SUMO-1: wrestling with a new ubiquitin-related modifier

Apple SUMO E3 ligase MdSIZ1 regulates cuticular wax biosynthesis by SUMOylating transcription factor MdMYB30

A key function of SUMOylation is the coordinated modification of numerous proteins to optimize plant growth and resistance to environmental stress. Plant cuticular wax is deposited on the surface of primary plant organs to form a barrier that provides protection against changes in terrestrial environments. Many recent studies have examined cuticular wax biosynthetic pathways and regulation. However, whether SUMOylation is involved in the regulation of cuticle wax deposition at the posttranslational level remains unclear. Here, we demonstrate that a small ubiquitin-like modifier (SUMO) E3 ligase, SAP AND MIZ1 DOMAIN CONTAINING LIGASE1 (MdSIZ1), regulates wax accumulation and cuticle permeability in apple (Malus domestica Borkh), SUMO E2 CONJUGATING ENZYME 1(MdSCE1) physically interacts with MdMYB30, a transcription factor involved in the regulation of cuticle wax accumulation. MdSIZ1 mediates the SUMOylation and accumulation of MdMYB30 by inhibiting its degradation through the 26S proteasome pathway. Furthermore, MdMYB30 directly binds to the β-KETOACYL-COA SYNTHASE 1 (MdKCS1) promoter to activate its expression and promote wax biosynthesis. These findings indicate that the MdSIZ1-MdMYB30-MdKCS1 module positively regulates cuticular wax biosynthesis in apples. Overall, the findings of our study provide insights into the regulation pathways involved in cuticular wax biosynthesis.

Differential effects of SUMO1 and SUMO2 on circadian protein PER2 stability and function

Posttranslational modification (PTM) of core circadian clock proteins, including Period2 (PER2), is required for proper circadian regulation. PER2 function is regulated by casein kinase 1 (CK1)-mediated phosphorylation and ubiquitination but little is known about other PER2 PTMs or their interaction with PER2 phosphorylation. We found that PER2 can be SUMOylated by both SUMO1 and SUMO2; however, SUMO1 versus SUMO2 conjugation had different effects on PER2 turnover and transcriptional suppressor function. SUMO2 conjugation facilitated PER2 interaction with β-TrCP leading to PER2 proteasomal degradation. In contrast, SUMO1 conjugation, mediated by E3 SUMO-protein ligase RanBP2, enhanced CK1-mediated PER2S662 phosphorylation, inhibited PER2 degradation and increased PER2 transcriptional suppressor function. PER2 K736 was critical for both SUMO1- and SUMO2-conjugation. A PER2K736R mutation was sufficient to alter PER2 protein oscillation and reduce PER2-mediated transcriptional suppression. Together, our data revealed that SUMO1 versus SUMO2 conjugation acts as a determinant of PER2 stability and function and thereby affects the circadian regulatory system and the expression of clock-controlled genes.

In-frame fusion of SUMO1 enhances βarrestin2's association with activated GPCRs as well as with nuclear pore complexes

Small ubiquitin like modifier (SUMO) conjugation or SUMOylation of βarrestin2 promotes its association with the clathrin adaptor protein AP2 and facilitates rapid β₂ adrenergic receptor (β₂AR) internalization. However, disruption of the consensus SUMOylation site in βarrestin2, did not prevent βarrestin2's association with activated β₂ARs, dopamine D₂ receptors (D₂Rs), angiotensin type 1a receptors (AT_1aRs) and V₂ vasopressin receptors (V₂Rs). To address the role of SUMOylation in the trafficking of βarrestin and GPCR complexes, we generated and characterized a yellow fluorescent protein (YFP) tagged βarrestin2-SUMO1 chimeric protein, which is resistant to de-SUMOylation. In HEK-293 cells, YFP-SUMO1 predominantly localized in the nucleus, whereas YFP-βarrestin2 is cytoplasmic. YFP-βarrestin2-SUMO1 in addition to being cytoplasmic, is localized at the nuclear membrane. Nonetheless, βarrestin2-SUMO1 associated robustly with agonist-activated β₂ARs as evaluated by co-immunoprecipitation, confocal microscopy and bioluminescence resonance energy transfer (BRET). βarrestin2-SUMO1 associated strongly with the D₂R, which forms transient complexes with βarrestin2. But, βarrestin2-SUMO1 and βarrestin2 showed equivalent binding with the V₂R, which forms stable complexes with βarrestin2. βarrestin2 expression level directly correlated with the steady state levels of the unmodified form of RanGAP1, which upon SUMOylation associates with nuclear membrane. On the other hand, βarrestin2-SUMO1 not only localized at the nuclear membrane, but also formed a macromolecular complex with RanGAP1. Taken together, our data suggest that SUMOylation of βarrestin2 promotes its protein interactions at both cell and nuclear membranes. Furthermore, βarrestin2-SUMO1 presents as a useful tool to characterize βarrestin2 recruitment to GPCRs, which form transient and unstable complex with βarrestin2.

