SUMO-1 controls the protein stability and the biological function of phosducin

TAK-981 enhances antitumor activity in ELT3 uterine leiomyoma cells through the modulation of apoptosis, cell cycle arrest, and autophagy

Uterine leiomyomas, commonly known as fibroids, are the most prevalent benign tumors in women of reproductive age and are characterized by abnormal smooth muscle cell proliferation in the uterine wall. TAK-981 (subasumstat), an investigational drug that inhibits SUMOylation by targeting SUMO-activating enzymes, has demonstrated high potential for the treatment of various cancers. However, its effects on uterine leiomyomas remain largely unexplored. In this study, we evaluated the therapeutic effects of TAK-981 on ELT3 uterine leiomyoma cells. TAK-981 significantly decreased the viability of ELT3 uterine leiomyoma cells and inhibited colony formation. It also induced apoptosis and caused G2/M phase cell cycle arrest, demonstrating a strong effect on cell proliferation and survival. Notably, although TAK-981 enhances reactive oxygen species production, it also induces apoptosis through a reactive oxygen species-independent mechanism, as evidenced by increased apoptosis rates upon co-treatment with antioxidants such as N-acetylcysteine. Furthermore, western blot analysis revealed that treatment with TAK-981 downregulated MEK-1 expression and inhibited ERK phosphorylation, leading to enhanced cleavage of caspase-3 and PARP, thereby promoting apoptosis in ELT3 cells. Additionally, TAK-981 reduced extracellular matrix accumulation by suppressing Collagen I and Acta2 expression and promoted autophagy in ELT3 cells, as indicated by increased levels of LC3. These findings suggest that TAK-981 can be used as a therapeutic option for managing uterine leiomyomas through multiple mechanisms, including apoptosis induction and autophagy promotion.

Tanshinone IIA destabilizes SLC7A11 by regulating PIAS4-mediated SUMOylation of SLC7A11 through KDM1A, and promotes ferroptosis in breast cancer

INTRODUCTION

Breast cancer (BC) is the most common malignancy in women with unfavorite prognosis.

OBJECTIVES

Tanshinone IIA (Tan IIA) inhibits BC progression, however, the underlying mechanism remains largely undefined.

METHODS

The cytotoxicity of Tan IIA was assessed by CCK-8 and LDH assays. Ferroptosis was monitored by the level of MDA, Fe2+, lipid ROS and GSH. IHC and western blot were employed to detect the localization and expression of SLC7A11, PIAS4, KDM1A and other key molecules. The SUMOylation of SLC7A11 was detected by Ni-beads pull-down assay and Co-IP. Luciferase and ChIP assays were employed to detect the direct association between KDM1A and PIAS4 promoter. The proliferative and metastatic properties of BC cells were assessed by colony formation, CCK-8 and Transwell assays, respectively. The in vitro findings were verified in xenograft and lung metastasis models.

RESULTS

Tan IIA promoted ferroptosis by suppressing SLC7A11 in BC cells. Silencing of PIAS4 or KDM1A inhibited cell growth and metastasis in BC. Mechanistically, PIAS4 facilitated the SUMOylation of SLC7A11 via direct binding to SLC7A11, and KDM1A acted as a transcriptional activator of PIAS4. Functional studies further revealed that Tan IIA decreased KDM1A expression, thus suppressing PIAS4 expression transcriptionally. The inhibition of PIAS4-dependent SUMOylation of SLC7A11 further induced ferroptosis, thereby inhibiting proliferation and metastasis in BC.

CONCLUSION

Tan IIA promoted ferroptosis and inhibited tumor growth and metastasis via suppressing KDM1A/PIAS4/SLC7A11 axis.

UBC9 stabilizes PFKFB3 to promote aerobic glycolysis and proliferation of glioblastoma cells

Cancer cells prefer to utilizing aerobic glycolysis to generate energy and anabolic metabolic intermediates for cell growth. However, whether the activities of glycolytic enzymes can be regulated by specific posttranslational modifications, such as SUMOylation, in response to oncogenic signallings, thereby promoting the Warburg effect, remain largely unclear. Here, we demonstrate that phosphofructo-2-kinase/fructose-2,6-bisphosphatase 3 (PFKFB3), a key glycolytic enzyme, interacts with SUMO-conjugating enzyme UBC9 and is SUMOylated at K302 in glioblastoma cells. Expression of UBC9, which competitively prevents the binding of ubiquitin E3 ligase APC/C to PFKFB3 and subsequent PFKFB3 polyubiquitination, increases PFKFB3 stability and expression. Importantly, EGFR activation increases the interaction between UBC9 and PFKFB3, leading to increased SUMOylation and expression of PFKFB3. This increase is blocked by inhibition of EGFR-induced AKT activation whereas expression of activate AKT by itself was sufficient to recapitulate EGF-induced effect. Knockout of PFKFB3 expression decreases EGF-enhanced lactate production and GBM cell proliferation and this decrease was fully rescued by reconstituted expression of WT PFKFB3 whereas PFKFB3 K302R mutant expression abrogates EGF- and UBC9-regulated lactate production and GBM cell proliferation. These findings reveal a previously unknown mechanism underlying the regulation of the Warburg effect through the EGFR activation-induced and UBC9-mediated SUMOylation and stabilization of PFKFB3.

Deciphering the role of cytoplasmic domain of Channelrhodopsin in modulating the interactome and SUMOylome of Chlamydomonas reinhardtii

Translocation of channelrhodopsins (ChRs) is mediated by the intraflagellar transport (IFT) machinery. However, the functional role of the network involving photoreceptors, IFT and other proteins in controlling algal ciliary motility is still not fully delineated. In the current study, we have identified two important motifs at the C-terminus of ChR1, VXPX and LKNE. VXPX is a known ciliary targeting sequence in animals, and LKNE is a well-known SUMOylation motif. To the best of our knowledge, this study gives prima facie insight into the role of SUMOylation in Chlamydomonas. We prove that VMPS of ChR1 is important for interaction with GTPase CrARL11. We show that SUMO motifs are present in the C-terminus of putative ChR1s from green algae. Performing experiments with n-Ethylmaleimide (NEM) and Ubiquitin-like protease 1 (ULP-1) we show that SUMOylation may modulate ChR1 protein in Chlamydomonas. Experiments with 2D08, a known sumoylation blocker, increased the concentration of ChR1 protein. Finally, we show the endogenous SUMOylated proteins (SUMOylome) of C. reinhardtii, identified by using immunoprecipitation followed by nano-LC-MS/MS detection. This report establishes a link between evolutionarily conserved SUMOylation, and ciliary machinery for the maintenance and functioning of cilia across the eukaryotes. Our enriched SUMOylome of C. reinhardtii comprehends the proteins related to ciliary development and, photo-signaling, along with orthologue(s) associated to human ciliopathies as SUMO targets.

SUMOylation of PDGF receptor α affects signaling via PLCγ and STAT3, and cell proliferation

BACKGROUND

The platelet-derived growth factor (PDGF) family of ligands exerts their cellular effects by binding to α- and β-tyrosine kinase receptors (PDGFRα and PDGFRβ, respectively). SUMOylation is an important posttranslational modification (PTM) which regulates protein stability, localization, activation and protein interactions. A mass spectrometry screen has demonstrated SUMOylation of PDGFRα. However, the functional role of SUMOylation of PDGFRα has remained unknown.

