The Fast-Growing Business of SUMO Chains

Solo or in Concert: SUMOylation in Pathogenic Fungi

SUMOylation plays a pivotal role in DNA replication and repair, transcriptional stability, and stress response. Although SUMOylation is a conserved posttranslational modification (PTM) in eukaryotes, the number, type, and function of SUMOylation-associated components vary among mammals, plants, and fungi. SUMOylation shares overlapping features with ubiquitination, another well-known PTM. However, comparative studies on the interplay between these two PTMs are largely limited to yeast among fungal species. Recently, the role of SUMOylation in pathogenicity and its potential for crosstalk with ubiquitination have gained attention in fungal pathogens. In this review, we summarize recent findings on the distinct components of SUMOylation across organisms and describe its critical functions in fungal pathogens. Furthermore, we propose new research directions for SUMOylation in fungal pathogens, both independently and in coordination with other PTMs. This review aims to illuminate the potential for advancing PTM crosstalk research in fungal systems.

SUMOylation and DeSUMOylation: Tug of War of Pain Signaling

SUMOylation is a post-translational modification that attaches a small ubiquitin-like modifier (SUMO) group to a target protein via SUMO ligases, while deSUMOylation refers to the removal of this SUMO group by sentrin-specific proteases (SENPs). Although the functions of these processes have been well described in the nucleus, the role of SUMOylation and deSUMOylation in regulating ion channels is emerging as a novel area of study. Despite this, their contributions to pain signaling remain less clear. Therefore, this review consolidates the current evidence on the link(s) between SUMOylation, deSUMOylation, and pain, with a specific focus on ion channels expressed in the sensory system. Additionally, we explore the role of SUMOylation in the expression and function of kinases, vesicle proteins, and transcription factors, which result in the modulation of certain ion channels contributing to pain. Altogether, this review aims to highlight the relationship between SUMOylation and deSUMOylation in the modulation of ion channels, ultimately exploring the potential therapeutic role of these processes in chronic pain.

SUMOylation of rice DELLA SLR1 modulates transcriptional responses and improves yield under salt stress

MAIN CONCLUSION

SUMOylation of SLR1 at K2 protects productivity under salt stress, possibly by modulation of SLR1 interactome. DELLA proteins modulate GA signaling and are major regulators of plant plasticity to endure stress. DELLAs are mostly regulated at the post-translational level, and their activity relies on the interaction with upstream regulators and transcription factors (TFs). SUMOylation is a post-translational modification (PTM) capable of changing protein interaction and has been found to influence DELLA activity in Arabidopsis. We determined that SUMOylation of the single rice DELLA, SLENDER RICE1 (SLR1), occurs in a lysine residue different from the one identified in Arabidopsis REPRESSOR OF GA (RGA). Artificially increasing the SUMOylated SLR1 levels attenuated the penalty of salt stress on rice yield. Gene expression analysis revealed that the overexpression of SUMOylated SLR1 can regulate GA biosynthesis, which could partially explain the sustained productivity upon salt stress imposition. Furthermore, SLR1 SUMOylation blocked the interaction with the growth regulator YAB4, which may fine-tune GA20ox2 expression. We also identified novel SLR1 interactors: bZIP23, bHLH089, bHLH094, and OSH1. All those interactions were impaired in the presence of SUMOylated SLR1. Mechanistically, we propose that SUMOylation of SLR1 disrupts its interaction with several transcription factors implicated in GA-dependent growth and ABA-dependent salinity tolerance to modulate downstream gene expression. We found that SLR1 SUMOylation represents a novel mechanism modulating DELLA activity, which attenuates the impact of stress on plant performance.

Recent Advances on the Regulations of Organic Anion Transporters

The organic anion transporter (OAT) family of over 10 members within the solute carrier (SLC) superfamily of membrane proteins plays critical roles in facilitating the flux of negatively charged molecules in and out of cell membranes. These anionic molecules include various endogenous and exogenous compounds such as signaling molecules, nutrients, metabolites, toxins, and drugs. Therefore, OATs actively contribute to the systemic homeostasis and efficacy of therapeutics. This article provides a brief overview on recent advances in the understanding of the regulatory mechanisms that control the expression and activity of OATs in both health and diseases.

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

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

SifA SUMOylation governs Salmonella Typhimurium intracellular survival via modulation of lysosomal function

One of the mechanisms shaping the pathophysiology during the infection of enteric pathogen Salmonella Typhimurium is host PTM machinery utilization by the pathogen encoded effectors. Salmonella Typhimurium (S. Tm) during infection in host cells thrives in a vacuolated compartment, Salmonella containing vacuole (SCV), which sequentially acquires host endosomal and lysosomal markers. Long tubular structures, called as Salmonella induced filaments (SIFs), are further generated by S. Tm, which are known to be required for SCV's nutrient acquisition, membrane maintenance and stability. A tightly coordinated interaction involving prominent effector SifA and various host adapters PLEKHM1, PLEKHM2 and Rab GTPases govern SCV integrity and SIF formation. Here, we report for the first time that the functional regulation of SifA is modulated by PTM SUMOylation at its 11th lysine. S. Tm expressing SUMOylation deficient lysine 11 mutants of SifA (SifAK11R) is defective in intracellular proliferation due to compromised SIF formation and enhanced lysosomal acidification. Furthermore, murine competitive index experiments reveal defective in vivo proliferation and weakened virulence of SifAK11R mutant. Concisely, our data reveal that SifAK11R mutant nearly behaves like a SifA knockout strain which impacts Rab9-MPR mediated lysosomal acidification pathway, the outcome of which culminates in reduced bacterial load in in vitro and in vivo infection model systems. Our results bring forth a novel pathogen-host crosstalk mechanism where the SUMOylation of effector SifA regulated S. Tm intracellular survival.

The regulation of human organic anion transporter 4 by insulin-like growth factor 1 and protein kinase B signaling

Human organic anion transporter 4 (hOAT4), mainly expressed in the kidney and placenta, is essential for the disposition of numerous drugs, toxins, and endogenous substances. Insulin-like growth factor 1 (IGF-1) is a hormone generated in the liver and plays important roles in systemic growth, development, and metabolism. In the current study, we explored the regulatory effects of IGF-1 and downstream signaling on the transport activity, protein expression, and SUMOylation of hOAT4. We showed that IGF-1 significantly increased the transport activity, expression, and maximal transport velocity Vmax of hOAT4 in kidney-derived cells. This stimulatory effect of IGF-1 on hOAT4 activity was also confirmed in cells derived from the human placenta. The increased activity and expression were correlated well with the reduced degradation rate of hOAT4 at the cell surface. Furthermore, IGF-1 significantly increased hOAT4 SUMOylation, and protein kinase B (PKB)-specific inhibitors blocked the IGF-1-induced regulations on hOAT4. In conclusion, our study demonstrates that the hepatic hormone IGF-1 regulates hOAT4 expressed in the kidney and placenta through the PKB signaling pathway. Our results support the remote sensing and signaling theory, where OATs play a central role in the remote communications among distal tissues.

Multi-SUMOylation of NAC1 is essential for the growth of prostate cancer cells

Nucleus accumbens-associated 1 (NAC1) is a member of pox virus and zinc finger/bric-a-brac tramtrack broad complex (BTB/POZ) gene family. Overexpression of NAC1 is implicated in cancer development, recurrence and chemotherapy resistance. In our previous study, we found NAC1 was a potential small ubiquitin-like modifier (SUMO) substrate in prostate cancer cells. However, there was still lack of evidences to further support and validate the result. In this work, we found that NAC1 is a multi-SUMO-sites acceptor. The SUMO acceptor lysines were K167, K318, K368, K483 and K498. SUMOylation didn't alter the localization of NAC1, but facilitated the formation of NAC1 nuclear bodies. Compared with NAC1 wild type (NAC1 WT), the SUMO-sites mutant of NAC1 (NAC1 SM) suppressed cell proliferation and tumor growth in cellular and animal levels. This work uncovered the function of SUMOylation of NAC1 in prostate cancer cells.

