB3-S variant, was cytotoxic and was accompanied by a decrease in proteasomal activity. Together, silencing Sm genes, but not SF3B1, causes a cytotoxic alternative splicing switch in the PSMB3 mRNA in NSCLC cells only.

Cellular Responses to Proteasome Inhibition: Molecular Mechanisms and Beyond

Proteasome inhibitors have been actively tested as potential anticancer drugs and in the treatment of inflammatory and autoimmune diseases. Unfortunately, cells adapt to survive in the presence of proteasome inhibitors activating a variety of cell responses that explain why these therapies have not fulfilled their expected results. In addition, all proteasome inhibitors tested and approved by the FDA have caused a variety of side effects in humans. Here, we describe the different types of proteasome complexes found within cells and the variety of regulators proteins that can modulate their activities, including those that are upregulated in the context of inflammatory processes. We also summarize the adaptive cellular responses activated during proteasome inhibition with special emphasis on the activation of the Autophagic-Lysosomal Pathway (ALP), proteaphagy, p62/SQSTM1 enriched-inclusion bodies, and proteasome biogenesis dependent on Nrf1 and Nrf2 transcription factors. Moreover, we discuss the role of IRE1 and PERK sensors in ALP activation during ER stress and the involvement of two deubiquitinases, Rpn11 and USP14, in these processes. Finally, we discuss the aspects that should be currently considered in the development of novel strategies that use proteasome activity as a therapeutic target for the treatment of human diseases.

Genome-wide identification and functional analysis of the splicing component SYF2/NTC31/p29 across different plant species

This study systematically identifies plant SYF2/NTC31/p29 genes from 62 plant species by a combinatory bioinformatics approach, revealing the importance of this gene family in phylogenetics, duplication, transcriptional, and post-transcriptional regulation. Alternative splicing is a post-transcriptional regulatory mechanism, which is critical for plant development and stress responses. The entire process is strictly attenuated by a complex of splicing-related proteins, designated splicing factors. Human p29, also referred to as synthetic lethal with cdc forty 2 (SYF2) or the NineTeen complex 31 (NTC31), is a core protein found in the NTC complex of humans and yeast. This splicing factor participates in a variety of biological processes, including DNA damage repair, control of the cell cycle, splicing, and tumorigenesis. However, its function in plants has been seldom reported. Thus, we have systematically identified 89 putative plant SYF2s from 62 plant species among the deposited entries in the Phytozome database. The phylogenetic relationships and evolutionary history among these plant SYF2s were carefully examined. The results revealed that plant SYF2s exhibited distinct patterns regarding their gene structure, promoter sequences, and expression levels, suggesting their functional diversity in response to developmental cues or stress treatments. Although local duplication events, such as tandem duplication and retrotransposition, were found among several plant species, most of the plant species contained only one copy of SYF2, suggesting the existence of additional mechanisms to confer duplication resistance. Further investigation using the model dicot and monocot representatives Arabidopsis and rice SYF2s indicated that the splicing pattern and resulting protein isoforms might play an alternative role in the functional diversity.

The Dual Role of Cellular Senescence in Developing Tumors and Their Response to Cancer Therapy

Cellular senescence describes an irreversible growth arrest characterized by distinct morphology, gene expression pattern, and secretory phenotype. The final or intermediate stages of senescence can be reached by different genetic mechanisms and in answer to different external and internal stresses. It has been maintained in the literature but never proven by clearcut experiments that the induction of senescence serves the evolutionary purpose of protecting the individual from development and growth of cancers. This hypothesis was recently scrutinized by new experiments and found to be partly true, but part of the gene activities now known to happen in senescence are also needed for cancer growth, leading to the view that senescence is a double-edged sword in cancer development. In current cancer therapy, cellular senescence is, on the one hand, intended to occur in tumor cells, as thereby the therapeutic outcome is improved, but might, on the other hand, also be induced unintentionally in non-tumor cells, causing inflammation, secondary tumors, and cancer relapse. Importantly, organismic aging leads to accumulation of senescent cells in tissues and organs of aged individuals. Senescent cells can occur transiently, e.g., during embryogenesis or during wound healing, with beneficial effects on tissue homeostasis and regeneration or accumulate chronically in tissues, which detrimentally affects the microenvironment by de- or transdifferentiation of senescent cells and their neighboring stromal cells, loss of tissue specific functionality, and induction of the senescence-associated secretory phenotype, an increased secretory profile consisting of pro-inflammatory and tissue remodeling factors. These factors shape their surroundings toward a pro-carcinogenic microenvironment, which fuels the development of aging-associated cancers together with the accumulation of mutations over time. We are presenting an overview of well-documented stress situations and signals, which induce senescence. Among them, oncogene-induced senescence and stress-induced premature senescence are prominent. New findings about the role of senescence in tumor biology are critically reviewed with respect to new suggestions for cancer therapy leveraging genetic and pharmacological methods to prevent senescence or to selectively kill senescent cells in tumors.

TaPUB1, a Putative E3 Ligase Gene from Wheat, Enhances Salt Stress Tolerance in Transgenic Nicotiana benthamiana

High salinity is one of the most severe environmental stresses and limits the growth and yield of diverse crop plants. We isolated a gene named TaPUB1 from wheat (Triticum aestivum L. cv HF9703) that encodes a novel protein containing a U-box domain, the precursor RNA processing 19p (Prp19) superfamily and WD-40 repeats. Real-time reverse transcription-PCR analysis showed that TaPUB1 transcript accumulation was up-regulated by high salinity, drought and phytohormones, suggesting that it plays a role in the abiotic-related defense response. We overexpressed TaPUB1 in Nicotiana benthamiana to evaluate the function of TaPUB1 in the regulation of the salt stress response. Transgenic N. benthamiana plants (OE) with constitutively overexpressed TaPUB1 under the control of the Cauliflower mosaic virus 35S (CaMV 35S) promoter exhibited a higher germination rate, less growth inhibition, less Chl loss and higher photosynthetic capacity than wild-type (WT) plants under salt stress conditions. These results demonstrated the increased tolerance of OE plants to salt stress compared with the WT. The OE plants had lower osmotic potential (OP), reduced Na+ toxicity and less reactive oxygen species accumulation compared with the WT, which may be related to their higher level of osmolytes, lower Na+/K+ ratio and higher antioxidant enzyme activities under salt stress conditions. Consistent with these results, the up-regulated expression of osmic- and antioxidant-related genes in OE plants indicated a role for TaPUB1 in plant salt tolerance.