RESULTS

In the present study, we validated that PDGFRα is SUMOylated on lysine residue 917 as was previously reported using a mass spectrometry approach. Mutation of lysine residue 917 to arginine (K917R) in PDGFRα substantially decreased SUMOylation, indicating that this amino acid residue is a major SUMOylation site. Whereas no difference in the stability of wild-type and mutant receptor was observed, the K917R mutant PDGFRα was less ubiquitinated than wild-type PDGFRα. The internalization and trafficking of the receptor to early and late endosomes were not affected by the mutation, neither was the localization of the PDGFRα to Golgi. However, the K917R mutant PDGFRα showed delayed activation of PLC-γ and enhanced activation of STAT3. Functional assays showed that the mutation of K917 of PDGFRα decreased cell proliferation in response to PDGF-BB stimulation.

CONCLUSIONS

SUMOylation of PDGFRα decreases ubiquitination of the receptor and affects ligand-induced signaling and cell proliferation.

SUMO1 hinders α-Synuclein fibrillation by inducing structural compaction

Small Ubiquitin-like Modifier 1 (SUMO1) is an essential protein for many cellular functions, including regulation, signaling, etc., achieved by a process known as SUMOylation, which involves covalent attachment of SUMO1 to target proteins. SUMO1 also regulates the function of several proteins via non-covalent interactions involving the hydrophobic patch in the target protein identified as SUMO Binding or Interacting Motif (SBM/SIM). Here, we demonstrate a crucial functional potential of SUMO1 mediated by its non-covalent interactions with α-Synuclein, a protein responsible for many neurodegenerative diseases called α-Synucleinopathies. SUMO1 hinders the fibrillation of α-Synuclein, an intrinsically disordered protein (IDP) that undergoes a transition to β-structures during the fibrillation process. Using a plethora of biophysical techniques, we show that SUMO1 transiently binds to the N-terminus region of α-Synuclein non-covalently and causes structural compaction, which hinders the self-association process and thereby delays the fibrillation process. On the one hand, this study demonstrates an essential functional role of SUMO1 protein concerning neurodegeneration; it also illustrates the commonly stated mechanism that IDPs carry out multiple functions by structural adaptation to suit specific target proteins, on the other. Residue-level details about the SUMO1-α-Synuclein interaction obtained here also serve as a reliable approach for investigating the detailed mechanisms of IDP functions.

Small-molecule inhibitors targeting small ubiquitin-like modifier pathway for the treatment of cancers and other diseases

SUMOylation is a key post-translational modification that involves the covalent attachment of small ubiquitin-like modifier (SUMO) to the lysine residues of target proteins. The well-balanced SUMOylation is essential for normal cellular behaviors, while disturbance of SUMOylation is associated with various cancers and other diseases. Herein, we summarize the structures and biological functions of proteins involved in the SUMOylation process, their dysregulation in human diseases, and the discovery of small-molecular inhibitors targeting this pathway. In addition, we highlight the emerging trends in this field.

SUMO conjugation regulates immune signalling

Post-translational modifications (PTMs) are critical drivers and attenuators for proteins that regulate immune signalling cascades in host defence. In this review, we explore functional roles for one such PTM, the small ubiquitin-like modifier (SUMO). Very few of the SUMO conjugation targets identified by proteomic studies have been validated in terms of their roles in host defence. Here, we compare and contrast potential SUMO substrate proteins in immune signalling for flies and mammals, with an emphasis on NFκB pathways. We discuss, using the few mechanistic studies that exist for validated targets, the effect of SUMO conjugation on signalling and also explore current molecular models that explain regulation by SUMO. We also discuss in detail roles of evolutionary conservation of mechanisms, SUMO interaction motifs, crosstalk of SUMO with other PTMs, emerging concepts such as group SUMOylation and finally, the potentially transforming roles for genome-editing technologies in studying the effect of PTMs.

Phosphine-Activated Lysine Analogues for Fast Chemical Control of Protein Subcellular Localization and Protein SUMOylation

The Staudinger reduction and its variants have exceptional compatibility with live cells but can be limited by slow kinetics. Herein we report new small-molecule triggers that turn on proteins through a Staudinger reduction/self-immolation cascade with substantially improved kinetics and yields. We achieved this through site-specific incorporation of a new set of azidobenzyloxycarbonyl lysine derivatives in mammalian cells. This approach allowed us to activate proteins by adding a nontoxic, bioorthogonal phosphine trigger. We applied this methodology to control a post-translational modification (SUMOylation) in live cells, using native modification machinery. This work significantly improves the rate, yield, and tunability of the Staudinger reduction-based activation, paving the way for its application in other proteins and organisms.

Dual Regulation of Host TRAIP Post-translation and Nuclear/Plasma Distribution by Porcine Reproductive and Respiratory Syndrome Virus Non-structural Protein 1α Promotes Viral Proliferation

In this study, we show that porcine reproductive and respiratory syndrome virus (PRRSV) non-structural protein 1α (nsp1α) facilitates PRRSV escape from innate immune by modulating nuclear to cytoplasmic translocation and distribution ratio of TRAIP to promote virus proliferation. Mechanistically, TRAIP interacts with PRRSV nsp1α via its K205 site, while NSP1α decreases the SUMOylation and K48 ubiquitination independent of the TRAIP interaction K205 site. Modulation of the dual modification of TRAIP by PRRSV nsp1α results in over-enrichment of TRAIP in the cytoplasm. Enrichment of nsp1α-induced cytoplasmic TRAIP in turn leads to excessive K48 ubiquitination and degradation of serine/threonine-protein kinase (TBK1), thereby antagonizing TBK1-IRF3-IFN signaling. This study proposes a novel mechanism by which PRRSV utilizes host proteins to regulate innate immunity. Findings from this study provides novel perspective to advance our understanding in the pathogenesis of PRRSV.

Ginkgolic Acid Rescues Lens Epithelial Cells from Injury Caused by Redox Regulated-Aberrant Sumoylation Signaling by Reviving Prdx6 and Sp1 Expression and Activities

Sumoylation is a downstream effector of aging/oxidative stress; excess oxidative stress leads to dysregulation of a specificity protein1 (Sp1) and its target genes, such as Peroxiredoxin 6 (Prdx6), resulting in cellular damage. To cope with oxidative stress, cells rely on a signaling pathway involving redox-sensitive genes. Herein, we examined the therapeutic efficacy of the small molecule Ginkgolic acid (GA), a Sumoylation antagonist, to disrupt aberrant Sumoylation signaling in human and mouse lens epithelial cells (LECs) facing oxidative stress or aberrantly expressing Sumo1 (small ubiquitin-like modifier). We found that GA globally reduced aberrant Sumoylation of proteins. In contrast, Betulinic acid (BA), a Sumoylation agonist, augmented the process. GA increased Sp1 and Prdx6 expression by disrupting the Sumoylation signaling, while BA repressed the expression of both molecules. In vitro DNA binding, transactivation, Sumoylation and expression assays revealed that GA enhanced Sp1 binding to GC-boxes in the Prdx6 promoter and upregulated its transcription. Cell viability and intracellular redox status assays showed that LECs pretreated with GA gained resistance against oxidative stress-driven aberrant Sumoylation signaling. Overall, our study revealed an unprecedented role for GA in LECs and provided new mechanistic insights into the use of GA in rescuing LECs from aging/oxidative stress-evoked dysregulation of Sp1/Prdx6 protective molecules.

