SUMO: getting it on

Therapeutic Potential of Targeting the SUMO Pathway in Cancer

SUMOylation is a dynamic and reversible post-translational modification, characterized more than 20 years ago, that regulates protein function at multiple levels. Key oncoproteins and tumor suppressors are SUMO substrates. In addition to alterations in SUMO pathway activity due to conditions typically present in cancer, such as hypoxia, the SUMO machinery components are deregulated at the genomic level in cancer. The delicate balance between SUMOylation and deSUMOylation is regulated by SENP enzymes possessing SUMO-deconjugation activity. Dysregulation of SUMO machinery components can disrupt the balance of SUMOylation, contributing to the tumorigenesis and drug resistance of various cancers in a context-dependent manner. Many molecular mechanisms relevant to the pathogenesis of specific cancers involve SUMO, highlighting the potential relevance of SUMO machinery components as therapeutic targets. Recent advances in the development of inhibitors targeting SUMOylation and deSUMOylation permit evaluation of the therapeutic potential of targeting the SUMO pathway in cancer. Finally, the first drug inhibiting SUMO pathway, TAK-981, is currently also being evaluated in clinical trials in cancer patients. Intriguingly, the inhibition of SUMOylation may also have the potential to activate the anti-tumor immune response. Here, we comprehensively and systematically review the recent developments in understanding the role of SUMOylation in cancer and specifically focus on elaborating the scientific rationale of targeting the SUMO pathway in different cancers.

HSP27 role in cardioprotection by modulating chemotherapeutic doxorubicin-induced cell death

The common phenomenon expected from any anti-cancer drug in use is to kill the cancer cells without any side effects to non-malignant cells. Doxorubicin is an anthracycline derivative anti-cancer drug active over different types of cancers with anti-cancer activity but attributed to unintended cytotoxicity and genotoxicity triggering mitogenic signals inducing apoptosis. Administration of doxorubicin tends to both acute and chronic toxicity resulting in cardiomyopathy (left ventricular dysfunction) and congestive heart failure (CHF). Cardiotoxicity is prevented through administration of different cardioprotectants along with the drug. This review elaborates on mechanism of drug-mediated cardiotoxicity and attenuation principle by different cardioprotectants, with a focus on Hsp27 as cardioprotectant by prevention of drug-induced oxidative stress, cell survival pathways with suppression of intrinsic cell death. In conclusion, Hsp27 may offer an exciting/alternating cardioprotectant, with a wider study being need of the hour, specifically on primary cell line and animal models in conforming its cardioprotectant behaviour.

Intrinsically Disordered SRC-3/AIB1 Protein Undergoes Homeostatic Nuclear Extrusion by Nuclear Budding While Ectopic Expression Induces Nucleophagy

SRC-3/AIB1 (Amplified in Breast Cancer-1) is a nuclear receptor coactivator for the estrogen receptor in breast cancer cells. It is also an intrinsically disordered protein when not engaged with transcriptional binding partners and degraded upon transcriptional coactivation. Given the amplified expression of SRC-3 in breast cancers, the objective of this study was to determine how increasing SRC-3 protein levels are regulated in MCF-7 breast cancer cells. We found that endogenous SRC-3 was expelled from the nucleus in vesicle-like spheres under normal growth conditions suggesting that this form of nuclear exclusion of SRC-3 is a homeostatic mechanism for regulating nuclear SRC-3 protein. Only SRC-3 not associated with CREB-binding protein (CBP) was extruded from the nucleus. We found that overexpression in MCF-7 cells results in aneuploid senescence and cell death with frequent formation of nuclear aggregates which were consistently juxtaposed to perinuclear microtubules. Transfected SRC-3 was SUMOylated and caused redistribution of nuclear promyelocytic leukemia (PML) bodies and perturbation of the nuclear membrane lamin B1, hallmarks of nucleophagy. Increased SRC-3 protein-induced autophagy and resulted in SUMO-1 localization to the nuclear membrane and formation of protrusions variously containing SRC-3 and chromatin. Aspects of SRC-3 overexpression and toxicity were recapitulated following treatment with clinically relevant agents that stabilize SRC-3 in breast cancer cells. We conclude that amplified SRC-3 levels have major impacts on nuclear protein quality control pathways and may mark cancer cells for sensitivity to protein stabilizing therapeutics.

CRMP2 and voltage-gated ion channels: potential roles in neuropathic pain

Neuropathic pain represents a significant and mounting burden on patients and society at large. Management of neuropathic pain, however, is both intricate and challenging, exacerbated by the limited quantity and quality of clinically available treatments. On this stage, dysfunctional voltage-gated ion channels, especially the presynaptic N-type voltage-gated calcium channel (VGCC) (Cav2.2) and the tetrodotoxin-sensitive voltage-gated sodium channel (VGSC) (Nav1.7), underlie the pathophysiology of neuropathic pain and serve as high profile therapeutic targets. Indirect regulation of these channels holds promise for the treatment of neuropathic pain. In this review, we focus on collapsin response mediator protein 2 (CRMP2), a protein with emergent roles in voltage-gated ion channel trafficking and discuss the therapeutic potential of targetting this protein.

SUMO2/3 modification of activating transcription factor 5 (ATF5) controls its dynamic translocation at the centrosome

Activating transcription factor 5 (ATF5) is a member of the ATF/cAMP response element-binding protein family of transcription factors. ATF5 regulates stress responses and cell survival, proliferation, and differentiation and also plays a role in viral infections, cancer, diabetes, schizophrenia, and the olfactory system. Moreover, it was found to also have a critical cell cycle-dependent structural function at the centrosome. However, the mechanism that controls the localization of ATF5 at the centrosome is unclear. Here we report that ATF5 is small ubiquitin-like modifier (SUMO) 2/3-modified at a conserved SUMO-targeting consensus site in various types of mammalian cells. We found that SUMOylation of ATF5 is elevated in the G₁ phase of the cell cycle and diminished in the G₂/M phase. ATF5 SUMOylation disrupted the interaction of ATF5 with several centrosomal proteins and dislodged ATF5 from the centrosome at the end of the M phase. Of note, blockade of ATF5 SUMOylation deregulated the centrosome cycle, impeded ATF5 translocation from the centrosome, and caused genomic instability and G₂/M arrest in HeLa cells. Our results indicate that ATF5 SUMOylation is an essential mechanism that regulates ATF5 localization and function at the centrosome.