The U-box family genes in Medicago truncatula: Key elements in response to salt, cold, and drought stresses

The ubiquitination pathway regulates growth, development, and stress responses in plants, and the U-box protein family of ubiquitin ligases has important roles in this pathway. Here, 64 putative U-box proteins were identified in the Medicago truncatula genome. In addition to the conserved U-box motif, other functional domains, such as the ARM, kinase, KAP, and WD40 domains, were also detected. Phylogenetic analysis of the M. truncatula U-box proteins grouped them into six subfamilies, and chromosomal mapping and synteny analyses indicated that tandem and segmental duplications may have contributed to the expansion and evolution of the U-box gene family in this species. Using RNA-seq data from M. truncatula seedlings subjected to three different abiotic stresses, we identified 33 stress-inducible plant U-box genes (MtPUBs). Specifically, 25 salinity-, 15 drought-, and 16 cold-regulated MtPUBs were detected. Among them, MtPUB10, MtPUB17, MtPUB18, MtPUB35, MtPUB42, and MtPUB44 responded to all three stress conditions. Expression profiling by qRT-PCR was consistent with the RNA-seq data, and stress-related elements were identified in the promoter regions. The present findings strongly indicate that U-box proteins play critical roles in abiotic stress response in M. truncatula.

Ubiquitous overexpression of the DNA repair factor dPrp19 reduces DNA damage and extends Drosophila life span

Mechanisms that ensure and maintain the stability of genetic information are fundamentally important for organismal function and can have a large impact on disease, aging, and life span. While a multi-layered cellular apparatus exists to detect and respond to DNA damage, various insults from environmental and endogenous sources continuously affect DNA integrity. Over time this can lead to the accumulation of somatic mutations, which is thought to be one of the major causes of aging. We have previously found that overexpression of the essential human DNA repair and splicing factor SNEV, also called PRP19 or hPso4, extends replicative life span of cultured human endothelial cells and impedes accumulation of DNA damage. Here, we show that adult-specific overexpression of dPrp19, the D. melanogaster ortholog of human SNEV/PRP19/hPso4, robustly extends life span in female fruit flies. This increase in life span is accompanied by reduced levels of DNA damage and improved resistance to oxidative and genotoxic stress. Our findings suggest that dPrp19 plays an evolutionarily conserved role in aging, life span modulation and stress resistance, and support the notion that superior DNA maintenance is key to longevity.

Increasing levels of DNA repair factor Prp19 in fruit flies extends their life span and protects against stress. Prp19 is a protein that is present in a wide range of organisms and enables human endothelial cells to live longer in vitro. In this article, an international team of scientists from Austria, Germany and Switzerland found that higher Prp19 levels also prolong the life span of a whole organism in fruit flies, reduce DNA damage and increase survival when exposed to DNA damaging compounds. In contrast to female flies, males were unaffected. Their findings support the long-held view that repair of DNA damage, one of the hallmarks of aging, is key to longevity. They also provide an intriguing but poorly understood connection between cellular aging and the survival of whole organisms.

SNEVhPrp19/hPso4 Regulates Adipogenesis of Human Adipose Stromal Cells

Aging is accompanied by loss of subcutaneous adipose tissue. This may be due to reduced differentiation capacity or deficiency in DNA damage repair (DDR) factors. Here we investigated the role of SNEV^hPrp19/hPso4, which was implicated in DDR and senescence evasion, in adipogenic differentiation of human adipose stromal cells (hASCs). We showed that SNEV is induced during adipogenesis and localized both in the nucleus and in the cytoplasm. Knockdown of SNEV perturbed adipogenic differentiation and led to accumulation of DNA damage in hASCs upon oxidative stress. In addition, we demonstrated that SNEV is required for fat deposition in Caenorhabditis elegans. Consequently, we tested other DDR factors and found that WRN is also required for adipogenesis in both models. These results demonstrate that SNEV regulates adipogenesis in hASCs and indicate that DDR capacity in general might be a pre-requisite for this process.

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SNEV is required for adipogenic differentiation of human adipose stromal cells

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SNEV modulates pro- and anti-adipogenic signaling pathways

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SNEV regulates DNA repair capacity of human adipose stromal cells

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SNEV modulates organismal fat deposition in Caenorhabditis elegans

hPso4/hPrp19: a critical component of DNA repair and DNA damage checkpoint complexes

Genome integrity is vital to cellular homeostasis and its forfeiture is linked to deleterious consequences-cancer, immunodeficiency, genetic disorders and premature aging. The human ubiquitin ligase Pso4/Prp19 has emerged as a critical component of multiple DNA damage response (DDR) signaling networks. It not only senses DNA damage, binds double-stranded DNA in a sequence-independent manner, facilitates processing of damaged DNA, promotes DNA end joining, regulates replication protein A (RPA2) phosphorylation and ubiquitination at damaged DNA, but also regulates RNA splicing and mitotic spindle formation in its integral capacity as a scaffold for a multimeric core complex. Accordingly, by virtue of its regulatory and structural interactions with key proteins critical for genome integrity-DNA double-strand break (DSB) repair, DNA interstrand crosslink repair, repair of stalled replication forks and DNA end joining-it fills a unique niche in restoring genomic integrity after multiple types of DNA damage and thus has a vital role in maintaining chromatin integrity and cellular functions. These properties may underlie its ability to thwart replicative senescence and, not surprisingly, have been linked to the self-renewal and colony-forming ability of murine hematopoietic stem cells. This review highlights recent advances in hPso4 research that provides a fascinating glimpse into the pleiotropic activities of a ubiquitously expressed multifunctional E3 ubiquitin ligase.

