A role for non-covalent SUMO interaction motifs in Pc2/CBX4 E3 activity

SUMO4 promotes SUMO deconjugation required for DNA double-strand-break repair

The amplitudes of small-modifier protein signaling through ubiquitin and the small ubiquitin-like modifiers, SUMO1-3, are critical to the correct phasing of DNA repair protein accumulation, activity, and clearance and for the completion of mammalian DNA double-strand-break (DSB) repair. However, how SUMO-conjugate signaling in the response is delineated is poorly understood. At the same time, the role of the non-conjugated SUMO protein, SUMO4, has remained enigmatic. Here, we reveal that human SUMO4 is required to prevent excessive DNA-damage-induced SUMOylation and deleterious over-accumulation of RAP80. Mechanistically we show that SUMO4 acts independently of its conjugation and potentiates SENP1 catalytic activity. These data identify SUMO4 as a SUMO deconjugation component and show that SUMO4:SENP1 are critical regulators of DNA-damage-induced SUMO signaling.

Characterizing the Conformational Dynamics of Human SUMO2: Insights into its Interaction with Metal Ions and SIMs

SUMO (Small Ubiquitin-like Modifiers) proteins are involved in a crucial post-translational modification commonly termed as SUMOylation. In this work, we have investigated the native-state conformational flexibility of human SUMO2 and its interaction with Cu2+ and Zn2+ ions using 15N-1H based 2D NMR spectroscopy. After SUMO1, SUMO2 is the most studied SUMO isoform in humans which shares 45 % and ~80 % similarity with SUMO1 in terms of sequence and structure, respectively. In this manuscript, we demonstrate that compared to SUMO1, several amino acids around the α1-helix region of SUMO2 access energetically similar near-native conformations. These conformations could play a crucial role in SUMO2's non-covalent interactions with SUMO interaction motifs (SIMs) on other proteins. The C-terminal of SUMO2 was found to bind strongly with Cu2+ ions resulting in a trimeric structure as observed by gel electrophoresis. This interaction seems to interfere in its non-covalent interaction with a V/I-x-V/I-V/I based SIM in Daxx protein.

SUMOylation and DeSUMOylation: Prospective therapeutic targets in cancer

The SUMO family is a type of ubiquitin-like protein modification molecule. Its protein modification mechanism is similar to that of ubiquitination: both involve modifier-activating enzyme E1, conjugating enzyme E2 and substrate-specific ligase E3. However, polyubiquitination can lead to the degradation of substrate proteins, while poly-SUMOylation only leads to the degradation of substrate proteins through the proteasome pathway after being recognized by ubiquitin as a signal factor. There are currently five reported subtypes in the SUMO family, namely SUMO1-5. As a reversible dynamic modification, intracellular sentrin/SUMO-specific proteases (SENPs) mainly regulate the reverse reaction pathway of SUMOylation. The SUMOylation modification system affects the localization, activation and turnover of proteins in cells and participates in regulating most nuclear and extranuclear molecular reactions. Abnormal expression of proteins related to the SUMOylation pathway is commonly observed in tumors, indicating that this pathway is closely related to tumor occurrence, metastasis and invasion. This review mainly discusses the composition of members in the protein family related to SUMOylation pathways, mutual connections between SUMOylation and other post-translational modifications on proteins as well as therapeutic drugs developed based on these pathways.

Synergistic roles of CBX4 chromo and SIM domains in regulating senescence of primary human osteoarthritic chondrocytes

BACKGROUND

Cellular senescence is a critical factor contributing to osteoarthritis (OA). Overexpression of chromobox homolog 4 (CBX4) in a mouse system was demonstrated to alleviate post-traumatic osteoarthritis (PTOA) by reducing cellular senescence. Additionally, replicative cellular senescence of WI-38 fibroblasts can be attenuated by CBX4. However, the mechanisms underlying this senomorphic function of CBX4 are not fully understood. In this study, we aimed to investigate the role of CBX4 in cellular senescence in human primary osteoarthritic chondrocytes and to identify the functional domains of CBX4 necessary for its function in modulating senescence.

METHODS

Chondrocytes, isolated from 6 individuals undergoing total knee replacement for OA, were transduced with wild-type CBX4, mutant CBX4, and control lentiviral constructs. Senescence-related phenotypic outcomes included the following: multiple flow cytometry-measured markers (p16INK4A, senescence-associated β-galactosidase [SA-β-gal] activity and dipeptidyl peptidase-4 [DPP4], and proliferation marker EdU), multiplex ELISA-measured markers in chondrocyte culture media (senescence-associated secretory phenotypes [SASPs], including IL-1β, IL-6, IL-8, TNF-α, MMP-1, MMP-3, and MMP-9), and PCR array-evaluated senescence-related genes.

RESULTS

Compared with control, CBX4 overexpression in OA chondrocytes decreased DPP4 expression and SASP secretion and increased chondrocyte proliferation confirming CBX4 senomorphic effects on primary human chondrocytes. Point mutations of the chromodomain domain (CDM, involved in chromatin modification) alone were sufficient to partially block the senomorphic activity of CBX4 (p16INK4A and DPP4 increased, and EdU decreased) but had minimal effect on SASP secretion. Although having no effect on p16INK4A, DPP4, and EdU, deletion of two small-ubiquitin-like-modifier-interaction motifs (CBX4 ΔSIMs) led to increased SASP secretion (IL-1β, TNF-α, IL-8). The combination CBX4 CDMΔSIMs altered all these measures adversely and to a greater degree than the single domain mutants. Deletion of the C-terminal (CBX4 ΔC-box) involved with transcriptional silencing of polycomb group proteins increased IL-1β slightly but significantly but altered none of the other senescence outcome measures.

CONCLUSIONS

CBX4 has a senomorphic effect on human osteoarthritic chondrocytes. CDM is critical for CBX4-mediated regulation of senescence. The SIMs are supportive but not indispensable for CBX4 senomorphic function while the C-box is dispensable.

Cbx4 governs HIF-1α to involve in Th9 cell differentiation promoting asthma by its SUMO E3 ligase activity

The potential role of polycomb chromobox 4 (Cbx4), as a small ubiquitin-like ligase (SUMO) E3 ligase, in the development and exacerbation of asthma remains unclear. Hypoxia inducible factor-1 (HIF-1) is a key transcription factor in the cellular response to hypoxia and contributes to the pathogenesis and progression of a range of diseases, including asthma. Here, we aimed to investigate the interaction of Cbx4 with Hypoxia inducible factor-1α (HIF-1α) and the potent mechanism of action in asthma progression. In present study, in vitro and ex vivo results demonstrated that Cbx4 interacts with HIF-1α protein through its SUMO E3 ligase activity and enhances the sumoylation, which increases HIF-1 transactivation through Cbx4 and promotes the differentiation of Th9 cells, then in turn promotes the process of asthma. Treatment of inhibitors targeting SUMO E3 ligase activity of Cbx4 or HIF-1α can effectively reduce HIF-1α activation and differentiation of Th9 cells, which further attenuates the asthma in mouse model. Current results collectively demonstrated Cbx4 can govern HIF-1α to involve in Th9 cell differentiation promoting asthma by its SUMO E3 ligase activity, providing a new direction for clinical treatment of asthma.

Cutting Edge: Polycomb Repressive Complex 1 Subunit Cbx4 Positively Regulates Effector Responses in CD8 T Cells

CTL differentiation is controlled by the crosstalk of various transcription factors and epigenetic modulators. Uncovering this process is fundamental to improving immunotherapy and designing novel therapeutic approaches. In this study, we show that polycomb repressive complex 1 subunit chromobox (Cbx)4 favors effector CTL differentiation in a murine model. Cbx4 deficiency in CTLs induced a transcriptional signature of memory cells and increased the memory CTL population during acute viral infection. It has previously been shown that besides binding to H3K27me3 through its chromodomain, Cbx4 functions as a small ubiquitin-like modifier (SUMO) E3 ligase in a SUMO-interacting motifs (SIM)-dependent way. Overexpression of Cbx4 mutants in distinct domains showed that this protein regulates CTL differentiation primarily in an SIM-dependent way and partially through its chromodomain. Our data suggest a novel role of a polycomb group protein Cbx4 controlling CTL differentiation and indicated SUMOylation as a key molecular mechanism connected to chromatin modification in this process.