Genetic alterations of the SUMO isopeptidase SENP6 drive lymphomagenesis and genetic instability in diffuse large B-cell lymphoma

SUMOylation is a post-translational modification of proteins that regulates these proteins’ localization, turnover or function. Aberrant SUMOylation is frequently found in cancers but its origin remains elusive. Using a genome-wide transposon mutagenesis screen in a MYC-driven B-cell lymphoma model, we here identify the SUMO isopeptidase (or deconjugase) SENP6 as a tumor suppressor that links unrestricted SUMOylation to tumor development and progression. Notably, SENP6 is recurrently deleted in human lymphomas and SENP6 deficiency results in unrestricted SUMOylation. Mechanistically, SENP6 loss triggers release of DNA repair- and genome maintenance-associated protein complexes from chromatin thereby impairing DNA repair in response to DNA damages and ultimately promoting genomic instability. In line with this hypothesis, SENP6 deficiency drives synthetic lethality to Poly-ADP-Ribose-Polymerase (PARP) inhibition. Together, our results link SENP6 loss to defective genome maintenance and reveal the potential therapeutic application of PARP inhibitors in B-cell lymphoma.

SUMOylation is a post-translational modification that has been shown to be altered in cancer. Here, the authors show that loss of the SUMO isopeptidase SENP6 leads to unrestricted SUMOylation and genomic instability promoting lymphomagenesis and generating vulnerability to PARP inhibition.

SUMO-Based Regulation of Nuclear Positioning to Spatially Regulate Homologous Recombination Activities at Replication Stress Sites

DNA lesions have properties that allow them to escape their nuclear compartment to achieve DNA repair in another one. Recent studies uncovered that the replication fork, when its progression is impaired, exhibits increased mobility when changing nuclear positioning and anchors to nuclear pore complexes, where specific types of homologous recombination pathways take place. In yeast models, increasing evidence points out that nuclear positioning is regulated by small ubiquitin-like modifier (SUMO) metabolism, which is pivotal to maintaining genome integrity at sites of replication stress. Here, we review how SUMO-based pathways are instrumental to spatially segregate the subsequent steps of homologous recombination during replication fork restart. In particular, we discussed how routing towards nuclear pore complex anchorage allows distinct homologous recombination pathways to take place at halted replication forks.

A Tale of Usurpation and Subversion: SUMO-Dependent Integrity of Promyelocytic Leukemia Nuclear Bodies at the Crossroad of Infection and Immunity

Promyelocytic leukemia nuclear bodies (PML NBs) are multi-protein assemblies representing distinct sub-nuclear structures. As phase-separated molecular condensates, PML NBs exhibit liquid droplet-like consistency. A key organizer of the assembly and dynamics of PML NBs is the ubiquitin-like SUMO modification system. SUMO is covalently attached to PML and other core components of PML NBs thereby exhibiting a glue-like function by providing multivalent interactions with proteins containing SUMO interacting motifs (SIMs). PML NBs serve as the catalytic center for nuclear SUMOylation and SUMO-SIM interactions are essential for protein assembly within these structures. Importantly, however, formation of SUMO chains on PML and other PML NB-associated proteins triggers ubiquitylation and proteasomal degradation which coincide with disruption of these nuclear condensates. To date, a plethora of nuclear activities such as transcriptional and post-transcriptional regulation of gene expression, apoptosis, senescence, cell cycle control, DNA damage response, and DNA replication have been associated with PML NBs. Not surprisingly, therefore, SUMO-dependent PML NB integrity has been implicated in regulating many physiological processes including tumor suppression, metabolism, drug-resistance, development, cellular stemness, and anti-pathogen immune response. The interplay between PML NBs and viral infection is multifaceted. As a part of the cellular antiviral defense strategy, PML NB components are crucial restriction factors for many viruses and a mutual positive correlation has been found to exist between PML NBs and the interferon response. Viruses, in turn, have developed counterstrategies for disarming PML NB associated immune defense measures. On the other end of the spectrum, certain viruses are known to usurp specific PML NB components for successful replication and disruption of these sub-nuclear foci has recently been linked to the stimulation rather than curtailment of antiviral gene repertoire. Importantly, the ability of invading virions to manipulate the host SUMO modification machinery is essential for this interplay between PML NB integrity and viruses. Moreover, compelling evidence is emerging in favor of bacterial pathogens to negotiate with the SUMO system thereby modulating PML NB-directed intrinsic and innate immunity. In the current context, we will present an updated account of the dynamic intricacies between cellular PML NBs as the nuclear SUMO modification hotspots and immune regulatory mechanisms in response to viral and bacterial pathogens.

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.

Peptide Hormone Insulin Regulates Function, Expression, and SUMOylation of Organic Anion Transporter 3

Organic anion transporter 3 (OAT3) plays an important role in the disposition of various anionic drugs which impacts the pharmacokinetics and pharmacodynamics of the therapeutics, thus influencing the pharmacological effects and toxicity of the drugs. In this study, we investigated the effect of insulin on the regulation of OAT3 function, expression, and SUMOylation. We demonstrated that insulin induced an increase in OAT3 transport activity through a dose- and time-dependent manner in COS-7 cells. The insulin-induced elevation in OAT3 function was blocked by PKA inhibitor H89, which correlated well with OAT3 protein expression. Moreover, both PKA activator Bt2-cAMP-induced increase and insulin-induced increase in OAT3 function were blocked by PKB inhibitor AKTi1/2. To further investigate the involvement of SUMOylation, we treated OAT3-expressing cells with insulin in presence or absence of H89 or AKTi1/2 followed by examining OAT3 SUMOylation. We showed that insulin enhanced OAT3 SUMOylation, and such enhancement was abrogated by H89 and AKTi1/2. Lastly, insulin increased OAT3 function and SUMOylation in rat kidney slice. In conclusion, our investigations demonstrated that insulin regulated OAT3 function, expression, and SUMOylation through PKA/PKB signaling pathway. Graphical abstract.

SUMOylation of α-tubulin is a novel modification regulating microtubule dynamics

Microtubules (MTs) are regulated by a number of known posttranslational modifications (PTMs) on α/β-tubulin to fulfill diverse cellular functions. Here, we showed that SUMOylation is a novel PTM on α-tubulin in vivo and in vitro. The SUMOylation on α-tubulin mainly occurred at Lys 96 (K96), K166, and K304 of soluble α-tubulin and could be removed by small ubiquitin-related modifier (SUMO)-specific peptidase 1. In vitro experiments showed that tubulin SUMOylation could reduce interprotofilament interaction, promote MT catastrophe, and impede MT polymerization. In cells, mutation of the SUMOylation sites on α-tubulin reduced catastrophe frequency and increased the proportion of polymerized α-tubulin, while upregulation of SUMOylation with fusion of SUMO1 reduced α-tubulin assembly into MTs. Additionally, overexpression of SUMOylation-deficient α-tubulin attenuated the neurite extension in Neuro-2a cells. Thus, SUMOylation on α-tubulin represents a new player in the regulation of MT properties.

