Targeting SUMO E1 to ubiquitin ligases: a viral strategy to counteract sumoylation

SUMOylation and Viral Infections of the Brain

The small ubiquitin-like modifier (SUMO) system regulates numerous biological processes, including protein localization, stability and/or activity, transcription, and DNA repair. SUMO also plays critical roles in innate immunity and antiviral defense by mediating interferon (IFN) synthesis and signaling, as well as the expression and function of IFN-stimulated gene products. Viruses including human immunodeficiency virus-1, Zika virus, herpesviruses, and coronaviruses have evolved to exploit the host SUMOylation system to counteract the antiviral activities of SUMO proteins and to modify their own proteins for viral persistence and pathogenesis. Understanding the exploitation of SUMO is necessary for the development of effective antiviral therapies. This review summarizes the interplay between viruses and the host SUMOylation system, with a special emphasis on viruses with neuro-invasive properties that have pathogenic consequences on the central nervous system.

SUMOylation in Viral Replication and Antiviral Defense

SUMOylation is a ubiquitination-like post-translational modification that plays an essential role in the regulation of protein function. Recent studies have shown that proteins from both RNA and DNA virus families can be modified by SUMO conjugation, which facilitates viral replication. Viruses can manipulate the entire process of SUMOylation through interplay with the SUMO pathway. By contrast, SUMOylation can eliminate viral infection by regulating host antiviral immune components. A deeper understanding of how SUMOylation regulates viral proteins and cellular antiviral components is necessary for the development of effective antiviral therapies. In the present review, the regulatory mechanism of SUMOylation in viral replication and infection and the antiviral immune response, and the consequences of this regulation for viral replication and engagement with antiviral innate immunity are summarized. The potential therapeutic applications of SUMOylation in diseases caused by viruses are also discussed.

Protein–Protein Interactions Facilitate E4orf6-Dependent Regulation of E1B-55K SUMOylation in HAdV-C5 Infection

The human adenovirus type C5 (HAdV-C5) E1B-55K protein is a multifunctional regulator of HAdV-C5 replication, participating in many processes required for maximal virus production. Its multifunctional properties are primarily regulated by post-translational modifications (PTMs). The most influential E1B-55K PTMs are phosphorylation at highly conserved serine and threonine residues at the C-terminus, and SUMO conjugation to lysines 104 (K104) and 101 (K101) situated in the N-terminal region of the protein, which have been shown to regulate each other. Reversible SUMO conjugation provides a molecular switch that controls key functions of the viral protein, including intracellular trafficking and viral immune evasion. Interestingly, SUMOylation at SUMO conjugation site (SCS) K104 is negatively regulated by another multifunctional HAdV-C5 protein, E4orf6, which is known to form a complex with E1B-55K. To further evaluate the role of E4orf6 in the regulation of SUMO conjugation to E1B-55K, we analyzed different virus mutants expressing E1B-55K proteins with amino acid exchanges in both SCS (K101 and K104) in the presence or absence of E4orf6. We could exclude phosphorylation as factor for E4orf6-mediated reduction of E1B-55K SUMOylation. In fact, we demonstrate that a direct interaction between E1B-55K and E4orf6 is required to reduce E1B-55K SUMOylation. Additionally, we show that an E4orf6-mediated decrease of SUMO conjugation to K101 and K104 result in impaired co-localization of E1B-55K and SUMO in viral replication compartments. These findings indicate that E4orf6 inhibits E1B-55K SUMOylation, which could favor assembly of E4orf6-dependent E3 ubiquitin ligase complexes that are known to degrade a variety of host restriction factors by proteasomal degradation and, thereby, promote viral replication.

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

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

Viruses, SUMO, and immunity: the interplay between viruses and the host SUMOylation system

The conjugation of small ubiquitin-like modifier (SUMO) proteins to substrates is a well-described post-translational modification that regulates protein activity, subcellular localization, and protein-protein interactions for a variety of downstream cellular activities. Several studies describe SUMOylation as an essential post-translational modification for successful viral infection across a broad range of viruses, including RNA and DNA viruses, both enveloped and un-enveloped. These viruses include but are not limited to herpes viruses, human immunodeficiency virus-1, and coronaviruses. In addition to the SUMOylation of viral proteins during infection, evidence shows that viruses manipulate the SUMO pathway for host protein SUMOylation. SUMOylation of host and viral proteins greatly impacts host innate immunity through viral manipulation of the host SUMOylation machinery to promote viral replication and pathogenesis. Other post-translational modifications like phosphorylation can also modulate SUMO function. For example, phosphorylation of COUP-TF interacting protein 2 (CTIP2) leads to its SUMOylation and subsequent proteasomal degradation. The SUMOylation of CTIP2 and subsequent degradation prevents CTIP2-mediated recruitment of a multi-enzymatic complex to the HIV-1 promoter that usually prevents the transcription of integrated viral DNA. Thus, the "SUMO switch" could have implications for CTIP2-mediated transcriptional repression of HIV-1 in latency and viral persistence. In this review, we describe the consequences of SUMO in innate immunity and then focus on the various ways that viral pathogens have evolved to hijack the conserved SUMO machinery. Increased understanding of the many roles of SUMOylation in viral infections can lead to novel insight into the regulation of viral pathogenesis with the potential to uncover new targets for antiviral therapies.

SUMO: a novel target for anti-coronavirus therapy

Over the past 20 years, humankind has encountered three severe coronavirus outbreaks. Currently ongoing, COVID-19 (coronavirus disease 2019) was declared a pandemic due to its massive impact on global health and the economy. Numerous scientists are working to identify efficacious therapeutic agents for COVID-19, although treatment ability has yet to be demonstrated. The SUMO (small ubiquitin-like modifier) system has diverse roles in viral manipulation, but the function of SUMO in coronaviruses is still unknown. The objective of this review article is to present recently published data suggesting contributions of the host SUMO system to coronavirus infection. These findings underscore the potential of SUMO as a novel target for anti-coronavirus therapy, and the need for a deeper understanding of coronavirus pathology to prepare and prevail against the current and emerging coronavirus outbreaks.

A tandem mass tag-based quantitative proteomic analysis of fowl adenovirus serotype 4-infected LMH cells

Fowl adenovirus serotype 4 (FAdV-4) is recognized as an economically important pathogen for the poultry industry worldwide. FAdV-4 infection causes a metabolic disturbance of hepatocytes, leading to hydropericardium-hepatitis syndrome (HHS) in poultry. However, the metabolic response of hepatocytes to FAdV-4 infection remains poorly investigated. Here, a tandem mass tag (TMT)-based approach was first used to quantitatively identify differentially expressed proteins (DEPs) in leghorn male hepatoma (LMH) cells infected with the virulent FAdV-4 strain GY. We identified 666 DEPs associated with many biological processes and pathways, according to Gene Ontology and Kyoto Encyclopedia of Genes and Genomes pathway analyses. Functional enrichment analysis revealed that three pathways, including metabolism-related signaling pathways, apoptosis, and autophagy responses, were enriched during FAdV-4 infection. Moreover, excessive induction of metabolism-related signaling pathways by FAdV-4 infection might be associated with HHS induced by the virus. Meanwhile, among the proteins in these pathways, RRM2, SAE1, AEN, and RAD50 were verified through western blotting to be markedly altered in FAdV-4-infected LMH cells. Notably, overexpression of SAE1 inhibited the replication of FAdV-4 in vitro, whereas silencing of SAE1 expression promoted the replication of the virus. Collectively, our findings show for the first time that SAE1 is a host cellular protein that plays roles in regulating the life cycle of FAdV-4.