Human Post-Translational SUMOylation Modification of SARS-CoV-2 Nucleocapsid Protein Enhances Its Interaction Affinity with Itself and Plays a Critical Role in Its Nuclear Translocation

Viruses, such as Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), infect hosts and take advantage of host cellular machinery for genome replication and new virion production. Identifying and elucidating host pathways for viral infection is critical for understanding the development of the viral life cycle and novel therapeutics. The SARS-CoV-2 N protein is critical for viral RNA (vRNA) genome packaging in new virion formation. Using our quantitative Förster energy transfer/Mass spectrometry (qFRET/MS) coupled method and immunofluorescence imaging, we identified three SUMOylation sites of the SARS-CoV-2 N protein. We found that (1) Small Ubiquitin-like modifier (SUMO) modification in Nucleocapsid (N) protein interaction affinity increased, leading to enhanced oligomerization of the N protein; (2) one of the identified SUMOylation sites, K65, is critical for its nuclear translocation. These results suggest that the host human SUMOylation pathway may be critical for N protein functions in viral replication and pathology in vivo. Thus, blocking essential host pathways could provide a novel strategy for future anti-viral therapeutics development, such as for SARS-CoV-2 and other viruses.

The Role of SUMO E3 Ligases in Signaling Pathway of Cancer Cells

Small ubiquitin-like modifier (SUMO)ylation is a reversible post-translational modification that plays a crucial role in numerous aspects of cell physiology, including cell cycle regulation, DNA damage repair, and protein trafficking and turnover, which are of importance for cell homeostasis. Mechanistically, SUMOylation is a sequential multi-enzymatic process where SUMO E3 ligases recruit substrates and accelerate the transfer of SUMO onto targets, modulating their interactions, localization, activity, or stability. Accumulating evidence highlights the critical role of dysregulated SUMO E3 ligases in processes associated with the occurrence and development of cancers. In the present review, we summarize the SUMO E3 ligases, in particular, the novel ones recently identified, and discuss their regulatory roles in cancer pathogenesis.

CBX4 Regulates Replicative Senescence of WI-38 Fibroblasts

Cellular senescence is characterized by cell cycle arrest and senescence-associated secretory phenotypes. Cellular senescence can be caused by various stress stimuli such as DNA damage, oxidative stress, and telomere attrition and is related to several chronic diseases, including atherosclerosis, Alzheimer's disease, and osteoarthritis. Chromobox homolog 4 (CBX4) has been shown to alleviate cellular senescence in human mesenchymal stem cells and is considered a possible target for senomorphic treatment. Here, we explored whether CBX4 expression is associated with replicative senescence in WI-38 fibroblasts, a classic human senescence model system. We also examined whether and how regulation of CBX4 modifies the senescence phenotype and functions as an antisenescence target in WI-38. During the serial culture of the WI-38 primary fibroblast cell line to a senescent state, we found increased expression of senescence markers, including senescence β-galactosidase (SA-βgal) activity, protein expression of p16, p21, and DPP4, and decreased proliferation marker EdU; moreover, CBX4 protein expression declined. With knockdown of CBX4, SA-βgal activity and p16 protein expression increased, and EdU decreased. With the activation of CBX4, SA-βgal activity, p16, and DPP4 protein decreased. In addition, CBX4 knockdown increased, while CBX4 activation decreased, gene expression of both CDKN2A (encoding the p16 protein) and DPP4. Genes related to DNA damage and cell cycle pathways were regulated by CBX4. These results demonstrate that CBX4 can regulate replicative senescence in a manner consistent with a senomorphic agent.

SALL1 Modulates CBX4 Stability, Nuclear Bodies, and Regulation of Target Genes

Development is orchestrated through a complex interplay of multiple transcription factors. The comprehension of this interplay will help us to understand developmental processes. Here we analyze the relationship between two key transcription factors: CBX4, a member of the Polycomb Repressive Complex 1 (PRC1), and SALL1, a member of the Spalt-like family with important roles in embryogenesis and limb development. Both proteins localize to nuclear bodies and are modified by the small ubiquitin-like modifier (SUMO). Our results show that CBX4 and SALL1 interact in the nucleoplasm and that increased SALL1 expression reduces ubiquitination of CBX4, enhancing its stability. This is accompanied by an increase in the number and size of CBX4-containing Polycomb bodies, and by a greater repression of CBX4 target genes. Thus, our findings uncover a new way of SALL1-mediated regulation of Polycomb bodies through modulation of CBX4 stability, with consequences in the regulation of its target genes, which could have an impact in cell differentiation and development.

SUMOylation of RepoMan during late telophase regulates dephosphorylation of lamin A

Dephosphorylation of lamin A, which triggers nuclear lamina reconstitution, is crucial for the completion of mitosis. However, the specific phosphatase and regulatory mechanism that allow timely lamin A dephosphorylation remain unclear. Here, we report that RepoMan (also known as CDCA2), a regulatory subunit of protein phosphatase 1γ (PP1γ) is transiently modified with SUMO-2 at K762 during late telophase. SUMOylation of RepoMan markedly enhanced its binding affinity with lamin A. Moreover, SUMOylated RepoMan contributes to lamin A recruitment to telophase chromosomes and dephosphorylation of the mitotic lamin A phosphorylation. Expression of a SUMO-2 mutant that has a defective interaction with the SUMO-interacting motif (SIM) resulted in failure of the lamin A and RepoMan association, along with abrogation of lamin A dephosphorylation and subsequent nuclear lamina formation. These findings strongly suggest that RepoMan recruits lamin A through SUMO–SIM interaction. Thus, transient SUMOylation of RepoMan plays an important role in the spatiotemporal regulation of lamin A dephosphorylation and the subsequent nuclear lamina formation at the end of mitosis.

Summary: Transient SUMOylation of RepoMan controls the recruitment of lamin A to telophase chromosomes, lamin A dephosphorylation and nuclear lamina formation.

Understanding and Exploiting Post-Translational Modifications for Plant Disease Resistance

Plants are constantly threatened by pathogens, so have evolved complex defence signalling networks to overcome pathogen attacks. Post-translational modifications (PTMs) are fundamental to plant immunity, allowing rapid and dynamic responses at the appropriate time. PTM regulation is essential; pathogen effectors often disrupt PTMs in an attempt to evade immune responses. Here, we cover the mechanisms of disease resistance to pathogens, and how growth is balanced with defence, with a focus on the essential roles of PTMs. Alteration of defence-related PTMs has the potential to fine-tune molecular interactions to produce disease-resistant crops, without trade-offs in growth and fitness.

Sumoylation as an Emerging Target in Therapeutics against Cancer

Sumoylation is the Post-translational modification gaining most of the research interest recently. Sumoylation is involved in various crucial functions of the cell such as regulation of cell cycle, DNA damage repair, apoptosis, etc. Oncology is advancing in radiotherapy, targeted chemotherapy, various forms of immunotherapy and targeted gene therapy. Researches are being conducted to prove its connotation with a variety of cancers and inhibitors are being developed to obstruct the fatal effect caused by misbalance of the SUMO-catalytic cycle. It has been shown that up-regulation of certain enzymes of Sumoylation correlates with cancer incidence in most of the cases. However, in some cases, down-regulation also associates with cancer invasion such as underexpression of UBC9 in initial stage breast cancer. This can aid in future study, treatment, and diagnosis of a variety of cancers including breast cancer, prostate cancer, lung adenocarcinoma, melanoma, multiple myeloma, etc. Various mechanistic assays are being developed and used to identify potential inhibitors against the dysregulated proteins of Sumoylation. This review summarizes the normal roles of the enzymes involved in the SUMOcatalytic cycle, their misbalanced regulation leading to tumorigenesis and nearly all the potent inhibitors identified to date, while after detailed studied it was observed that ML-792 could be a promising inhibitor in treating cancers by inhibiting Sumoylation enzymes.