Profiling the Murine SUMO Proteome in Response to Cardiac Ischemia and Reperfusion Injury

SUMOylation is a reversible posttranslational modification pathway catalyzing the conjugation of small ubiquitin-related modifier (SUMO) proteins to lysine residues of distinct target proteins. SUMOylation modifies a wide variety of cellular regulators thereby affecting a multitude of key processes in a highly dynamic manner. The SUMOylation pathway displays a hallmark in cellular stress-adaption, such as heat or redox stress. It has been proposed that enhanced cellular SUMOylation protects the brain during ischemia, however, little is known about the specific regulation of the SUMO system and the potential target proteins during cardiac ischemia and reperfusion injury (I/R). By applying left anterior descending (LAD) coronary artery ligation and reperfusion in mice, we detect dynamic changes in the overall cellular SUMOylation pattern correlating with decreased SUMO deconjugase activity during I/R injury. Further, unbiased system-wide quantitative SUMO-proteomics identified a sub-group of SUMO targets exhibiting significant alterations in response to cardiac I/R. Notably, transcription factors that control hypoxia- and angiogenesis-related gene expression programs, exhibit altered SUMOylation during ischemic stress adaptation. Moreover, several components of the ubiquitin proteasome system undergo dynamic changes in SUMO conjugation during cardiac I/R suggesting an involvement of SUMO signaling in protein quality control and proteostasis in the ischemic heart. Altogether, our study reveals regulated candidate SUMO target proteins in the mouse heart, which might be important in coping with hypoxic/proteotoxic stress during cardiac I/R injury.

Regulation of organic anion transporters: Role in physiology, pathophysiology, and drug elimination

The members of the organic anion transporter (OAT) family are mainly expressed in kidney, liver, placenta, intestine, and brain. These transporters play important roles in the disposition of clinical drugs, pesticides, signaling molecules, heavy metal conjugates, components of phytomedicines, and toxins, and therefore critical for maintaining systemic homeostasis. Alterations in the expression and function of OATs contribute to the intra- and inter-individual variability of the therapeutic efficacy and the toxicity of many drugs, and to many pathophysiological conditions. Consequently, the activity of these transporters must be highly regulated to carry out their normal functions. This review will present an update on the recent advance in understanding the cellular and molecular mechanisms underlying the regulation of renal OATs, emphasizing on the post-translational modification (PTM), the crosstalk among these PTMs, and the remote sensing and signaling network of OATs. Such knowledge will provide significant insights into the roles of these transporters in health and disease.

The Nuclear SUMO-Targeted Ubiquitin Quality Control Network Regulates the Dynamics of Cytoplasmic Stress Granules

Exposure of cells to heat or oxidative stress causes misfolding of proteins. To avoid toxic protein aggregation, cells have evolved nuclear and cytosolic protein quality control (PQC) systems. In response to proteotoxic stress, cells also limit protein synthesis by triggering transient storage of mRNAs and RNA-binding proteins (RBPs) in cytosolic stress granules (SGs). We demonstrate that the SUMO-targeted ubiquitin ligase (StUbL) pathway, which is part of the nuclear proteostasis network, regulates SG dynamics. We provide evidence that inactivation of SUMO deconjugases under proteotoxic stress initiates SUMO-primed, RNF4-dependent ubiquitylation of RBPs that typically condense into SGs. Impairment of SUMO-primed ubiquitylation drastically delays SG resolution upon stress release. Importantly, the StUbL system regulates compartmentalization of an amyotrophic lateral sclerosis (ALS)-associated FUS mutant in SGs. We propose that the StUbL system functions as surveillance pathway for aggregation-prone RBPs in the nucleus, thereby linking the nuclear and cytosolic axis of proteotoxic stress response.

SUMO system - a key regulator in sarcomere organization

Skeletal muscles constitute roughly 40% of human body mass. Muscles are specialized tissues that generate force to drive movements through ATP-driven cyclic interactions between the protein filaments, namely actin and myosin filaments. The filaments are organized in an intricate structure called the 'sarcomere', which is a fundamental contractile unit of striated skeletal and cardiac muscle, hosting a fine assembly of macromolecular protein complexes. The micrometer-sized sarcomere units are arranged in a reiterated array within myofibrils of muscle cells. The precise spatial organization of sarcomere is tightly controlled by several molecular mechanisms, indispensable for its force-generating function. Disorganized sarcomeres, either due to erroneous molecular signaling or due to mutations in the sarcomeric proteins, lead to human diseases such as cardiomyopathies and muscle atrophic conditions prevalent in cachexia. Protein post-translational modifications (PTMs) of the sarcomeric proteins serve a critical role in sarcomere formation (sarcomerogenesis), as well as in the steady-state maintenance of sarcomeres. PTMs such as phosphorylation, acetylation, ubiquitination, and SUMOylation provide cells with a swift and reversible means to adapt to an altered molecular and therefore cellular environment. Over the past years, SUMOylation has emerged as a crucial modification with implications for different aspects of cell function, including organizing higher-order protein assemblies. In this review, we highlight the fundamentals of the small ubiquitin-like modifiers (SUMO) pathway and its link specifically to the mechanisms of sarcomere assembly. Furthermore, we discuss recent studies connecting the SUMO pathway-modulated protein homeostasis with sarcomere organization and muscle-related pathologies.

SUMO Chains Rule on Chromatin Occupancy

The dynamic and reversible post-translational modification of proteins and protein complexes with the ubiquitin-related SUMO modifier regulates a wide variety of nuclear functions, such as transcription, replication and DNA repair. SUMO can be attached as a monomer to its targets, but can also form polymeric SUMO chains. While monoSUMOylation is generally involved in the assembly of protein complexes, multi- or polySUMOylation may have very different consequences. The evolutionary conserved paradigmatic signaling process initiated by multi- or polySUMOylation is the SUMO-targeted Ubiquitin ligase (StUbL) pathway, where the presence of multiple SUMO moieties primes ubiquitylation by the mammalian E3 ubiquitin ligases RNF4 or RNF111, or the yeast Slx5/8 heterodimer. The mammalian SUMO chain-specific isopeptidases SENP6 or SENP7, or yeast Ulp2, counterbalance chain formation thereby limiting StUbL activity. Many facets of SUMO chain signaling are still incompletely understood, mainly because only a limited number of polySUMOylated substrates have been identified. Here we summarize recent work that revealed a highly interconnected network of candidate polySUMO modified proteins functioning in DNA damage response and chromatin organization. Based on these datasets and published work on distinct polySUMO-regulated processes we discuss overarching concepts in SUMO chain function. We propose an evolutionary conserved role of polySUMOylation in orchestrating chromatin dynamics and genome stability networks by balancing chromatin-residency of protein complexes. This concept will be exemplified in processes, such as centromere/kinetochore organization, sister chromatid cohesion, DNA repair and replication.

Loss of the E2 SUMO-conjugating enzyme Ube2i in oocytes during ovarian folliculogenesis causes infertility in mice

The number and quality of oocytes within the ovarian reserve largely determines fertility and reproductive lifespan in mammals. An oocyte-specific transcription factor cascade controls oocyte development, and some of these transcription factors, such as newborn ovary homeobox gene (NOBOX), are candidate genes for primary ovarian insufficiency in women. Transcription factors are frequently modified by the post-translational modification SUMOylation, but it is not known whether SUMOylation is required for function of the oocyte-specific transcription factors or if SUMOylation is required in oocytes during their development within the ovarian follicle. To test this, the sole E2 SUMO-conjugating enzyme, Ube2i, was ablated in mouse oocytes beginning in primordial follicles. Loss of oocyte Ube2i resulted in female infertility with major defects in stability of the primordial follicle pool, ovarian folliculogenesis, ovulation and meiosis. Transcriptomic profiling of ovaries suggests that loss of oocyte Ube2i caused defects in both oocyte- and granulosa cell-expressed genes, including NOBOX and some of its known target genes. Together, these studies show that SUMOylation is required in the mammalian oocyte during folliculogenesis for both oocyte development and communication with ovarian somatic cells.