RAS GTPases are modified by SUMOylation

RAS proteins are GTPases that participate in multiple signal cascades, regulating crucial cellular processes including cell survival, proliferation, differentiation, and autophagy. Mutations or deregulated activities of RAS are frequently the driving force for oncogenic transformation and tumorigenesis. Given the important roles of the small ubiquitin-related modifier (SUMO) pathway in controlling the stability, activity, or subcellular localization of key cellular regulators, we investigated here whether RAS proteins are posttranslationally modified (i.e. SUMOylated) by the SUMO pathway. We observed that all three RAS protein isoforms (HRAS, KRAS, and NRAS) were modified by the SUMO3 protein. SUMOylation of KRAS protein, either endogenous or ectopically expressed, was observed in multiple cell lines. The SUMO3 modification of KRAS proteins could be removed by SUMO1/sentrin-specific peptidase 1 (SENP1) and SENP2, but not by SENP6, indicating that RAS SUMOylation is a reversible process. A conserved residue in RAS, Lys-42, was a site that mediates SUMOylation. Results from biochemical and molecular studies indicated that the SUMO-E3 ligase PIASγ specifically interacts with RAS and promotes its SUMOylation. Moreover, SUMOylation of RAS appeared to be associated with its activation. In summary, our study reveals a new posttranslational modification for RAS proteins. Since we found that HRAS, KRAS, and NRAS can all be SUMOylated, we propose that SUMOylation might represent a mechanism by which RAS activities are controlled.

Sumoylation and Its Contribution to Cancer

Post-translational modifications play an important role in regulating protein activity by altering their functions. Sumoylation is a highly dynamic process which is tightly regulated by a fine balance between conjugating and deconjugating enzyme activities. It affects intracellular localization and their interaction with their binding partners, thereby changing gene expression. Consequently, these changes in turn affect signaling mechanisms that regulate many cellular functions, such as cell growth, proliferation, apoptosis , DNA repair , and cell survival. It is becoming apparent that deregulation in the SUMO pathway contributes to oncogenic transformation by affecting sumoylation/desumoylation of many oncoproteins and tumor suppressors. Loss of balance between sumoylation and desumoylation has been reported in a number of studies in a variety of disease types including cancer. This chapter summarizes the mechanisms and functions of the deregulated SUMO pathway affecting oncogenes and tumor suppressor genes.

Mechanisms of Hsp90 regulation

Heat shock protein 90 (Hsp90) is a molecular chaperone that is involved in the activation of disparate client proteins. This implicates Hsp90 in diverse biological processes that require a variety of co-ordinated regulatory mechanisms to control its activity. Perhaps the most important regulator is heat shock factor 1 (HSF1), which is primarily responsible for upregulating Hsp90 by binding heat shock elements (HSEs) within Hsp90 promoters. HSF1 is itself subject to a variety of regulatory processes and can directly respond to stress. HSF1 also interacts with a variety of transcriptional factors that help integrate biological signals, which in turn regulate Hsp90 appropriately. Because of the diverse clientele of Hsp90 a whole variety of co-chaperones also regulate its activity and some are directly responsible for delivery of client protein. Consequently, co-chaperones themselves, like Hsp90, are also subject to regulatory mechanisms such as post translational modification. This review, looks at the many different levels by which Hsp90 activity is ultimately regulated.

The critical role of SENP1-mediated GATA2 deSUMOylation in promoting endothelial activation in graft arteriosclerosis

Data from clinical research and our previous study have suggested the potential involvement of SENP1, the major protease of post-translational SUMOylation, in cardiovascular disorders. Here, we investigate the role of SENP1-mediated SUMOylation in graft arteriosclerosis (GA), the major cause of allograft failure. We observe an endothelial-specific induction of SENP1 and GATA2 in clinical graft rejection specimens that show endothelial activation-mediated vascular remodelling. In mouse aorta transplantation GA models, endothelial-specific SENP1 knockout grafts demonstrate limited neointima formation with attenuated leukocyte recruitment, resulting from diminished induction of adhesion molecules in the graft endothelium due to increased GATA2 SUMOylation. Mechanistically, inflammation-induced SENP1 promotes the deSUMOylation of GATA2 and IκBα in endothelial cells, resulting in increased GATA2 stability, promoter-binding capability and NF-κB activity, which leads to augmented endothelial activation and inflammation. Therefore, upon inflammation, endothelial SENP1-mediated SUMOylation drives GA by regulating the synergistic effect of GATA2 and NF-κB and consequent endothelial dysfunction.

Regulatory Mechanisms of Hsp90

The ability of Hsp90 to activate a disparate clientele implicates this chaperone in diverse biological processes. To accommodate such varied roles, Hsp90 requires a variety of regulatory mechanisms that are coordinated in order to modulate its activity appropriately. Amongst these, the master-regulator heat shock factor 1 (HSF1) is critically important in upregulating Hsp90 during stress, but is also responsible, through interaction with specific transcription factors (such as STAT1 and Strap/p300) for the integration of a variety of biological signals that ultimately modulate Hsp90 expression. Additionally, transcription factors, such as STAT1, STAT3 (including STAT1-STAT3 oligomers), NF-IL6, and NF-kB, are known to influence Hsp90 expression directly. Co-chaperones offer another mechanism for Hsp90 regulation, and these can modulate the chaperone cycle appropriately for specific clientele. Co-chaperones include those that deliver specific clients to Hsp90, and others that regulate the chaperone cycle for specific Hsp90-client complexes by modulating Hsp90s ATPase activity. Finally, post-translational modification (PTM) of Hsp90 and its co-chaperones helps too further regulate the variety of different Hsp90 complexes found in cells.

Detection of ORF6 protein associated with latent KHV infection

Koi herpesvirus (KHV) is highly pathogenic to Cyprinus carpio. KHV can also become latent in recovered fish and reactivate from latency under stressful conditions. Understanding KHV latency is important for development of strategies against herpesvirus latent infection. Our previous studies found KHV ORF6 mRNA is detectable during latent infection. In this study, ORF6 protein expression was investigated by a polyclonal antibody specific to ORF6 peptide. Positive staining by an immunofluorescence assay was observed in both KHV infected CCB (common carp brain) cells and IgM⁺ white blood cells (WBCs) from recovered KHV⁺ koi. Proteins at the expected size, 68kDa, and several different sizes can be detected during productive infection. Five potential sumoylation sites were identified in the ORF6 protein. Our study demonstrated that ORF6 protein is expressed in both productive infection and latent infection and may have different post-translational modifications during productive infection.

Intense Resistance Exercise Promotes the Acute and Transient Nuclear Translocation of Small Ubiquitin-Related Modifier (SUMO)-1 in Human Myofibres

Protein sumoylation is a posttranslational modification triggered by cellular stress. Because general information concerning the role of small ubiquitin-related modifier (SUMO) proteins in adult skeletal muscle is sparse, we investigated whether SUMO-1 proteins will be subjected to time-dependent changes in their subcellular localization in sarcoplasmic and nuclear compartments of human type I and II skeletal muscle fibers in response to acute stimulation by resistance exercise (RE). Skeletal muscle biopsies were taken at baseline (PRE), 15, 30, 60, 240 min and 24 h post RE from 6 male subjects subjected to a single bout of one-legged knee extensions. SUMO-1 localization was determined via immunohistochemistry and confocal laser microscopy. At baseline SUMO-1 was localized in perinuclear regions of myonuclei. Within 15 and up to 60 min post exercise, nuclear SUMO-1 localization was significantly increased (p < 0.01), declining towards baseline levels within 240 min post exercise. Sarcoplasmic SUMO-1 localization was increased at 15 min post exercise in type I and up to 30 min post RE in type II myofibres. The changing localization of SUMO-1 proteins acutely after intense muscle contractions points to a role for SUMO proteins in the acute regulation of the skeletal muscle proteome after exercise.