Simultaneous EGFP and tag labeling of the β7 subunit for live imaging and affinity purification of functional human proteasomes

The proteasome is a multi-subunit protein complex that serves as a major pathway for intracellular protein degradation, playing important functions in various biological processes. The C-terminus of the β7 (PSMB4) proteasome subunit was tagged with EGFP and with a composite element for affinity purification and TEV cleavage elution (HTBH). When the construct was retrovirally delivered into HeLa cells, virtually all of the β7-EGFP-HTBH fusion protein was found to be incorporated into fully functional proteasomes. This ensured that subcellular localization of the EGFP signal in living HeLa cells could be attributed to β7-EGFP-HTBH within the proteasome complex rather than to free protein. The β7-EGFP-HTBH fusion can, therefore, serve as a valuable tool for in vivo imaging of proteasomes as well as for high-affinity purification of these complexes and associated molecules for subsequent analyses.

The PSO4 protein complex associates with replication protein A (RPA) and modulates the activation of ataxia telangiectasia-mutated and Rad3-related (ATR)

The PSO4 core complex is composed of PSO4/PRP19/SNEV, CDC5L, PLRG1, and BCAS2/SPF27. Besides its well defined functions in pre-mRNA splicing, the PSO4 complex has been shown recently to participate in the DNA damage response. However, the specific role for the PSO4 complex in the DNA damage response pathways is still not clear. Here we show that both the BCAS2 and PSO4 subunits of the PSO4 complex directly interact and colocalize with replication protein A (RPA). Depletion of BCAS2 or PSO4 impairs the recruitment of ATR-interacting protein (ATRIP) to DNA damage sites and compromises CHK1 activation and RPA2 phosphorylation. Moreover, we demonstrate that both the RPA1-binding ability of BCAS2 and the E3 ligase activity of PSO4 are required for efficient accumulation of ATRIP at DNA damage sites and the subsequent CHK1 activation and RPA2 phosphorylation. Our results suggest that the PSO4 complex functionally interacts with RPA and plays an important role in the DNA damage response.

Regulation of proteasome activity in health and disease

The ubiquitin-proteasome system (UPS) is the primary selective degradation system in the nuclei and cytoplasm of eukaryotic cells, required for the turnover of myriad soluble proteins. The hundreds of factors that comprise the UPS include an enzymatic cascade that tags proteins for degradation via the covalent attachment of a poly-ubiquitin chain, and a large multimeric enzyme that degrades ubiquitinated proteins, the proteasome. Protein degradation by the UPS regulates many pathways and is a crucial component of the cellular proteostasis network. Dysfunction of the ubiquitination machinery or the proteolytic activity of the proteasome is associated with numerous human diseases. In this review we discuss the contributions of the proteasome to human pathology, describe mechanisms that regulate the proteolytic capacity of the proteasome, and discuss strategies to modulate proteasome function as a therapeutic approach to ameliorate diseases associated with altered UPS function. This article is part of a Special Issue entitled: Ubiquitin-Proteasome System. Guest Editors: Thomas Sommer and Dieter H. Wolf.

Splicing and beyond: the many faces of the Prp19 complex

The conserved Prp19 complex (Prp19C) - also known as NineTeen Complex (NTC) - functions in several processes of paramount importance for cellular homeostasis. NTC/Prp19C was discovered as a complex that functions in splicing and more specifically during the catalytic activation of the spliceosome. More recent work revealed that NTC/Prp19C plays a role in transcription elongation in Saccharomyces cerevisiae and in genome maintenance in higher eukaryotes. In addition, mouse PRP19 might ubiquity late proteins targeted for degradation and guide them to the proteasome. Furthermore, NTC/Prp19C has been implicated in lipid droplet biogenesis. In the future, the molecular function of NTC/Prp19C in all of these processes needs to be refined or elucidated. Most of NTC/Prp19C's functions have been shown in only one or few organisms. However, since this complex is highly conserved it is likely that it has the same functions across all species. Moreover, one NTC/Prp19C or different subcomplexes could function in the above-mentioned processes. Intriguingly, NTC/Prp19C might link these different processes to ensure an optimal coordination of cellular processes. Thus, many important questions about the functions of this interesting complex remain to be investigated. In this review we discuss the different functions of NTC/Prp19C focusing on the novel and emerging ones as well as open questions.

The MLL recombinome of acute leukemias in 2013

Chromosomal rearrangements of the human MLL (mixed lineage leukemia) gene are associated with high-risk infant, pediatric, adult and therapy-induced acute leukemias. We used long-distance inverse-polymerase chain reaction to characterize the chromosomal rearrangement of individual acute leukemia patients. We present data of the molecular characterization of 1590 MLL-rearranged biopsy samples obtained from acute leukemia patients. The precise localization of genomic breakpoints within the MLL gene and the involved translocation partner genes (TPGs) were determined and novel TPGs identified. All patients were classified according to their gender (852 females and 745 males), age at diagnosis (558 infant, 416 pediatric and 616 adult leukemia patients) and other clinical criteria. Combined data of our study and recently published data revealed a total of 121 different MLL rearrangements, of which 79 TPGs are now characterized at the molecular level. However, only seven rearrangements seem to be predominantly associated with illegitimate recombinations of the MLL gene (≈ 90%): AFF1/AF4, MLLT3/AF9, MLLT1/ENL, MLLT10/AF10, ELL, partial tandem duplications (MLL PTDs) and MLLT4/AF6, respectively. The MLL breakpoint distributions for all clinical relevant subtypes (gender, disease type, age at diagnosis, reciprocal, complex and therapy-induced translocations) are presented. Finally, we present the extending network of reciprocal MLL fusions deriving from complex rearrangements.