SUMO E3 ligase CBX4 regulates hTERT-mediated transcription of CDH1 and promotes breast cancer cell migration and invasion

hTERT, the catalytic component of the human telomerase enzyme, is regulated by post-translational modifications, like phosphorylation and ubiquitination by multiple proteins which remarkably affects the overall activity of the enzyme. Here we report that hTERT gets SUMOylated by SUMO1 and polycomb protein CBX4 acts as the SUMO E3 ligase of hTERT. hTERT SUMOylation positively regulates its telomerase activity which can be inhibited by SENP3-mediated deSUMOylation. Interestingly, we have established a new role of hTERT SUMOylation in the repression of E-cadherin gene expression and consequent triggering on the epithelial-mesenchymal-transition (EMT) program in breast cancer cells. We also observed that catalytically active CBX4, leads to retention of hTERT/ZEB1 complex onto E-cadherin promoter leading to its repression through hTERT-SUMOylation. Further through wound healing and invasion assays in breast cancer cells, we showed the tumor promoting ability of hTERT was significantly compromised upon overexpression of SUMO-defective mutant of hTERT. Thus our findings establish a new post-translational modification of hTERT which on one hand is involved in telomerase activity maintenance and on the other hand plays a crucial role in the regulation of gene expression thereby promoting migration and invasion of breast cancer cells.

SUMO and Transcriptional Regulation: The Lessons of Large-Scale Proteomic, Modifomic and Genomic Studies

One major role of the eukaryotic peptidic post-translational modifier SUMO in the cell is transcriptional control. This occurs via modification of virtually all classes of transcriptional actors, which include transcription factors, transcriptional coregulators, diverse chromatin components, as well as Pol I-, Pol II- and Pol III transcriptional machineries and their regulators. For many years, the role of SUMOylation has essentially been studied on individual proteins, or small groups of proteins, principally dealing with Pol II-mediated transcription. This provided only a fragmentary view of how SUMOylation controls transcription. The recent advent of large-scale proteomic, modifomic and genomic studies has however considerably refined our perception of the part played by SUMO in gene expression control. We review here these developments and the new concepts they are at the origin of, together with the limitations of our knowledge. How they illuminate the SUMO-dependent transcriptional mechanisms that have been characterized thus far and how they impact our view of SUMO-dependent chromatin organization are also considered.

Global non-covalent SUMO interaction networks reveal SUMO-dependent stabilization of the non-homologous end joining complex

In contrast to our extensive knowledge on covalent small ubiquitin-like modifier (SUMO) target proteins, we are limited in our understanding of non-covalent SUMO-binding proteins. We identify interactors of different SUMO isoforms-monomeric SUMO1, monomeric SUMO2, or linear trimeric SUMO2 chains-using a mass spectrometry-based proteomics approach. We identify 379 proteins that bind to different SUMO isoforms, mainly in a preferential manner. Interestingly, XRCC4 is the only DNA repair protein in our screen with a preference for SUMO2 trimers over mono-SUMO2, as well as the only protein in our screen that belongs to the non-homologous end joining (NHEJ) DNA double-strand break repair pathway. A SUMO interaction motif (SIM) in XRCC4 regulates its recruitment to sites of DNA damage and phosphorylation of S320 by DNA-PKcs. Our data highlight the importance of non-covalent and covalent sumoylation for DNA double-strand break repair via the NHEJ pathway and provide a resource of SUMO isoform interactors.

Characterization of a C-Terminal SUMO-Interacting Motif Present in Select PIAS-Family Proteins

The human PIAS proteins are small ubiquitin-like modifier (SUMO) E3 ligases that participate in important cellular functions. Several of these functions depend on a conserved SUMO-interacting motif (SIM) located in the central region of all PIAS proteins (SIM1). Recently, it was determined that Siz2, a yeast homolog of PIAS proteins, possesses a second SIM at its C terminus (SIM2). Sequence alignment indicates that a SIM2 is also present in PIAS1-3, but not PIAS4. Using biochemical and structural studies, we demonstrate PIAS-SIM2 binds to SUMO1, but that phosphorylation of the PIAS-SIM2 or acetylation of SUMO1 alter this interaction in a manner distinct from what is observed for the PIAS-SIM1. We also show that the PIAS-SIM2 plays a key role in formation of a UBC9-PIAS1-SUMO1 complex. These results provide insights into how post-translational modifications selectively regulate the specificity of multiple SIMs found in the PIAS proteins by exploiting the plasticity built into the SUMO-SIM binding interface.

Noncovalent SUMO-interaction motifs in HIV integrase play important roles in SUMOylation, cofactor binding, and virus replication

Background

HIV integrase (IN) and its cellular cofactors, including lens-epithelium-derived growth factor (LEDGF/p75), Ku70, p300, and Rad52, are subject to small ubiquitin-like modifier (SUMO) modification. In addition to covalent SUMOylation, SUMO paralogs can also noncovalently bind proteins through SUMO-interacting motifs (SIMs). However, little is known about whether HIV IN contains SIMs and the roles of these motifs.

Results

We searched for the amino acid sequence of HIV IN and investigated three putative SIMs of IN: SIM1 72VILV75, SIM2 200IVDI203 and SIM3 257IKVV260. Our mutational analysis showed that 200IVDI203 and 257IKVV260 are two bona fide SIMs that mediate IN-SUMO noncovalent interactions. Additionally, a cell-based SUMOylation assay revealed that IN SIMs negatively regulate the SUMOylation of IN, as well as the interaction between IN and SUMO E2 conjugation enzyme Ubc9. Conversely, IN SIMs are required for its interactions with LEDGF/p75 but not with Ku70. Furthermore, our study reveals that SIM2 and SIM3 are required for the nuclear localization of IN. Finally, we investigated the impact of IN SIM2 and SIM3 on HIV single cycle replication in CD4⁺ C8166 T cells, and the results showed that viruses carrying IN SIM mutants are replication defective at the steps of the early viral life cycle, including reverse transcription, nuclear import and integration.

Conclusion

Our data suggested that the IN^SIM-SUMO interaction constitutes a new regulatory mechanism of IN functions and might be important for HIV-1 replication.

Functional impairment of the HIPK2 small ubiquitin-like modifier (SUMO)-interacting motif in acute myeloid leukemia

Covalent conjugations of the SUMO-1 moiety on a target protein play important roles in the regulation of cellular protein function. SUMO-conjugation of PML is a regulatory step for PML nuclear body (PML-NB) formation, and HIPK2 is SUMO-conjugated and recruited into the PML-NBs. Although HIPK2 mutations (R861W and N951I) were found in acute myeloid leukemia (AML) and myelodysplastic syndrome (MDS) patients, little is known about the underlying mechanisms by which HIPK2 mutations are associated with the pathogenesis of leukemia. Here we show that HIPK2 mutants found in AML and MDS patients are defective in SUMO-interacting motif (SIM) function. Due to defective SIM function, the HIPK2 mutants were not modified with SUMO-1, and not recruited to the PML-NBs. However, the HIPK2 mutants can normally bind to and phosphorylate AML1b. Therefore, the HIPK2 mutants can sequestrate the AML1 complex out of the PML-NBs, resulting in the disruption of AML1-mediated activation of target genes for myeloid differentiation. In addition, the differentiation of K562 blast cells was impaired by the expression of the HIPK2 SIM-defective mutants. These results suggest that HIPK2 targeting into the PML-NBs via the SIMs is crucial for HIPK2-mediated induction of myeloid differentiation, and is associated with AML pathogenesis.