SUMOylation mediates CtIP's functions in DNA end resection and replication fork protection

Double-strand breaks and stalled replication forks are a significant threat to genomic stability that can lead to chromosomal rearrangements or cell death. The protein CtIP promotes DNA end resection, an early step in homologous recombination repair, and has been found to protect perturbed forks from excessive nucleolytic degradation. However, it remains unknown how CtIP's function in fork protection is regulated. Here, we show that CtIP recruitment to sites of DNA damage and replication stress is impaired upon global inhibition of SUMOylation. We demonstrate that CtIP is a target for modification by SUMO-2 and that this occurs constitutively during S phase. The modification is dependent on the activities of cyclin-dependent kinases and the PI-3-kinase-related kinase ATR on CtIP's carboxyl-terminal region, an interaction with the replication factor PCNA, and the E3 SUMO ligase PIAS4. We also identify residue K578 as a key residue that contributes to CtIP SUMOylation. Functionally, a CtIP mutant where K578 is substituted with a non-SUMOylatable arginine residue is defective in promoting DNA end resection, homologous recombination, and in protecting stalled replication forks from excessive nucleolytic degradation. Our results shed further light on the tightly coordinated regulation of CtIP by SUMOylation in the maintenance of genome stability.

Knockdown of OsSAE1a affects the growth and development and phosphate homeostasis in rice

SUMOylation is a post-translational modification process that comprises a tandem enzymatic cascade, i.e., maturation, activation, conjugation, and ligation of a small ubiquitin-like modifier, which triggers the modulated activities and transport of the cellular proteins to other areas of the cell. In Oryza sativa (rice), OsSIZ1/2 encoding E3 SUMO ligase exerts regulatory influences on Pi homeostasis and developmental responses. However, the role of OsSAE1a, SUMO E1 activating enzyme, in regulating phosphate (Pi) utilization and/or growth and development is not known in rice and was thus investigated in this study. The qRT-PCR assay revealed a constitutive and variable spatiotemporal expression pattern of OsSAE1a in the vegetative and reproductive tissues and was comparable in the root and shoot grown under different Pi regimes. RNAi-mediated suppression of OsSAE1a exerted variable effects on the concentrations of Pi and total P in different tissues, uptake and distribution of ³²Pi, and relative expression levels of several genes that play pivotal roles in the maintenance of Pi homeostasis. The effects of the mutation in OsSAE1a were also evident in the vegetative and reproductive traits of rice during growth in a hydroponic system and pot soil, respectively. Overall, these results suggest a broad-spectrum role of OsSAE1a in the maintenance of Pi homeostasis and regulating growth and development.

SUMO proteins in the cardiovascular system: friend or foe?

Post-translational modifications (PTMs) are crucial for the adaptation of various signalling pathways to ensure cellular homeostasis and proper adaptation to stress. PTM is a covalent addition of a small chemical functional group such as a phosphate group (phosphorylation), methyl group (methylation), or acetyl group (acetylation); lipids like hydrophobic isoprene polymers (isoprenylation); sugars such as a glycosyl group (glycosylation); or even small peptides such as ubiquitin (ubiquitination), SUMO (SUMOylation), NEDD8 (neddylation), etc. SUMO modification changes the function and/or fate of the protein especially under stress conditions, and the consequences of this conjugation can be appreciated from development to diverse disease processes. The impact of SUMOylation in disease has not been monotonous, rather SUMO is found playing a role on both sides of the coin either facilitating or impeding disease progression. Several recent studies have implicated SUMO proteins as key regulators in various cardiovascular disorders. The focus of this review is thus to summarize the current knowledge on the role of the SUMO family in the pathophysiology of cardiovascular diseases.

SUMO: From Bench to Bedside

Sentrin/small ubiquitin-like modifier (SUMO) is protein modification pathway that regulates multiple biological processes, including cell division, DNA replication/repair, signal transduction, and cellular metabolism. In this review, we will focus on recent advances in the mechanisms of disease pathogenesis, such as cancer, diabetes, seizure, and heart failure, which have been linked to the SUMO pathway. SUMO is conjugated to lysine residues in target proteins through an isopeptide linkage catalyzed by SUMO-specific activating (E1), conjugating (E2), and ligating (E3) enzymes. In steady state, the quantity of SUMO-modified substrates is usually a small fraction of unmodified substrates due to the deconjugation activity of the family Sentrin/SUMO-specific proteases (SENPs). In contrast to the complexity of the ubiquitination/deubiquitination machinery, the biochemistry of SUMOylation and de-SUMOylation is relatively modest. Specificity of the SUMO pathway is achieved through redox regulation, acetylation, phosphorylation, or other posttranslational protein modification of the SUMOylation and de-SUMOylation enzymes. There are three major SUMOs. SUMO-1 usually modifies a substrate as a monomer; however, SUMO-2/3 can form poly-SUMO chains. The monomeric SUMO-1 or poly-SUMO chains can interact with other proteins through SUMO-interactive motif (SIM). Thus SUMO modification provides a platform to enhance protein-protein interaction. The consequence of SUMOylation includes changes in cellular localization, protein activity, or protein stability. Furthermore, SUMO may join force with ubiquitin to degrade proteins through SUMO-targeted ubiquitin ligases (STUbL). After 20 yr of research, SUMO has been shown to play critical roles in most, if not all, biological pathways. Thus the SUMO enzymes could be targets for drug development to treat human diseases.

SUMOylation Targets Adeno-associated Virus Capsids but Mainly Restricts Transduction by Cellular Mechanisms

Adeno-associated virus (AAV) has proven to be a promising candidate for gene therapy due to its nonpathogenic nature, ease of production, and broad tissue tropism. However, its transduction capabilities are not optimal due to the interaction with various host factors within the cell. In a previous study, we identified members of the small ubiquitin-like modifier (SUMO) pathway as significant restriction factors in AAV gene transduction. In the present study, we explored the scope of this restriction by focusing on the AAV capsid and host cell proteins as targets. We show that during vector production, the capsid protein VP2 becomes SUMOylated, as indicated by deletion and point mutations of VP2 or the obstruction of its N terminus via the addition of a tag. We observed that SUMOylated AAV capsids display higher stability than non-SUMOylated capsids. Prevention of capsid SUMOylation by VP2 mutations did not abolish transduction restriction by SUMOylation; however, it reduced activation of gene transduction by shutdown of the cellular SUMOylation pathway. This indicates a link between capsid SUMOylation and SUMOylation of cellular proteins in restricting gene transduction. Infection with AAV triggers general SUMOylation of cellular proteins. In particular, the DAXX protein, a putative host cell restriction factor that can become SUMOylated, is able to restrict AAV gene transduction by reducing the intracellular accumulation of AAV vectors. We also observe that the coexpression of a SUMOylation inhibitor with an AAV2 reporter gene vector increased gene transduction significantly.IMPORTANCE Host factors within the cell are the major mode of restriction of adeno-associated virus (AAV) and keep it from fulfilling its maximum potential as a gene therapy vector. A better understanding of the intricacies of restriction would enable the engineering of better vectors. Via a genome-wide short interfering RNA screen, we identified that proteins of the small ubiquitin-like modifier (SUMO) pathway play an important role in AAV restriction. In this study, we investigate whether this restriction is targeted to the AAV directly or indirectly through host cell factors. The results indicate that both targets act in concert to restrict AAV.