SUMOylation and phosphorylation cross-talk in hepatocellular carcinoma

Hepatocellular carcinoma (HCC) is a primary malignancy of the liver and occurs predominantly in patients with underlying chronic liver disease and cirrhosis. The large spectrum of protein post-translational modification (PTM) includes numerous critical signaling events that occur during neoplastic transformation. PTMs occur to nearly all proteins and increase the functional diversity of proteins. We have reviewed the role of two major PTMs, SUMOylation and phosphorylation, in the altered signaling of key players in HCC. SUMOylation is a PTM that involves addition of a small ubiquitin-like modifiers (SUMO) group to proteins. It is known to regulate protein stability, protein-protein interactions, trafficking and transcriptional activity. The major pathways that are regulated by SUMOylation and may influence HCC are regulation of transcription, cell growth pathways associated with B-cell lymphoma 2 (Bcl-2) and methionine adenosyltransferases (MAT), oxidative stress pathways [nuclear erythroid 2-related factor 2 (Nrf2)], tumor suppressor pathways (p53), hypoxia-inducible signaling [hypoxia-inducible factor-1 (HIF-1)], glucose and lipid metabolism, nuclear factor kappa B (NF-κB) and β-Catenin signaling. Phosphorylation is an extensively studied PTM in HCC. The mitogen-activated protein kinase (MAPK), phosphatidyl inositol/AK-strain transforming (PI3K/AKT), and C-SRC pathways have been extensively studied for deregulation of kinases and alteration in signaling of targets through phosphorylation of their substrates. Cross-talk between phosphorylation and SUMOylation is known to influence transcriptional activity of proteins and protein-protein interactions. In HCC, several SUMOylation-dependent phosphorylation events have been studied such as MAPK activation and c-SRC activity that have been reviewed in this work. The drastic effects of site-specific phosphorylation or SUMOylation on enzyme activity of signaling players and its effect on growth and tumorigenesis suggests that these PTMs are novel targets for therapeutic intervention in HCC.

SUMOylation regulates cytochrome P450 2E1 expression and activity in alcoholic liver disease

Alcohol acts through numerous pathways leading to alcoholic liver disease (ALD). Cytochrome P450 (CYP2E1), an ethanol-inducible enzyme, metabolizes ethanol-producing toxic reactive oxygen species (ROS) and is regulated at the posttranslational level. Small ubiquitin-like modifier (SUMO)ylation is a posttranslational modification that involves the addition of SUMOs, which modulate protein stability, activity, and localization. We demonstrated that ubiquitin-conjugation enzyme 9, the SUMO-conjugating enzyme, is induced in the livers of an intragastric ethanol mouse model. Our aim is to examine whether SUMOylation could regulate ethanol-induced CYP2E1 expression in ALD and to elucidate the molecular mechanism(s). CYP2E1 and UBC9 expression in vitro and in vivo was detected by real-time PCR and immunoblotting/immunostaining. SUMOylation was assayed by mass spectrometry and coimmunoprecipitation. Ubc9 expression was induced in ethanol-fed mouse livers, and silencing inhibited ethanol-mediated CYP2E1 microsomal retention and enzymatic activity. CYP2E1 SUMOylation was found to be induced by ethanol in vitro and in vivo. Ubc9 silencing prevents ethanol-induced lipid accumulation and ROS production. UBC9 was highly expressed in human ALD livers. Finally, we found that lysine 410 is a key SUMOylated residue contributing to CYP2E1 protein stability and activity preventing CYP2E1 SUMOylation. Ethanol-mediated up-regulation of CYP2E1 via SUMOylation enhancing its protein stability and activity and may have important implications in ALD.-Tomasi, M. L., Ramani, K., Ryoo, M., Cossu, C., Floris, A., Murray, B. J., Iglesias-Ara, A., Spissu, Y., Mavila, N. SUMOylation regulates cytochrome P450 2E1 expression and activity in alcoholic liver disease.

The Latency-Associated Nuclear Antigen of Kaposi's Sarcoma-Associated Herpesvirus Inhibits Expression of SUMO/Sentrin-Specific Peptidase 6 To Facilitate Establishment of Latency

Kaposi's sarcoma-associated herpesvirus (KSHV), which belongs to the Gammaherpesviridae, typically displays two different phases in its life cycle, the latent phase and the lytic phase. Latency-associated nuclear antigen (LANA), the primary viral product during latency, has been reported to bind to a series of cellular gene promoters to modulate gene transcription. To systemically elucidate the cellular genes regulated by LANA, we identified genome-wide LANA binding sites by chromatin immunoprecipitation coupled with sequencing (ChIP-seq). We stratified ChIP-seq data and found that LANA might be involved in the macromolecule catabolic process. Specifically, we found and verified that LANA could directly bind to the promoter of the SUMO/sentrin-specific peptidase 6 (SENP6) gene in vivo and in vitro LANA could repress SENP6 promoter activity in a dose-dependent manner in a reporter gene assay. LANA expression was sufficient to inhibit endogenous SENP6 expression at both the RNA and protein levels. Moreover, SENP6 overexpression in KSHV-infected cells reduced LANA at the protein level. Mechanistically, we found that SENP6 could interact with LANA and reduce the formation of sumoylated LANA, which relies on the desumoylation ability of SENP6. During de novo infection, SENP6 overexpression would decrease the abundance of LANA and enhance viral gene expression, which would hamper the establishment of latency. Taken together, these data suggest that KSHV-encoded LANA could inhibit SENP6 expression to regulate the abundance of itself, which may play an important role in controlling the establishment of latency.IMPORTANCE LANA, as a key latent protein produced by KSHV, is responsible for episome persistence and regulates viral reactivation. In the present study, our results demonstrated that LANA could bind to the promoter region of the SENP6 gene and inhibit SENP6 expression while the regulated SENP6 could in turn modulate the abundance of LANA through desumoylation. This delicate regulation may provide important insights to explain the abundance of LANA during KSHV latency.

Pias3 is necessary for dorso-ventral patterning and visual response of retinal cones but is not required for rod photoreceptor differentiation

Protein inhibitor of activated Stat 3 (Pias3) is implicated in guiding specification of rod and cone photoreceptors through post-translational modification of key retinal transcription factors. To investigate its role during retinal development, we deleted exon 2-5 of the mouse Pias3 gene, which resulted in complete loss of the Pias3 protein. Pias3^−/− mice did not show any overt phenotype, and retinal lamination appeared normal even at 18 months. We detected reduced photopic b-wave amplitude by electroretinography following green light stimulation of postnatal day (P)21 Pias3^−/− retina, suggesting a compromised visual response of medium wavelength (M) cones. No change was evident in response of short wavelength (S) cones or rod photoreceptors until 7 months. Increased S-opsin expression in the M-cone dominant dorsal retina suggested altered distribution of cone photoreceptors. Transcriptome profiling of P21 and 18-month-old Pias3^−/− retina revealed aberrant expression of a subset of photoreceptor genes. Our studies demonstrate functional redundancy in SUMOylation-associated transcriptional control mechanisms and identify a specific, though limited, role of Pias3 in modulating spatial patterning and optimal function of cone photoreceptor subtypes in the mouse retina.

Summary: Loss of Pias3 in mice results in altered dorso-ventral patterning of retinal cone photoreceptors by modulating the expression of a subset of genes, but does not affect rod development.