Fifty shades of SUMO: its role in immunity and at the fulcrum of the growth-defence balance

The sessile nature of plants requires them to cope with an ever-changing environment. Effective adaptive responses require sophisticated cellular mechanisms at the post-transcriptional and post-translational levels. Post-translational modification by small ubiquitin-like modifier (SUMO) proteins is emerging as a key player in these adaptive responses. SUMO conjugation can rapidly change the overall fate of target proteins by altering their stability or interaction with partner proteins or DNA. SUMOylation entails an enzyme cascade that leads to the activation, conjugation and ligation of SUMO to lysine residues of target proteins. In addition to their SUMO processing activities, SUMO proteases also possess de-conjugative activity capable of cleaving SUMO from target proteins, providing reversibility and buffering to the pathway. These proteases play critical roles in the maintenance of the SUMO machinery in equilibrium. We hypothesize that SUMO proteases provide the all-important substrate specificity within the SUMO system. Furthermore, we provide an overview of the role of SUMO in plant innate immunity. SUMOylation also overlaps with multiple growth-promoting and defence-related hormone signalling pathways, and hence is pivotal for the maintenance of the growth-defence balance. This review aims to highlight the intricate molecular mechanisms utilized by SUMO to regulate plant defence and to stabilize the growth-defence equilibrium.

The post-translational modification, SUMOylation, and cancer (Review)

SUMOylation is a reversible post-translational modification which has emerged as a crucial molecular regulatory mechanism, involved in the regulation of DNA damage repair, immune responses, carcinogenesis, cell cycle progression and apoptosis. Four SUMO isoforms have been identified, which are SUMO1, SUMO2/3 and SUMO4. The small ubiquitin-like modifier (SUMO) pathway is conserved in all eukaryotes and plays pivotal roles in the regulation of gene expression, cellular signaling and the maintenance of genomic integrity. The SUMO catalytic cycle includes maturation, activation, conjugation, ligation and de-modification. The dysregulation of the SUMO system is associated with a number of diseases, particularly cancer. SUMOylation is widely involved in carcinogenesis, DNA damage response, cancer cell proliferation, metastasis and apoptosis. SUMO can be used as a potential therapeutic target for cancer. In this review, we briefly outline the basic concepts of the SUMO system and summarize the involvement of SUMO proteins in cancer cells in order to better understand the role of SUMO in human disease.

Concepts and Methodologies to Study Protein SUMOylation: An Overview

Protein modification by the small ubiquitin-related modifier (SUMO) was simultaneously discovered by several groups at the middle of the 1990s. Although distinct names were proposed including Sentrin, GMP1, PIC1, or SMT3, SUMO became the most popular. Early studies on the functions of SUMOylation focused on activities in the nucleus, including transcription activation, chromatin structure, and DNA repair. However, it is now recognized that SUMOylation affects a large diversity of cellular processes both in the nucleus and the cytoplasm and functions of SUMOylation appear to have undefined limits. SUMO-conjugating enzymes and specific proteases actively regulate the modification status of target proteins. The recent discoveries of ubiquitin-SUMO hybrid chains, multiple SUMO-interacting motifs, and macromolecular complexes regulated by SUMOylation underscore the high complexity of this dynamic reversible system. New conceptual frameworks suggested by these findings have motivated the development of new methodologies to study pre- and post-SUMOylation events in vitro and in vivo, using distinct model organisms. Here we summarize some of the new developments and methodologies in the field, particularly those that will be further elaborated on in the chapters integrating this book.

SUMOylation of HP1α supports association with ncRNA to define responsiveness of breast cancer cells to chemotherapy

Epigenetic reprogramming allows cancer cells to bypass normal checkpoints and potentiate aberrant proliferation. Several chromatin regulators are subject to reversible SUMO-modification but little is known about how SUMOylation of chromatin-remodelers modulates the cancer epigenome. Recently, we demonstrated that SUMO-protease SENP7L is upregulated in aggressive BCa and maintains hypoSUMOylated heterochromatin protein 1-α (HP1α). Canonical models define HP1α as a "reader" of repressive H3K9m3 marks that supports constitutive heterochromatin. It is unclear how SUMOylation affects HP1α function in BCa cells. This report shows HP1α SUMO-dynamics are closely regulated in a complex with SENP7L and SUMO-E3 Polycomb-2 (PC2/CBX4). This complex accumulates at H3K9m3 sites, hypoSUMOylates HP1α and PC2, and reduces PC2's SUMO-E3 activity. HyperSUMO conditions cause complex dissociation, SUMOylation of PC2 and HP1α, and recruitment of SUMOylated HP1α to multiple DNA-repair genes including Rad51C. SUMOylated HP1α's enrichment at euchromatin requires chromatin-bound non-coding RNA (ncRNA), reduces Rad51C protein, and increases DNA-breaks in BCa cells. Hence, HP1α SUMOylation and consistently low SENP7L increase efficacy of DNA-damaging chemotherapeutic agents. BCa patients on chemotherapy that express low SENP7L exhibit greater survival rates than patients with high SENP7L. Collectively, these studies suggest that SUMOylated HP1α is a critical epigenetic-regulator of DNA-repair in BCa that could define chemotherapy responsiveness.

SUMO and Chromatin Remodeling

Many of the known SUMO substrates are nuclear proteins, which regulate gene expression and chromatin dynamics. Sumoylation, in general, appears to correlate with decreased transcriptional activity, and in many cases modulation of the chromatin template is implicated. Sumoylation of the core histones is associated with transcriptional silencing, and transcription factor sumoylation can decrease gene expression by promoting recruitment of chromatin modifying enzymes. Additionally, sumoylation of transcriptional corepressors and chromatin remodeling enzymes can influence interactions with other transcriptional regulators, and alter their enzymatic activity. In some cases, proteins that are components of transcriptional corepressor complexes have been shown to be SUMO E3 ligases, further emphasizing the integration of sumoylation with the regulation of chromatin remodeling. Despite the evidence suggesting that sumoylation is primarily repressive for access to chromatin, recent analyses suggest that protein sumoylation on the chromatin template may play important roles at highly expressed genes. Elucidating the dynamic interplay of sumoylation with other post-translational modifications of histones and chromatin associated proteins will be key to fully understanding the regulation of access to the chromatin template.

Identification of a new small ubiquitin-like modifier (SUMO)-interacting motif in the E3 ligase PIASy

Small ubiquitin-like modifier (SUMO) conjugation is a reversible post-translational modification process implicated in the regulation of gene transcription, DNA repair, and cell cycle. SUMOylation depends on the sequential activities of E1 activating, E2 conjugating, and E3 ligating enzymes. SUMO E3 ligases enhance transfer of SUMO from the charged E2 enzyme to the substrate. We have previously identified PIASy, a member of the Siz/protein inhibitor of activated STAT (PIAS) RING family of SUMO E3 ligases, as essential for mitotic chromosomal SUMOylation in frog egg extracts and demonstrated that it can mediate effective SUMOylation. To address how PIASy catalyzes SUMOylation, we examined various truncations of PIASy for their ability to mediate SUMOylation. Using NMR chemical shift mapping and mutagenesis, we identified a new SUMO-interacting motif (SIM) in PIASy. The new SIM and the currently known SIM are both located at the C terminus of PIASy, and both are required for the full ligase activity of PIASy. Our results provide novel insights into the mechanism of PIASy-mediated SUMOylation. PIASy adds to the growing list of SUMO E3 ligases containing multiple SIMs that play important roles in the E3 ligase activity.