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.

Enrichments of post-translational modifications in proteomic studies

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

SUMOylation of Vps34 by SUMO1 promotes phenotypic switching of vascular smooth muscle cells by activating autophagy in pulmonary arterial hypertension

INTRODUCTION

Pulmonary arterial hypertension (PAH) is a life-threatening disease without effective therapies. PAH is associated with a progressive increase in pulmonary vascular resistance and irreversible pulmonary vascular remodeling. SUMO1 (small ubiquitin-related modifier 1) can bind to target proteins and lead to protein SUMOylation, an important post-translational modification with a key role in many diseases. However, the contribution of SUMO1 to PAH remains to be fully characterized.

METHODS

In this study, we explored the role of SUMO1 in the dedifferentiation of vascular smooth muscle cells (VSMCs) involved in hypoxia-induced pulmonary vascular remodeling and PAH in vivo and in vitro.

RESULTS

In a mouse model of hypoxic PAH, SUMO1 expression was significantly increased, which was associated with activation of autophagy (increased LC3b and decreased p62), dedifferentiation of pulmonary arterial VSMCs (reduced α-SMA, SM22 and SM-MHC), and pulmonary vascular remodeling. Similar results were obtained in a MCT-induced PAH model. Overexpression of SUMO1 significantly increased VSMCs proliferation, migration, hypoxia-induced VSMCs dedifferentiation, and autophagy, but these effects were abolished by inhibition of autophagy by 3-MA in aortic VSMCs. Furthermore, SUMO1 knockdown reversed hypoxia-induced proliferation and migration of PASMCs. Mechanistically, SUMO1 promotes Vps34 SUMOylation and the assembly of the Beclin-1-Vps34-Atg14 complex, thereby inducing autophagy, whereas Vps34 mutation K840R reduces Vps34 SUMOylation and inhibits VSMCs dedifferentiation.

DISCUSSION

Our data uncovers an important role of SUMO1 in VSMCs proliferation, migration, autophagy, and phenotypic switching (dedifferentiation) involved in pulmonary vascular remodeling and PAH. Targeting of the SUMO1-Vps34-autophagy signaling axis may be exploited to develop therapeutic strategies to treat PAH.

Exploiting protein modification systems to boost crop productivity: SUMO proteases in focus

In recent years, post-translational modification (PTM) of proteins has emerged as a key process that integrates plant growth and response to a changing environment. During the processes of domestication and breeding, plants were selected for various yield and adaptational characteristics. The post-translational modifier small ubiquitin-like modifier (SUMO) protein is known to have a role in the regulation of a number of these characteristics. Using bioinformatics, we mined the genomes of cereal and Brassica crops and their non-crop relatives Arabidopsis thaliana and Brachypodium distachyon for ubiquitin-like protease (ULP) SUMO protease sequences. We discovered that the SUMO system in cereal crops is disproportionately elaborate in comparison with that in B. distachyon. We use these data to propose deSUMOylation as a mechanism for specificity in the SUMO system.