SUMO-regulated mitochondrial function in Parkinson's disease

Parkinson's disease (PD) is the second most common neurodegenerative disorder characterized by cardinal motor signs such as rigidity, bradykinesia or rest tremor that arise from a significant death of dopaminergic neurons. Non-dopaminergic degeneration also occurs and it seems to induce the deficits in olfactory, emotional, and memory functions that precede the classical motor symptoms in PD. Despite the majority of PD cases being sporadic, several genes have previously been associated with the hereditary forms of the disease. The proteins encoded by some of these genes, including α-synuclein, DJ-1, and parkin, are modified by small ubiquitin-like modifier (SUMO), a post-translational modification that regulates a variety of cellular processes. Among the several pathogenic mechanisms proposed for PD is mitochondrial dysfunction. Recent studies suggest that SUMOylation can interfere with mitochondrial dynamics, which is essential for neuronal function, and may play a pivotal role in PD pathogenesis. Here, we present an overview of recent studies on mitochondrial disturbance in PD and the potential SUMO-modified proteins and pathways involved in this process. SUMOylation, a post-translational modification, interferes with mitochondrial dynamics, and may play a pivotal role in Parkinson's disease (PD). SUMOylation maintains α-synuclein (α-syn) in a soluble form and activates DJ-1, decreasing mitochondrial oxidative stress. SUMOylation may reduce the amount of parkin available for mitochondrial recruitment and decreases mitochondrial biogenesis through suppression of peroxisomal proliferator-activated receptor-γ co-activator 1 α (PGC-1α). Mitochondrial fission can be regulated by dynamin-related protein 1 SUMO-1- or SUMO-2/3-ylation. A fine balance for the SUMOylation/deSUMOylation of these proteins is required to ensure adequate mitochondrial function in PD.

Post-translational Modifications in Heart Failure: Small Changes, Big Impact

Heart failure is a complex disease process with various aetiologies and is a significant cause of morbidity and death world-wide. Post-translational modifications (PTMs) alter protein structure and provide functional diversity in terms of physiological functions of the heart. In addition, alterations in protein PTMs have been implicated in human disease pathogenesis. Small ubiquitin-like modifier mediated modification (SUMOylation) pathway was found to play essential roles in cardiac development and function. Abnormal SUMOylation has emerged as a new feature of heart failure pathology. In this review, we will highlight the importance of SUMOylation as a regulatory mechanism of SERCA2a function, and its therapeutic potential for the treatment of heart failure.

SUMOylation regulates p27Kip1 stability and localization in response to TGFβ

Exposure of normal and tumor-derived cells to TGFβ results in different outcomes, depending on the regulation of key targets. The CDK inhibitor p27(Kip1) is one of these TGFβ targets and is essential for the TGFβ-induced cell cycle arrest. TGFβ treatment inhibits p27(Kip1) degradation and induces its nuclear translocation, through mechanisms that are still unknown. Recent evidences suggest that SUMOylation, a post-translational modification able to modulate the stability and subcellular localization of target proteins, critically modifies members of the TGFβ signaling pathway. Here, we demonstrate that p27(Kip1) is SUMOylated in response to TGFβ treatment. Using different p27(Kip1) point mutants, we identified lysine 134 (K134) as the residue modified by small ubiquitin-like modifier 1 (SUMO1) in response to TGFβ treatment. TGFβ-induced K134 SUMOylation increased protein stability and nuclear localization of both endogenous and exogenously expressed p27(Kip1). We observed that SUMOylation regulated p27(Kip1) binding to CDK2, thereby governing its nuclear proteasomal degradation through the phosphorylation of threonine 187. Importantly, p27(Kip1) SUMOylation was necessary for proper cell cycle exit following TGFβ treatment. These data indicate that SUMOylation is a novel regulatory mechanism that modulates p27(Kip1) function in response to TGFβ stimulation. Given the involvement of TGFβ signaling in cancer cell proliferation and invasion, our data may shed light on an important aspect of this pathway during tumor progression.

SUMOylation of EHD3 Modulates Tubulation of the Endocytic Recycling Compartment

Endocytosis defines the entry of molecules or macromolecules through the plasma membrane as well as membrane trafficking in the cell. It depends on a large number of proteins that undergo protein-protein and protein-phospholipid interactions. EH Domain containing (EHDs) proteins formulate a family, whose members participate in different stages of endocytosis. Of the four mammalian EHDs (EHD1-EHD4) EHD1 and EHD3 control traffic to the endocytic recycling compartment (ERC) and from the ERC to the plasma membrane, while EHD2 modulates internalization. Recently, we have shown that EHD2 undergoes SUMOylation, which facilitates its exit from the nucleus, where it serves as a co-repressor. In the present study, we tested whether EHD3 undergoes SUMOylation and what is its role in endocytic recycling. We show, both in-vitro and in cell culture, that EHD3 undergoes SUMOylation. Localization of EHD3 to the tubular structures of the ERC depends on its SUMOylation on lysines 315 and 511. Absence of SUMOylation of EHD3 has no effect on its dimerization, an important factor in membrane localization of EHD3, but has a dominant negative effect on its appearance in tubular ERC structures. Non-SUMOylated EHD3 delays transferrin recycling from the ERC to the cell surface. Our findings indicate that SUMOylation of EHD3 is involved in tubulation of the ERC membranes, which is important for efficient recycling.

LMP1-Induced Sumoylation Influences the Maintenance of Epstein-Barr Virus Latency through KAP1

As a herpesvirus, Epstein-Barr virus (EBV) establishes a latent infection that can periodically undergo reactivation, resulting in lytic replication and the production of new infectious virus. Latent membrane protein-1 (LMP1), the principal viral oncoprotein, is a latency-associated protein implicated in regulating viral reactivation and the maintenance of latency. We recently found that LMP1 hijacks the SUMO-conjugating enzyme Ubc9 via its C-terminal activating region-3 (CTAR3) and induces the sumoylation of cellular proteins. Because protein sumoylation can promote transcriptional repression, we hypothesized that LMP1-induced protein sumoylation induces the repression of EBV lytic promoters and helps maintain the viral genome in its latent state. We now show that with inhibition of LMP1-induced protein sumoylation, the latent state becomes less stable or leakier in EBV-transformed lymphoblastoid cell lines. The cells are also more sensitive to viral reactivation induced by irradiation, which results in the increased production and release of infectious virus, as well as increased susceptibility to ganciclovir treatment. We have identified a target of LMP1-mediated sumoylation that contributes to the maintenance of latency in this context: KRAB-associated protein-1 (KAP1). LMP1 CTAR3-mediated sumoylation regulates the function of KAP1. KAP1 also binds to EBV OriLyt and immediate early promoters in a CTAR3-dependent manner, and inhibition of sumoylation processes abrogates the binding of KAP1 to these promoters. These data provide an additional line of evidence that supports our findings that CTAR3 is a distinct functioning regulatory region of LMP1 and confirm that LMP1-induced sumoylation may help stabilize the maintenance of EBV latency.