SUMO Modification Stabilizes Enterovirus 71 Polymerase 3D To Facilitate Viral Replication

Accumulating evidence suggests that viruses hijack cellular proteins to circumvent the host immune system. Ubiquitination and SUMOylation are extensively studied posttranslational modifications (PTMs) that play critical roles in diverse biological processes. Cross talk between ubiquitination and SUMOylation of both host and viral proteins has been reported to result in distinct functional consequences. Enterovirus 71 (EV71), an RNA virus belonging to the family Picornaviridae, is a common cause of hand, foot, and mouth disease. Little is known concerning how host PTM systems interact with enteroviruses. Here, we demonstrate that the 3D protein, an RNA-dependent RNA polymerase (RdRp) of EV71, is modified by small ubiquitin-like modifier 1 (SUMO-1) both during infection and in vitro Residues K159 and L150/D151/L152 were responsible for 3D SUMOylation as determined by bioinformatics prediction combined with site-directed mutagenesis. Also, primer-dependent polymerase assays indicated that mutation of SUMOylation sites impaired 3D polymerase activity and virus replication. Moreover, 3D is ubiquitinated in a SUMO-dependent manner, and SUMOylation is crucial for 3D stability, which may be due to the interplay between the two PTMs. Importantly, increasing the level of SUMO-1 in EV71-infected cells augmented the SUMOylation and ubiquitination levels of 3D, leading to enhanced replication of EV71. These results together suggested that SUMO and ubiquitin cooperatively regulated EV71 infection, either by SUMO-ubiquitin hybrid chains or by ubiquitin conjugating to the exposed lysine residue through SUMOylation. Our study provides new insight into how a virus utilizes cellular pathways to facilitate its replication.

IMPORTANCE

Infection with enterovirus 71 (EV71) often causes neurological diseases in children, and EV71 is responsible for the majority of fatalities. Based on a better understanding of interplay between virus and host cell, antiviral drugs against enteroviruses may be developed. As a dynamic cellular process of posttranslational modification, SUMOylation regulates global cellular protein localization, interaction, stability, and enzymatic activity. However, little is known concerning how SUMOylation directly influences virus replication by targeting viral polymerase. Here, we found that EV71 polymerase 3D was SUMOylated during EV71 infection and in vitro Moreover, the SUMOylation sites were determined, and in vitro polymerase assays indicated that mutations at SUMOylation sites could impair polymerase synthesis. Importantly, 3D is ubiquitinated in a SUMOylation-dependent manner that enhances the stability of the viral polymerase. Our findings indicate that the two modifications likely cooperatively enhance virus replication. Our study may offer a new therapeutic strategy against virus replication.

A comprehensive compilation of SUMO proteomics

Small ubiquitin-like modifiers (SUMOs) are essential for the regulation of several cellular processes and are potential therapeutic targets owing to their involvement in diseases such as cancer and Alzheimer disease. In the past decade, we have witnessed a rapid expansion of proteomic approaches for identifying sumoylated proteins, with recent advances in detecting site-specific sumoylation. In this Analysis, we combined all human SUMO proteomics data currently available into one cohesive database. We provide proteomic evidence for sumoylation of 3,617 proteins at 7,327 sumoylation sites, and insight into SUMO group modification by clustering the sumoylated proteins into functional networks. The data support sumoylation being a frequent protein modification (on par with other major protein modifications) with multiple nuclear functions, including in transcription, mRNA processing, DNA replication and the DNA-damage response.

SUMO proteases: uncovering the roles of deSUMOylation in plants

Plants have evolved to cope with changing environmental conditions. One way plants achieve this is through post-translational modification of target proteins by ubiquitination and SUMOylation. These post-translational modifiers (PMs) can alter stability, protein-protein interactions, and the overall fate of the protein. Both of these systems have remarkable similarities in terms of the process leading to attachment of the PM to its substrate : having to undertake activation, conjugation, and finally ligation to the target. In the ubiquitin system, there are a vast number of ubiquitin ligase enzymes (E3s) that provide specificity for the attachment of ubiquitin. With the SUMO system, only a small number of SUMO E3 ligases have so far been identified in the fully sequenced plant genomes. In Arabidopsis thaliana, there are only two SUMO E3s, compared to over 1400 ubiquitin E3s, a trend also observed in crop species such as Oryza sativa and Zea mays Recent research indicates that removing SUMO from its substrate by the enzymatically active SUMO proteases is a vital part of this system. A class of SUMO proteases called ubiquitin-like proteases (ULPs) are widespread in all eukaryotes; within plants, both monocot and dicot kingdoms have conserved and divergent ULPs and ULP-like proteases. This paper examines the roles ULPs have in stress responses and highlights the 'fine-tuning' of SUMO attachment/removal in balancing growth versus stress.

Cooperativity of the SUMO and Ubiquitin Pathways in Genome Stability

Covalent attachment of ubiquitin (Ub) or SUMO to DNA repair proteins plays critical roles in maintaining genome stability. These structurally related polypeptides can be viewed as distinct road signs, with each being read by specific protein interaction motifs. Therefore, via their interactions with selective readers in the proteome, ubiquitin and SUMO can elicit distinct cellular responses, such as directing DNA lesions into different repair pathways. On the other hand, through the action of the SUMO-targeted ubiquitin ligase (STUbL) family proteins, ubiquitin and SUMO can cooperate in the form of a hybrid signal. These mixed SUMO-ubiquitin chains recruit “effector” proteins such as the AAA⁺ ATPase Cdc48/p97-Ufd1-Npl4 complex that contain both ubiquitin and SUMO interaction motifs. This review will summarize recent key findings on collaborative and distinct roles that ubiquitin and SUMO play in orchestrating DNA damage responses.

Chromobox Homolog 4 is Positively Correlated to Tumor Growth, Survival and Activation of HIF-1α Signaling in Human Osteosarcoma under Normoxic Condition

Objectives: The clinical significance and tumorigenesis of Chromobox homolog 4 (CBX4) have been reported in hepatocellular carcinoma. The purpose of this study is to confirm the expression, elucidate the biological function and investigate the potential mechanism of CBX4 in osteosarcoma (OS). Methods: The expression of CBX4 in OS samples and cell lines was measured by RT-PCR and western blot test. Cell cycle, CCK8 and colony-forming assays were used to detect changes of cells growth. Cell apoptosis assay was used to measure cell survival capacity. Trans-well assay was used to test the activities of migration and invasion. The expression of genes regulated by CBX4 was detected by qRT-PCT test. Results: The expression of CBX4 was up-regulated in multiple OS cell lines and clinical samples. Overexpression of CBX4 was correlated with advanced clinical stage, high degree of malignancy and low tumor necrosis rate. Moreover, knockdown of CBX4 resulted in significant inhibition of cell growth and cell survival in OS cells under normoxic condition. In addition, we found that knockdown of CBX4 lead to down-regulating of HIF-1α-targeted genes without changing HIF-1α expression itself. Conclusion: Taken together, CBX4 is up-regulated and has a pro-tumor effect in OS with an activation of HIF-1α signaling pathway under normoxic condition. Therefore, targeting CBX4 may provide a new therapeutic method for OS.

Cbx4 maintains the epithelial lineage identity and cell proliferation in the developing stratified epithelium

During development, multipotent progenitor cells establish lineage-specific programmers of gene activation and silencing underlying their differentiation into specialized cell types. We show that the Polycomb component Cbx4 serves as a critical determinant that maintains the epithelial identity in the developing epidermis by repressing nonepidermal gene expression programs. Cbx4 ablation in mice results in a marked decrease of the epidermal thickness and keratinocyte (KC) proliferation associated with activation of numerous neuronal genes and genes encoding cyclin-dependent kinase inhibitors (p16/p19 and p57). Furthermore, the chromodomain- and SUMO E3 ligase-dependent Cbx4 activities differentially regulate proliferation, differentiation, and expression of nonepidermal genes in KCs. Finally, Cbx4 expression in KCs is directly regulated by p63 transcription factor, whereas Cbx4 overexpression is capable of partially rescuing the effects of p63 ablation on epidermal development. These data demonstrate that Cbx4 plays a crucial role in the p63-regulated program of epidermal differentiation, maintaining the epithelial identity and proliferative activity in KCs via repression of the selected nonepidermal lineage and cell cycle inhibitor genes.