Cullin-RING Ligase 5: Functional characterization and its role in human cancers

Cullin-RING ligase 5 (CRL5) is a multi-protein complex and consists of a scaffold protien cullin 5, a RING protein RBX2 (also known as ROC2 or SAG), adaptor proteins Elongin B/C, and a substrate receptor protein SOCS. Through targeting a variety of substrates for proteasomal degradation or modulating various protein-protein interactions, CRL5 is involved in regulation of many biological processes, such as cytokine signal transduction, inflammation, viral infection, and oncogenesis. As many substrates of CRL5 are well-known oncoproteins or tumor suppressors, abnormal regulation of CRL5 is commonly found in human cancers. In this review, we first briefly introduce each of CRL5 components, and then discuss the biological processes regulated by four members of SOCS-box-containing substrate receptor family through substrate degradation. We next describe how CRL5 is hijacked by a variety of viral proteins to degrade host anti-viral proteins, which facilitates virus infection. We further discuss the regulation of CUL5 and its various roles in human cancers, acting as either a tumor suppressor or an oncoprotein in a context-dependent manner. Finally, we propose novel insights for future perspectives on the validation of cullin5 and other CRL5 components as potential targets, and possible targeting strategies to discover CRL5 inhibitors for anti-cancer and anti-virus therapies.

A deficiency in SUMOylation activity disrupts multiple pathways leading to neural tube and heart defects in Xenopus embryos

Background

Adenovirus protein, Gam1, triggers the proteolytic destruction of the E1 SUMO-activating enzyme. Microinjection of an empirically determined amount of Gam1 mRNA into one-cell Xenopus embryos can reduce SUMOylation activity to undetectable, but nonlethal, levels, enabling an examination of the role of this post-translational modification during early vertebrate development.

Results

We find that SUMOylation-deficient embryos consistently exhibit defects in neural tube and heart development. We have measured differences in gene expression between control and embryos injected with Gam1 mRNA at three developmental stages: early gastrula (immediately following the initiation of zygotic transcription), late gastrula (completion of the formation of the three primary germ layers), and early neurula (appearance of the neural plate). Although changes in gene expression are widespread and can be linked to many biological processes, three pathways, non-canonical Wnt/PCP, snail/twist, and Ets-1, are especially sensitive to the loss of SUMOylation activity and can largely account for the predominant phenotypes of Gam1 embryos. SUMOylation appears to generate different pools of a given transcription factor having different specificities with this post-translational modification involved in the regulation of more complex, as opposed to housekeeping, processes.

Conclusions

We have identified changes in gene expression that underlie the neural tube and heart phenotypes resulting from depressed SUMOylation activity. Notably, these developmental defects correspond to the two most frequently occurring congenital birth defects in humans, strongly suggesting that perturbation of SUMOylation, either globally or of a specific protein, may frequently be the origin of these pathologies.

Electronic supplementary material

The online version of this article (10.1186/s12864-019-5773-3) contains supplementary material, which is available to authorized users.

Shigella entry unveils a calcium/calpain-dependent mechanism for inhibiting sumoylation

Disruption of the sumoylation/desumoylation equilibrium is associated with several disease states such as cancer and infections, however the mechanisms regulating the global SUMO balance remain poorly defined. Here, we show that infection by Shigella flexneri, the causative agent of human bacillary dysentery, switches off host sumoylation during epithelial cell infection in vitro and in vivo and that this effect is mainly mediated by a calcium/calpain-induced cleavage of the SUMO E1 enzyme SAE2, thus leading to sumoylation inhibition. Furthermore, we describe a mechanism by which Shigella promotes its own invasion by altering the sumoylation state of RhoGDIα, a master negative regulator of RhoGTPase activity and actin polymerization. Together, our data suggest that SUMO modification is essential to restrain pathogenic bacterial entry by limiting cytoskeletal rearrangement induced by bacterial effectors. Moreover, these findings identify calcium-activated calpains as powerful modulators of cellular sumoylation levels with potentially broad implications in several physiological and pathological situations.

Viral Interplay with the Host Sumoylation System

Viruses have evolved elaborate means to regulate diverse cellular pathways in order to create a cellular environment that facilitates viral survival and reproduction. This includes enhancing viral macromolecular synthesis and assembly, as well as preventing antiviral responses, including intrinsic, innate, and adaptive immunity. There are numerous mechanisms by which viruses mediate their effects on the host cell, and this includes targeting various cellular post-translational modification systems, including sumoylation. The wide-ranging impact of sumoylation on cellular processes such as transcriptional regulation, apoptosis, stress response, and cell cycle control makes it an attractive target for viral dysregulation. To date, proteins from both RNA and DNA virus families have been shown to be modified by SUMO conjugation, and this modification appears critical for viral protein function. More interestingly, members of the several viral families have been shown to modulate sumoylation, including papillomaviruses, adenoviruses, herpesviruses, orthomyxoviruses, filoviruses, and picornaviruses. This chapter will focus on mechanisms by which sumoylation both impacts human viruses and is used by viruses to promote viral infection and disease.

Viral manipulation of the cellular sumoylation machinery

Viruses exploit various cellular processes for their own benefit, including counteracting anti-viral responses and regulating viral replication and propagation. In the past 20 years, protein sumoylation has emerged as an important post-translational modification that is manipulated by viruses to modulate anti-viral responses, viral replication, and viral pathogenesis. The process of sumoylation is a multi-step cascade where a small ubiquitin-like modifier (SUMO) is covalently attached to a conserved ΨKxD/E motif within a target protein, altering the function of the modified protein. Here we review how viruses manipulate the cellular machinery at each step of the sumoylation process to favor viral survival and pathogenesis.

SUMOylation Inhibition Mediated by Disruption of SUMO E1-E2 Interactions Confers Plant Susceptibility to Necrotrophic Fungal Pathogens

Protein modification by SUMO modulates essential biological processes in eukaryotes. SUMOylation is facilitated by sequential action of the E1-activating, E2-conjugating, and E3-ligase enzymes. In plants, SUMO regulates plant development and stress responses, which are key determinants in agricultural productivity. To generate additional tools for advancing our knowledge about the SUMO biology, we have developed a strategy for inhibiting in vivo SUMO conjugation based on disruption of SUMO E1-E2 interactions through expression of E1 SAE2^UFDCt domain. Targeted mutagenesis and phylogenetic analyses revealed that this inhibition involves a short motif in SAE2^UFDCt highly divergent across kingdoms. Transgenic plants expressing the SAE2^UFDCt domain displayed dose-dependent inhibition of SUMO conjugation, and have revealed the existence of a post-transcriptional mechanism that regulates SUMO E2 conjugating enzyme levels. Interestingly, these transgenic plants displayed increased susceptibility to necrotrophic fungal infections by Botrytis cinerea and Plectosphaerella cucumerina. Early after fungal inoculation, host SUMO conjugation was post-transcriptionally downregulated, suggesting that targeting SUMOylation machinery could constitute a novel mechanism for fungal pathogenicity. These findings support the role of SUMOylation as a mechanism involved in plant protection from environmental stresses. In addition, the strategy for inhibiting SUMO conjugation in vivo described in this study might be applicable in important crop plants and other non-plant organisms regardless of their genetic complexity.