IMPORTANCE

Epstein-Barr virus (EBV) latent membrane protein-1 (LMP1) plays an important role in the maintenance of viral latency. Previously, we documented that LMP1 targets cellular proteins to be modified by a ubiquitin-like protein (SUMO). We have now identified a function for this LMP1-induced modification of cellular proteins in the maintenance of EBV latency. Because latently infected cells have to undergo viral reactivation in order to be vulnerable to antiviral drugs, these findings identify a new way to increase the rate of EBV reactivation, which increases cell susceptibility to antiviral therapies.

Fever, immunity, and molecular adaptations

The heat shock response (HSR) is an ancient and highly conserved process that is essential for coping with environmental stresses, including extremes of temperature. Fever is a more recently evolved response, during which organisms temporarily subject themselves to thermal stress in the face of infections. We review the phylogenetically conserved mechanisms that regulate fever and discuss the effects that febrile-range temperatures have on multiple biological processes involved in host defense and cell death and survival, including the HSR and its implications for patients with severe sepsis, trauma, and other acute systemic inflammatory states. Heat shock factor-1, a heat-induced transcriptional enhancer is not only the central regulator of the HSR but also regulates expression of pivotal cytokines and early response genes. Febrile-range temperatures exert additional immunomodulatory effects by activating mitogen-activated protein kinase cascades and accelerating apoptosis in some cell types. This results in accelerated pathogen clearance, but increased collateral tissue injury, thus the net effect of exposure to febrile range temperature depends in part on the site and nature of the pathologic process and the specific treatment provided.

SUMO modification of TBK1 at the adaptor-binding C-terminal coiled-coil domain contributes to its antiviral activity

The non-canonical IKK kinase TBK1 serves as an important signal transmitter of the antiviral interferon response, but is also involved in the regulation of further processes such as autophagy. The activity of TBK1 is regulated by posttranslational modifications comprising phosphorylation and ubiquitination. This study identifies SUMOylation as a novel posttranslational TBK1 modification. TBK1 kinase activity is required to allow the attachment of SUMO1 or SUMO2/3 proteins. Since TBK1 does not bind to the E2 enzyme Ubc9, this modification most likely proceeds via trans-SUMOylation. Mass spectrometry allowed identifying K694 as the SUMO acceptor site, a residue located in the C-terminal coiled-coil domain which is exclusively responsible for the association with the adaptor proteins NAP1, Sintbad and TANK. SUMO modification at K694 contributes to the antiviral function of TBK1 and accordingly the viral protein Gam1 antagonizes this posttranslational modification.

Transcellular chaperone signaling: an organismal strategy for integrated cell stress responses

The ability of each cell within a metazoan to adapt to and survive environmental and physiological stress requires cellular stress-response mechanisms, such as the heat shock response (HSR). Recent advances reveal that cellular proteostasis and stress responses in metazoans are regulated by multiple layers of intercellular communication. This ensures that an imbalance of proteostasis that occurs within any single tissue 'at risk' is protected by a compensatory activation of a stress response in adjacent tissues that confers a community protective response. While each cell expresses the machinery for heat shock (HS) gene expression, the HSR is regulated cell non-autonomously in multicellular organisms, by neuronal signaling to the somatic tissues, and by transcellular chaperone signaling between somatic tissues and from somatic tissues to neurons. These cell non-autonomous processes ensure that the organismal HSR is orchestrated across multiple tissues and that transmission of stress signals between tissues can also override the neuronal control to reset cell- and tissue-specific proteostasis. Here, we discuss emerging concepts and insights into the complex cell non-autonomous mechanisms that control stress responses in metazoans and highlight the importance of intercellular communication for proteostasis maintenance in multicellular organisms.

SUMOylation and deimination of proteins: two epigenetic modifications involved in Giardia encystation

SUMOylation, a posttranslational modification of proteins, has been recently described as vital in eukaryotic cells. In a previous work, we analyzed the role of SUMO protein and the genes encoding the putative enzymes of the SUMOylation pathway in the parasite Giardia lamblia. Although we observed several SUMOylated proteins, only the enzyme Arginine Deiminase (ADI) was confirmed as a SUMOylated substrate. ADI is involved in the survival of the parasite and, besides its role in ATP production, it also catalyzes the modification of arginine residues to citrulline in the cytoplasmic tail of surface proteins. During encystation, however, ADI translocates to the nuclei and downregulates the expression of the Cyst Wall Protein 2 (CWP2). In this work, we made site-specific mutation of the ADI SUMOylation site (Lys101) and observed that transgenic trophozoites did not translocate to the nuclei at the first steps of encystation but shuttled in the nuclei late during this process through classic nuclear localization signals. Inside the nuclei, ADI acts as a peptidyl arginine deiminase, being probably involved in the downregulation of CWPs expression and cyst wall formation. Our results strongly indicate that ADI plays a regulatory role during encystation in which posttranslational modifications of proteins are key players.

Post-translational modifications of the progesterone receptors

Progesterone plays a key role in the development, differentiation and maintenance of female reproductive tissues and has multiple non-reproductive neural functions. Depending on the cell and tissue, the hormonal environment, growth conditions and the developmental stage, progesterone can either stimulate cell growth or inhibit it while promoting differentiation. Progesterone receptors (PRs) belong to the steroid hormone receptor superfamily of ligand-dependent transcription factors. PR proteins are subject to extensive post-translational modifications that include phosphorylation, acetylation, ubiquitination and SUMOylation. The interplay among these modifications is complex with alteration of the receptors by one factor influencing the impact of another. Control over these modifications is species-, tissue- and cell-specific. They in turn regulate multiple functions including PR stability, their subcellular localization, protein-protein interactions and transcriptional activity. These complexities may explain how tissue- and gene-specific differences in regulation are achieved in the same organism, by the same receptor protein and hormone. Here we review current knowledge of PR post-translational modifications and discuss how these may influence receptor function focusing on human breast cancer cells. There is much left to be learned. However, our understanding of this may help to identify therapeutic agents that target PR activity in tissue-specific, even gene-specific ways.

The BTB-containing protein Kctd15 is SUMOylated in vivo

Potassium Channel Tetramerization Domain containing 15 (Kctd15) has a role in regulating the neural crest (NC) domain in the embryo. Kctd15 inhibits NC induction by antagonizing Wnt signaling and by interaction with the transcription factor AP-2α activation domain blocking its activity. Here we demonstrate that Kctd15 is SUMOylated by SUMO1 and SUMO2/3. Kctd15 contains a classical SUMO interacting motif, ψKxE, at the C-terminal end, and variants of the motif within the molecule. Kctd15 SUMOylation occurs exclusively in the C-terminal motif. Inability to be SUMOylated did not affect Kctd15's subcellular localization, or its ability to repress AP-2 transcriptional activity and to inhibit NC formation in zebrafish embryos. In contrast, a fusion of Kctd15 and SUMO had little effectiveness in AP-2 inhibition and in blocking of NC formation. These data suggest that the non-SUMOylated form of Kctd15 functions in NC development.