Inhibition of pre-mRNA splicing by a synthetic Blom7α-interacting small RNA

Originally the novel protein Blom7α was identified as novel pre-mRNA splicing factor that interacts with SNEV^Prp19/Pso4, an essential protein involved in extension of human endothelial cell life span, DNA damage repair, the ubiquitin-proteasome system, and pre-mRNA splicing. Blom7α belongs to the heteronuclear ribonucleoprotein K homology (KH) protein family, displaying 2 KH domains, a well conserved and widespread RNA-binding motif. In order to identify specific sequence binding motifs, we here used Systematic Evolution of Ligands by Exponential Enrichment (SELEX) with a synthetic RNA library. Besides sequence motifs like (U/A)_1–4 C_2–6 (U/A)_1–5, we identified an AC-rich RNA-aptamer that we termed AK48 (Aptamer KH-binding 48), binding to Blom7α with high affinity. Addition of AK48 to pre-mRNA splicing reactions in vitro inhibited the formation of mature spliced mRNA and led to a slight accumulation of the H complex of the spliceosome. These results suggest that the RNA binding activity of Blom7α might be required for pre-mRNA splicing catalysis. The inhibition of in-vitro splicing by the small RNA AK48 indicates the potential use of small RNA molecules in targeting the spliceosome complex as a novel target for drug development.

Cereblon inhibits proteasome activity by binding to the 20S core proteasome subunit beta type 4

In humans, mutations in the gene encoding cereblon (CRBN) are associated with mental retardation. Although CRBN has been investigated in several cellular contexts, its function remains unclear. Here, we demonstrate that CRBN plays a role in regulating the ubiquitin-proteasome system (UPS). Heterologous expression of CRBN inhibited proteasome activity in a human neuroblastoma cell line. Furthermore, proteasome subunit beta type 4 (PSMB4), the β7 subunit of the 20S core complex, was identified as a direct binding partner of CRBN. These findings suggest that CRBN may modulate proteasome activity by directly interacting with the β7 subunit.

ATM-dependent phosphorylation of SNEVhPrp19/hPso4 is involved in extending cellular life span and suppression of apoptosis

Defective DNA repair is widely acknowledged to negatively impact on healthy aging, since mutations in DNA repair factors lead to accelerated and premature aging. However, the opposite, namely if improved DNA repair will also increase the life or health span is less clear, and only few studies have tested if overexpression of DNA repair factors modulates life and health span in cells or organisms. Recently, we identified and characterized SNEV^hPrp19/hPso4, a protein that plays a role in DNA repair and pre-mRNA splicing, and observed a doubling of the replicative life span upon ectopic overexpression, accompanied by lower basal DNA damage and apoptosis levels as well as an increased resistance to oxidative stress. Here we find that SNEV^hPrp19/hPso4 is phosphorylated at S149 in an ataxia telangiectasia mutated protein (ATM)-dependent manner in response to oxidative stress and DNA double strand break inducing agents. By overexpressing wild-type SNEV^hPrp19/hPso4 and a phosphorylation-deficient point-mutant, we found that S149 phosphorylation is necessary for mediating the resistance to apoptosis upon oxidative stress and is partially necessary for elongating the cellular life span. Therefore, ATM dependent phosphorylation of SNEV^hPrp19/hPso4 upon DNA damage or oxidative stress might represent a novel axis capable of modulating cellular life span.

The spliceosome-activating complex: molecular mechanisms underlying the function of a pleiotropic regulator

Correct interpretation of the coding capacity of RNA polymerase II transcribed eukaryotic genes is determined by the recognition and removal of intronic sequences of pre-mRNAs by the spliceosome. Our current knowledge on dynamic assembly and subunit interactions of the spliceosome mostly derived from the characterization of yeast, Drosophila, and human spliceosomal complexes formed on model pre-mRNA templates in cell extracts. In addition to sequential structural rearrangements catalyzed by ATP-dependent DExH/D-box RNA helicases, catalytic activation of the spliceosome is critically dependent on its association with the NineTeen Complex (NTC) named after its core E3 ubiquitin ligase subunit PRP19. NTC, isolated recently from Arabidopsis, occurs in a complex with the essential RNA helicase and GTPase subunits of the U5 small nuclear RNA particle that are required for both transesterification reactions of splicing. A compilation of mass spectrometry data available on the composition of NTC and spliceosome complexes purified from different organisms indicates that about half of their conserved homologs are encoded by duplicated genes in Arabidopsis. Thus, while mutations of single genes encoding essential spliceosome and NTC components lead to cell death in other organisms, differential regulation of some of their functionally redundant Arabidopsis homologs permits the isolation of partial loss of function mutations. Non-lethal pleiotropic defects of these mutations provide a unique means for studying the roles of NTC in co-transcriptional assembly of the spliceosome and its crosstalk with DNA repair and cell death signaling pathways.

The evolutionarily conserved MOS4-associated complex

Recent work in plant immunity has shown that MOS4, a known intermediate in R protein mediated resistance, is a core member of the nuclear MOS4-associated complex (MAC). This complex is highly conserved in eukaryotes, as orthologous complexes known as the CDC5L-SNEVPrp19-Pso4 complex and the Nineteen complex (NTC) were previously identified in human and yeast, respectively. The involvement of these complexes in pre-mRNA splicing and spliceosome assembly suggests that the MAC probably has a similar function in plants. Double mutants of any two MAC components are lethal, whereas single mutants of the MAC core components mos4, Atcdc5, mac3, and prl1 are all viable and display pleiotropic defects. This suggests that while the MAC is required for some essential biological function such as splicing, individual MAC components are not crucial for complex functionality and likely have regulatory roles in other biological processes such as plant immunity and flowering time control. Future studies on MAC components in Arabidopsis will provide further insight into the regulatory mechanisms of the MAC on specific biological processes.