A post-translational balancing act: the good and the bad of SUMOylation in pancreatic islets

Post-translational modification of proteins contributes to the control of cell function and survival. The balance of these in insulin-producing pancreatic beta cells is important for the maintenance of glucose homeostasis. Protection from the damaging effects of reactive oxygen species is required for beta cell survival, but if this happens at the expense of insulin secretory function then the ability of islets to respond to changing metabolic conditions may be compromised. In this issue of Diabetologia, He et al ( https://doi.org/10.1007/s00125-017-4523-9 ) show that post-translational attachment of small ubiquitin-like modifier (SUMO) to target lysine residues (SUMOylation) strikes an important balance between the protection of beta cells from oxidative stress and the maintenance of insulin secretory function. They show that SUMOylation is required to stabilise nuclear factor erythroid 2-related factor 2 (NRF2) and increase antioxidant gene expression. Decreasing SUMOylation in beta cells impairs their antioxidant capacity, causes cell death, hyperglycaemia, and increased sensitivity to streptozotocin-induced diabetes, while increasing SUMOylation is protective. However, this protection from overt diabetes occurs in concert with glucose intolerance due to impaired beta cell function. A possible role for SUMOylation as a key factor balancing beta cell protection vs beta cell responsiveness to metabolic cues is discussed in this Commentary.

The strategies for identification and quantification of SUMOylation

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

Roles for SUMO in pre-mRNA processing

When the small ubiquitin-like modifier (SUMO)-1 protein is localized on the genome, it is found on proteins bound to the promoters of the most highly active genes and on proteins bound to the DNA-encoding exons. Inhibition of the SUMO-1 modification leads to reductions in initiation of messenger RNA (mRNA) synthesis and splicing. In this review, we discuss what is known about the SUMOylation of factors involved in transcription initiation, pre-mRNA processing, and polyadenylation. We suggest a mechanism by which SUMO modifications of factors at the promoters of high-activity genes trigger the formation of an RNA polymerase II complex that coordinates and integrates the stimulatory signals for each process to catalyze an extremely high level of gene expression. WIREs RNA 2016, 7:105-112. doi: 10.1002/wrna.1318 For further resources related to this article, please visit the WIREs website.

Increased SUMO-1 expression in response to hypoxia: Interaction with HIF-1α in hypoxic pulmonary hypertension

Pulmonary hypertension (PH) develops in 30-70% of chronic obstructive pulmonary disease patients and increases morbidity and mortality. The present study aimed to investigate the regulation of small ubiquitin‑related modifier‑1 (SUMO‑1) expression in response to hypoxia. The experiments were carried out in vitro in rat pulmonary arterial smooth muscle cells (PASMCs) and in vivo using a rat hypoxic PH (HPH) model. A significant increase in SUMO‑1 mRNA and protein levels was observed following hypoxic stimulation in vivo and in vitro. SUMO‑1 is known to interact with various transcription factors, including hypoxia‑inducible factor‑1α (HIF‑1α) in vitro. Notably, the expression of HIF‑1α and its target gene, vascular endothelial growth factor, was increased by hypoxia in HPH. In addition, the present data suggest that SUMO‑1 regulated HIF‑1α in response to hypoxia (gene silencing and overexpression). Finally, the co‑immunoprecipitation assays suggest a direct and specific interaction between SUMO‑1 and HIF‑1α. In conclusion, SUMO‑1 may participate in the modulation of HIF‑1α through sumoylation in HPH. However, further studies are required to confirm this.

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

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

Novel roles of SUMO in pancreatic β-cells: thinking outside the nucleus

The endocrine pancreas is critically important in the regulation of energy metabolism, with defective insulin secretion from pancreatic islet β-cells a major contributing factor to the development of type 2 diabetes. Small ubiquitin-like modifier (SUMO) proteins have been demonstrated to covalently modify a wide range of target proteins, mediating a broad range of cellular processes. While the effects of SUMOylation on β-cell gene transcription have been previously reviewed, recent reports indicate roles for SUMO outside of the nucleus. In this review we shall focus on the reported non-nuclear roles of SUMOylation in the regulation of β-cells, including SUMOylation as a novel signaling pathway in the acute regulation of insulin secretion.

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.