Nuclear PTEN controls DNA repair and sensitivity to genotoxic stress

Loss of function of the phosphatase and tensin homolog deleted on chromosome 10 (PTEN) tumor suppressor gene is associated with many human cancers. In the cytoplasm, PTEN antagonizes the phosphatidylinositol 3-kinase (PI3K) signaling pathway. PTEN also accumulates in the nucleus, where its function remains poorly understood. We demonstrate that SUMOylation (SUMO, small ubiquitin-like modifier) of PTEN controls its nuclear localization. In cells exposed to genotoxic stress, SUMO-PTEN was rapidly excluded from the nucleus dependent on the protein kinase ataxia telangiectasia mutated (ATM). Cells lacking nuclear PTEN were hypersensitive to DNA damage, whereas PTEN-deficient cells were susceptible to killing by a combination of genotoxic stress and a small-molecule PI3K inhibitor both in vitro and in vivo. Our findings may have implications for individualized therapy for patients with PTEN-deficient tumors.

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.

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.

Inhibition of SENP6-induced radiosensitization of human hepatocellular carcinoma cells by blocking radiation-induced NF-κB activation

Hepatocellular carcinoma is the most common type of liver cancer. Radiotherapy combined with chemotherapy is the treatment of choice for hepatocellular carcinoma, but radioresistance of the cancer remains a significant therapeutic hindrance. Here, we provided several lines of evidence that small ubiquitin-like modifier (SUMO)-specific protease 6 (SENP6) could be an attractive molecular target for the treatment of hepatocellular carcinoma. By using immunohistochemical and real-time PCR, we showed that SENP6 was overexpressed in more than half of the hepatocellular carcinoma tissues. The growth retardation and radiosensitization were caused by silencing of SENP6 in the hepatocellular carcinoma cell lines using lentiviral shRNA. Moreover, SENP6 was required for radiation-induced NF-κB activation and the half-life of IκBα, a well-known inhibitor of NF-κB, and was extended by SENP6 silencing. Thus, our data demonstrated that SENP6 is an attractive drug target for anticancer therapy and radiosensitization.

WITHDRAWN: Protein sumoylation and human diseases

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SUMOylation of claudin-2

The C-terminal cytoplasmic tails of claudins are likely sites for interaction with proteins that regulate their function. We performed a yeast two-hybrid screen with the tail of human claudin-2 against a human kidney cDNA library and identified interactions with the PDZ3 domain of ZO-2 as well as ubiquitin-conjugating enzyme E2I (SUMO ligase-1) and E3 SUMO-protein ligase PIAS; the first is a predicted interaction, while the latter two are novel and suggest that claudin-2 is a substrate for SUMOylation. Using an in vitro SUMOylation assay, we identified K218 as a conjugation site on claudin-2; mutation of that lysine to arginine blocked SUMOylation. Stable expression of inducible GFP-SUMO-1 in MDCK cells resulted in decreased levels of claudin-2 protein by immunoblot and decreased claudin-2 membrane expression by immunofluorescence microscopy. We conclude that the cellular levels of claudin-2 may be modulated by SUMOylation, warranting further investigation of cellular pathways that regulate this modification in vivo.

Role of ubiquitin in parainfluenza virus 5 particle formation

Ubiquitin is important for the budding of many retroviruses and other enveloped viruses, but the precise role of ubiquitin in virus budding remains unclear. Here, we characterized the ubiquitination of the matrix (M) protein of a paramyxovirus, parainfluenza virus 5 (PIV5). The PIV5 M protein (but not the PIV5 nucleocapsid protein) was found to be targeted for monoubiquitination in transfected mammalian cells. Major sites of ubiquitin attachment identified by mass spectrometry analysis were lysine residues at amino acid positions 79/80, 130, and 247. The cumulative mutation of lysine residues 79, 80, and 130 to arginines led to an altered pattern of M protein ubiquitination and impaired viruslike particle (VLP) production. However, the cumulative mutation of lysine residues 79, 80, 130, and 247 to arginines restored M protein ubiquitination and VLP production, suggesting that ubiquitin is attached to alternative sites on the M protein when the primary ones have been removed. Additional lysine residues were targeted for mutagenesis based on the UbiPred algorithm. An M protein with seven lysine residues changed to arginines exhibited altered ubiquitination and poor VLP production. A recombinant virus encoding an M protein with seven lysines mutated was generated, and this virus exhibited a 6-fold-reduced maximum titer, with the defect being attributed mainly to the budding of noninfectious particles. The recombinant virus was assembly deficient, as judged by the redistribution of viral M and hemagglutinin-neuraminidase proteins in infected cells. Similar assembly defects were observed for the wild-type (wt) virus after treatment with a proteasome inhibitor. Collectively, these findings suggest that the monoubiquitination of the PIV5 M protein is important for proper virus assembly and for the budding of infectious particles.

Global SUMO Proteome Responses Guide Gene Regulation, mRNA Biogenesis, and Plant Stress Responses

Small Ubiquitin-like MOdifier (SUMO) is a key regulator of abiotic stress, disease resistance, and development in plants. The identification of >350 plant SUMO targets has revealed many processes modulated by SUMO and potential consequences of SUMO on its targets. Importantly, highly related proteins are SUMO-modified in plants, yeast, and metazoans. Overlapping SUMO targets include heat-shock proteins (HSPs), transcription regulators, histones, histone-modifying enzymes, proteins involved in DNA damage repair, but also proteins involved in mRNA biogenesis and nucleo-cytoplasmic transport. Proteomics studies indicate key roles for SUMO in gene repression by controlling histone (de)acetylation activity at genomic loci. The responsible heavily sumoylated transcriptional repressor complexes are recruited by plant transcription factors (TFs) containing an (ERF)-associated Amphiphilic Repression (EAR) motif. These TFs are not necessarily themselves a SUMO target. Conversely, SUMO acetylation (Ac) prevents binding of downstream partners by blocking binding of their SUMO-interaction peptide motifs to Ac-SUMO. In addition, SUMO acetylation has emerged as a mechanism to recruit specifically bromodomains. Bromodomains are generally linked with gene activation. These findings strengthen the idea of a bi-directional sumo-acetylation switch in gene regulation. Quantitative proteomics has highlighted that global sumoylation provides a dynamic response to protein damage involving SUMO chain-mediated protein degradation, but also SUMO E3 ligase-dependent transcription of HSP genes. With these insights in SUMO function and novel technical advancements, we can now study SUMO dynamics in responses to (a)biotic stress in plants.