Introduction to Sumoylation

Reversible post-translational modification is a rapid and efficient system to control the activity of pre-existing proteins. Modifiers range from small chemical moieties, such as phosphate groups, to proteins themselves as the modifier. The patriarch of the protein modifiers is ubiquitin which plays a central role in protein degradation and protein targeting. Over the last 20 years, the ubiquitin family has expanded to include a variety of ubiquitin-related small modifier proteins that are all covalently attached to a lysine residue on target proteins via series of enzymatic reactions. Of these more recently discovered ubiquitin-like proteins, the SUMO family has gained prominence as a major regulatory component that impacts numerous aspects of cell growth, differentiation, and response to stress. Unlike ubiquitinylation which often leads to proteins turn over, sumoylation performs a variety of function such as altering protein stability, modulating protein trafficking, directing protein-protein interactions, and regulating protein activity. This chapter will introduce the basic properties of SUMO proteins and the general tenets of sumoylation.

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

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

IMPORTANCE

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

Methionine adenosyltransferase α2 sumoylation positively regulate Bcl-2 expression in human colon and liver cancer cells

Ubiquitin-conjugating enzyme 9 (Ubc9) is required for sumoylation and inhibits apoptosis via Bcl-2 by unknown mechanism. Methionine adenosyltransferase 2A (MAT2A) encodes for MATα2, the catalytic subunit of the MATII isoenzyme that synthesizes S-adenosylmethionine (SAMe). Ubc9, Bcl-2 and MAT2A expression are up-regulated in several malignancies. Exogenous SAMe decreases Ubc9 and MAT2A expression and is pro-apoptotic in liver and colon cancer cells. Here we investigated whether there is interplay between Ubc9, MAT2A and Bcl-2. We used human colon and liver cancer cell lines RKO and HepG2, respectively, and confirmed key finding in colon cancer specimens. We found MATα2 can regulate Bcl-2 expression at multiple levels. MATα2 binds to Bcl-2 promoter to activate its transcription. This effect is independent of SAMe as MATα2 catalytic mutant was also effective. MATα2 also directly interacts with Bcl-2 to enhance its protein stability. MATα2's effect on Bcl-2 requires Ubc9 as MATα2's stability is influenced by sumoylation at K340, K372 and K394. Overexpressing wild type (but not less stable MATα2 sumoylation mutants) protected from 5-fluorouracil-induced apoptosis in both colon and liver cancer cells. Colon cancer have higher levels of sumoylated MATα2, total MATα2, Ubc9 and Bcl-2 and higher MATα2 binding to the Bcl-2 P2 promoter. Taken together, Ubc9's protective effect on apoptosis may be mediated at least in part by sumoylating and stabilizing MATα2 protein, which in turn positively maintains Bcl-2 expression. These interactions feed forward to further enhance growth and survival of the cancer cell.

SUMOylation Confers Posttranslational Stability on NPM-ALK Oncogenic Protein

Nucleophosmin-anaplastic lymphoma kinase–expressing (NPM-ALK⁺) T-cell lymphoma is an aggressive form of cancer that commonly affects children and adolescents. The expression of NPM-ALK chimeric oncogene results from the chromosomal translocation t(2;5)(p23;q35) that causes the fusion of the ALK and NPM genes. This translocation generates the NPM-ALK protein tyrosine kinase that forms the constitutively activated NPM-ALK/NPM-ALK homodimers. In addition, NPM-ALK is structurally associated with wild-type NPM to form NPM/NPM-ALK heterodimers, which can translocate to the nucleus. The mechanisms that sustain the stability of NPM-ALK are not fully understood. SUMOylation is a posttranslational modification that is characterized by the reversible conjugation of small ubiquitin-like modifiers (SUMOs) with target proteins. SUMO competes with ubiquitin for substrate binding and therefore, SUMOylation is believed to protect target proteins from proteasomal degradation. Moreover, SUMOylation contributes to the subcellular distribution of target proteins. Herein, we found that the SUMOylation pathway is deregulated in NPM-ALK⁺ T-cell lymphoma cell lines and primary lymphoma tumors from patients. We also identified Lys²⁴ and Lys³² within the NPM domain as the sites where NPM-ALK conjugates with SUMO-1 and SUMO-3. Importantly, antagonizing SUMOylation by the SENP1 protease decreased the accumulation of NPM-ALK and suppressed lymphoma cell viability, proliferation, and anchorage-independent colony formation. One possible mechanism for the SENP1-mediated decrease in NPM-ALK levels was the increase in NPM-ALK association with ubiquitin, which facilitates its degradation. Our findings propose a model in which aberrancies in SUMOylation contribute to the pathogenesis of NPM-ALK⁺ T-cell lymphoma. Unraveling such pathogenic mechanisms may lead to devising novel strategies to eliminate this aggressive neoplasm.

The Nucleus-Localized Epidermal Growth Factor Receptor Is SUMOylated

The epidermal growth factor receptor (EGFR) plays important roles in normal and cancer cell growth. The EGFR has principally two different signaling pathways: the canonical kinase route induced at the plasma membrane resulting in an intracellular phosphorylation cascade via MAPKs and PI3K and the more recently discovered pathway by which the receptor functions as a transcriptional co-activator inside the cell nucleus. Full length EGFR translocates to the inner nuclear membrane, via the endoplasmic reticulum, through association with the sec61β translocon. The c-myc (MYC) and cyclin D1 (CNND1) genes represent two target genes for nuclear EGFR (nEGFR). Here we show that EGFR is SUMOylated and that the SUMO-1-modified receptors are almost unexceptionally nuclear. Co-immunoprecipitation experiments suggest that EGFR is multi-SUMOylated. Using two mass spectrometry-based strategies (matrix-assisted laser desorption ionization time of flight and electrospray ionization liquid chromatography with tandem mass spectrometry), lysine 37 was identified as a SUMO-1-modified residue by both methods. A lysine 37 site mutant (K37R) was transfected into EGFR deficient cells. Total SUMOylation of EGFR was not altered in the K37R-transfected cells, confirming the presence of other SUMOylation sites. To gain preliminary insight into the possible functional role of EGFR SUMOylation, we compared the effect of expression of the wild-type EGFR with the K37R mutant on promoter activity and expression of CMYC and CNND1. Our results indicate that SUMO-1 modification may affect the transcriptional activity of EGFR, which might have additional impact on, e.g., cancer progression.

The role of ubiquitin and ubiquitin-like modification systems in papillomavirus biology

Human papillomaviruses (HPVs) are small DNA viruses that are important etiological agents of a spectrum of human skin lesions from benign to malignant. Because of their limited genome coding capacity they express only a small number of proteins, only one of which has enzymatic activity. Additionally, the HPV productive life cycle is intimately tied to the epithelial differentiation program and they must replicate in what are normally non-replicative cells, thus, these viruses must reprogram the cellular environment to achieve viral reproduction. Because of these limitations and needs, the viral proteins have evolved to co-opt cellular processes primarily through protein-protein interactions with critical host proteins. The ubiquitin post-translational modification system and the related ubiquitin-like modifiers constitute a widespread cellular regulatory network that controls the levels and functions of thousands of proteins, making these systems an attractive target for viral manipulation. This review describes the interactions between HPVs and the ubiquitin family of modifiers, both to regulate the viral proteins themselves and to remodel the host cell to facilitate viral survival and reproduction.