Small ubiquitin-related modifier ligase activity of Mms21 is required for maintenance of chromosome integrity during the unperturbed mitotic cell division cycle in Saccharomyces cerevisiae

The SUMO ligase activity of Mms21/Nse2, a conserved member of the Smc5/6 complex, is required for resisting extrinsically induced genotoxic stress. We report that the Mms21 SUMO ligase activity is also required during the unchallenged mitotic cell cycle in Saccharomyces cerevisiae. SUMO ligase-defective cells were slow growing and spontaneously incurred DNA damage. These cells required caffeine-sensitive Mec1 kinase-dependent checkpoint signaling for survival even in the absence of extrinsically induced genotoxic stress. SUMO ligase-defective cells were sensitive to replication stress and displayed synthetic growth defects with DNA damage checkpoint-defective mutants such as mec1, rad9, and rad24. MMS21 SUMO ligase and mediator of replication checkpoint 1 gene (MRC1) were epistatic with respect to hydroxyurea-induced replication stress or methyl methanesulfonate-induced DNA damage sensitivity. Subjecting Mms21 SUMO ligase-deficient cells to transient replication stress resulted in enhancement of cell cycle progression defects such as mitotic delay and accumulation of hyperploid cells. Consistent with the spontaneous activation of the DNA damage checkpoint pathway observed in the Mms21-mediated sumoylation-deficient cells, enhanced frequency of chromosome breakage and loss was detected in these mutant cells. A mutation in the conserved cysteine 221 that is engaged in coordination of the zinc ion in Loop 2 of the Mms21 SPL-RING E3 ligase catalytic domain resulted in strong replication stress sensitivity and also conferred slow growth and Mec1 dependence to unchallenged mitotically dividing cells. Our findings establish Mms21-mediated sumoylation as a determinant of cell cycle progression and maintenance of chromosome integrity during the unperturbed mitotic cell division cycle in budding yeast.

A semisynthetic Eph receptor tyrosine kinase provides insight into ligand-induced kinase activation

We have developed a methodology for generating milligram amounts of functional Eph tyrosine kinase receptor using the protein engineering approach of expressed protein ligation. Stimulation with ligand induces efficient autophosphorylation of the semisynthetic Eph construct. The in vitro phosphorylation of key Eph tyrosine residues upon ligand-induced activation was monitored via time-resolved, quantitative phosphoproteomics, suggesting a precise and unique order of phosphorylation of the Eph tyrosines in the kinase activation process. To our knowledge, this work represents the first reported semisynthesis of a receptor tyrosine kinase and provides a potentially general method for producing single-pass membrane proteins for structural and biochemical characterization.

High-stringency tandem affinity purification of proteins conjugated to ubiquitin-like moieties

The post-translational modification of proteins with ubiquitin and ubiquitin-like proteins (Ubl) is vital to many cellular functions, and thus the identification of Ubl targets is key to understanding their function. In most cases, only a small proportion of the cellular pool of proteins is found conjugated to a particular Ubl, making identification of Ubl targets technically challenging. For the purposes of proteomic analyses, we have developed a protocol for the large-scale purification of Ubl-linked proteins that minimizes sample contamination with noncovalent interactors and prevents the cleavage of Ubl-substrate bonds catalyzed by Ubl-specific proteases. This is achieved by introducing a denaturing lysis step (in the presence of sodium dodecyl sulfate and alkylating agents that irreversibly inhibit Ubl proteases) before TAP (tandem affinity purification) that allows for efficient purification of putative Ubl-specific substrates in a form suitable for proteomic analysis. The timescale from cell lysis to purified protein sample is 5-6 d.

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.

Preparation of SUMO proteases and kinetic analysis using endogenous substrates

SUMO proteases catalyze two reactions, deconjugation of SUMO from substrates and processing of precursor SUMO isoforms to prepare SUMO for conjugation. The SUMO protease family includes two members in yeast (Ulp1 and Ulp2) and as many as six members in human (SENP1-3, SENP5-7). SENP/Ulp proteases each contain conserved C-terminal domains that catalyze protease activity. The C-terminal protease domains exhibit unique specificities during SUMO processing and deconjugation in vitro. While there are many available reagents to assess these activities, including fusion proteins and chemically modified SUMO isoforms, our studies have indicated that the composition of substrates C-terminal to the scissile bond can substantively influence the activity of the protease. As such, we have relied extensively on assays that utilize endogenous substrates, namely wild-type SUMO precursors and SUMO conjugated substrates. In this chapter, we will describe methodological details for purification and characterization of SUMO precursors, SUMO conjugated substrates, and SUMO proteases. We will also describe methods for kinetic analysis of SUMO deconjugation and maturation using endogenous substrates.