The role of cullin 5-containing ubiquitin ligases

The suppressor of cytokine signaling (SOCS) box consists of the BC box and the cullin 5 (Cul5) box, which interact with Elongin BC and Cul5, respectively. SOCS box-containing proteins have ubiquitin ligase activity mediated by the formation of a complex with the scaffold protein Cul5 and the RING domain protein Rbx2, and are thereby members of the cullin RING ligase superfamily. Cul5-type ubiquitin ligases have a variety of substrates that are targeted for polyubiquitination and proteasomal degradation. Here, we review the current knowledge on the identification of Cul5 and the regulation of its expression, as well as the signaling pathways regulated by Cul5 and how viruses highjack the Cul5 system to overcome antiviral responses.

Activating transcription factor 3 SUMOylation is involved in angiotensin II-induced endothelial cell inflammation and dysfunction

Activating transcription factor 3 (ATF3) is an adaptive-response protein induced by various environmental stresses and is implicated in the pathogenesis of many disease states. However, the role of ATF3 SUMOylation in hypertension-induced vascular injury remains poorly understood. Here we investigated the function of ATF3 SUMOylation in vascular endothelial cells (ECs). The expression of ATF3 and small ubiquitin-like modifier 1 (SUMO1) was increased in angiotensin II (Ang II)-induced human umbilical vein endothelial cells (HUVECs). Microscopic analyses further revealed that the expression of ATF3 and SUMO1 is upregulated and colocalized in the endothelium of thoracic aortas from Ang II-induced hypertensive mice. However, Ang II-induced upregulation of ATF3 and SUMO1 in vitro and in vivo was blocked by Ang II type I receptor antagonist olmesartan. Moreover, Ang II induced ATF3 SUMOylation at lysine 42, which is SUMO1 dependent. ATF3 SUMOylation attenuated ATF3 ubiquitination and in turn promoted ATF3 protein stability. ATF3 or SUMO1 knockdown inhibited Ang II-induced expression of inflammatory molecules such as tumor necrosis factor (TNF)-α, interleukin (IL)-6 and IL-8. Wild type ATF3 but not ATF3-K42R (SUMOylation defective mutant) reduced the production of nitric oxide (NO), a key indicator of EC function. Consistently, ginkgolic acid, an inhibitor of SUMOylation, increased NO production in HUVECs and significantly improved vasodilatation of aorta from Ang II-induced hypertensive mice. Our findings demonstrated that ATF3 SUMOylation is involved in Ang II-induced EC inflammation and dysfunction in vitro and in vivo through inhibiting ATF3 ubiquitination and increasing ATF3 protein stability.

Socs36E limits STAT signaling via Cullin2 and a SOCS-box independent mechanism in the Drosophila egg chamber

The Suppressor of Cytokine Signaling (SOCS) proteins are critical, highly conserved feedback inhibitors of signal transduction cascades. The family of SOCS proteins is divided into two groups: ancestral and vertebrate-specific SOCS proteins. Vertebrate-specific SOCS proteins have been heavily studied as a result of their strong mutant phenotypes. However, the ancestral clade remains less studied, a potential result of genetic redundancies in mammals. Use of the genetically tractable organism Drosophila melanogaster enables in vivo assessment of signaling components and mechanisms with less concern about the functional redundancy observed in mammals. In this study, we investigated how the SOCS family member Suppressor of Cytokine Signaling at 36E (Socs36E) attenuates Janus Kinase/Signal Transducer and Activator of Transcription (Jak/STAT) activation during specification of motile border cells in Drosophila oogenesis. We found that Socs36E genetically interacts with the Cullin2 (Cul2) scaffolding protein. Like Socs36E, Cul2 is required to limit the number of motile cells in egg chambers. We demonstrated that loss of Cul2 in the follicle cells significantly increased nuclear STAT protein levels, which resulted in additional cells acquiring invasive properties. Further, reduction of Cul2 suppressed border cell migration defects that occur in a Stat92E-sensitized genetic background. Our data incorporated Cul2 into a previously described Jak/STAT-directed genetic regulatory network that is required to generate a discrete boundary between cell fates. We also found that Socs36E is able to attenuate STAT activity in the egg chamber when it does not have a functional SOCS box. Collectively, this work contributes mechanistic insight to a Jak/STAT regulatory genetic circuit, and suggests that Socs36E regulates Jak/STAT signaling via a Cul2-dependent mechanism, as well as by a Cullin-independent manner, in vivo.

Sumo and the cellular stress response

The ubiquitin family member Sumo has important functions in many cellular processes including DNA repair, transcription and cell division. Numerous studies have shown that Sumo is essential for maintaining cell homeostasis when the cell encounters endogenous or environmental stress, such as osmotic stress, hypoxia, heat shock, genotoxic stress, and nutrient stress. Regulation of transcription is a key component of the Sumo stress response, and multiple mechanisms have been described by which Sumo can regulate transcription. Although many individual substrates have been described that are sumoylated during the Sumo stress response, an emerging concept is modification of entire complexes or pathways by Sumo. This review focuses on the function and regulation of Sumo during the stress response.

The SUMO conjugating enzyme UBC9 as a biomarker for cervical HPV infections

Human papillomaviruses (HPVs) infect stratified epithelium and are the causative agents of cervical cancer, the second most common cause of cancer-related death in women. A critical aspect that still persists in the HPV field is the selection of very sensitive and specific HPV diagnostic assays. Here, we provide evidence that the crucial small ubiquitin-like modifier (SUMO) E2-conjugating enzyme Ubc9 is strongly upregulated in cervical lesions. Ubc9 detection could thus be used in diagnosing and/or monitoring the progression of an HPV oncogenic infection.

Identification of sumoylated proteins in the silkworm Bombyx mori

Small ubiquitin-like modifier (SUMO) modification (SUMOylation) is an important and widely used reversible modification system in eukaryotic cells. It regulates various cell processes, including protein targeting, transcriptional regulation, signal transduction, and cell division. To understand its role in the model lepidoptera insect Bombyx mori, a recombinant baculovirus was constructed to express an enhanced green fluorescent protein (eGFP)-SUMO fusion protein along with ubiquitin carrier protein 9 of Bombyx mori (BmUBC9). SUMOylation substrates from Bombyx mori cells infected with this baculovirus were isolated by immunoprecipitation and identified by LC-ESI-MS/MS. A total of 68 candidate SUMOylated proteins were identified, of which 59 proteins were functionally categorized to gene ontology (GO) terms. Analysis of kyoto encyclopedia of genes and genomes (KEGG) pathways showed that 46 of the identified proteins were involved in 76 pathways that mainly play a role in metabolism, spliceosome and ribosome functions, and in RNA transport. Furthermore, SUMOylation of four candidates (polyubiquitin-C-like isoform X1, 3-hydroxyacyl-CoA dehydrogenase, cyclin-related protein FAM58A-like and GTP-binding nuclear protein Ran) were verified by co-immunoprecipitation in Drosophila schneide 2 cells. In addition, 74% of the identified proteins were predicted to have at least one SUMOylation site. The data presented here shed light on the crucial process of protein sumoylation in Bombyx mori.