Enrichments of post-translational modifications in proteomic studies

More than 300 different protein post-translational modifications are currently known, but only a few have been extensively investigated because modified proteoforms are commonly present in sub-stoichiometry amount. For this reason, improvement of specific enrichment techniques is particularly useful for the proteomic characterization of post-translationally modified proteins. Enrichment proteomic strategies could help the researcher in the challenging issue to decipher the complex molecular cross-talk existing between the different factors influencing the cellular pathways. In this review the state of art of the platforms applied for the enrichment of specific and most common post-translational modifications, such as glycosylation and glycation, phosphorylation, sulfation, redox modifications (i.e. sulfydration and nitrosylation), methylation, acetylation, and ubiquitinylation, are described. Enrichments strategies applied to characterize less studied post-translational modifications are also briefly discussed.

The SUMO Isopeptidase SENP6 Functions as a Rheostat of Chromatin Residency in Genome Maintenance and Chromosome Dynamics

Signaling by the ubiquitin-related SUMO pathway relies on coordinated conjugation and deconjugation events. SUMO-specific deconjugating enzymes counterbalance SUMOylation, but comprehensive insight into their substrate specificity and regulation is missing. By characterizing SENP6, we define an N-terminal multi-SIM domain as a critical determinant in targeting SENP6 to SUMO chains. Proteomic profiling reveals a network of SENP6 functions at the crossroads of chromatin organization and DNA damage response (DDR). SENP6 acts as a SUMO eraser at telomeric and centromeric chromatin domains and determines the SUMOylation status and chromatin association of the cohesin complex. Importantly, SENP6 is part of the hPSO4/PRP19 complex that drives ATR-Chk1 activation. SENP6 deficiency impairs chromatin association of the ATR cofactor ATRIP, thereby compromising the activation of Chk1 signaling in response to aphidicolin-induced replicative stress and sensitizing cells to DNA damage. We propose a general role of SENP6 in orchestrating chromatin dynamics and genome stability networks by balancing chromatin residency of protein complexes.

Modification of the base excision repair enzyme MBD4 by the small ubiquitin-like molecule SUMO1

The base excision repair DNA N-glycosylase MBD4 (also known as MED1), an interactor of the DNA mismatch repair protein MLH1, plays a central role in the maintenance of genomic stability of CpG sites by removing thymine and uracil from G:T and G:U mismatches, respectively. MBD4 is also involved in DNA damage response and transcriptional regulation. The interaction with other proteins is likely critical for understanding MBD4 functions. To identify novel proteins that interact with MBD4, we used tandem affinity purification (TAP) from HEK-293 cells. The MBD4-TAP fusion and its co-associated proteins were purified sequentially on IgG and calmodulin affinity columns; the final eluate was shown to contain MLH1 by western blotting, and MBD4-associated proteins were identified by mass spectrometry. Bands with molecular weight higher than that expected for MBD4 (˜66 kD) yielded peptides corresponding to MBD4 itself and the small ubiquitin-like molecule-1 (SUMO1), suggesting that MBD4 is sumoylated in vivo. MBD4 sumoylation was validated by co-immunoprecipitation in HEK-293 and MCF7 cells, and by an in vitrosumoylation assay. Sequence and mutation analysis identified three main sumoylation sites: MBD4 is sumoylated preferentially on K137, with additional sumoylation at K215 and K377. Patterns of MBD4 sumoylation were altered, in a DNA damage-specific way, by the anti-metabolite 5-fluorouracil, the alkylating agent N-Methyl-N-nitrosourea and the crosslinking agent cisplatin. MCF7 extract expressing sumoylated MBD4 displays higher thymine glycosylase activity than the unmodified species. Of the 67 MBD4 missense mutations reported in The Cancer Genome Atlas, 14 (20.9%) map near sumoylation sites. These results indicate that MBD4 is sumoylated in vivo in a DNA damage-specific manner, and suggest that sumoylation serves to regulate its repair activity and could be compromised in cancer. This study expands the role played by sumoylation in fine-tuning DNA damage response and repair.

The poly-SUMO2/3 protease SENP6 enables assembly of the constitutive centromere-associated network by group deSUMOylation

In contrast to our extensive knowledge on ubiquitin polymer signaling, we are severely limited in our understanding of poly-SUMO signaling. We set out to identify substrates conjugated to SUMO polymers, using knockdown of the poly-SUMO2/3 protease SENP6. We identify over 180 SENP6 regulated proteins that represent highly interconnected functional groups of proteins including the constitutive centromere-associated network (CCAN), the CENP-A loading factors Mis18BP1 and Mis18A and DNA damage response factors. Our results indicate a striking protein group de-modification by SENP6. SENP6 deficient cells are severely compromised for proliferation, accumulate in G2/M and frequently form micronuclei. Accumulation of CENP-T, CENP-W and CENP-A to centromeres is impaired in the absence of SENP6. Surprisingly, the increase of SUMO chains does not lead to ubiquitin-dependent proteasomal degradation of the CCAN subunits. Our results indicate that SUMO polymers can act in a proteolysis-independent manner and consequently, have a more diverse signaling function than previously expected.

While the biological roles of ubiquitin chains are well studied, little is known about the functions of SUMO polymers. Here, the authors identify poly-SUMOylation substrates and provide evidence that SUMO polymers regulate the accumulation of CCAN subunits at chromatin and centromeres.

The SUMO-Specific Protease Senp2 Regulates SUMOylation, Expression and Function of Human Organic Anion Transporter 3

Organic anion transporter 3 (OAT3) plays a vital role in removing a broad array of anionic drugs from kidney, thereby avoiding their possibly toxic side effects in the body. We earlier demonstrated that OAT3 is subjected to a specific type of post-translational modification called SUMOylation. SUMOylation is a dynamic event, where de-SUMOylation is catalyzed by a class of SUMO-specific proteases. In the present investigation, we assessed the role of SUMO-specific protease Senp2 in OAT3 SUMOylation, expression and function. We report here that overexpression of Senp2 in COS-7 cells led to a reduced OAT3 SUMOylation, which correlated well with a decreased OAT3 expression and transport activity. Such phenomenon was not observed in cells overexpressing an inactive mutant of Senp2. Furthermore, transfection of cells with Senp2-specific siRNA to knockdown the endogenous Senp2 resulted in an increased OAT3 SUMOylation, which correlated well with an enhanced OAT3 expression and transport activity. Coimmunoprecipitation experiments showed that Senp2 directly interacted with OAT3 in the kidneys of rats. Together these results provided first demonstration that Senp2 is a significant regulator for OAT3-mediated organic anion/drug transport.