Non-nuclear function of sumoylated proteins

Post-translational modification by the SUMO moiety is now regarded as one of the key regulatory modifications in eukaryotic cells. Up to now, plenty of sumoylated proteins have been found to be involved in nuclear processes such as chromatin organization, transcription and DNA repair as well as in other cellular functions. Since the number of data concerning sumoylated proteins and their function outside the nucleus has grown rapidly, in this review we summarized the results describing the non-nuclear role of SUMO substrates. In particular, we focused on the role of sumoylation in the regulation of channel activity, receptor function, G-protein signaling, activity of enzymes, cytoskeletal organization, exocytosis, autophagy and mitochondrial dynamics.

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

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

SUMOylation determines turnover and localization of nephrin at the plasma membrane

Podocyte effacement and the reformation of foot processes and slit diaphragms can be induced within minutes experimentally. Therefore, it seems likely that the slit diaphragm proteins underlie orchestrated recycling mechanisms under the control of posttranslational modifiers. One of these modifiers, SUMO (small ubiquitin-like modifier), is an ubiquitin-like protein with a 20% corresponding identity to ubiquitin. Modification by SUMOs to proteins on lysine residues can block the ubiquitination of the same site leading to the stabilization of the target protein. Here we found in vitro and in vivo that nephrin is a substrate modified by SUMO proteins thereby increasing its steady-state level and expression at the plasma membrane. A conversion of lysines to arginines at positions 1114 and 1224 of the intracellular tail of murine nephrin led to decreased stability of nephrin, decreased expression at the plasma membrane, and decreased PI3K/AKT signaling. Furthermore, treatment of podocytes with the SUMOylation inhibitor ginkgolic acid led to reduced membrane expression of nephrin. Similarly, the conversion of lysine to arginine at position 1100 of human nephrin caused decreased stability and expression at the plasma membrane. As SUMOylation is a reversible process, our results suggest that SUMOylation participates in the tight orchestration of nephrin turnover at the slit diaphragm.

Sumoylation of influenza A virus nucleoprotein is essential for intracellular trafficking and virus growth

Viruses take advantage of host posttranslational modifications for their own benefit. It was recently reported that influenza A virus proteins interact extensively with the host sumoylation system. Thereby, several viral proteins, including NS1 and M1, are sumoylated to facilitate viral replication. However, to what extent sumoylation is exploited by influenza A virus is not fully understood. In this study, we found that influenza A virus nucleoprotein (NP) is a bona fide target of sumoylation in both NP-transfected cells and virus-infected cells. We further found that NP is sumoylated at the two most N-terminal residues, lysines 4 and 7, and that sumoylation at lysine 7 of NP is highly conserved across different influenza A virus subtypes and strains, including the recently emerged human H7N9 virus. While NP stability and polymerase activity are little affected by sumoylation, the NP sumoylation-defective WSN-NPK4,7R virus exhibited early cytoplasmic localization of NP. The growth of the WSN-NPK4,7R virus was highly attenuated compared to that of the wild-type WSN virus, and the lysine residue at position 7 is indispensable for the virus's survival, as illustrated by the rapid emergence of revertant viruses. Thus, sumoylation of influenza A virus NP is essential for intracellular trafficking of NP and for virus growth, illustrating sumoylation as a crucial strategy extensively exploited by influenza A virus for survival in its host.

IMPORTANCE

Host posttranslational modifications are heavily targeted by viruses for their own benefit. We and others previously reported that influenza A virus interacts extensively with the host sumoylation system. However, the functional outcomes of viral sumoylation are not fully understood. Here we found that influenza A virus nucleoprotein (NP), an essential component for virus replication, is a new target of SUMO. This is the first study to find that NP from different influenza A viruses, including recently emerged H7N9, is sumoylated at conserved lysine 7. Our data further illustrated that sumoylation of influenza A virus NP is essential for intracellular trafficking of NP and virus growth, indicating that influenza A virus relies deeply on sumoylation to survive in host cells. Strategies to downregulate viral sumoylation could thus be a potential antiviral treatment.

Nuclear translocation of IGF-1R via p150(Glued) and an importin-β/RanBP2-dependent pathway in cancer cells

Mounting evidence has shown that the insulin-like growth factor-1 receptor (IGF-1R) has critical roles in cancer cell growth. This has prompted pharmacological companies to develop agents targeting the receptor. Surprisingly, clinical trials using specific IGF-1R antibodies have, however, revealed disappointing results. Further understanding of the role of IGF-1R in cancer cells is therefore necessary for development of efficient therapeutic strategies. Recently, we showed that IGF-1R is sumoylated and translocated into the cell nucleus where it activates gene transcription. Several other studies have confirmed our findings and it has been reported that nuclear IGF-1R (nIGF-1R) has prognostic and predictive impact in cancer. To increase the understanding of IGF-1R in cancer cells, we here present the first study that proposes a pathway by which IGF-1R translocates into the cell nucleus. We could demonstrate that IGF-1R first associates with the dynactin subunit p150(Glued), which transports the receptor to the nuclear pore complex, where it co-localizes with importin-β followed by association with RanBP2. Sumoylation of IGF-1R seems to be required for interaction with RanBP2, which in turn may serve as the SUMO E3 ligase. In the context of sumoylation, we provided evidence that it may favor nIGF-1R accumulation by increasing the stability of the receptor. Taken together, topographic and functional interactions between dynactin, importin-β and RanBP2 are involved in nuclear translocation of IGF-1R. Our results provide new understanding of IGF-1R in cancer, which in turn may contribute to development of new therapeutic strategies.

Functional swapping between transmembrane proteins TMEM16A and TMEM16F

The transmembrane proteins TMEM16A and -16F each carry eight transmembrane regions with cytoplasmic N and C termini. TMEM16A carries out Ca(2+)-dependent Cl(-) ion transport, and TMEM16F is responsible for Ca(2+)-dependent phospholipid scrambling. Here we established assay systems for the Ca(2+)-dependent Cl(-) channel activity using 293T cells and for the phospholipid scramblase activity using TMEM16F(-/-) immortalized fetal thymocytes. Chemical cross-linking analysis showed that TMEM16A and -16F form homodimers in both 293T cells and immortalized fetal thymocytes. Successive deletion from the N or C terminus of both proteins and the swapping of regions between TMEM16A and -16F indicated that their cytoplasmic N-terminal (147 amino acids for TMEM16A and 95 for 16F) and C-terminal (88 amino acids for TMEM16A and 68 for 16F) regions were essential for their localization at plasma membranes and protein stability, respectively, and could be exchanged. Analyses of TMEM16A and -16F mutants with point mutations in the pore region (located between the fifth and sixth transmembrane regions) indicated that the pore region is essential for both the Cl(-) channel activity of TMEM16A and the phospholipid scramblase activity of TMEM16F. Some chemicals such as epigallocatechin-3-gallate and digallic acid inhibited the Cl(-) channel activity of TMEM16A and the scramblase activity of TMEM16F with an opposite preference. These results indicate that TMEM16A and -16F use a similar mechanism for sorting to plasma membrane and protein stabilization, but their functional domains significantly differ.