SUMO-mimicking peptides inhibiting protein SUMOylation

The ubiquitin-like protein SUMO is transferred through a core E1-E2 cascade composed of the SUMO-activating enzyme (SAE) and Ubc9 to modify cellular proteins and transmit important biological signals. SAE primarily recognizes the C-terminal tail of SUMO and catalyzes ATP condensation with the SUMO C-terminal carboxylate to activate its transfer through the cascade. Here, we used phage display to show that a broad profile of SUMO C-terminal sequences could be activated by SAE. Based on this, we developed heptamer peptides that could 1) form thioester conjugates with SAE, 2) be transferred from SAE to Ubc9, and 3) be further transferred to the SUMOylation target protein RanGAP1. As these peptides recapitulate the action of SUMO in protein modification, we refer to them as "SUMO-mimicking peptides". We found that, once the peptides were conjugated to SAE and Ubc9, they blocked full-length SUMO from entering the cascade. These peptides can thus function as mechanism-based inhibitors of the protein SUMOylation reaction.

Small ubiquitin-like modifier (SUMO)-mediated repression of the Xenopus Oocyte 5 S rRNA genes

The 5 S rRNA gene-specific transcription factor IIIA (TFIIIA) interacts with the small ubiquitin-like modifier (SUMO) E3 ligase PIAS2b and with one of its targets, the transcriptional corepressor, XCtBP. PIAS2b is restricted to the cytoplasm of Xenopus oocytes but relocates to the nucleus immediately after fertilization. Following the midblastula transition, PIAS2b and XCtBP are present on oocyte-type, but not somatic-type, 5 S rRNA genes up through the neurula stage, as is a limiting amount of TFIIIA. Histone H3 methylation, coincident with the binding of XCtBP, also occurs exclusively on the oocyte-type genes. Immunohistochemical staining of embryos confirms the occupancy of a subset of the oocyte-type genes by TFIIIA that become positioned at the nuclear periphery shortly after the midblastula transition. Inhibition of SUMOylation activity relieves repression of oocyte-type 5 S rRNA genes and is correlated with a decrease in methylation of H3K9 and H3K27 and disruption of subnuclear localization. These results reveal a novel function for TFIIIA as a negative regulator that recruits histone modification activity through the CtBP repressor complex exclusively to the oocyte-type 5 S rRNA genes, leading to their terminal repression.

Expression and in vitro functional analyses of recombinant Gam1 protein

Gam1, an early gene product of an avian adenovirus, is essential for viral replication. Gam1 is the first viral protein found to globally inhibit cellular SUMOylation, a critical posttranslational modification that alters the function and cellular localization of proteins. The interaction details at the interface between Gam1 and its cellular targets remain unclear due to the lack of structural information. Although Gam1 has been previously characterized, the purity of the protein was not suitable for structural investigations. In the present study, the gene of Gam1 was cloned and expressed in various bacterial expression systems to obtain pure and soluble recombinant Gam1 protein for in vitro functional and structural studies. While Gam1 was insoluble in most expression systems tested, it became soluble when it was expressed as a fusion protein with trigger factor (TF), a ribosome associated bacterial chaperone, under the control of a cold shock promoter. Careful optimization indicates that both low temperature induction and the chaperone function of TF play critical roles in increasing Gam1 solubility. Soluble Gam1 was purified to homogeneity through sequential chromatography techniques. Monomeric Gam1 was obtained via size exclusion chromatography and analyzed by dynamic light scattering. The SUMOylation inhibitory function of the purified Gam1 was confirmed in an in vitro assay. These results have built the foundation for further structural investigations that will broaden our understanding of Gam1's roles in viral replication.

Cullin E3 ligases and their rewiring by viral factors

The ability of viruses to subvert host pathways is central in disease pathogenesis. Over the past decade, a critical role for the Ubiquitin Proteasome System (UPS) in counteracting host immune factors during viral infection has emerged. This counteraction is commonly achieved by the expression of viral proteins capable of sequestering host ubiquitin E3 ligases and their regulators. In particular, many viruses hijack members of the Cullin-RING E3 Ligase (CRL) family. Viruses interact in many ways with CRLs in order to impact their ligase activity; one key recurring interaction involves re-directing CRL complexes to degrade host targets that are otherwise not degraded within host cells. Removal of host immune factors by this mechanism creates a more amenable cellular environment for viral propagation. To date, a small number of target host factors have been identified, many of which are degraded via a CRL-proteasome pathway. Substantial effort within the field is ongoing to uncover the identities of further host proteins targeted in this fashion and the underlying mechanisms driving their turnover by the UPS. Elucidation of these targets and mechanisms will provide appealing anti-viral therapeutic opportunities. This review is focused on the many methods used by viruses to perturb host CRLs, focusing on substrate sequestration and viral regulation of E3 activity.

Sumoylation controls host anti-bacterial response to the gut invasive pathogen Shigella flexneri

Shigella flexneri, the etiological agent of bacillary dysentery, invades the human colonic epithelium and causes its massive inflammatory destruction. Little is known about the post-translational modifications implicated in regulating the host defense pathway against Shigella. Here, we show that SUMO-2 impairs Shigella invasion of epithelial cells in vitro. Using mice haploinsufficient for the SUMO E2 enzyme, we found that sumoylation regulates intestinal permeability and is required to restrict epithelial invasion and control mucosal inflammation. Quantitative proteomics reveals that Shigella infection alters the sumoylation status of a restricted set of transcriptional regulators involved in intestinal functions and inflammation. Consistent with this, sumoylation restricts the pro-inflammatory transcriptional response of Shigella-infected guts. Altogether, our results show that the SUMO pathway is an essential component of host innate protection, as it reduces the efficiency of two key steps of shigellosis: invasion and inflammatory destruction of the intestinal epithelium.

Analysis of human cytomegalovirus-encoded SUMO targets and temporal regulation of SUMOylation of the immediate-early proteins IE1 and IE2 during infection

Post-translational modification of proteins by members of the small ubiquitin-like modifier (SUMO) is involved in diverse cellular functions. Many viral proteins are SUMO targets and also interact with the cellular SUMOylation system. During human cytomegalovirus (HCMV) infection, the immediate-early (IE) proteins IE1 and IE2 are covalently modified by SUMO. IE2 SUMOylation promotes its transactivation activity, whereas the role of IE1 SUMOylation is not clear. We performed in silico, genome-wide analysis to identify possible SUMOylation sites in HCMV-encoded proteins and evaluated their modification using the E. coli SUMOylation system and in vitro assays. We found that only IE1 and IE2 are substantially modified by SUMO in E. coli, although US34A was also identified as a possible SUMO target in vitro. We also found that SUMOylation of IE1 and IE2 is temporally regulated during viral infection. Levels of SUMO-modified form of IE1 were increased during the early phase of infection, but decreased in the late phase when IE2 and its SUMO-modified forms were expressed at high levels. IE2 expression inhibited IE1 SUMOylation in cotransfection assays. As in IE2 SUMOylation, PIAS1, a SUMO E3 ligase, interacted with IE1 and enhanced IE1 SUMOylation. In in vitro assays, an IE2 fragment that lacked covalent and non-covalent SUMO attachment sites, but was sufficient for PIAS1 binding, effectively inhibited PIAS1-mediated SUMOylation of IE1, indicating that IE2 expression negatively regulates IE1 SUMOylation. We also found that the IE2-mediated downregulation of IE1 SUMOylation correlates with the IE1 activity to repress the promoter containing the interferon stimulated response elements. Taken together, our data demonstrate that IE1 and IE2 are the main viral SUMO targets in HCMV infection and that temporal regulation of their SUMOylation may be important in the progression of this infection.