Exo70, a subunit of the exocyst complex, interacts with SNEV(hPrp19/hPso4) and is involved in pre-mRNA splicing

The Cdc5L (cell division cycle 5-like) complex is a spliceosomal subcomplex that also plays a role in DNA repair. The complex contains the splicing factor hPrp19, also known as SNEV or hPso4, which is involved in cellular life-span regulation and proteasomal breakdown. In a recent large-scale proteomics analysis for proteins associated with this complex, proteins involved in transcription, cell-cycle regulation, DNA repair, the ubiquitin-proteasome system, chromatin remodelling, cellular aging, the cytoskeleton and trafficking, including four members of the exocyst complex, were identified. In the present paper we report that Exo70 interacts directly with SNEV(hPrp19/hPso4) and shuttles to the nucleus, where it associates with the spliceosome. We mapped the interaction site to the N-terminal 100 amino acids of Exo70, which interfere with pre-mRNA splicing in vitro. Furthermore, Exo70 influences the splicing of a model substrate as well as of its own pre-mRNA in vivo. In addition, we found that Exo70 is alternatively spliced in a cell-type- and cell-age- dependent way. These results suggest a novel and unexpected role of Exo70 in nuclear mRNA splicing, where it might signal membrane events to the splicing apparatus.

An intranucleolar body associated with rDNA

The nucleolus is the subnuclear organelle responsible for ribosome subunit biogenesis and can also act as a stress sensor. It forms around clusters of ribosomal DNA (rDNA) and is mainly organised in three subcompartments, i.e. fibrillar centre, dense fibrillar component and granular component. Here, we describe the localisation of 21 protein factors to an intranucleolar region different to these main subcompartments, called the intranucleolar body (INB). These factors include proteins involved in DNA maintenance, protein turnover, RNA metabolism, chromatin organisation and the post-translational modifiers SUMO1 and SUMO2/3. Increase in the size and number of INBs is promoted by specific types of DNA damage and depends on the functional integrity of the nucleolus. INBs are abundant in nucleoli of unstressed cells during S phase and localise in close proximity to rDNA with heterochromatic features. The data suggest the INB is linked with regulation of rDNA transcription and/or maintenance of rDNA.

Ubiquitin-proteasome genes as targets for modulation of cisplatin sensitivity in fission yeast

Background

The ubiquitin(Ub)-proteasome pathway is implicated in the regulation of a variety of cellular functions and plays a major role in stress response in eukaryotic cells, by targeting misfolded and damaged proteins for degradation. In addition, in the presence of DNA damage, the Ub-proteasome system regulates proteins involved in sensing, repairing, and/or tolerating the damage. Antitumor agents such as cisplatin can activate the pathway, but the role of specific pathway components in cell sensitivity/response to the drug is not known. Since platinum compounds represent clinically relevant antitumor agents and a major limitation to their use is the development of drug resistance, there is an urgent need for identifying targets for improving their efficacy.

Results

In the present study, we performed a genome-wide screening for sensitivity to cisplatin using non-essential haploid deletion mutants of the fission yeast Schizosaccharomyces pombe, belonging to a collection of haploid strains constructed through homologous recombination. Using this approach, we identified three Ub-proteasome mutants exhibiting hypersensitivity to cisplatin (ubp16, ubc13 and pmt3) and ten mutants (including ufd2, beta7 20S, rpt6/let1) resistant to the drug. In addition, the importance of lub1 gene emerged from the comparison between the present screening and gene expression profile data previously obtained in fission yeast.

Conclusions

The factors identified in the present study allowed us to highlight most finely the close relationship between the Ub-proteasome system and DNA damage response mechanisms, thus establishing a comprehensive framework of regulators likely relevant also in higher eukaryotes. Our results provide the proof of principle of the involvement of specific genes modulated by cisplatin treatment in cell response to the drug, suggesting their potential role as targets for modulating cisplatin sensitivity. In this regard, the prospective identification of novel targets for modulation of cisplatin sensitivity in an eukaryotic model organism appears particularly intriguing towards the discovery of strategies to overcome cisplatin resistance in human tumors.

Post-translational modification of cellular proteins by ubiquitin and ubiquitin-like molecules: role in cellular senescence and aging

Ubiquitination ofendogenous proteins is one of the key regulatory steps that guides protein degradation through regulation of proteasome activity. During the last years evidence has accumulated that proteasome activity is decreased during the aging process in various model systems and that these changes might be causally related to aging and age-associated diseases. Since in most instances ubiquitination is the primary event in target selection, the system ofubiquitination and deubiquitination might be of similar importance. Furthermore, ubiquitination and proteasomal degradation are not completely congruent, since ubiquitination confers also functions different from targeting proteins for degradation. Depending on mono- and polyubiquitination and on how ubiquitin chains are linked together, post-translational modifications of cellular proteins by covalent attachment of ubiquitin and ubiquitin-like proteins are involved in transcriptional regulation, receptor internalization, DNA repair, stabilization of protein complexes and autophagy. Here, we summarize the current knowledge regarding the ubiquitinome and the underlying ubiquitin ligases and deubiquitinating enzymes in replicative senescence, tissue aging as well as in segmental progeroid syndromes and discuss potential causes and consequences for aging.

Regulation of Metnase's TIR binding activity by its binding partner, Pso4

Metnase (also known as SETMAR) is a SET and transposase fusion protein in humans and plays a positive role in double-strand break (DSB) repair. While the SET domain possesses histone lysine methyltransferase activity, the transposase domain is responsible for 5'-terminal inverted repeat (TIR)-specific binding, DNA looping, and DNA cleavage activities. We recently demonstrated that human homolog of Pso4 (hPso4) is a Metnase binding partner that mediates Metnase binding to non-TIR DNA such as DNA damage sites. Here we show that Metnase functions as a dimer in its TIR binding. While both Metnase and hPso4 can independently interact with TIR DNA, Metnase's DNA binding activity is not required for formation of the Metnase-hPso4-DNA complex. A further stoichiometric analysis indicated that only one protein is involved in interaction with dsDNA when Metnase-hPso4 forms a stable complex. Interaction of the Metnase-hPso4 complex with TIR DNA was competitively inhibited by both TIR and non-TIR DNA, suggesting that hPso4 is solely responsible for binding to DNA in the Metnase-hPso4-DNA complex. Together, our study suggests that hPso4, once it forms a complex with Metnase, negatively regulates Metnase's TIR binding activity, which is perhaps necessary for Metnase localization at non-TIR sites such as DSBs.