Structural basis for catalytic activation by the human ZNF451 SUMO E3 ligase

E3 protein ligases enhance transfer of ubiquitin-like (Ubl) proteins from E2 conjugating enzymes to substrates by stabilizing the thioester-charged E2~Ubl in a closed configuration optimally aligned for nucleophilic attack. Here, we report biochemical and structural data that define the N-terminal domain of the Homo sapiens ZNF451 as the catalytic module for SUMO E3 ligase activity. ZNF451 catalytic module contains tandem SUMO interaction motifs (SIMs) bridged by a Proline-Leucine-Arginine-Proline (PLRP) motif. The first SIM and PLRP motif engage thioester charged E2~SUMO while the next SIM binds a second molecule of SUMO bound to the backside of E2. We show that ZNF451 is SUMO2 specific and that SUMO-modification of ZNF451 may contribute to activity by providing a second molecule of SUMO that interacts with E2. Our results are consistent with ZNF451 functioning as a bona fide SUMO E3 ligase.

Wss1 metalloprotease partners with Cdc48/Doa1 in processing genotoxic SUMO conjugates

Sumoylation during genotoxic stress regulates the composition of DNA repair complexes. The yeast metalloprotease Wss1 clears chromatin-bound sumoylated proteins. Wss1 and its mammalian analog, DVC1/Spartan, belong to minigluzincins family of proteases. Wss1 proteolytic activity is regulated by a cysteine switch mechanism activated by chemical stress and/or DNA binding. Wss1 is required for cell survival following UV irradiation, the smt3-331 mutation and Camptothecin-induced formation of covalent topoisomerase 1 complexes (Top1cc). Wss1 forms a SUMO-specific ternary complex with the AAA ATPase Cdc48 and an adaptor, Doa1. Upon DNA damage Wss1/Cdc48/Doa1 is recruited to sumoylated targets and catalyzes SUMO chain extension through a newly recognized SUMO ligase activity. Activation of Wss1 results in metalloprotease self-cleavage and proteolysis of associated proteins. In cells lacking Tdp1, clearance of topoisomerase covalent complexes becomes SUMO and Wss1-dependent. Upon genotoxic stress, Wss1 is vacuolar, suggesting a link between genotoxic stress and autophagy involving the Doa1 adapter.

The SLX4 complex is a SUMO E3 ligase that impacts on replication stress outcome and genome stability

The SLX4 Fanconi anemia protein is a tumor suppressor that may act as a key regulator that engages the cell into specific genome maintenance pathways. Here, we show that the SLX4 complex is a SUMO E3 ligase that SUMOylates SLX4 itself and the XPF subunit of the DNA repair/recombination XPF-ERCC1 endonuclease. This SLX4-dependent activity is mediated by a remarkably specific interaction between SLX4 and the SUMO-charged E2 conjugating enzyme UBC9 and relies not only on newly identified SUMO-interacting motifs (SIMs) in SLX4 but also on its BTB domain. In contrast to its ubiquitin-binding UBZ4 motifs, SLX4 SIMs are dispensable for its DNA interstrand crosslink repair functions. Instead, while detrimental in response to global replication stress, the SUMO E3 ligase activity of the SLX4 complex is critical to prevent mitotic catastrophe following common fragile site expression.

Aberrant sumoylation signaling evoked by reactive oxygen species impairs protective function of Prdx6 by destabilization and repression of its transcription

Loss of the cytoprotective protein peroxiredoxin 6 (Prdx6) in cells that are aging or under oxidative stress is known to be linked to the pathobiology of many age-related diseases. However, the mechanism by which Prdx6 activity goes awry is largely unknown. Using Prdx6-deficient (Prdx6(-/-) ) cells as a model for aging or redox active cells, human/mouse lens epithelial cells (LECs) facing oxidative stress and aging lenses, we found a significant increase in the levels of small ubiquitin-like modifier (Sumo)1 conjugates. These cells displayed increased levels of Sumo1 and reduced the expression of Prdx6. Specifically, we observed that Prdx6 is a target for aberrant sumoylation signaling, and that Sumo1 modification reduces its cellular abundance. LECs overexpressing Sumo1 showed reduced expression and activity of Prdx6 and its transactivator specificity protein 1 (Sp1), mRNA and protein with increased levels of reactive oxygen species; those cells were vulnerable to oxidative stress-induced cell death. A significant reduction in Prdx6, Sp1 protein and mRNA expression was observed in redox active Prdx6(-/-) cells and in aging lenses/LECs. The reduction was correlated with increased expression of Sumo1 and enrichment of the inactive form (dimeric) of Sumo-specific protease (Senp)1. Experiments with Sumo1-fused Prdx6 and Prdx6 promoter-linked to chloramphenicol acetyltransferase reporter gene constructs indicated that Sumo1 dysregulated Prdx6 activity by reducing its abundance and attenuating its transcription; in contrast, the delivery of Senp1 or Prdx6 reversed the process. The data show that reactive oxygen species-evoked aberrant sumoylation signaling affects Prdx6 activity by reducing Prdx6 abundance, as well as transcription. The findings of the present study may provide a foundation for a strategy to repair deleterious oxidative signaling generated by a reduced activity of Prdx6.

Multiple Arkadia/RNF111 structures coordinate its Polycomb body association and transcriptional control

The RING domain protein Arkadia/RNF111 is a ubiquitin ligase in the transforming growth factor β (TGFβ) pathway. We previously identified Arkadia as a small ubiquitin-like modifier (SUMO)-binding protein with clustered SUMO-interacting motifs (SIMs) that together form a SUMO-binding domain (SBD). However, precisely how SUMO interaction contributes to the function of Arkadia was not resolved. Through analytical molecular and cell biology, we found that the SIMs share redundant function with Arkadia's M domain, a region distinguishing Arkadia from its paralogs ARKL1/ARKL2 and the prototypical SUMO-targeted ubiquitin ligase (STUbL) RNF4. The SIMs and M domain together promote both Arkadia's colocalization with CBX4/Pc2, a component of Polycomb bodies, and the activation of a TGFβ pathway transcription reporter. Transcriptome profiling through RNA sequencing showed that Arkadia can both promote and inhibit gene expression, indicating that Arkadia's activity in transcriptional control may depend on the epigenetic context, defined by Polycomb repressive complexes and DNA methylation.

Polycomb chromobox 4 enhances migration and pulmonary metastasis of hepatocellular carcinoma cell line MHCC97L

We recently report that the expression of polycomb chromobox 4 (Cbx4) is significantly correlated with the overall survival of a great cohort of hepatocellular carcinoma (HCC) patients and it enhances hypoxia-induced vascular endothelial growth factor (VEGF) expression and angiogenesis in HCC cells through enhancing sumoylation of hypoxia inducible factor-1alpha (HIF-1α). Here we continue to investigate the potential effects of Cbx4 on the migration and metastasis of the metastatic HCC cell line MHCC97L. Our results show that Cbx4 overexpression in the cell line increases the in vitro vessel formation of vascular endothelial cells in its SUMO interaction motifs-dependent manner, and promotes the in vitro migration of the cancer cell, which can be effectively abrogated by anti-VEGF antibody. Although Cbx4 expression does not impact the in vitro growth of MHCC97L cells, it still promotes the progression and metastasis of orthotopically transplanted tumors in nude mice. These results further support the role of Cbx4 as a SUMO E3 ligase in the progression and metastasis of HCC.