Structural modulation of phosducin by phosphorylation and 14-3-3 protein binding

Phosducin (Pdc), a highly conserved phosphoprotein, plays an important role in the regulation of G protein signaling, transcriptional control, and modulation of blood pressure. Pdc is negatively regulated by phosphorylation followed by binding to the 14-3-3 protein, whose role is still unclear. To gain insight into the role of 14-3-3 in the regulation of Pdc function, we studied structural changes of Pdc induced by phosphorylation and 14-3-3 protein binding using time-resolved fluorescence spectroscopy. Our data show that the phosphorylation of the N-terminal domain of Pdc at Ser-54 and Ser-73 affects the structure of the whole Pdc molecule. Complex formation with 14-3-3 reduces the flexibility of both the N- and C-terminal domains of phosphorylated Pdc, as determined by time-resolved tryptophan and dansyl fluorescence. Therefore, our data suggest that phosphorylated Pdc undergoes a conformational change when binding to 14-3-3. These changes involve the G(t)βγ binding surface within the N-terminal domain of Pdc, and thus could explain the inhibitory effect of 14-3-3 on Pdc function.

The ubiquitin-proteasome system in retinal health and disease

The ubiquitin-proteasome system (UPS) is the main intracellular pathway for modulated protein turnover, playing an important role in the maintenance of cellular homeostasis. It also exerts a protein quality control through degradation of oxidized, mutant, denatured, or misfolded proteins and is involved in many biological processes where protein level regulation is necessary. This system allows the cell to modulate its protein expression pattern in response to changing physiological conditions and provides a critical protective role in health and disease. Impairments of UPS function in the central nervous system (CNS) underlie an increasing number of genetic and idiopathic diseases, many of which affect the retina. Current knowledge on the UPS composition and function in this tissue, however, is scarce and dispersed. This review focuses on UPS elements reported in the retina, including ubiquitinating and deubiquitinating enzymes (DUBs), and alternative proteasome assemblies. Known and inferred roles of protein ubiquitination, and of the related, SUMO conjugation (SUMOylation) process, in normal retinal development and adult homeostasis are addressed, including modulation of the visual cycle and response to retinal stress and injury. Additionally, the relationship between UPS dysfunction and human neurodegenerative disorders affecting the retina, including Alzheimer's, Parkinson's, and Huntington's diseases, are dealt with, together with numerous instances of retina-specific illnesses with UPS involvement, such as retinitis pigmentosa, macular degenerations, glaucoma, diabetic retinopathy (DR), and aging-related impairments. This information, though still basic and limited, constitutes a suitable framework to be expanded in incoming years and should prove orientative toward future therapy design targeting sight-affecting diseases with a UPS underlying basis.

SUMOylation of pancreatic glucokinase regulates its cellular stability and activity

Glucokinase is the predominant hexokinase expressed in hepatocytes and pancreatic β-cells, with a pivotal role in regulating glucose-stimulated insulin secretion, illustrated by glucokinase gene mutations causing monogenic diabetes and congenital hyperinsulinemic hypoglycemia. A complex tissue-specific network of mechanisms regulates this enzyme, and a major unanswered question in glucokinase biology is how post-translational modifications control the function of the enzyme. Here, we show that the pancreatic isoform of human glucokinase is SUMOylated in vitro, using recombinant enzymes, and in insulin-secreting model cells. Three N-terminal lysines unique for the pancreatic isoform (Lys-12/Lys-13 and/or Lys-15) may represent one SUMOylation site, with an additional site (Lys-346) common for the pancreatic and the liver isoform. SUMO-1 and E2 overexpression stabilized preferentially the wild-type human pancreatic enzyme in MIN6 β-cells, and SUMOylation increased the catalytic activity of recombinant human glucokinase in vitro and also of glucokinase in target cells. Small ubiquitin-like modifier conjugation represents a novel form of post-translational modification of the enzyme, and it may have an important regulatory function in pancreatic β-cells.

Forkhead box protein A2 (FOXA2) protein stability and activity are regulated by sumoylation

The forkhead box protein A2 (FOXA2) is an important regulator of glucose and lipid metabolism and organismal energy balance. Little is known about how FOXA2 protein expression and activity are regulated by post-translational modifications. We have identified that FOXA2 is post-translationally modified by covalent attachment of a small ubiquitin related modifier-1 (SUMO-1) and mapped the sumoylation site to the amino acid lysine 6 (K6). Preventing sumoylation by mutating the SUMO acceptor K6 to arginine resulted in downregulation of FOXA2 protein but not RNA expression in INS-1E insulinoma cells. K6R mutation also downregulated FOXA2 protein levels in HepG2 hepatocellular carcinoma cells, HCT116 colon cancer cells and LNCaP and DU145 prostate cancer cells. Further, interfering with FOXA2 sumoylation through siRNA mediated knockdown of UBC9, an essential SUMO E2 conjugase, resulted in downregulation of FOXA2 protein levels. Stability of sumoylation deficient FOXA2K6R mutant protein was restored when SUMO-1 was fused in-frame. FOXA2 sumoylation and FOXA2 protein levels were increased by PIAS1 SUMO ligase but not a SUMO ligase activity deficient PIAS1 mutant. Although expressed at lower levels, sumoylation deficient FOXA2K6R mutant protein was detectable in the nucleus indicating that FOXA2 nuclear localization is independent of sumoylation. Sumoylation increased the transcriptional activity of FOXA2 on Pdx-1 area I enhancer. Together, our results show that sumoylation regulates FOXA2 protein expression and activity.

SUMOylation and SUMO-interacting motif (SIM) of metastasis tumor antigen 1 (MTA1) synergistically regulate its transcriptional repressor function

Metastasis tumor antigen 1 (MTA1), a component of the Mi-2·nucleosome remodeling and deacetylase complex, plays a crucial role in gene transcription, but the mechanism involved remains largely unknown. Here, we report that MTA1 is a substrate for small ubiquitin-related modifier 2/3 (SUMO2/3) in vivo. Protein inhibitor of activated STAT (PIAS) proteins enhance SUMOylation of MTA1 and may participate in paralog-selective SUMOylation, whereas sentrin/SUMO-specific protease 1 (SENP1) and 2 may act as deSUMOylation enzymes for MTA1. Moreover, MTA1 contains a functional SUMO-interacting motif (SIM) at its C terminus, and SIM is required for the efficient SUMOylation of MTA1. SUMO conjugation on Lys-509, which is located within the SUMO consensus site, together with SIM synergistically regulates the co-repressor activity of MTA1 on PS2 transcription, probably by recruiting HDAC2 onto the PS2 promoter. Interestingly, MTA1 may up-regulate the expression of SUMO2 via interaction with RNA polymerase II and SP1 at the SUMO2 promoter. These findings not only provide novel mechanistic insights into the regulation of the transcriptional repressor function of MTA1 by SUMOylation and SIM but also uncover a potential function of MTA1 in modulating the SUMOylation pathway.

SUMO-SIM interactions regulate the activity of RGSZ2 proteins

The RGSZ2 gene, a regulator of G protein signaling, has been implicated in cognition, Alzheimer's disease, panic disorder, schizophrenia and several human cancers. This 210 amino acid protein is a GTPase accelerating protein (GAP) on Gαi/o/z subunits, binds to the N terminal of neural nitric oxide synthase (nNOS) negatively regulating the production of nitric oxide, and binds to the histidine triad nucleotide-binding protein 1 at the C terminus of different G protein-coupled receptors (GPCRs). We now describe a novel regulatory mechanism of RGS GAP function through the covalent incorporation of Small Ubiquitin-like MOdifiers (SUMO) into RGSZ2 RGS box (RH) and the SUMO non covalent binding with SUMO-interacting motifs (SIM): one upstream of the RH and a second within this region. The covalent attachment of SUMO does not affect RGSZ2 binding to GPCR-activated GαGTP subunits but abolishes its GAP activity. By contrast, non-covalent binding of SUMO with RH SIM impedes RGSZ2 from interacting with GαGTP subunits. Binding of SUMO to the RGSZ2 SIM that lies outside the RH does not affect GαGTP binding or GAP activity, but it could lead to regulatory interactions with sumoylated proteins. Thus, sumoylation and SUMO-SIM interactions constitute a new regulatory mechanism of RGS GAP function and therefore of GPCR cell signaling as well.