Blom7alpha is a novel heterogeneous nuclear ribonucleoprotein K homology domain protein involved in pre-mRNA splicing that interacts with SNEVPrp19-Pso4

The removal of introns from pre-mRNA is performed by the spliceosome that stepwise assembles on the pre-mRNA before performing two catalytic steps. The spliceosome-associated CDC5L-SNEV(Prp19-Pso4) complex is implicated in activation of the second catalytic step of pre-mRNA splicing, and one of its members, SNEV(Prp19-Pso4), is also implicated in spliceosome assembly. To identify interaction partners of SNEVPrp19-Pso4, we have performed yeast two-hybrid screenings. Among the putative binding partners was a so far uncharacterized protein carrying two heterogeneous nuclear ribonucleoprotein K homology domains that we termed Blom7alpha. Blom7alpha is expressed in all tissues tested, and at least three splice variants exist. After confirming direct and physical interaction of SNEV and Blom7alpha, we investigated if it plays a functional role during pre-mRNA splicing. Indeed, Blom7alpha co-localizes and co-precipitates with splicing factors and pre-mRNA and is present in affinity-purified spliceosomes. More importantly, addition of Blom7alpha to HeLa nuclear extracts increased splicing activity in a dose-dependent manner. Furthermore, we tested if Blom7alpha influences splice site selection using two different minigene constructs. Indeed, both 5'- as well as 3'-site selection was altered upon Blom7alpha overexpression. Thus we suggest that Blom7alpha is a novel splicing factor of the K homology domain family that might be implicated in alternative splicing by helping to position the CDC5L-SNEV(Prp19-Pso4) complex at the splice sites.

Cdc5L interacts with ATR and is required for the S-phase cell-cycle checkpoint

Cell division cycle 5-like protein (Cdc5L) is a core component of the putative E3 ubiquitin ligase complex containing Prp19/Pso4, Plrg1 and Spf27. This complex has been shown to have a role in pre-messenger RNA splicing from yeast to humans; however, more recent studies have described a function for this complex in the cellular response to DNA damage. Here, we show that Cdc5L interacts physically with the cell-cycle checkpoint kinase ataxia-telangiectasia and Rad3-related (ATR). Depletion of Cdc5L by RNA-mediated interference methods results in a defective S-phase cell-cycle checkpoint and cellular sensitivity in response to replication-fork blocking agents. Furthermore, we show that Cdc5L is required for the activation of downstream effectors or mediators of ATR checkpoint function such as checkpoint kinase 1 (Chk1), cell cycle checkpoint protein Rad 17 (Rad17) and Fanconi anaemia complementation group D2 protein (FancD2). In addition, we have mapped the ATR-binding region in Cdc5L and show that a deletion mutant that is unable to interact with ATR is defective in the rescue of the checkpoint deficiency in Cdc5L-depleted cells. These findings show a new function for Cdc5L in the regulation of the ATR-mediated cell-cycle checkpoint in response to genotoxic agents.

Two Prp19-like U-box proteins in the MOS4-associated complex play redundant roles in plant innate immunity

Plant Resistance (R) proteins play an integral role in defense against pathogen infection. A unique gain-of-function mutation in the R gene SNC1, snc1, results in constitutive activation of plant immune pathways and enhanced resistance against pathogen infection. We previously found that mutations in MOS4 suppress the autoimmune phenotypes of snc1, and that MOS4 is part of a nuclear complex called the MOS4-Associated Complex (MAC) along with the transcription factor AtCDC5 and the WD-40 protein PRL1. Here we report the immuno-affinity purification of the MAC using HA-tagged MOS4 followed by protein sequence analysis by mass spectrometry. A total of 24 MAC proteins were identified, 19 of which have predicted roles in RNA processing based on their homology to proteins in the Prp19-Complex, an evolutionarily conserved spliceosome-associated complex containing homologs of MOS4, AtCDC5, and PRL1. Among these were two highly similar U-box proteins with homology to the yeast and human E3 ubiquitin ligase Prp19, which we named MAC3A and MAC3B. MAC3B was recently shown to exhibit E3 ligase activity in vitro. Through reverse genetics analysis we show that MAC3A and MAC3B are functionally redundant and are required for basal and R protein-mediated resistance in Arabidopsis. Like mos4-1 and Atcdc5-1, mac3a mac3b suppresses snc1-mediated autoimmunity. MAC3 localizes to the nucleus and interacts with AtCDC5 in planta. Our results suggest that MAC3A and MAC3B are members of the MAC that function redundantly in the regulation of plant innate immunity.

The RNA binding protein Cwc2 interacts directly with the U6 snRNA to link the nineteen complex to the spliceosome during pre-mRNA splicing

Intron removal during pre-messenger RNA (pre-mRNA) splicing involves arrangement of snRNAs into conformations that promote the two catalytic steps. The Prp19 complex [nineteen complex (NTC)] can specify U5 and U6 snRNA interactions with pre-mRNA during spliceosome activation. A candidate for linking the NTC to the snRNAs is the NTC protein Cwc2, which contains motifs known to bind RNA, a zinc finger and RNA recognition motif (RRM). In yeast cells mutation of either the zinc finger or RRM destabilize Cwc2 and are lethal. Yeast cells depleted of Cwc2 accumulate pre-mRNA and display reduced levels of U1, U4, U5 and U6 snRNAs. Cwc2 depletion also reduces U4/U6 snRNA complex levels, as found with depletion of other NTC proteins, but without increase in free U4. Purified Cwc2 displays general RNA binding properties and can bind both snRNAs and pre-mRNA in vitro. A Cwc2 RRM fragment alone can bind RNA but with reduced efficiency. Under splicing conditions Cwc2 can associate with U2, U5 and U6 snRNAs, but can only be crosslinked directly to the U6 snRNA. Cwc2 associates with U6 both before and after the first step of splicing. We propose that Cwc2 links the NTC to the spliceosome during pre-mRNA splicing through the U6 snRNA.