Global SUMO Proteome Responses Guide Gene Regulation, mRNA Biogenesis, and Plant Stress Responses

Small Ubiquitin-like MOdifier (SUMO) is a key regulator of abiotic stress, disease resistance, and development in plants. The identification of >350 plant SUMO targets has revealed many processes modulated by SUMO and potential consequences of SUMO on its targets. Importantly, highly related proteins are SUMO-modified in plants, yeast, and metazoans. Overlapping SUMO targets include heat-shock proteins (HSPs), transcription regulators, histones, histone-modifying enzymes, proteins involved in DNA damage repair, but also proteins involved in mRNA biogenesis and nucleo-cytoplasmic transport. Proteomics studies indicate key roles for SUMO in gene repression by controlling histone (de)acetylation activity at genomic loci. The responsible heavily sumoylated transcriptional repressor complexes are recruited by plant transcription factors (TFs) containing an (ERF)-associated Amphiphilic Repression (EAR) motif. These TFs are not necessarily themselves a SUMO target. Conversely, SUMO acetylation (Ac) prevents binding of downstream partners by blocking binding of their SUMO-interaction peptide motifs to Ac-SUMO. In addition, SUMO acetylation has emerged as a mechanism to recruit specifically bromodomains. Bromodomains are generally linked with gene activation. These findings strengthen the idea of a bi-directional sumo-acetylation switch in gene regulation. Quantitative proteomics has highlighted that global sumoylation provides a dynamic response to protein damage involving SUMO chain-mediated protein degradation, but also SUMO E3 ligase-dependent transcription of HSP genes. With these insights in SUMO function and novel technical advancements, we can now study SUMO dynamics in responses to (a)biotic stress in plants.

Glycine fluoromethylketones as SENP-specific activity based probes

We report here the synthesis and biochemical properties of a new peptidyl activity-based probe 1 for SUMO proteases, SENPs. The activity-based probe has at its C terminus a glycine-derived fluoromethylketone moiety as a reactive group designed to target the active-site cysteine of SENPs. Based on a study of the interactions between SENPs and SUMOs, we introduced further design elements that allow the activity-based probe to selectively target SENPs at low micromolar to high nanomolar concentrations. Moreover, 1 out-competes SUMO1 from the reversible SUMO1-SENP1 complex, thus suggesting that 1 and SUMO1 share a common binding site on SENP1.

Hormesis, cellular stress response and vitagenes as critical determinants in aging and longevity

Understanding mechanisms of aging and determinants of life span will help to reduce age-related morbidity and facilitate healthy aging. Average lifespan has increased over the last centuries, as a consequence of medical and environmental factors, but maximal life span remains unchanged. Extension of maximal life span is currently possible in animal models with measures such as genetic manipulations and caloric restriction (CR). CR appears to prolong life by reducing reactive oxygen species (ROS)-mediated oxidative damage. But ROS formation, which is positively implicated in cellular stress response mechanisms, is a highly regulated process controlled by a complex network of intracellular signaling pathways. By sensing the intracellular nutrient and energy status, the functional state of mitochondria, and the concentration of ROS produced in mitochondria, the longevity network regulates life span across species by co-ordinating information flow along its convergent, divergent and multiply branched signaling pathways, including vitagenes which are genes involved in preserving cellular homeostasis during stressful conditions. Vitagenes encode for heat shock proteins (Hsp) Hsp32, Hsp70, the thioredoxin and the sirtuin protein systems. Dietary antioxidants, such as carnosine, carnitines or polyphenols, have recently been demonstrated to be neuroprotective through the activation of hormetic pathways, including vitagenes. The hormetic dose-response, challenges long-standing beliefs about the nature of the dose-response in a lowdose zone, having the potential to affect significantly the design of pre-clinical studies and clinical trials as well as strategies for optimal patient dosing in the treatment of numerous diseases. Given the broad cytoprotective properties of the heat shock response there is now strong interest in discovering and developing pharmacological agents capable of inducing stress responses. In this review we discuss the most current and up to date understanding of the possible signaling mechanisms by which caloric restriction, as well hormetic caloric restriction-mimetics compounds by activating vitagenes can enhance defensive systems involved in bioenergetic and stress resistance homeostasis with consequent impact on longevity processes.

Regulation of HSF1 function in the heat stress response: implications in aging and disease

To dampen proteotoxic stresses and maintain protein homeostasis, organisms possess a stress-responsive molecular machinery that detects and neutralizes protein damage. A prominent feature of stressed cells is the increased synthesis of heat shock proteins (Hsps) that aid in the refolding of misfolded peptides and restrain protein aggregation. Transcriptional activation of the heat shock response is orchestrated by heat shock factor 1 (HSF1), which rapidly translocates to hsp genes and induces their expression. Although the role of HSF1 in protecting cells and organisms against severe stress insults is well established, many aspects of how HSF1 senses qualitatively and quantitatively different forms of stresses have remained poorly understood. Moreover, recent discoveries that HSF1 controls life span have prompted new ways of thinking about an old transcription factor. Here, we review the established role of HSF1 in counteracting cell stress and prospect the role of HSF1 as a regulator of disease states and aging.

Interaction between geminivirus replication protein and the SUMO-conjugating enzyme is required for viral infection

Geminiviruses are small DNA viruses that replicate in nuclei of infected plant cells by using plant DNA polymerases. These viruses encode a protein designated AL1, Rep, or AC1 that is essential for viral replication. AL1 is an oligomeric protein that binds to double-stranded DNA, catalyzes the cleavage and ligation of single-stranded DNA, and induces the accumulation of host replication machinery. It also interacts with several host proteins, including the cell cycle regulator retinoblastoma-related protein (RBR), the DNA replication protein PCNA (proliferating cellular nuclear antigen), and the sumoylation enzyme that conjugates SUMO to target proteins (SUMO-conjugating enzyme [SCE1]). The SCE1-binding motif was mapped by deletion to a region encompassing AL1 amino acids 85 to 114. Alanine mutagenesis of lysine residues in the binding region either reduced or eliminated the interaction with SCE1, but no defects were observed for other AL1 functions, such as oligomerization, DNA binding, DNA cleavage, and interaction with AL3 or RBR. The lysine mutations reduced or abolished virus infectivity in plants and viral DNA accumulation in transient-replication assays, suggesting that the AL1-SCE1 interaction is required for viral DNA replication. Ectopic AL1 expression did not result in broad changes in the sumoylation pattern of plant cells, but specific changes were detected, indicating that AL1 modifies the sumoylation state of selected host proteins. These results established the importance of AL1-SCE1 interactions during geminivirus infection of plants and suggested that AL1 alters the sumoylation of selected host factors to create an environment suitable for viral infection.