Assays of SUMO protease/isopeptidase activity and function in mammalian cells and tissues

Covalent conjugation of the ubiquitin-related SUMO modifier to lysine residues of cellular proteins (SUMOylation) is a prevalent posttranslational modification. SUMOs are synthesized as precursor proteins that require carboxy-terminal processing prior to conjugation. Subsequently, a multistep enzymatic pathway is used for conjugation to target proteins. SUMOylation generally impacts protein-protein interactions and the assembly of multiprotein complexes. Cellular processes regulated by SUMOylation include DNA damage responses, cell cycle progression, or the control of gene expression. SUMOylation is reversible and commonly only a small fraction of a particular SUMO target is modified at a given time. Deconjugation of SUMO is catalyzed by a group of cysteine proteases termed SUMO proteases or SUMO isopeptidases. In human cells nine SUMO proteases, belonging to three separate families of cysteine proteases have been identified so far. The regulation and target specificity of individual SUMO proteases have not been dissected in detail, but the current view is that each protease controls the modification of subsets of proteins that are functionally and/or physically linked. Importantly, some SUMO proteases/isopeptidases not only function in deconjugation of SUMO from proteins, but also act in C-terminal processing of the SUMO precursors. Here we describe general methods for monitoring SUMO protease/isopeptidase activities in cell or tissue extracts.

Activation of Protein Kinase A Stimulates SUMOylation, Expression, and Transport Activity of Organic Anion Transporter 3

Organic anion transporter 3 (OAT3) plays a vital role in removing a broad variety of anionic drugs from kidney, thus avoiding their possible toxicity in the body. We earlier established that activation of protein kinase C (PKC) enhances OAT3 ubiquitination, which promotes OAT3 internalization from the cell plasma membrane to intracellular endosomes and consequent degradation. As a result, OAT3 expression and transport activity are reduced. In the current study, we discovered that protein kinase A (PKA) had an opposite effect to PKC on the regulation of OAT3. We showed that activation of PKA by Bt2-cAMP stimulated OAT3 transport activity, which was largely caused by an enhanced plasma membrane expression of the transporter, kinetically reflected as an augmented maximal transport velocity V_max without notable alteration in substrate-binding affinity K_m. Additionally, we showed that PKA activation accelerated the rate of OAT3 recycling from intracellular compartments to the plasma membrane and decelerated the rate of OAT3 degradation. We further showed that OAT3 is subjected to post-translational modification by SUMO-2 and SUMO-3 not by SUMO-1. PKA activation enhanced OAT3 SUMOylation, which was accompanied by a reduced OAT3 ubiquitination. Finally, insulin-like growth factor 1 significantly stimulated OAT3 transport activity and SUMOylation through PKA signaling pathway. In conclusion, this is the first demonstration that PKA stimulated OAT3 expression and transport activity by altering the trafficking kinetics of OAT3 possibly through the crosstalk between SUMOylation and ubiquitination. Our studies are consistent with a remote sensing and signaling model for transporters (Wu et al. in Mol Pharmacol. 79(5):795-805, 2011).

SUMOylation by SUMO2 is implicated in the degradation of misfolded ataxin-7 via RNF4 in SCA7 models

Perturbation of protein homeostasis and aggregation of misfolded proteins is a major cause of many human diseases. A hallmark of the neurodegenerative disease spinocerebellar ataxia type 7 (SCA7) is the intranuclear accumulation of mutant, misfolded ataxin-7 (polyQ-ATXN7). Here, we show that endogenous ATXN7 is modified by SUMO proteins, thus also suggesting a physiological role for this modification under conditions of proteotoxic stress caused by the accumulation of polyQ-ATXN7. Co-immunoprecipitation experiments, immunofluorescence microscopy and proximity ligation assays confirmed the colocalization and interaction of polyQ-ATXN7 with SUMO2 in cells. Moreover, upon inhibition of the proteasome, both endogenous SUMO2/3 and the RNF4 ubiquitin ligase surround large polyQ-ATXN7 intranuclear inclusions. Overexpression of RNF4 and/or SUMO2 significantly decreased levels of polyQ-ATXN7 and, upon proteasomal inhibition, led to a marked increase in the polyubiquitination of polyQ-ATXN7. This provides a mechanism for the clearance of polyQ-ATXN7 from affected cells that involves the recruitment of RNF4 by SUMO2/3-modified polyQ-ATXN7, thus leading to its ubiquitination and proteasomal degradation. In a SCA7 knock-in mouse model, we similarly observed colocalization of SUMO2/3 with polyQ-ATXN7 inclusions in the cerebellum and retina. Furthermore, we detected accumulation of SUMO2/3 high-molecular-mass species in the cerebellum of SCA7 knock-in mice, compared with their wild-type littermates, and changes in SUMO-related transcripts. Immunohistochemical analysis showed the accumulation of SUMO proteins and RNF4 in the cerebellum of SCA7 patients. Taken together, our results show that the SUMO pathway contributes to the clearance of aggregated ATXN7 and suggest that its deregulation might be associated with SCA7 disease progression.

Rhabdoviruses, Antiviral Defense, and SUMO Pathway

Small Ubiquitin-like MOdifier (SUMO) conjugation to proteins has essential roles in several processes including localization, stability, and function of several players implicated in intrinsic and innate immunity. In human, five paralogs of SUMO are known of which three are ubiquitously expressed (SUMO1, 2, and 3). Infection by rhabdoviruses triggers cellular responses through the activation of pattern recognition receptors, which leads to the production and secretion of interferon. This review will focus on the effects of the stable expression of the different SUMO paralogs or Ubc9 depletion on rhabdoviruses-induced interferon production and interferon signaling pathways as well as on the expression and functions of restriction factors conferring the resistance to rhabdoviruses.

SUMO-Targeted Ubiquitin Ligases (STUbLs) Reduce the Toxicity and Abnormal Transcriptional Activity Associated With a Mutant, Aggregation-Prone Fragment of Huntingtin

Cell viability and gene expression profiles are altered in cellular models of neurodegenerative disorders such as Huntington’s Disease (HD). Using the yeast model system, we show that the SUMO-targeted ubiquitin ligase (STUbL) Slx5 reduces the toxicity and abnormal transcriptional activity associated with a mutant, aggregation-prone fragment of huntingtin (Htt), the causative agent of HD. We demonstrate that expression of an aggregation-prone Htt construct with 103 glutamine residues (103Q), but not the non-expanded form (25Q), results in severe growth defects in slx5Δ and slx8Δ cells. Since Slx5 is a nuclear protein and because Htt expression affects gene transcription, we assessed the effect of STUbLs on the transcriptional properties of aggregation-prone Htt. Expression of Htt 25Q and 55Q fused to the Gal4 activation domain (AD) resulted in reporter gene auto-activation. Remarkably, the auto-activation of Htt constructs was abolished by expression of Slx5 fused to the Gal4 DNA-binding domain (BD-Slx5). In support of these observations, RNF4, the human ortholog of Slx5, curbs the aberrant transcriptional activity of aggregation-prone Htt in yeast and a variety of cultured human cell lines. Functionally, we find that an extra copy of SLX5 specifically reduces Htt aggregates in the cytosol as well as chromatin-associated Htt aggregates in the nucleus. Finally, using RNA sequencing, we identified and confirmed specific targets of Htt’s transcriptional activity that are modulated by Slx5. In summary, this study of STUbLs uncovers a conserved pathway that counteracts the accumulation of aggregating, transcriptionally active Htt (and possibly other poly-glutamine expanded proteins) on chromatin in both yeast and in mammalian cells.