The chicken adenovirus Gam1 protein, an inhibitor of the sumoylation pathway, partially complements ICP0-null mutant herpes simplex virus 1

Herpes simplex virus 1 (HSV-1) regulatory protein ICP0 stimulates efficient infection via its E3 ubiquitin ligase activity that causes degradation of several cellular proteins, some of which are sumoylated. Chicken adenovirus Gam1 protein also interferes with the sumoylation pathway, and both proteins disrupt promyelocytic leukemia protein (PML) nuclear bodies (NBs). We report that Gam1 increases the infection efficiency of ICP0-null mutant HSV-1 by approximately 100-fold, thus strengthening the hypothesis that PML NB- and sumoylation-related mechanisms are important factors in the control of HSV-1 infection.

Analysis of the Cullin binding sites of the E4orf6 proteins of human adenovirus E3 ubiquitin ligases

E4orf6 proteins of human adenoviruses form Cullin-based E3 ubiquitin ligase complexes that degrade cellular proteins, which impedes efficient viral replication. These complexes also include the viral E1B55K product, which is believed to recruit most substrates for ubiquitination. Heterogeneity in the composition of these ligases exists, as serotypes representing some species form Cul5-based complexes (species B2, C, D, and E), whereas others utilize Cul2 (species A and F). Adenovirus type 16 (Ad16; species B1) binds significant levels of both. In this report, we show that the Cul2 binding sequence in E4orf6 of Ad12 (species A) and Ad40 (species F) resembles the cellular consensus Cul2 box. Mutation within this Cul2 box prevents binding not only of Cul2 but also in some cases Elongin C and reduces the ability to degrade target proteins, such as Mre11 and p53. A comparable Cul2 box is not present in E4orf6 of Ad5 and other serotypes that bind Cul5; however, creation of this Cul2 box sequence in Ad5 E4orf6 promoted binding to Cul2 and Cul2-dependent degradation of Mre11. E4orf6 of Ad16 also binds Cul2; however, unlike Ad40, it does not contain an Ad12-like Cul2 box, suggesting that Ad16 binds Cul2 in a unique but perhaps nonfunctional manner, as only Cul5 binding complexes appeared able to degrade Mre11. Our extensive analyses have thus far failed to identify a consensus Cul5 binding sequence, suggesting that association occurs via a novel and perhaps complex pattern of protein-protein interactions. Nevertheless, the identification of the Cul2 box may allow prediction of Cullin specificity for all E4orf6-containing Adenoviridae.

IMPORTANCE

The work described in this paper is a continuation of our in-depth studies on the Cullin-based E3 ligase complexes formed by the viral E4orf6 and E1B55K proteins of all human adenoviruses. This complex induces the degradation of a growing series of cellular proteins that impede efficient viral replication. Some human adenovirus species utilize Cul5, whereas others bind Cul2. In this paper, we are the first to identify the E4orf6 Cul2 binding site, which conforms in sequence to a classic cellular Cul2 box. Ours is the first detailed biochemical and genetic analysis of a Cul2-based adenovirus ligase and provides insights into both the cooperative interactions in forming Cullin-based ligases as well as the universality of formation of all adenovirus ligase complexes. Our work now permits future analysis of the evolutionary significance of the ligase complex, work that is currently in progress in our lab.

SUMO Ubc9 enzyme as a viral target

Viruses alter specific host cell targets to counteract possible defense mechanisms aimed at eliminating infectivity and viral propagation. The SUMO conjugating enzyme Ubc9 functions as a hub for protein sumoylation, whilst also providing an interactive surface for sumoylated proteins through noncovalent interactions. The targeting of Ubc9 by viruses and viral proteins is thus highly beneficial for the disruption of both protein modification and protein-protein interaction mechanisms with which proteins increase their functional repertoire in cells. This review explores some of the clever mechanisms adopted by viruses to deregulate Ubc9, influence effector pathways and positively impact viral persistence consequently.

Sumoylation: a regulatory protein modification in health and disease

Posttranslational modification with small ubiquitin-related modifier (SUMO) proteins is now established as one of the key regulatory protein modifications in eukaryotic cells. Hundreds of proteins involved in processes such as chromatin organization, transcription, DNA repair, macromolecular assembly, protein homeostasis, trafficking, and signal transduction are subject to reversible sumoylation. Hence, it is not surprising that disease links are beginning to emerge and that interference with sumoylation is being considered for intervention. Here, we summarize basic mechanisms and highlight recent developments in the physiology of sumoylation.

BC-box protein domain-related mechanism for VHL protein degradation

The tumor suppressor VHL (von Hippel-Lindau) protein is a substrate receptor for Ubiquitin Cullin Ring Ligase complexes (CRLs), containing a BC-box domain that associates to the adaptor Elongin B/C. VHL targets hypoxia-inducible factor 1α to proteasome-dependent degradation. Gam1 is an adenoviral protein, which also possesses a BC-box domain that interacts with the host Elongin B/C, thereby acting as a viral substrate receptor. Gam1 associates with both Cullin2 and Cullin5 to form CRL complexes targeting the host protein SUMO enzyme SAE1 for proteasomal degradation. We show that Gam1 protein expression induces VHL protein degradation leading to hypoxia-inducible factor 1α stabilization and induction of its downstream targets. We also characterize the CRL-dependent mechanism that drives VHL protein degradation via proteasome. Interestingly, expression of Suppressor of Cytokine Signaling (SOCS) domain-containing viral proteins and cellular BC-box proteins leads to VHL protein degradation, in a SOCS domain-containing manner. Our work underscores the exquisite ability of viral domains to uncover new regulatory mechanisms by hijacking key cellular proteins.

SUMO rules: regulatory concepts and their implication in neurologic functions

Posttranslational modification of proteins by the small ubiquitin-like modifier (SUMO) is a potent regulator of various cellular events. Hundreds of substrates have been identified, many of them involved in vital processes like transcriptional regulation, signal transduction, protein degradation, cell cycle regulation, DNA repair, chromatin organization, and nuclear transport. In recent years, protein sumoylation increasingly attracted attention, as it could be linked to heart failure, cancer, and neurodegeneration. However, underlying mechanisms involving how modification by SUMO contributes to disease development are still scarce thus necessitating further research. This review aims to critically discuss currently available concepts of the SUMO pathway, thereby highlighting regulation in the healthy versus diseased organism, focusing on neurologic aspects. Better understanding of differential regulation in health and disease may finally allow to uncover pathogenic mechanisms and contribute to the development of disease-specific therapies.