Estrogen-responsive genes in macrophages of the bony fish gilthead seabream: a transcriptomic approach

The role of sex steroids in the modulation of fish immune responses has received little attention. Previous studies have demonstrated that 17β-estradiol (E(2)) is able to alter the response of gilthead seabream leukocytes to infectious agents. We have used suppression subtractive hybridization to identify genes upregulated by E(2) (50 ng/ml) in macrophage cultures from gilthead seabream. We isolated 393 up-regulated cDNA fragments that led to the identification of 162 candidate estrogen-responsive genes. Functional analyses revealed the presence of several enriched immune processes and molecular pathways. The E(2) up-regulation of some immune-relevant genes was further confirmed by real time RT-PCR. Bioinformatics analysis revealed the ability of E(2) to orchestrate profound alterations in the macrophage expression profile, especially immune-related processes and pathways. This is the first report on E(2)-dependent modifications of fish macrophage transcriptome and lends weight to a suggested role for estrogen in the immune system, the possible significance of which is discussed.

Distinct roles for Arabidopsis SUMO protease ESD4 and its closest homolog ELS1

SUMO conjugation affects a broad range of processes in Arabidopsis thaliana, including flower initiation, pathogen defense, and responses to cold, drought and salt stress. We investigated two sequence-related SUMO-specific proteases that are both widely expressed and show that they differ significantly in their properties. The closest homolog of SUMO protease ESD4, ESD4-LIKE SUMO PROTEASE 1 (ELS1, alternatively called AtULP1a) has SUMO-specific proteolytic activity, but is functionally distinct from ESD4, as shown by intracellular localization, mutant phenotype and heterologous expression in yeast mutants. Furthermore, we show that the growth defects caused by loss of ESD4 function are not due to increased synthesis of the stress signal salicylic acid, as was previously shown for a SUMO ligase, indicating that impairment of the SUMO system affects plant growth in different ways. Our results demonstrate that two A. thaliana SUMO proteases showing close sequence similarity have distinct in vivo functions.

SUMO and its role in human diseases

The covalent attachment of small ubiquition-like modifier (SUMO) polypeptides, or sumoylation, is an important regulator of the functional properties of many proteins. Among these are many proteins implicated in human diseases including cancer and Huntington's, Alzheimer's, and Parkinson's diseases, as well as spinocerebellar ataxia 1 and amyotrophic lateral sclerosis. The results of two more recent studies identify two additional human disease-associated proteins that are sumoylated, amyloid precursor protein (APP), and lamin A. APP sumoylation modulates Aβ peptide levels, suggesting a potential role in Alzheimer's disease, and decreased lamin A sumoylation due to mutations near its SUMO site has been implicated in causing some forms of familial dilated cardiomyopathy.

Sumoylation regulates nuclear localization of repressor DREAM

DREAM is a Ca(2+)-binding protein with specific functions in different cell compartments. In the nucleus, DREAM acts as a transcriptional repressor, although the mechanism that controls its nuclear localization is unknown. Yeast two-hybrid assay revealed the interaction between DREAM and the SUMO-conjugating enzyme Ubc9 and bioinformatic analysis identified four sumoylation-susceptible sites in the DREAM sequence. Single K-to-R mutations at positions K26 and K90 prevented in vitro sumoylation of recombinant DREAM. DREAM sumoylation mutants retained the ability to bind to the DRE sequence but showed reduced nuclear localization and failed to regulate DRE-dependent transcription. In PC12 cells, sumoylated DREAM is present exclusively in the nucleus and neuronal differentiation induced nuclear accumulation of sumoylated DREAM. In fully differentiated trigeminal neurons, DREAM and SUMO-1 colocalized in nuclear domains associated with transcription. Our results show that sumoylation regulates the nuclear localization of DREAM in differentiated neurons. This article is part of a Special Issue entitled: 11th European Symposium on Calcium.

Protein kinase A binds and activates heat shock factor 1

BACKGROUND

Many inducible transcription factors are regulated through batteries of posttranslational modifications that couple their activity to inducing stimuli. We have studied such regulation of Heat Shock Factor 1 (HSF1), a key protein in control of the heat shock response, and a participant in carcinogenisis, neurological health and aging. As the mechanisms involved in the intracellular regulation of HSF1 in good health and its dysregulation in disease are still incomplete we are investigating the role of posttranslational modifications in such regulation.

METHODOLOGY/PRINCIPAL FINDINGS

In a proteomic study of HSF1 binding partners, we have discovered its association with the pleiotropic protein kinase A (PKA). HSF1 binds avidly to the catalytic subunit of PKA, (PKAcα) and becomes phosphorylated on a novel serine phosphorylation site within its central regulatory domain (serine 320 or S320), both in vitro and in vivo. Intracellular PKAcα levels and phosphorylation of HSF1 at S320 were both required for HSF1 to be localized to the nucleus, bind to response elements in the promoter of an HSF1 target gene (hsp70.1) and activate hsp70.1 after stress. Reduction in PKAcα levels by small hairpin RNA led to HSF1 exclusion from the nucleus, its exodus from the hsp70.1 promoter and decreased hsp70.1 transcription. Likewise, null mutation of HSF1 at S320 by alanine substitution for serine led to an HSF1 species excluded from the nucleus and deficient in hsp70.1 activation.

CONCLUSIONS

These findings of PKA regulation of HSF1 through S320 phosphorylation add to our knowledge of the signaling networks converging on this factor and may contribute to elucidating its complex roles in the stress response and understanding HSF1 dysregulation in disease.

Regulation of the members of the mammalian heat shock factor family

Regulation of gene expression is fundamental in all living organisms and is facilitated by transcription factors, the single largest group of proteins in humans. For cell- and stimulus-specific gene regulation, strict control of the transcription factors themselves is crucial. Heat shock factors are a family of transcription factors best known as master regulators of induced gene expression during the heat shock response. This evolutionary conserved cellular stress response is characterized by massive production of heat shock proteins, which function as cytoprotective molecular chaperones against various proteotoxic stresses. In addition to promoting cell survival under stressful conditions, heat shock factors are involved in the regulation of life span and progression of cancer and they are also important for developmental processes such as gametogenesis, neurogenesis and maintenance of sensory organs. Here, we review the regulatory mechanisms steering the activities of the mammalian heat shock factors 1–4.

Sumoylation regulates interaction of FOG1 with C-terminal-binding protein (CTBP)

Erythropoietic and megakaryocytic programs are specified from multipotential progenitors by the transcription factor GATA1. FOG1, a GATA1-interaction partner, is critical for GATA1 function in several contexts by bringing multiple complexes into association with GATA1 to facilitate activation or repression of target genes. To further elucidate regulation of these associations by cellular and extracellular cues, we examined FOG1 for post-translational modifications. We found that FOG1 is SUMOylated and phosphorylated in erythroid cells in a differentiation-dependent manner. Removal of the SUMOylation sites in FOG1 does not impair nuclear localization, protein stability, or chromatin occupancy. However, SUMOylation of FOG1 modulates interactions with C-terminal binding protein family members, specifically promoting CTBP1 binding. Phosphorylation of FOG1 modulates SUMOylation and, therefore, indirectly regulates the CTBP interaction. Post-translational modification of FOG1 may contribute to control of co-occupancy by CTBP family members, the NuRD complex, and GATA1 at differentially regulated genes.