Acetylation of SUMO2 at lysine 11 favors the formation of non-canonical SUMO chains

Post-translational modifications by ubiquitin-related SUMO modifiers regulate cellular signaling networks and protein homeostasis. While SUMO1 is mainly conjugated to proteins as a monomer, SUMO2/3 can form polymeric chains. Poly-SUMOylation is best understood in the SUMO-targeted ubiquitin ligase (StUbL) pathway, where chains prime proteins for subsequent ubiquitylation by StUbLs. SUMO chains typically form in response to genotoxic or proteotoxic stress and are preferentially linked via lysine 11 of SUMO2/3. Here, we report that K11 of SUMO2/3 undergoes reversible acetylation with SIRT1 being the K11 deacetylase. In a purified in vitro system, acetylation of SUMO2/3 impairs chain formation and restricts chain length. In a cellular context, however, K11 acetyl-mimicking SUMO2 does not affect the StUbL pathway, indicating that in cells non-canonical chains are more prevalent. MS-based SUMO proteomics indeed identified non-canonical chain types under basal and stress conditions. Importantly, mimicking K11 acetylation alters chain architecture by favoring K5- and K35-linked chains, while inhibiting K7 and K21 linkages. These data provide insight into SUMO chain signaling and point to a role of K11 acetylation as a modulator of SUMO2/3 chains.

Site-specific characterization of endogenous SUMOylation across species and organs

Small ubiquitin-like modifiers (SUMOs) are post-translational modifications that play crucial roles in most cellular processes. While methods exist to study exogenous SUMOylation, large-scale characterization of endogenous SUMO2/3 has remained technically daunting. Here, we describe a proteomics approach facilitating system-wide and in vivo identification of lysines modified by endogenous and native SUMO2. Using a peptide-level immunoprecipitation enrichment strategy, we identify 14,869 endogenous SUMO2/3 sites in human cells during heat stress and proteasomal inhibition, and quantitatively map 1963 SUMO sites across eight mouse tissues. Characterization of the SUMO equilibrium highlights striking differences in SUMO metabolism between cultured cancer cells and normal tissues. Targeting preferences of SUMO2/3 vary across different organ types, coinciding with markedly differential SUMOylation states of all enzymes involved in the SUMO conjugation cascade. Collectively, our systemic investigation details the SUMOylation architecture across species and organs and provides a resource of endogenous SUMOylation sites on factors important in organ-specific functions.

Proteomics is a powerful method to study protein SUMOylation, but system-wide insights into endogenous SUMO2/3 modification events are still sparse. Here, the authors develop a more sensitive SUMO proteomics approach, providing detailed maps of endogenous SUMO2/3 sites in human cells and mouse tissues.

SUMO-specific proteases and isopeptidases of the SENP family at a glance

The ubiquitin-related SUMO system controls many cellular signaling networks. In mammalian cells, three SUMO forms (SUMO1, SUMO2 and SUMO3) act as covalent modifiers of up to thousands of cellular proteins. SUMO conjugation affects cell function mainly by regulating the plasticity of protein networks. Importantly, the modification is reversible and highly dynamic. Cysteine proteases of the sentrin-specific protease (SENP) family reverse SUMO conjugation in mammalian cells. In this Cell Science at a Glance article and the accompanying poster, we will summarize how the six members of the mammalian SENP family orchestrate multifaceted deconjugation events to coordinate cell processes, such as gene expression, the DNA damage response and inflammation.

The strategies for identification and quantification of SUMOylation

SUMOylation is a post-translational modification that plays critical roles in a multitude of cellular processes including transcription, cellular localization, DNA repair and cell cycle progression. Similar to ubiquitin, the small ubiquitin-like modifiers (SUMOs) are covalently attached to the epsilon amino group of lysine residues in the substrates. To understand the regulation and the dynamics of post-translational modifications (PTMs), the identification and quantification of SUMOylation is strictly needed. Although numerous proteomic approaches have been developed to identify hundreds of SUMO target proteins, the number of SUMOylation signatures identified from endogenous modified proteins is limited, and the identification of precise acceptor sites remains a challenge due to the low abundance of in vivo SUMO-modified proteins and the high activity of SUMO-specific proteases in cell lysates. In particular, very few sensitive strategies are available for accurate quantification of SUMO target proteins. Within the past decade, mass spectrometry-based strategies have been the most popular technologies for proteome-wide studies of SUMOylation. Recently, some new approaches such as single-molecule detection have been introduced. In this review, we summarize the strategies that have been exploited for enrichment, purification and identification of SUMOylation substrates and acceptor sites as well as ultrasensitive quantification of SUMOylation. We highlight the emerging trends in this field as well.

SUMO2 modification of Aurora B and its impact on follicular development and atresia in the mouse ovary

In the mammalian ovary, >99% follicles fail to ovulate due to apoptosis in granulosa cells. Aurora B, a core subunit enzyme of the chromosomal passenger complex, exerts a crucial role in microtubule‑kinetochore attachment, and has been reported to be modified by small ubiquitin‑related modifier (SUMO) proteins. However, the details of how Aurora B and its SUMOylation impact on follicular development have yet to be fully elucidated. The aim of the present study was to explore the roles, and possible molecular mechanism, of Aurora B and its SUMOylation in the granulosa cells of the mouse follicle. It was revealed that the protein level of Aurora B increased with follicular development and the growth of the granulosa cells. Aurora B impacted follicular development and atresia through mediating the p38 mitogen‑activated protein kinase and FasL/Fas pathways, and caused the downregulation of cyclin‑dependent kinase 4, proliferating cell nuclear antigen, Bcl‑2, and upregulation of caspases‑3 and ‑8 to modulate the viability of the granulosa cells. In addition, Aurora B undergoes modification by SUMO2, but not by SUMO1, in vivo and in vitro, and Lys‑207 is a major modification site. SUMOylation modulates follicular development through an increase in Aurora B localization in the nucleus, and by stabilizing the protein level of Aurora B and keeping the viability of the granulosa cells. Taken together, Aurora B and its SUMOylation are important for follicular development and atresia in the ovaries of mice.

SUMO polymeric chains are involved in nuclear foci formation and chromatin organization in Trypanosoma brucei procyclic forms

SUMOylation is a post-translational modification conserved in eukaryotic organisms that involves the covalent attachment of the small ubiquitin-like protein SUMO to internal lysine residues in target proteins. This tag usually alters the interaction surface of the modified protein and can be translated into changes in its biological activity, stability or subcellular localization, among other possible outputs. SUMO can be attached as a single moiety or as SUMO polymers in case there are internal acceptor sites in SUMO itself. These chains have been shown to be important for proteasomal degradation as well as for the formation of subnuclear structures such as the synaptonemal complex in Saccharomyces cerevisiae or promyelocytic leukemia nuclear bodies in mammals. In this work, we have examined SUMO chain formation in the protozoan parasite Trypanosoma brucei. Using a recently developed bacterial strain engineered to produce SUMOylated proteins we confirmed the ability of TbSUMO to form polymers and determined the type of linkage using site-directed mutational analysis. By generating transgenic procyclic parasites unable to form chains we demonstrated that although not essential for normal growth, SUMO polymerization determines the localization of the modified proteins in the nucleus. In addition, FISH analysis of telomeres showed a differential positioning depending on the polySUMOylation abilities of the cells. Thus, our observations suggest that TbSUMO chains might play a role in establishing interaction platforms contributing to chromatin organization.

Functions of SUMO in the Maintenance of Genome Stability

Like in most other areas of cellular metabolism, the functions of the ubiquitin-like modifier SUMO in the maintenance of genome stability are manifold and varied. Perturbations of global sumoylation causes a wide spectrum of phenotypes associated with defects in DNA maintenance, such as hypersensitivity to DNA-damaging agents, gross chromosomal rearrangements and loss of entire chromosomes. Consistent with these observations, many key factors involved in various DNA repair pathways have been identified as SUMO substrates. However, establishing a functional connection between a given SUMO target, the cognate SUMO ligase and a relevant phenotype has remained a challenge, mainly because of the difficulties involved in identifying important modification sites and downstream effectors that specifically recognize the target in its sumoylated state. This review will give an overview over the major pathways of DNA repair and genome maintenance influenced by the SUMO system and discuss selected examples of SUMO's actions in these pathways where the biological consequences of the modification have been elucidated.