Kaposi's sarcoma-associated herpesvirus K-Rta exhibits SUMO-targeting ubiquitin ligase (STUbL) like activity and is essential for viral reactivation

The small ubiquitin-like modifier (SUMO) is a protein that regulates a wide variety of cellular processes by covalent attachment of SUMO moieties to a diverse array of target proteins. Sumoylation also plays an important role in the replication of many viruses. Previously, we showed that Kaposi's sarcoma-associated herpesvirus (KSHV) encodes a SUMO-ligase, K-bZIP, which catalyzes sumoylation of host and viral proteins. We report here that this virus also encodes a gene that functions as a SUMO-targeting ubiquitin-ligase (STUbL) which preferentially targets sumoylated proteins for degradation. K-Rta, the major transcriptional factor which turns on the entire lytic cycle, was recently found to have ubiquitin ligase activity toward a selected set of substrates. We show in this study that K-Rta contains multiple SIMs (SUMO interacting motif) and binds SUMOs with higher affinity toward SUMO-multimers. Like RNF4, the prototypic cellular STUbL, K-Rta degrades SUMO-2/3 and SUMO-2/3 modified proteins, including promyelocytic leukemia (PML) and K-bZIP. PML-NBs (nuclear bodies) or ND-10 are storage warehouses for sumoylated proteins, which negatively regulate herpesvirus infection, as part of the intrinsic immune response. Herpesviruses have evolved different ways to degrade or disperse PML bodies, and KSHV utilizes K-Rta to inhibit PML-NBs formation. This process depends on K-Rta's ability to bind SUMO, as a K-Rta SIM mutant does not effectively degrade PML. Mutations in the K-Rta Ring finger-like domain or SIM significantly inhibited K-Rta transactivation activity in reporter assays and in the course of viral reactivation. Finally, KSHV with a mutation in the Ring finger-like domain or SIM of K-Rta replicates poorly in culture, indicating that reducing SUMO-conjugates in host cells is important for viral replication. To our knowledge, this is the first virus which encodes both a SUMO ligase and a SUMO-targeting ubiquitin ligase that together may generate unique gene regulatory programs.

Viral manipulation of cellular protein conjugation pathways: The SUMO lesson

Small ubiquitin-like modifier (SUMO)ylation is a key post-translational modification mechanism that controls the function of a plethora of proteins and biological processes. Given its central regulatory role, it is not surprising that it is widely exploited by viruses. A number of viral proteins are known to modify and/or be modified by the SUMOylation system to exert their function, to create a cellular environment more favorable for virus survival and propagation, and to prevent host antiviral responses. Since the SUMO pathway is a multi-step cascade, viral proteins engage with it at many levels, to advance and favor each stage of a typical infection cycle: replication, viral assembly and immune evasion. Here we review the current knowledge on the interplay between the host SUMO system and viral lifecycle.

Interplay between viruses and host sumoylation pathways

Post-translational modification by members of the small ubiquitin-like modifier (SUMO) family of proteins is important for the regulation of many cellular proteins and pathways. As obligate parasites, viruses must engage with the host cell throughout their replication cycles, and it is therefore unsurprising that there are many examples of interplay between viral proteins and the host sumoylation system. This article reviews recent advances in this field, summarizing information on sumoylated viral proteins, the varied ways in which viruses engage with SUMO-related pathways, and the consequences of these interactions for viral replication and engagement with innate and intrinsic immunity.

New insights into the role of the small ubiquitin-like modifier (SUMO) in plants

Small ubiquitin-like modifier (SUMO) is a small (∼12kDa) protein that occurs in all eukaryotes and participates in the reversible posttranslational modification of target cellular proteins. The three-dimensional structure of SUMO and ubiquitin (Ub) are superimposable although there is very little similarity in their primary amino acid sequences. In all organisms, conjugation and deconjugation of Ub and SUMO proceed by the same reactions while using pathway-specific enzymes. SUMO conjugation in plants is a part of the controls governing important biological processes such as growth, development, flowering, environmental (abiotic) stress responses, and response to pathogen infection. Most of the evidence for this comes from genetic analyses. Recent efforts to dissect the function of sumoylation have focused on uncovering targets of SUMO conjugation by using either a yeast two-hybrid screen employing components of the SUMO cycle as bait or by using affinity purification of SUMO-conjugated proteins followed by identification of these proteins by mass spectrometry. This chapter reviews the current knowledge regarding sumoylation in plants, with special focus on the model plant Arabidopsis thaliana.

Small heat shock proteins target mutant cystic fibrosis transmembrane conductance regulator for degradation via a small ubiquitin-like modifier-dependent pathway

Selective degradation of the mutant protein responsible for most cystic fibrosis, F508del cystic fibrosis transmembrane conductance regulator (CFTR), is initiated by Hsp27, which associates with the small ubiquitin-like modifier (SUMO) E2, Ubc9. They modify F508del with SUMO-2/3, directing F508del to a SUMO-targeted ubiquitin ligase, RNF4. This work implicates SUMO and RNF4 in quality control of a cytosolic transmembrane protein.

Small heat shock proteins (sHsps) bind destabilized proteins during cell stress and disease, but their physiological functions are less clear. We evaluated the impact of Hsp27, an sHsp expressed in airway epithelial cells, on the common protein misfolding mutant that is responsible for most cystic fibrosis. F508del cystic fibrosis transmembrane conductance regulator (CFTR), a well-studied protein that is subject to cytosolic quality control, selectively associated with Hsp27, whose overexpression preferentially targeted mutant CFTR to proteasomal degradation. Hsp27 interacted physically with Ubc9, the small ubiquitin-like modifier (SUMO) E2 conjugating enzyme, implying that F508del SUMOylation leads to its sHsp-mediated degradation. Enhancing or disabling the SUMO pathway increased or blocked Hsp27’s ability to degrade mutant CFTR. Hsp27 promoted selective SUMOylation of F508del NBD1 in vitro and of full-length F508del CFTR in vivo, which preferred endogenous SUMO-2/3 paralogues that form poly-chains. The SUMO-targeted ubiquitin ligase (STUbL) RNF4 recognizes poly-SUMO chains to facilitate nuclear protein degradation. RNF4 overexpression elicited F508del degradation, whereas Hsp27 knockdown blocked RNF4’s impact on mutant CFTR. Similarly, the ability of Hsp27 to degrade F508del CFTR was lost during overexpression of dominant-negative RNF4. These findings link sHsp-mediated F508del CFTR degradation to its SUMOylation and to STUbL-mediated targeting to the ubiquitin–proteasome system and thereby implicate this pathway in the disposal of an integral membrane protein.

Contribution of SUMO-interacting motifs and SUMOylation to the antiretroviral properties of TRIM5α

Recent findings suggested that the SUMO-interacting motifs (SIMs) present in the human TRIM5α (TRIM5α(hu)) protein play an important role in the ability of TRIM5α(hu) to restrict N-MLV. Here we explored the role of SIMs in the ability of rhesus TRIM5α (TRIM5α(rh)) to restrict HIV-1, and found that TRIM5α(rh) SIM mutants IL376KK (SIM1mut) and VI405KK (SIM2mut) completely lost their ability to block HIV-1 infection. Interestingly, these mutants also lost the recently described property of TRIM5α(rh) to shuttle into the nucleus. Analysis of these variants revealed that they are unable to interact with the HIV-1 core, which might explain the reason that these variants are not active against HIV-1. Furthermore, NMR titration experiments to assay the binding between the PRYSPRY domain of TRIM5α(rh) and the small ubiquitin-like modifier 1(SUMO-1) revealed no interaction. In addition, we examined the role of SUMOylation in restriction, and find out that inhibition of SUMOylation by the adenoviral protein Gam1 did not alter the retroviral restriction ability of TRIM5α. Overall, our results do not support a role for SIMs or SUMOylation in the antiviral properties of TRIM5α.