Human Pso4 is a metnase (SETMAR)-binding partner that regulates metnase function in DNA repair

Metnase, also known as SETMAR, is a SET and transposase fusion protein with an undefined role in mammalian DNA repair. The SET domain is responsible for histone lysine methyltransferase activity at histone 3 K4 and K36, whereas the transposase domain possesses 5'-terminal inverted repeat (TIR)-specific DNA binding, DNA looping, and DNA cleavage activities. Although the transposase domain is essential for Metnase function in DNA repair, it is not clear how a protein with sequence-specific DNA binding activity plays a role in DNA repair. Here, we show that human homolog of the ScPSO4/PRP19 (hPso4) forms a stable complex with Metnase on both TIR and non-TIR DNA. The transposase domain essential for Metnase-TIR interaction is not sufficient for its interaction with non-TIR DNA in the presence of hPso4. In vivo, hPso4 is induced and co-localized with Metnase following ionizing radiation treatment. Cells treated with hPso4-siRNA failed to show Metnase localization at DSB sites and Metnase-mediated stimulation of DNA end joining coupled to genomic integration, suggesting that hPso4 is necessary to bring Metnase to the DSB sites for its function(s) in DNA repair.

Contributions of DNA interstrand cross-links to aging of cells and organisms

Impaired DNA damage repair, especially deficient transcription-coupled nucleotide excision repair, leads to segmental progeroid syndromes in human patients as well as in rodent models. Furthermore, DNA double-strand break signalling has been pinpointed as a key inducer of cellular senescence. Several recent findings suggest that another DNA repair pathway, interstrand cross-link (ICL) repair, might also contribute to cell and organism aging. Therefore, we summarize and discuss here that (i) systemic administration of anti-cancer chemotherapeutics, in many cases DNA cross-linking drugs, induces premature progeroid frailty in long-term survivors; (ii) that ICL-inducing 8-methoxy-psoralen/UVA phototherapy leads to signs of premature skin aging as prominent long-term side effect and (iii) that mutated factors involved in ICL repair like ERCC1/XPF, the Fanconi anaemia proteins, WRN and SNEV lead to reduced replicative life span in vitro and segmental progeroid syndromes in vivo. However, since ICL-inducing drugs cause damage different from ICL and since all currently known ICL repair factors work in more than one pathway, further work will be needed to dissect the actual contribution of ICL damage to aging.

Regulation of plant innate immunity by three proteins in a complex conserved across the plant and animal kingdoms

Innate immunity against pathogen infection is an evolutionarily conserved process among multicellular organisms. Arabidopsis SNC1 encodes a Resistance protein that combines attributes of multiple mammalian pattern recognition receptors. Utilizing snc1 as an autoimmune model, we identified a discrete protein complex containing at least three members--MOS4 (Modifier Of snc1, 4), AtCDC5, and PRL1 (Pleiotropic Regulatory Locus 1)--that are all essential for plant innate immunity. AtCDC5 has DNA-binding activity, suggesting that this complex probably regulates defense responses through transcriptional control. Since the complex components along with their interactions are highly conserved from fission yeast to Arabidopsis and human, they may also have a yet-to-be-identified function in mammalian innate immunity.

Mouse homologue of yeast Prp19 interacts with mouse SUG1, the regulatory subunit of 26S proteasome

Yeast Prp19 has been shown to involve in pre-mRNA splicing and DNA repair as well as being an ubiquitin ligase. Mammalian homologue of yeast Prp19 also plays on similar functional activities in cells. In the present study, we isolated mouse SUG1 (mSUG1) as binding partner of mouse Prp19 (mPrp19) by the yeast two-hybrid system. We confirmed the interaction of mPrp9 with mSUG1 by GST pull-down assay and co-immunoprecipitation assay. The N-terminus of mPrp19 including U-box domain was associated with the C-terminus of mSUG1. Although, mSUG1 is a regulatory subunit of 26S proteasome, mPrp19 was not degraded in the proteasome-dependent pathway. Interestingly, GFP-mPrp19 fusion protein was co-localized with mSUG1 protein in cytoplasm as the formation of the speckle-like structures in the presence of a proteasome inhibitor MG132. In addition, the activity of proteasome was increased in cells transfected with mPrp19. Taken together, these results suggest that mPrp19 involves the regulation of protein turnover and may transport its substrates to 26S proteasome through mSUG1 protein.

Early embryonic lethality of mice lacking the essential protein SNEV

SNEV (Prp19, Pso4, NMP200) is a nuclear matrix protein known to be involved in pre-mRNA splicing, ubiquitylation, and DNA repair. In human umbilical vein endothelial cells, SNEV overexpression delayed the onset of replicative senescence. Here we analyzed the function of the mouse SNEV gene in vivo by employing homologous recombination in mice and conclude that SNEV is indispensable for early mouse development. Mutant preimplantation embryos initiated blastocyst formation but died shortly thereafter. Outgrowth of SNEV-null blastocysts showed a lack of proliferation of cells of the inner cell mass, which subsequently underwent cell death. While SNEV-heterozygous mice showed no overt phenotype, heterozygous mouse embryonic fibroblast cell lines with reduced SNEV levels displayed a decreased proliferative potential in vitro. Our experiments demonstrate that the SNEV protein is essential, functionally nonredundant, and indispensable for mouse development.

The Prp19/Pso4 core complex undergoes ubiquitylation and structural alterations in response to DNA damage

Prp19/Pso4, a U-box containing E3 ligase, has a demonstrated role in pre-mRNA splicing, but has also been implicated in both yeast and mammalian cells as having a direct role in DNA damage processing. In this report, we provide further evidence in support of this latter assertion. We show that hPrp19 forms an ubiquitylated oligomeric species that is resistant to disruption by SDS gel electrophoresis under nonreducing conditions suggesting that is mediated by a thiolester between ubiquitin and a cysteine residue in Prp19. The level of this species is significantly enhanced upon treatment of cells with DNA damaging agents, and its association with chromatin is increased. In addition, hPrp19 is known to form a stable core complex with Cdc5L, Plrg1, and Spf27; however, ubiquitylated hPrp19 fails to interact with either Cdc5L or Plrg1 indicating that DNA damage can induce profound alterations to the hPrp19 core complex. Finally, we show that overexpression of hPrp19 in human cells provides a pro-survival affect in that it reduces the levels of apoptosis observed after exposure of cells to DNA damage.