SUMO E3 ligases are expressed in the retina and regulate SUMOylation of the metabotropic glutamate receptor 8b

The central nervous system regulates neuronal excitability by macromolecular signalling complexes that consist of functionally related proteins, including neurotransmitter receptors, enzymes and scaffolds. The composition of these signal complexes is regulated by post-translational modifications, such as phosphorylation and SUMOylation (SUMO is small ubiquitin-related modifier). In the present study, we searched for proteins interacting with the intracellular C-termini of the metabotropic glutamate receptors mGluR8a and mGluR8b and identified proteins of the SUMOylation and NEDDylation machinery. The SUMO E3 ligases Pias1 [Pias is protein inhibitor of activated STAT (signal transducer and activator of transcription)] and Pias3L interacted strongly with mGluR8b, and were co-localized with the E2-conjugating Ubc9, SUMO1 and mGluR8b in cell bodies present in the ganglion cell layer of the mammalian retina. SUMO1 conjugation of Lys882, present in a bona fide consensus sequence for SUMOylation (VKSE) in the mGluR8b C-terminus, was enhanced by addition of Pias1, consistent with an interaction between both proteins. Mutation of Lys882 to arginine reduced, but did not abolish, mGluR8b SUMOylation. Co-mutating a second lysine residue (Lys903) located in the mGluR8b isoform-specific C-terminus largely prevented SUMO1 conjugation by Ubc9. Modelling studies suggested that Lys903 contacts Ubc9 and thus is part of the non-canonical SUMOylation site VKSG. In summary, the results of the present study show in vivo SUMOylation of the complete mGluR8b and co-localize proteins of the SUMOylation machinery in the retina.

Small ubiquitin-like modifier modification of arrestin-3 regulates receptor trafficking

Nonvisual arrestins are regulated by direct post-translational modifications, such as phosphorylation, ubiquitination, and nitrosylation. However, whether arrestins are regulated by other post-translational modifications remains unknown. Here we show that nonvisual arrestins are modified by small ubiquitin-like modifier 1 (SUMO-1) upon activation of β(2)-adrenergic receptor (β(2)AR). Lysine residues 295 and 400 in arrestin-3 fall within canonical SUMO consensus sites, and mutagenic analysis reveals that Lys-400 represents the main SUMOylation site. Depletion of the SUMO E2 modifying enzyme Ubc9 blocks arrestin-3 SUMOylation and attenuates β(2)AR internalization, suggesting that arrestin SUMOylation mediates G protein-coupled receptor endocytosis. Consistent with this, expression of a SUMO-deficient arrestin mutant failed to promote β(2)AR internalization as compared with wild-type arrestin-3. Our data reveal an unprecedented role for SUMOylation in mediating GPCR endocytosis and provide novel mechanistic insight into arrestin function and regulation.

The physiological roles of phosducin: from retinal function to stress-dependent hypertension

In the time since its discovery, phosducin's functions have been intensively studied both in vivo and in vitro. Phosducin's most important biochemical feature in in vitro studies is its binding to heterotrimeric G protein βγ-subunits. Data on phosducin's in vivo relevance, however, have only recently been published but expand the range of biological actions, as shown both in animal models as well as in human studies. This review gives an overview of different aspects of phosducin biology ranging from structure, phylogeny of phosducin family members, posttranscriptional modification, biochemical features, localization and levels of expression to its physiological functions. Special emphasis will be placed on phosducin's function in the regulation of blood pressure. In the second part of this article, findings concerning cardiovascular regulation and their clinical relevance will be discussed on the basis of recently published data from gene-targeted mouse models and human genetic studies.

Phosducin and monomeric β-actin have common epitope recognized by anti-phosducin antibodies

Phosducin family proteins are regulators of cytoplasmic processes. The main function ascribed to phosducin is the binding and sequestration of the β subunit of heterotrimeric G proteins. Phosducin-like protein 1, longer than phosducin by 37 amino-acids, is involved in chaperoning of newly synthesized proteins. β-Actin, a component of the cytoskeleton, participates in cell movement. There is no apparent evolutionary relationship between phosducin and β-actin nor structure similarity. Nevertheless we obtained the polyclonal antibodies named ap33, originally directed against a phosducin-derived peptide (SQSLEEDFEGQATHTGPK), that also recognized β-actin. The epitope on the β-actin molecule was characterized. It is a conformational epitope grouping some of the L-D-F-E-Q-A-T-K amino-acids found in the peptide originally used to obtain the antibodies. The main part of the epitope is localized on the actin-actin interface of polymerized actin, so it is accessible only on monomeric actin. The existence of a common epitope on the molecules of phosducin and β-actin may reflect a topological similarity of a small region of their surfaces.

Modification of nonstructural protein 1 of influenza A virus by SUMO1

Nonstructural protein 1 (NS1) is one of the major factors resulting in the efficient infection rate and high level of virulence of influenza A virus. Although consisting of only approximately 230 amino acids, NS1 has the ability to interfere with several systems of the host viral defense. In the present study, we demonstrate that NS1 of the highly pathogenic avian influenza A/Duck/Hubei/L-1/2004 (H5N1) virus interacts with human Ubc9, which is the E2 conjugating enzyme for sumoylation, and we show that SUMO1 is conjugated to H5N1 NS1 in both transfected and infected cells. Furthermore, two lysine residues in the C terminus of NS1 were identified as SUMO1 acceptor sites. When the SUMO1 acceptor sites were removed by mutation, NS1 underwent rapid degradation. Studies of different influenza A virus strains of human and avian origin showed that the majority of viruses possess an NS1 protein that is modified by SUMO1, except for the recently emerged swine-origin influenza A virus (S-OIV) (H1N1). Interestingly, growth of a sumoylation-deficient WSN virus mutant was retarded compared to that of wild-type virus. Together, these results indicate that sumoylation enhances NS1 stability and thus promotes rapid growth of influenza A virus.

Targets and consequences of protein SUMOylation in neurons

The post-translational modification of proteins is critical for the spatial and temporal regulation of signalling cascades. This is especially important in the CNS where the processes affecting differentiation, growth, targeting and communication between neurones are highly complex and very tightly regulated. In recent years it has emerged that modification of proteins by members of the SUMO (small ubiquitin-related modifier) family of proteins play key roles in neuronal function. SUMOylation involves the covalent conjugation of a member of the SUMO family to lysine residues in target proteins. Multiple nuclear and perinuclear SUMOylation targets have been reported to be involved in nuclear organisation and transcriptional regulation. In addition, a growing number of extranuclear SUMO substrates have been identified that can have important acute effects on neuronal function. The SUMOylation of both intra- and extranuclear proteins have been implicated in a diverse array of processes that have far-reaching implications for neuronal function and pathophysiology. Here we review the current understanding of the targets and consequences of protein SUMOylation in the brain and examine its established and potential involvement in a wide range of neurological and neurodegenerative diseases.