SUMO-modified nuclear cyclin D1 bypasses Ras-induced senescence

Oncogene-induced senescence represents a key tumor suppressive mechanism. Here, we show that Ras oncogene-induced senescence can be mediated by the recently identified haploinsufficient tumor suppressor apoptosis-stimulating protein of p53 (ASPP) 2 through a novel and p53/p19(Arf)/p21(waf1/cip1)-independent pathway. ASPP2 suppresses Ras-induced small ubiquitin-like modifier (SUMO)-modified nuclear cyclin D1 and inhibits retinoblastoma protein (Rb) phosphorylation. The lysine residue, K33, of cyclin D1 is a key site for this newly identified regulation. In agreement with the fact that its nuclear localization is required for its oncogenic activity, we show that nuclear cyclin D1 is far more potent than wild-type (WT) cyclin D1 in bypassing Ras-induced senescence. Thus, this study identifies SUMO modification as a positive regulator of nuclear cyclin D1, and reveals a new way by which cell cycle entry and senescence are regulated.

Modification of human papillomavirus minor capsid protein L2 by sumoylation

The human papillomavirus (HPV) minor capsid protein L2 plays important roles in the generation of infectious viral particles and in the initial steps of infection. Here we show that HPV-16 L2 protein is sumoylated at lysine 35 and that sumoylation affects its stability. Interestingly, the sumoylated form of L2 cannot bind to the major capsid protein L1, suggesting a mechanism by which capsid assembly may be modulated in an infected cell. Additionally, L2 appears to modulate the overall sumoylation status of the host cell. These observations indicate a complex interplay between the HPV L2 protein and the host sumoylation machinery.

Modulation of CP2 family transcriptional activity by CRTR-1 and sumoylation

CRTR-1 is a member of the CP2 family of transcription factors. Unlike other members of the family which are widely expressed, CRTR-1 expression shows specific spatio-temporal regulation. Gene targeting demonstrates that CRTR-1 plays a central role in the maturation and function of the salivary glands and the kidney. CRTR-1 has also recently been identified as a component of the complex transcriptional network that maintains pluripotency in embryonic stem (ES) cells. CRTR-1 was previously shown to be a repressor of transcription. We examine the activity of CRTR-1 in ES and other cells and show that CRTR-1 is generally an activator of transcription and that it modulates the activity of other family members, CP2, NF2d9 and altNF2d9, in a cell specific manner. We also demonstrate that CRTR-1 activity is regulated by sumoylation at a single major site, residue K30. These findings imply that functional redundancy with other family members may mask important roles for CRTR-1 in other tissues, including the blastocyst stage embryo and embryonic stem cells.

Arabidopsis small ubiquitin-like modifier paralogs have distinct functions in development and defense

Posttranslational modifications allow dynamic and reversible changes to protein function. In Arabidopsis thaliana, a small gene family encodes paralogs of the small ubiquitin-like posttranslational modifier. We studied the function of these paralogs. Single mutants of the SUM1 and SUM2 paralogs do not exhibit a clear phenotype. However, the corresponding double knockdown mutant revealed that SUM1 and SUM2 are essential for plant development, floral transition, and suppression of salicylic acid (SA)-dependent defense responses. The SUM1 and SUM2 genes are constitutively expressed, but their spatial expression patterns do not overlap. Tight transcriptional regulation of these two SUM genes appears to be important, as overexpression of either wild-type or conjugation-deficient mutants resulted in activation of SA-dependent defense responses, as did the sum1 sum2 knockdown mutant. Interestingly, expression of the paralog SUM3 is strongly and widely induced by SA and by the defense elicitor Flg22, whereas its expression is otherwise low and restricted to a few specific cell types. Loss of SUM3 does not result in an aberrant developmental phenotype except for late flowering, while SUM3 overexpression causes early flowering and activates plant defense. Apparently, SUM3 promotes plant defense downstream of SA, while SUM1 and SUM2 together prevent SA accumulation in noninfected plants.

SENP1 participates in the dynamic regulation of Elk-1 SUMOylation

The modification of proteins with SUMO (small ubiquitin-related modifier) plays an important role in determining their functional properties. Importantly though, SUMOylation is a highly dynamic process enabling transient responses to be elicited. This dynamism is controlled by two competing conjugating and deconjugating activities. The latter activity is mediated by the SENP [SUMO1/sentrin/SMT3 (suppressor of mif two 3 homologue 1)-specific peptidase] family of SUMO-specific proteases. The transcription factor Elk-1 [ETS (E twenty-six)-like 1] undergoes rapid de-SUMOylation following cellular stimulation with growth factors, and this contributes to its conversion from a SUMO-dependent repressor into a potent transcriptional activator. In the present study we demonstrate an important role for SENP1 in the de-SUMOylation of Elk-1, and therefore an integral role in determining the Elk-1-dependent transcriptional programme. Among the SENPs, Elk-1 preferentially forms a complex with SENP1. This preferential binding is reflected by the higher efficiency of SENP1 in promoting Elk-1 transactivation. Moreover, depletion of SENP1 causes a reciprocal effect and reduces the transactivation properties of Elk-1. Partial redundancy of function with SENP2 is revealed by combinatorial knockdown studies. Importantly, depletion of SENP1 also reduces the activation of the Elk-1 target gene c-FOS. Taken together, these results therefore reveal an important role for SENP1 in the regulation of Elk-1-mediated gene expression in response to mitogenic signalling cues.

Phosphorylation-dependent interaction of SATB1 and PIAS1 directs SUMO-regulated caspase cleavage of SATB1

Special AT-rich sequence-binding protein 1 (SATB1) is a tissue-restricted genome organizer that provides a key link between DNA loop organization, chromatin modification/remodeling, and transcription factor association at matrix attachment regions (MARs). The SUMO E3 ligase PIAS1 enhances SUMO conjugation to SATB1 lysine-744, and this modification regulates caspase-6 mediated cleavage of SATB1 at promyelocytic leukemia nuclear bodies (PML NBs). Since this regulated caspase cleavage occurs on only a subset of SATB1, and the products are relatively stable, proteolysis likely mediates cellular processes other than programmed cell death. However, the mechanism for the spatial and temporal regulation of SATB1 sumoylation and caspase cleavage is not known. Here we report that these processes are controlled by SATB1 phosphorylation; specifically, PIAS1 interaction with SATB1 is inhibited by phosphorylation. Mutagenesis studies identified interaction of the PIAS SAP (scaffold attachment factor-A/B/acinus/PIAS) motif with SATB1 N-terminal sequences. Notably, phosphorylation of SATB1 at threonine-188 regulates its interaction with PIAS1. Sequences near this phosphorylation site, LXXLL (residues 193 to 197), appear to be conserved among a subset of SUMO substrate proteins. Thus, this motif may be commonly involved in interaction with the PIAS SAP, and phosphorylation may similarly inhibit some of these substrates by preventing their interaction with the ligase.