Aging and the ubiquitinome: traditional and non-traditional functions of ubiquitin in aging cells and tissues

Ubiquitination of endogenous proteins is one of the key regulatory steps of protein degradation followed by regulation of proteasome activity. During the last years evidence has increased that proteasome activity is decreased during the aging process in various model systems and that these changes might be causally related to aging and aging associated diseases. Since in most instances ubiquitination is the primary event in target selection, the system of ubiquitination and deubiquitination might be of similar importance. Furthermore, ubiquitination and proteasomal degradation are not completely congruent, since ubiquitination also confers functions different from giving "the kiss of death" to proteins. Depending on mono- and polyubiquitination and on how ubiquitin chains are linked together, ubiquitination is involved in transcriptional regulation, receptor internalization, DNA repair, and stabilization of protein complexes. This review is therefore the first to summarize the current knowledge regarding the ubiquitinome and the underlying ubiquitin ligases and deubiquitinating enzymes in replicative senescence, tissue aging as well as in segmental progeroid syndromes and to discuss potential causes and consequences for aging.

SNEV overexpression extends the life span of human endothelial cells

In a recent screening for genes down regulated in replicatively senescent human umbilical vein endothelial cells (HUVECs), we have isolated the novel protein SNEV. Since then SNEV has proven as a multifaceted protein playing a role in pre-mRNA splicing, DNA repair, and the ubiquitin/proteosome system. Here, we report that SNEV mRNA decreases in various cell types during replicative senescence, and that it is increased in various immortalized cell lines, as well as in breast tumors, where SNEV transcript levels also correlate with the survival of breast cancer patients. Since these mRNA profiles suggested a role of SNEV in the regulation of cell proliferation, the effect of its overexpression was tested. Thereby, a significant extension of the cellular life span was observed, which was not caused by altered telomerase activity or telomere dynamics but rather by enhanced stress resistance. When SNEV overexpressing cells were treated with bleomycin or bleomycin combined with BSO, inducing DNA damage as well as reactive oxygen species, a significantly lower fraction of apoptotic cells was found in comparison to vector control cells. These data suggest that high levels of SNEV might extend the cellular life span by increasing the resistance to stress or by improving the DNA repair capacity of the cells.

SNEV is an evolutionarily conserved splicing factor whose oligomerization is necessary for spliceosome assembly

We have isolated the human protein SNEV as downregulated in replicatively senescent cells. Sequence homology to the yeast splicing factor Prp19 suggested that SNEV might be the orthologue of Prp19 and therefore might also be involved in pre-mRNA splicing. We have used various approaches including gene complementation studies in yeast using a temperature sensitive mutant with a pleiotropic phenotype and SNEV immunodepletion from human HeLa nuclear extracts to determine its function. A human-yeast chimera was indeed capable of restoring the wild-type phenotype of the yeast mutant strain. In addition, immunodepletion of SNEV from human nuclear extracts resulted in a decrease of in vitro pre-mRNA splicing efficiency. Furthermore, as part of our analysis of protein-protein interactions within the CDC5L complex, we found that SNEV interacts with itself. The self-interaction domain was mapped to amino acids 56-74 in the protein's sequence and synthetic peptides derived from this region inhibit in vitro splicing by surprisingly interfering with spliceosome formation and stability. These results indicate that SNEV is the human orthologue of yeast PRP19, functions in splicing and that homo-oligomerization of SNEV in HeLa nuclear extract is essential for spliceosome assembly and that it might also be important for spliceosome stability.

Establishment of a strategy for the rapid generation of a monoclonal antibody against the human protein SNEV (hNMP200) by flow-cytometric cell sorting

The screening for antigen-specific hybridoma cells with adequate production rates is still a time-, labour- and money-consuming procedure. A reduction in cell culture testing by specifically selecting those fused cells that produce antibody could therefore make hybridoma technology more attractive, even for small research groups or for newly discovered proteins at an early stage of research. Additional problems, such as the requirement to produce sufficient amounts of the unknown protein at a purity that allows specific immunisation of mice and testing of the resulting hybridoma clones, also need to be overcome. Here we present a new strategy to isolate rapidly and efficiently monoclonal antibodies against new proteins, for which only sequence information at the DNA level is known. The strategy consists of fusion of the protein to a hexa-His-tag to allow easy purification, production in yeast and insect cells to reduce background immunisation with host cell proteins and the selection of IgG-producing hybridoma cells by flow-cytometric cell sorting using the affinity matrix secretion assay technique.

The Pso4 mRNA splicing and DNA repair complex interacts with WRN for processing of DNA interstrand cross-links

DNA interstrand cross-links (ICLs) are perhaps the most formidable lesion encountered by the cellular DNA repair machinery, and the elucidation of the process by which they are removed in eukaryotic cells has proved a daunting task. In particular, the early stages of adduct recognition and uncoupling of the cross-link have remained elusive principally because genetic studies have not been highly revealing. We have developed a biochemical assay in which processing of a DNA substrate containing a site-specific psoralen ICL can be monitored in vitro. Using this assay we have shown previously that the mismatch repair factor MutSbeta, the nucleotide excision repair heterodimer Ercc1-Xpf, and the replication proteins RPA and PCNA are involved in an early stage of psoralen ICL processing. Here, we report the identification of two additional factors required in the ICL repair process, a previously characterized pre-mRNA splicing complex composed of Pso4/Prp19, Cdc5L, Plrg1, and Spf27 (Pso4 complex), and WRN the protein deficient in Werner syndrome. Analysis of the WRN protein indicates that its DNA helicase function, but not its exonuclease activity, is required for ICL processing in vitro. In addition, we show that WRN and the Pso4 complex interact through a direct physical association between WRN and Cdc5L. A putative model for uncoupling of ICLs in mammalian cells is presented.