Sumoylation at the host-pathogen interface

Many viral proteins have been shown to be sumoylated with corresponding regulatory effects on their protein function, indicating that this host cell modification process is widely exploited by viral pathogens to control viral activity. In addition to using sumoylation to regulate their own proteins, several viral pathogens have been shown to modulate overall host sumoylation levels. Given the large number of cellular targets for SUMO addition and the breadth of critical cellular processes that are regulated via sumoylation, viral modulation of overall sumoylation presumably alters the cellular environment to ensure that it is favorable for viral reproduction and/or persistence. Like some viruses, certain bacterial plant pathogens also target the sumoylation system, usually decreasing sumoylation to disrupt host anti-pathogen responses. The recent demonstration that Listeria monocytogenes also disrupts host sumoylation, and that this is required for efficient infection, extends the plant pathogen observations to a human pathogen and suggests that pathogen modulation of host sumoylation may be more widespread than previously appreciated. This review will focus on recent aspects of how pathogens modulate the host sumoylation system and how this benefits the pathogen.

Post-Translational Modifications of Kaposi's Sarcoma-Associated Herpesvirus Regulatory Proteins - SUMO and KSHV

KSHV latency can be envisioned as an outcome that is balanced between factors that promote viral gene expression and lytic replication against those that facilitate gene silencing and establish or maintain latency. A large body of work has focused on the activities of the key viral regulatory proteins involved in KSHV latent or lytic states. Moreover, recent studies have also begun to document the importance of epigenetic landscape evolution of the KSHV viral genome during latency and reactivation. However, one area of KSHV molecular virology that remains largely unanswered is the precise role of post-translational modifications on the activities of viral factors that function during latency and reactivation. In this review, we will summarize the post-translational modifications associated with three viral factors whose activities contribute to the viral state. The viral proteins discussed are the two major KSHV encoded transcription factors, K-Rta (KSHV replication and transcriptional activator) and K-bZIP (KSHV basic leucine zipper) and the viral latency-associated nuclear antigen (LANA). A special emphasis will be placed on the role of the sumoylation pathway in the modulation of the KSHV lifecycle. Newly uncovered small ubiquitin-like modifier (SUMO)-associated properties of LANA and K-Rta will also be presented, namely LANA histone targeting SUMO E3 ligase activity and K-Rta SUMO-targeted ubiquitin ligase function.

Changes in the striatal proteome of YAC128Q mice exhibit gene-environment interactions between mutant huntingtin and manganese

Huntington's disease (HD) is a neurodegenerative disorder caused by expansion of a CAG repeat within the Huntingtin (HTT) gene, though the clinical presentation of disease and age-of-onset are strongly influenced by ill-defined environmental factors. We recently reported a gene-environment interaction wherein expression of mutant HTT is associated with neuroprotection against manganese (Mn) toxicity. Here, we are testing the hypothesis that this interaction may be manifested by altered protein expression patterns in striatum, a primary target of both neurodegeneration in HD and neurotoxicity of Mn. To this end, we compared striatal proteomes of wild-type and HD (YAC128Q) mice exposed to vehicle or Mn. Principal component analysis of proteomic data revealed that Mn exposure disrupted a segregation of WT versus mutant proteomes by the major principal component observed in vehicle-exposed mice. Identification of altered proteins revealed novel markers of Mn toxicity, particularly proteins involved in glycolysis, excitotoxicity, and cytoskeletal dynamics. In addition, YAC128Q-dependent changes suggest that axonal pathology may be an early feature in HD pathogenesis. Finally, for several proteins, genotype-specific responses to Mn were observed. These differences include increased sensitivity to exposure in YAC128Q mice (UBQLN1) and amelioration of some mutant HTT-induced alterations (SAE1, ENO1). We conclude that the interaction of Mn and mutant HTT may suppress proteomic phenotypes of YAC128Q mice, which could reveal potential targets in novel treatment strategies for HD.

Human pathogens and the host cell SUMOylation system

Since posttranslational modification (PTM) by the small ubiquitin-related modifiers (SUMOs) was discovered over a decade ago, a huge number of cellular proteins have been found to be reversibly modified, resulting in alteration of differential cellular pathways. Although the molecular consequences of SUMO attachment are difficult to predict, the underlying principle of SUMOylation is altering inter- and/or intramolecular interactions of the modified substrate, changing localization, stability, and/or activity. Unsurprisingly, many different pathogens have evolved to exploit the cellular SUMO modification system due to its functional flexibility and far-reaching functional downstream consequences. Although the extensive knowledge gained so far is impressive, a definitive conclusion about the role of SUMO modification during virus infection in general remains elusive and is still restricted to a few, yet promising concepts. Based on the available data, this review aims, first, to provide a detailed overview of the current state of knowledge and, second, to evaluate the currently known common principles/molecular mechanisms of how human pathogenic microbes, especially viruses and their regulatory proteins, exploit the host cell SUMO modification system.

HPV E6 proteins target Ubc9, the SUMO conjugating enzyme

The human papillomavirus oncogenic protein, E6, interacts with a number of cellular proteins, and for some targets, E6 directs their degradation through the ubiquitin-proteasome pathway. Post-translational modification with ubiquitin-like modifiers, such as SUMO, also influences protein activities, protein-protein interactions, and protein stability. We report that the high risk HPVE6 proteins reduce the intracellular quantity of the sole SUMO conjugation enzyme, Ubc9, concomitant with decreased host sumoylation. E6 did not significantly influence transcription of Ubc9, indicating that the effects were likely at the protein level. Consistent with typical E6-mediated proteasomal degradation, E6 bound to Ubc9 in vitro, and required E6AP for reduction of Ubc9 levels. Under stable E6 expression conditions in differentiating keratinocytes there was a decrease in Ubc9 and a loss of numerous sumoylated targets indicating a significant perturbation of the normal sumoylation profile. While E6 is known to inhibit PIASy, a SUMO ligase, our results suggest that HPV E6 also targets the Ubc9 protein to modulate host cell sumoylation, suggesting that the sumoylation system may be an important target during viral reproduction and possibly the subsequent development of cervical cancer.

The SUMO pathway: emerging mechanisms that shape specificity, conjugation and recognition

Proteins of the small ubiquitin-related modifier (SUMO) family are conjugated to proteins to regulate such cellular processes as nuclear transport, transcription, chromosome segregation and DNA repair. Recently, numerous insights into regulatory mechanisms of the SUMO modification pathway have emerged. Although SUMO-conjugating enzymes can discriminate between SUMO targets, many substrates possess characteristics that facilitate their modification. Other post-translational modifications also regulate SUMO conjugation, suggesting that SUMO signalling is integrated with other signal transduction pathways. A better understanding of SUMO regulatory mechanisms will lead to improved approaches for analysing the function of SUMO and substrate conjugation in distinct cellular pathways.