Agonist-regulated cleavage of the extracellular domain of parathyroid hormone receptor type 1

The receptor for parathyroid hormone (PTHR) is a main regulator of calcium homeostasis and bone maintenance. As a member of class B of G protein-coupled receptors, it harbors a large extracellular domain, which is required for ligand binding. Here, we demonstrate that the PTHR extracellular domain is cleaved by a protease belonging to the family of extracellular metalloproteinases. We show that the cleavage takes place in a region of the extracellular domain that belongs to an unstructured loop connecting the ligand-binding parts and that the N-terminal 10-kDa fragment is connected to the receptor core by a disulfide bond. Cleaved receptor revealed reduced protein stability compared with noncleaved receptor, suggesting degradation of the whole receptor. In the presence of the agonistic peptides PTH(1-34), PTH(1-14), or PTH(1-31), the processing of the PTHR extracellular domain was inhibited, and receptor protein levels were stabilized. A processed form of the PTHR was also detected in human kidney. These findings suggest a new model of PTHR processing and regulation of its stability.

Phosducin influences sympathetic activity and prevents stress-induced hypertension in humans and mice

Hypertension and its complications represent leading causes of morbidity and mortality. Although the cause of hypertension is unknown in most patients, genetic factors are recognized as contributing significantly to an individual's lifetime risk of developing the condition. Here, we investigated the role of the G protein regulator phosducin (Pdc) in hypertension. Mice with a targeted deletion of the gene encoding Pdc (Pdc-/- mice) had increased blood pressure despite normal cardiac function and vascular reactivity, and displayed elevated catecholamine turnover in the peripheral sympathetic system. Isolated postganglionic sympathetic neurons from Pdc-/- mice showed prolonged action potential firing after stimulation with acetylcholine and increased firing frequencies during membrane depolarization. Furthermore, Pdc-/- mice displayed exaggerated increases in blood pressure in response to post-operative stress. Candidate gene-based association studies in 2 different human populations revealed several SNPs in the PDC gene to be associated with stress-dependent blood pressure phenotypes. Individuals homozygous for the G allele of an intronic PDC SNP (rs12402521) had 12-15 mmHg higher blood pressure than those carrying the A allele. These findings demonstrate that PDC is an important modulator of sympathetic activity and blood pressure and may thus represent a promising target for treatment of stress-dependent hypertension.

Enhanced retinal insulin receptor-activated neuroprotective survival signal in mice lacking the protein-tyrosine phosphatase-1B gene

Protein-tyrosine phosphatase 1B (PTP1B) has been implicated in the negative regulation of insulin signaling. We previously demonstrated that light-induced tyrosine phosphorylation of the retinal insulin receptor (IR) results in the activation of phosphoinositide 3-kinase/Akt survival pathway in rod photoreceptor cells. The molecular mechanism behind light-induced activation of IR is not known. We investigated the in vivo mechanism of IR activation and found that PTP1B activity in dark-adapted retinas was significantly higher than in light-adapted retinas. We made a novel finding in this study that the light-dependent regulation of PTP1B activity is signaled through photobleaching of rhodopsin. Conditional deletion of PTP1B in rod photoreceptors by the Cre-loxP system resulted in enhanced IR signaling. Further PTP1B activity negatively regulated the neuroprotective survival signaling in the retina. One of the challenging questions in the retina research is how mutations in human rhodopsin gene slowly disable and eventually disrupt photoreceptor functions. Our studies suggest that a defect in the photobleaching of rhodopsin and mutation in rhodopsin gene enhances the activity of PTP1B, and this activated activity could down-regulate the IR survival signaling. Our studies suggest that PTP1B antagonists could be potential therapeutic agents to treat stress-induced photoreceptor degenerations and provide further evidence that rhodopsin photoexcitation may trigger signaling events alternative to the classic phototransduction.

SUMO proteases: redox regulation and biological consequences

Small-ubiquitin modifier (SUMO) has emerged as a novel modification system that governs the activities of a wide spectrum of protein substrates. SUMO-specific proteases (SENP) are of particular interest, as they are responsible for both the maturation of SUMO precursors and for their deconjugation. The interruption of SENPs has been implicated in embryonic defects and carcinoma cells, indicating that a proper balance of SUMO conjugation and deconjugation is crucial. Recent advances in molecular and cellular biology have highlighted the distinct subcellular localization, and endopeptidase and isopeptidase activities of SENPs, suggesting that they are nonredundant. A better understanding of the molecular basis of SUMO recognition and hydrolytic cleavage has been obtained from the crystal structures of SENP-substrate complexes. While a number of proteomic studies have shown an upregulation of sumoylation, attention is now increasingly being directed towards the regulatory mechanism of sumoylation, in particular the oxidative effect. Findings on the oxidation-induced intermolecular disulfide of E1-E2 ligases and SENP1/2 have improved our understanding of the mechanism by which modification is switched up or down. More intriguingly, a growing body of evidence suggests that sumoylation cross-talks with other modifications, and that the upstream and downstream signaling pathway is co-regulated by more than one modifier.

Mechanisms underlying the control of progesterone receptor transcriptional activity by SUMOylation

Posttranslational modification by small ubiquitin-like modifier (SUMO) is a major regulator of transcription. We previously showed that progesterone receptors (PR) have a single consensus psiKXE SUMO-conjugation motif centered at Lys-388 in the N-terminal domain of PR-B and a homologous site of PR-A. SUMOylation of the PR is hormone-dependent and has a suppressive effect on transcription of an exogenous promoter. Here we show that repression of PR activity by SUMOylation at Lys-388 is uncoupled from phosphorylation, involves synergy between tandem progesterone response elements, and is associated with lowered ligand sensitivity and slowed ligand-dependent down-regulation. However, paradoxically, cellular overexpression of SUMO-1 increases PR transcriptional activity even if Lys-388 is mutated, suggesting that the receptors are activated indirectly by other SUMOylated proteins. One of these is the coactivator SRC-1, whose binding to PR and enhancement of agonist-dependent N-/C-terminal interactions is augmented by the presence of SUMO-1. Increased transcription due to SRC-1 is independent of PR SUMOylation based on assays with the Lys-388 mutants and the pure antiprogestin ZK98299, which blocks N-/C-terminal interactions. In summary, SUMOylation tightly regulates the transcriptional activity of PR by repressing the receptors directly while activating them indirectly through augmented SRC-1 coactivation.

Pias3-dependent SUMOylation directs rod photoreceptor development

Specification of retinal rod photoreceptors is determined by several different transcription factors that activate expression of rod-specific genes and repress expression of cone photoreceptor-specific genes. The mechanism by which this dual regulation occurs is unclear. We have found that Pias3, a transcriptional coregulator and E3 SUMO ligase that is selectively expressed in developing photoreceptors, promotes the differentiation of rod photoreceptors while preventing rods from adopting cone photoreceptor-like characteristics. Pias3 binds the photoreceptor-specific transcription factors Crx and Nr2e3 and is specifically targeted to the promoters of photoreceptor-specific genes. Pias3 SUMOylates Nr2e3, converting it into a potent repressor of cone-specific gene expression. Rod- and cone-specific promoters are bound by hyperSUMOylated proteins in rod photoreceptors, and blocking SUMOylation in photoreceptors results in cells with morphological and molecular features of cones and an absence of rod-specific markers. Our data thus identify Pias3-mediated SUMOylation of photoreceptor-specific transcription factors as a key mechanism of rod specification.