Evidence Implicating CCNB1IP1, a RING Domain-Containing Protein Required for Meiotic Crossing Over in Mice, as an E3 SUMO Ligase

The RING domain-containing protein CCNB1IP1 (Cyclin B1 Interacting Protein 1) is a putative ubiquitin E3 ligase that is essential for chiasmata formation, and hence fertility, in mice. Previous studies in cultured cells indicated that CCNB1IP1 targets Cyclin B for degradation, thus playing a role in cell cycle regulation. Mice homozygous for a mutant allele (mei4) of Ccnb1ip1 display no detectable phenotype other than meiotic failure from an absence of chiasmata. CCNB1IP1 is not conserved in key model organisms such as yeast and Drosophila, and there are no features of the protein that implicate clear mechanisms for a role in recombination. To gain insight into CCNB1IP1’s function in meiotic cells, we raised a specific antibody and determined that the protein appears in pachynema. This indicates that CCNB1IP1 is involved with crossover intermediate maturation, rather than early (leptotene) specification of a subset of SPO11-induced double strand breaks towards the crossover pathway. Additionally, a yeast 2-hybrid (Y2H) screen revealed that CCNB1IP1 interacts with SUMO2 and a set of proteins enriched for consensus sumoylation sites. The Y2H studies, combined with scrutiny of CCNB1IP1 domains, implicate this protein as an E3 ligase of the sumoylation cascade. We hypothesize CCNB1IP1 represents a novel meiosis-specific SUMO E3 ligase critical to resolution of recombination intermediates into mature chiasmata.

PRC1 components exhibit different binding kinetics in Polycomb bodies

BACKGROUND INFORMATION

Polycomb group (PcG) proteins keep the memory of cell identity by maintaining the repression of numerous target genes. They accumulate into nuclear foci called Polycomb bodies, which function in Drosophila cells as silencing compartments where PcG target genes convene. PcG proteins also exert their activities elsewhere in the nucleoplasm. In mammalian cells, the dynamic organisation and function of Polycomb bodies remain to be determined.

RESULTS

Fluorescently tagged PcG proteins CBXs and their partners BMI1 and RING1 form foci of different sizes and intensities in human U2OS cells. Fluorescence recovery after photobleaching (FRAP) analysis reveals that PcG dynamics outside foci is governed by diffusion as complexes and transient binding. In sharp contrast, recovery curves inside foci are substantially slower and exhibit large variability. The slow binding component amplitudes correlate with the intensities and sizes of these foci, suggesting that foci contained varying numbers of binding sites. CBX4-green fluorescent protein (GFP) foci were more stable than CBX8-GFP foci; yet the presence of CBX4 or CBX8-GFP in the same focus had a minor impact on BMI1 and RING1 recovery kinetics.

CONCLUSION

We propose that FRAP recovery curves provide information about PcG binding to their target genes outside foci and about PcG spreading onto chromatin inside foci.

Cbx4 governs HIF-1α to potentiate angiogenesis of hepatocellular carcinoma by its SUMO E3 ligase activity

Cbx4 is a polycomb group protein that is also a SUMO E3 ligase, but its potential roles in tumorigenesis remain to be explored. Here, we report that Cbx4, but not other members of the Cbx family, enhances hypoxia-induced vascular endothelial growth factor (VEGF) expression and angiogenesis in hepatocellular carcinoma (HCC) cells through enhancing HIF-1α sumoylations at K391 and K477 in its two SUMO-interacting motifs-dependent mechanisms and increasing transcriptional activity of HIF-1. The Cbx4 expression is significantly correlated with VEGF expression, angiogenesis, and the overall survival of HCC patients and also in subcutaneously and orthotopically transplanted mice HCC models. Collectively, our findings demonstrate that Cbx4 plays a critical role in tumor angiogenesis by governing HIF-1α protein.

Contribution of CBX4 to cumulus oophorus cell phenotype in mice and attendant effects in cumulus cell cloned embryos

Cumulus oophorus cells play an essential role in oocyte development. They are also widely employed as donor cells for cloning by somatic cell nuclear transfer. Our previous studies revealed that Cbx4 mRNA was overexpressed in cloned two-cell embryos. These data indicated that CBX4 may regulate normal cumulus cell differentiation and that its overexpression in clones could contribute to aberrant gene regulation. We used siRNA-mediated knockdown of Cbx4 to assess its role in determining cumulus cell phenotype and compared the effects of this knockdown to published data for aberrant gene regulation in cloned embryos. We observed widespread effects on the expression of genes related to diverse processes in cultured cumulus cells, including cell assembly/proliferation and DNA replication/repair, endocrine function, carbohydrate and lipid metabolism, inflammation, and cell morphology, with apparent effects of CBX4 in promoting cumulus cell proliferation and survival and inhibiting differentiation. Overall, the data implicate CBX4 as a key component in the pathway integrating endocrine signals, intraovarian paracrine factors, and oocyte-derived factors in the control of cumulus cell functions. We also observed altered expression of 25 cumulus cell markers of oocyte quality, indicating an important role of CBX4 in production of high quality oocytes. Finally, we found that about one-quarter of the genes showing aberrant transcription in cloned embryos are sensitive to Cbx4 knockdown in cumulus cells, consistent with a role for aberrant Cbx4 regulation in elaborating abnormal cloned embryo characteristics.

Identification of a non-covalent ternary complex formed by PIAS1, SUMO1, and UBC9 proteins involved in transcriptional regulation

Post-translational modifications with ubiquitin-like proteins require three sequentially acting enzymes (E1, E2, and E3) that must unambiguously recognize each other in a coordinated fashion to achieve their functions. Although a single E2 (UBC9) and few RING-type E3s (PIAS) operate in the SUMOylation system, the molecular determinants regulating the interactions between UBC9 and the RING-type E3 enzymes are still not well defined. In this study we use biochemical and functional experiments to characterize the interactions between PIAS1 and UBC9. Our results reveal that UBC9 and PIAS1 are engaged both in a canonical E2·E3 interaction as well as assembled into a previously unidentified non-covalent ternary complex with SUMO as evidenced by bioluminescence resonance energy transfer, nuclear magnetic resonance spectroscopy, and isothermal titration calorimetry studies. In this ternary complex, SUMO functions as a bridge by forming non-overlapping interfaces with UBC9 and PIAS1. Moreover, our data suggest that phosphorylation of serine residues adjacent to the PIAS1 SUMO-interacting motif favors formation of the non covalent PIAS1·SUMO·UBC9 ternary complex. Finally, our results also indicate that the non-covalent ternary complex is required for the known transcriptional repression activities mediated by UBC9 and SUMO1. Taken together, the data enhance our knowledge concerning the mode of interaction of enzymes of the SUMOylation machinery as well as their role in transcriptional regulation and establishes a framework for investigations of other ubiquitin-like protein systems.

CRMP2 protein SUMOylation modulates NaV1.7 channel trafficking

Voltage-gated sodium channel (NaV) trafficking is incompletely understood. Post-translational modifications of NaVs and/or auxiliary subunits and protein-protein interactions have been posited as NaV-trafficking mechanisms. Here, we tested if modification of the axonal collapsin response mediator protein 2 (CRMP2) by a small ubiquitin-like modifier (SUMO) could affect NaV trafficking; CRMP2 alters the extent of NaV slow inactivation conferred by the anti-epileptic (R)-lacosamide, implying NaV-CRMP2 functional coupling. Expression of a CRMP2 SUMOylation-incompetent mutant (CRMP2-K374A) in neuronal model catecholamine A differentiated (CAD) cells did not alter lacosamide-induced NaV slow inactivation compared with CAD cells expressing wild type CRMP2. Like wild type CRMP2, CRMP2-K374A expressed robustly in CAD cells. Neurite outgrowth, a canonical CRMP2 function, was moderately reduced by the mutation but was still significantly higher than enhanced GFP-transfected cortical neurons. Notably, huwentoxin-IV-sensitive NaV1.7 currents, which predominate in CAD cells, were significantly reduced in CAD cells expressing CRMP2-K374A. Increasing deSUMOylation with sentrin/SUMO-specific protease SENP1 or SENP2 in wild type CRMP2-expressing CAD cells decreased NaV1.7 currents. Consistent with a reduction in current density, biotinylation revealed a significant reduction in surface NaV1.7 levels in CAD cells expressing CRMP2-K374A; surface NaV1.7 expression was also decreased by SENP1 + SENP2 overexpression. Currents in HEK293 cells stably expressing NaV1.7 were reduced by CRMP2-K374A in a manner dependent on the E2-conjugating enzyme Ubc9. No decrement in current density was observed in HEK293 cells co-expressing CRMP2-K374A and NaV1.1 or NaV1.3. Diminution of sodium currents, largely NaV1.7, was recapitulated in sensory neurons expressing CRMP2-K374A. Our study elucidates a novel regulatory mechanism that utilizes CRMP2 SUMOylation to choreograph NaV1.7 trafficking.