RNF4 interacts with multiSUMOylated ETV4

Protein SUMOylation represents an important regulatory event that changes the activities of numerous proteins. Recent evidence demonstrates that polySUMO chains can act as a trigger to direct the ubiquitin ligase RNF4 to substrates to cause their turnover through the ubiquitin pathway. RNF4 uses multiple SUMO interaction motifs (SIMs) to bind to these chains. However, in addition to polySUMO chains, a multimeric binding surface created by the simultaneous SUMOylation of multiple residues on a protein or complex could also provide a platform for the recruitment of multi-SIM proteins like RNF4. Here we demonstrate that multiSUMOylated ETV4 can bind to RNF4 and that a unique combination of SIMs is required for RNF4 to interact with this multiSUMOylated platform. Thus RNF4 can bind to proteins that are either polySUMOylated through a single site or multiSUMOylated on several sites and raises the possibility that such multiSIM-multiSUMO interactions might be more widespread.

Molecular Imaging Biosensor Monitors p53 Sumoylation in Cells and Living Mice

Small molecule mediated stabilization of p53 tumor suppressor protein through sumoylation is a promising new strategy for improving cancer chemotherapy. A molecular tool that monitors p53 sumoylation status and expedites screening for drugs that enhance p53 sumoylation would be beneficial. We report a molecularly engineered reporter fragment complementation biosensor based on optical imaging of Firefly luciferase (FLuc), to quantitatively image p53 sumoylation and desumoylation in cells and living mice. We initially characterized this biosensor by successfully imaging sumoylation of several target proteins, achieving significant FLuc complementation for ERα (p < 0.01), p53 (p < 0.005), FKBP12 (p < 0.03), ID (p < 0.03), and HDAC1 (p < 0.002). We then rigorously tested the sensitivity and specificity of the biosensor using several variants of p53 and SUMO1, including deletion mutants, and those with modified sequences containing the SUMO-acceptor site of target proteins. Next we evaluated the performance of the biosensor in HepG2 cells by treatment with ginkgolic acid, a drug that reduces p53 sumoylation, as well as trichostatin A, a potential inducer of p53 sumoylation by enhancement of its nuclear export. Lastly, we demonstrated the in vivo utility of this biosensor in monitoring and quantifying the effects of these drugs on p53 sumoylation in living mice using bioluminescence imaging. Adoption of this biosensor in future high throughput drug screening has the important potential to help identify new and repurposed small molecules that alter p53 sumoylation, and to preclinically evaluate candidate anticancer drugs in living animals.

Regulation of Ebola virus VP40 matrix protein by SUMO

The matrix protein of Ebola virus (EBOV) VP40 regulates viral budding, nucleocapsid recruitment, virus structure and stability, viral genome replication and transcription, and has an intrinsic ability to form virus-like particles. The elucidation of the regulation of VP40 functions is essential to identify mechanisms to inhibit viral replication and spread. Post-translational modifications of proteins with ubiquitin-like family members are common mechanisms for the regulation of host and virus multifunctional proteins. Thus far, no SUMOylation of VP40 has been described. Here we demonstrate that VP40 is modified by SUMO and that SUMO is included into the viral like particles (VLPs). We demonstrate that lysine residue 326 in VP40 is involved in SUMOylation, and by analyzing a mutant in this residue we show that SUMO conjugation regulates the stability of VP40 and the incorporation of SUMO into the VLPs. Our study indicates for the first time, to the best of our knowledge, that EBOV hijacks the cellular SUMOylation system in order to modify its own proteins. Modulation of the VP40-SUMO interaction may represent a novel target for the therapy of Ebola virus infection.

RNF4 interacts with multiSUMOylated ETV4

Protein SUMOylation represents an important regulatory event that changes the activities of numerous proteins. Recent evidence demonstrates that polySUMO chains can act as a trigger to direct the ubiquitin ligase RNF4 to substrates to cause their turnover through the ubiquitin pathway. RNF4 uses multiple SUMO interaction motifs (SIMs) to bind to these chains. However, in addition to polySUMO chains, a multimeric binding surface created by the simultaneous SUMOylation of multiple residues on a protein or complex could also provide a platform for the recruitment of multi-SIM proteins like RNF4. Here we demonstrate that multiSUMOylated ETV4 can bind to RNF4 and that a unique combination of SIMs is required for RNF4 to interact with this multiSUMOylated platform. Thus RNF4 can bind to proteins that are either polySUMOylated through a single site or multiSUMOylated on several sites and raises the possibility that such multiSIM-multiSUMO interactions might be more widespread.

Functional Crosstalk between the PP2A and SUMO Pathways Revealed by Analysis of STUbL Suppressor, razor 1-1

Posttranslational modifications (PTMs) provide dynamic regulation of the cellular proteome, which is critical for both normal cell growth and for orchestrating rapid responses to environmental stresses, e.g. genotoxins. Key PTMs include ubiquitin, the Small Ubiquitin-like MOdifier SUMO, and phosphorylation. Recently, SUMO-targeted ubiquitin ligases (STUbLs) were found to integrate signaling through the SUMO and ubiquitin pathways. In general, STUbLs are recruited to target proteins decorated with poly-SUMO chains to ubiquitinate them and drive either their extraction from protein complexes, and/or their degradation at the proteasome. In fission yeast, reducing or preventing the formation of SUMO chains can circumvent the essential and DNA damage response functions of STUbL. This result indicates that whilst some STUbL "targets" have been identified, the crucial function of STUbL is to antagonize SUMO chain formation. Herein, by screening for additional STUbL suppressors, we reveal crosstalk between the serine/threonine phosphatase PP2A-Pab1^B55 and the SUMO pathway. A hypomorphic Pab1^B55 mutant not only suppresses STUbL dysfunction, but also mitigates the phenotypes associated with deletion of the SUMO protease Ulp2, or mutation of the STUbL cofactor Rad60. Together, our results reveal a novel role for PP2A-Pab1^B55 in modulating SUMO pathway output, acting in parallel to known critical regulators of SUMOylation homeostasis. Given the broad evolutionary functional conservation of the PP2A and SUMO pathways, our results could be relevant to the ongoing attempts to therapeutically target these factors.

Posttranslational modifiers (PTMs) orchestrate the proteins and processes that control genome stability and cell growth. Accordingly, deregulation of PTMs causes disease, but can also be harnessed therapeutically. Crosstalk between PTMs is widespread, and acts to increase specificity and selectivity in signal transduction. Such crosstalk exists between two major PTMs, SUMO and ubiquitin, wherein a SUMO-targeted ubiquitin ligase (STUbL) can additionally mark SUMO-modified proteins with ubiquitin. Thereby, STUbL generates a hybrid SUMO-ubiquitin signal that is recognized by selective effectors, which can extract proteins from complexes and/or direct their degradation at the proteasome. STUbL function is critical to maintain genome stability, and it also mediates the therapeutic effects of arsenic trioxide in leukemia treatment. Therefore, a full appreciation of STUbL regulation and integration with other PTMs is warranted. Unexpectedly, we find that reduced activity of PP2A, a major cellular phosphatase, compensates for STUbL inactivation. Our results indicate that PP2A-regulated phosphorylation reduces the SUMO chain output of the SUMO pathway, thus reducing cellular dependency on STUbL and the functionally related factors Ulp2 and Rad60. Our data not only reveal a striking level of plasticity in signaling through certain PTMs, but also highlight potential "escape" mechanisms for SUMO pathway-based therapies.