C-Terminal Binding Protein: A Molecular Link between Metabolic Imbalance and Epigenetic Regulation in Breast Cancer

The prevalence of obesity has given rise to significant global concerns as numerous population-based studies demonstrate an incontrovertible association between obesity and breast cancer. Mechanisms proposed to account for this linkage include exaggerated levels of carbohydrate substrates, elevated levels of circulating mitogenic hormones, and inflammatory cytokines that impinge on epithelial programming in many tissues. Moreover, recently many scientists have rediscovered the observation, first described by Otto Warburg nearly a century ago, that most cancer cells undergo a dramatic metabolic shift in energy utilization and expenditure that fuels and supports the cellular expansion associated with malignant proliferation. This shift in substrate oxidation comes at the cost of sharp changes in the levels of the high energy intermediate, nicotinamide adenine dinucleotide (NADH). In this review, we discuss a novel example of how shifts in the concentration and flux of substrates metabolized and generated during carbohydrate metabolism represent components of a signaling network that can influence epigenetic regulatory events in the nucleus. We refer to this regulatory process as "metabolic transduction" and describe how the C-terminal binding protein (CtBP) family of NADH-dependent nuclear regulators represents a primary example of how cellular metabolic status can influence epigenetic control of cellular function and fate.

SUMOylation alters CRMP2 regulation of calcium influx in sensory neurons

The axon/dendrite specification collapsin response mediator protein 2 (CRMP2) bidirectionally modulates N-type voltage-gated Ca ( 2+) channels (CaV2.2). Here we demonstrate that small ubiquitin-like modifier (SUMO) protein modifies CRMP2 via the SUMO E2-conjugating enzyme Ubc9 in vivo. Removal of a SUMO conjugation site KMD in CRMP2 (K374A/M375A/D376A; CRMP2AAA) resulted in loss of SUMOylated CRMP2 without compromising neurite branching, a canonical hallmark of CRMP2 function. Increasing SUMOylation levels correlated inversely with calcium influx in sensory neurons. CRMP2 deSUMOylation by SUMO proteases SENP1 and SENP2 normalized calcium influx to those in the CRMP2AAA mutant. Thus, our results identify a novel role for SUMO modification in CRMP2/CaV2.2 signaling pathway.

UHRF2, a ubiquitin E3 ligase, acts as a small ubiquitin-like modifier E3 ligase for zinc finger protein 131

Small ubiquitin-like modifier (SUMO), a member of the ubiquitin-related protein family, is covalently conjugated to lysine residues of its substrates in a process referred to as SUMOylation. SUMOylation occurs through a series of enzymatic reactions analogous to that of the ubiquitination pathway, resulting in modification of the biochemical and functional properties of substrates. To date, four mammalian SUMO isoforms, a single heterodimeric SUMO-activating E1 enzyme SAE1/SAE2, a single SUMO-conjugating E2 enzyme ubiquitin-conjugating enzyme E2I (UBC9), and a few subgroups of SUMO E3 ligases have been identified. Several SUMO E3 ligases such as topoisomerase I binding, arginine/serine-rich (TOPORS), TNF receptor-associated factor 7 (TRAF7), and tripartite motif containing 27 (TRIM27) have dual functions as ubiquitin E3 ligases. Here, we demonstrate that the ubiquitin E3 ligase UHRF2 also acts as a SUMO E3 ligase. UHRF2 effectively enhances zinc finger protein 131 (ZNF131) SUMOylation but does not enhance ZNF131 ubiquitination. In addition, the SUMO E3 activity of UHRF2 on ZNF131 depends on the presence of SET and RING finger-associated and nuclear localization signal-containing region domains, whereas the critical ubiquitin E3 activity RING domain is dispensable. Our findings suggest that UHRF2 has independent functional domains and regulatory mechanisms for these two distinct enzymatic activities.

DNA damage-induced heterogeneous nuclear ribonucleoprotein K sumoylation regulates p53 transcriptional activation

Heterogeneous nuclear ribonucleoprotein (hnRNP) K is a nucleocytoplasmic shuttling protein that is a key player in the p53-triggered DNA damage response, acting as a cofactor for p53 in response to DNA damage. hnRNP K is a substrate of the ubiquitin E3 ligase MDM2 and, upon DNA damage, is de-ubiquitylated. In sharp contrast with the role and consequences of the other post-translational modifications, nothing is known about the role of SUMO conjugation to hnRNP K in p53 transcriptional co-activation. In the present work, we show that hnRNP K is modified by SUMO in lysine 422 within its KH3 domain, and sumoylation is regulated by the E3 ligase Pc2/CBX4. Most interestingly, DNA damage stimulates hnRNP K sumoylation through Pc2 E3 activity, and this modification is required for p53 transcriptional activation. Abrogation of hnRNP K sumoylation leads to an aberrant regulation of the p53 target gene p21. Our findings link the DNA damage-induced Pc2 activation to the p53 transcriptional co-activation through hnRNP K sumoylation.

Strategies to Identify Recognition Signals and Targets of SUMOylation

SUMOylation contributes to the regulation of many essential cellular factors. Diverse techniques have been used to explore the functional consequences of protein SUMOylation. Most approaches consider the identification of sequences on substrates, adaptors, or receptors regulating the SUMO conjugation, recognition, or deconjugation. The large majority of the studied SUMOylated proteins contain the sequence [IVL]KxE. SUMOylated proteins are recognized by at least 3 types of hydrophobic SUMO-interacting motifs (SIMs) that contribute to coordinate SUMO-dependent functions. Typically, SIMs are constituted by a hydrophobic core flanked by one or two clusters of negatively charged amino acid residues. Multiple SIMs can integrate SUMO binding domains (SBDs), optimizing binding, and control over SUMO-dependent processes. Here, we present a survey of the methodologies used to study SUMO-regulated functions and provide guidelines for the identification of cis and trans sequences controlling SUMOylation. Furthermore, an integrative analysis of known and putative SUMO substrates illustrates an updated landscape of several SUMO-regulated events. The strategies and analysis presented here should contribute to the understanding of SUMO-controlled functions and provide rational approach to identify biomarkers or choose possible targets for intervention in processes where SUMOylation plays a critical role.

Small ubiquitin-like modifier (SUMO) modification of zinc finger protein 131 potentiates its negative effect on estrogen signaling

Like ubiquitin, small ubiquitin-like modifier (SUMO) covalently attaches to specific target proteins and modulates their functional properties, including subcellular localization, protein dimerization, DNA binding, and transactivation of transcription factors. Diverse transcriptional co-regulator complexes regulate the ability of estrogen receptors to respond to positive and negative acting hormones. Zinc finger protein 131 (ZNF131) is poorly characterized but may act as a repressor of estrogen receptor α (ERα)-mediated trans-activation. Here, we identify ZNF131 as a target for SUMO modification and as a substrate for the SUMO E3 ligase human polycomb protein 2 (hPc2). We report that the SUMO-interacting motif 1 (SIM1) and the C-box of hPc2 are critical regions required for ZNF131 SUMOylation and define the ZNF131 SUMOylation site as lysine 567. We further show that SUMO modification potentiates the negative effect of ZNF131 on estrogen signaling and consequently attenuates estrogen-induced cell growth in a breast cancer cell line. Our findings suggest that SUMOylation is a novel regulator of ZNF131 action in estrogen signaling and breast cancer cell proliferation.