Sumoylation of eIF4A2 affects stress granule formation

Regulation of protein synthesis is crucial for cells to maintain viability and to prevent unscheduled proliferation that could lead to tumorigenesis. Exposure to stress results in stalling of translation, with many translation initiation factors, ribosomal subunits and mRNAs being sequestered into stress granules or P bodies. This allows the re-programming of the translation machinery. Many aspects of translation are regulated by post-translational modification. Several proteomic screens have identified translation initiation factors as targets for sumoylation, although in many cases the role of this modification has not been determined. We show here that eIF4A2 is modified by SUMO, with sumoylation occurring on a single residue (K226). We demonstrate that sumoylation of eIF4A2 is modestly increased in response to arsenite and ionising radiation, but decreases in response to heat shock or hippuristanol. In arsenite-treated cells, but not in hippuristanol-treated cells, eIF4A2 is recruited to stress granules, suggesting sumoylation of eIF4A2 correlates with its recruitment to stress granules. Furthermore, we demonstrate that the inability to sumoylate eIF4A2 results in impaired stress granule formation, indicating a new role for sumoylation in the stress response.

Summary: In response to stress, proteins required to initiate protein synthesis are modified; we demonstrate that sumoylation of eIF4A2 correlates with its recruitment to stress granules.

Regulatory Functions of Ubiquitin and SUMO in DNA Repair Pathways

Ubiquitin and SUMO are structurally related protein modifiers that are covalently attached to lysine residues of target proteins. While ubiquitin is traditionally known as a signal for proteasomal degradation, its nondegradative actions are equally important in the control of cellular key processes. Similarly, the SUMO system primarily acts in a nondegradative manner. Accumulating evidence indicates that these nonproteolytic functions of ubiquitin and SUMO are particularly important in the control of the DNA damage response network, which coordinates a set of DNA repair pathways and allows cells to cope with different types of genotoxic stress. In this chapter we will illustrate some key functions of ubiquitin and SUMO in the control of selected DNA repair pathways.

c-Myc is targeted to the proteasome for degradation in a SUMOylation-dependent manner, regulated by PIAS1, SENP7 and RNF4

c-Myc is the most frequently overexpressed oncogene in tumors, including breast cancer, colon cancer and lung cancer. Post-translational modifications comprising phosphorylation, acetylation and ubiquitylation regulate the activity of c-Myc. Recently, it was shown that c-Myc-driven tumors are strongly dependent on the SUMO pathway. Currently, the relevant SUMO target proteins in this pathway are unknown. Here we show that c-Myc is a target protein for SUMOylation, and that SUMOylated c-Myc is subsequently ubiquitylated and degraded by the proteasome. SUMO chains appeared to be dispensable for this process, polymerization-deficient SUMO mutants supported proteolysis of SUMOylated c-Myc. These results indicate that multiple SUMO monomers conjugated to c-Myc could be sufficient to direct SUMOylated c-Myc to the ubiquitin-proteasome pathway. Knocking down the SUMO-targeted ubiquitin ligase RNF4 enhanced the levels of SUMOylated c-Myc, indicating that RNF4 could recognize a multi-SUMOylated protein as a substrate in addition to poly-SUMOylated proteins. Knocking down the SUMO E3 ligase PIAS1 resulted in reduced c-Myc SUMOylation and increased c-Myc transcriptional activity, indicating that PIAS1 mediates c-Myc SUMOylation. Increased SUMOylation of c-Myc was noted upon knockdown of the SUMO protease SENP7, indicating that it also could regulate a multi-SUMOylated protein in addition to poly-SUMOylated proteins. C-Myc lacks KxE-type SUMOylation consensus motifs. We used mass spectrometry to identify 10 SUMO acceptor lysines: K52, K148, K157, K317, K323, K326, K389, K392, K398 and K430. Intriguingly, mutating all 10 SUMO acceptor lysines did not reduce c-Myc SUMOylation, suggesting that SUMO acceptor lysines in c-Myc act promiscuously. Our results provide novel insight into the complexity of c-Myc post-translational regulation.

S. cerevisiae Mre11 recruits conjugated SUMO moieties to facilitate the assembly and function of the Mre11-Rad50-Xrs2 complex

Double-strand breaks (DSBs) in chromosomes are the most challenging type of DNA damage. The yeast and mammalian Mre11-Rad50-Xrs2/Nbs1 (MRX/N)-Sae2/Ctp1 complex catalyzes the resection of DSBs induced by secondary structures, chemical adducts or covalently-attached proteins. MRX/N also initiates two parallel DNA damage responses-checkpoint phosphorylation and global SUMOylation-to boost a cell's ability to repair DSBs. However, the molecular mechanism of this SUMO-mediated response is not completely known. In this study, we report that Saccharomyces cerevisiae Mre11 can non-covalently recruit the conjugated SUMO moieties, particularly the poly-SUMO chain. Mre11 has two evolutionarily-conserved SUMO-interacting motifs, Mre11(SIM1) and Mre11(SIM2), which reside on the outermost surface of Mre11. Mre11(SIM1) is indispensable for MRX assembly. Mre11(SIM2) non-covalently links MRX with the SUMO enzymes (E2/Ubc9 and E3/Siz2) to promote global SUMOylation of DNA repair proteins. Mre11(SIM2) acts independently of checkpoint phosphorylation. During meiosis, the mre11(SIM2) mutant, as for mre11S, rad50S and sae2Δ, allows initiation but not processing of Spo11-induced DSBs. Using MRX and DSB repair as a model, our work reveals a general principle in which the conjugated SUMO moieties non-covalently facilitate the assembly and functions of multi-subunit protein complexes.

Identification and Characterization of SUMO-SIM Interactions

The covalent attachment of SUMO to lysine residues of cellular proteins serves as an important mechanism for the dynamic control of protein networks. SUMO conjugates typically mediate selected protein-protein interactions by binding to specific recognition modules. Identification of SUMO-binding proteins and the characterization of the binding motifs are key to understanding SUMO signaling. Here we describe two complementary approaches that are used to tackle these questions.

SUMOylation represses SnRK1 signaling in Arabidopsis

The SnRK1 protein kinase balances cellular energy levels in accordance with extracellular conditions and is thereby key for plant stress tolerance. In addition, SnRK1 has been implicated in numerous growth and developmental processes from seed filling and maturation to flowering and senescence. Despite its importance, the mechanisms that regulate SnRK1 activity are poorly understood. Here, we demonstrate that the SnRK1 complex is SUMOylated on multiple subunits and identify SIZ1 as the E3 Small Ubiquitin-like Modifier (SUMO) ligase responsible for this modification. We further show that SnRK1 is ubiquitinated in a SIZ1-dependent manner, causing its degradation through the proteasome. In consequence, SnRK1 degradation is deficient in siz1-2 mutants, leading to its accumulation and hyperactivation of SnRK1 signaling. Finally, SnRK1 degradation is strictly dependent on its activity, as inactive SnRK1 variants are aberrantly stable but recover normal degradation when expressed as SUMO mimetics. Altogether, our data suggest that active SnRK1 triggers its own SUMOylation and degradation, establishing a negative feedback loop that attenuates SnRK1 signaling and prevents detrimental hyperactivation of stress responses.