Ubiquitin proteolytic system: focus on SUMO

The small ubiquitin-like modifier proteins (Smt3 in yeast and SUMOs 1-4 in vertebrates) are members of the ubiquitin super family. Like ubiquitin, the SUMOs are protein modifiers that are covalently attached to the epsilon-amino group of lysine residues in the substrates. The application of proteomics to the SUMO field has greatly expanded both the number of known targets and the number of identified target lysines. As new refinements of proteomic techniques are developed and applied to sumoylation, an explosion of novel data is likely in the next 5 years. This ability to examine sumoylated proteins globally, rather than individually, will lead to new insights into both the functions of the individual SUMO types, and how dynamic changes in overall sumoylation occur in response to alterations in cellular environment. In addition, there is a growing appreciation for the existence of cross-talk mechanisms between the sumoylation and ubiquitinylation processes. Rather than being strictly parallel, these two systems have many points of intersection, and it is likely that the coordination of these two systems is a critical contributor to the regulation of many fundamental cellular events.

Sumoylation of the transcriptional intermediary factor 1beta (TIF1beta), the Co-repressor of the KRAB Multifinger proteins, is required for its transcriptional activity and is modulated by the KRAB domain

Small ubiquitin-related modifier (SUMO) has emerged as a key post-translational modulator of protein functions. Here we show that TIF1beta, a developmental regulator proposed to act as a universal co-repressor for the large family of KRAB domain-containing zinc finger proteins, is a heavily SUMO-modified substrate. A combined analysis of deletion and punctual mutants identified TIF1beta as a multilysine acceptor for SUMO which specifically targets six lysine residues (Lys(554), Lys(575), Lys(676), Lys(750), Lys(779), and Lys(804)) within the TIF1beta C-terminal repressive region. Reporter gene assays indicate that TIF1beta requires SUMO-modification for its repressive activity. Indeed, sumoylation-less mutants failed to recapitulate TIF1beta-dependent repression. TIF1beta homodimerization properties and interaction with the KRAB domain are preserved in the mutants with lysine to arginine substitutions as confirmed by in vivo bioluminescence resonance energy transfer (BRET). Using histone deacetylase (HDAC) inhibitors, we also demonstrate that TIF1beta sumoylation is a prerequisite for the recruitment of HDAC and that TIF1beta SUMO-dependent repressive activity involves both HDAC-dependent and HDAC-independent components. Finally, we report that, in addition to relying on the integrity of its PHD finger and on its self-oligomerization, TIF1beta sumoylation is positively regulated by its interaction with KRAB domain-containing proteins. Altogether, our results provide new mechanistic insights into TIF1beta transcriptional repression and suggest that KRAB multifinger proteins not only recruit TIF1beta co-repressor to target genes but also increase its repressive activity through enhancement of its sumoylation.

Structural basis for SENP2 protease interactions with SUMO precursors and conjugated substrates

SUMO processing and deconjugation are essential proteolytic activities for nuclear metabolism and cell-cycle progression in yeast and higher eukaryotes. To elucidate the mechanisms used during substrate lysine deconjugation, SUMO isoform processing and SUMO isoform interactions, X-ray structures were determined for a catalytically inert SENP2 protease domain in complex with conjugated RanGAP1-SUMO-1 or RanGAP1-SUMO-2, or in complex with SUMO-2 or SUMO-3 precursors. Common features within the active site include a 90 degrees kink proximal to the scissile bond that forces C-terminal amino acid residues or the lysine side chain toward a protease surface that appears optimized for lysine deconjugation. Analysis of this surface reveals SENP2 residues, particularly Met497, that mediate, and in some instances reverse, in vitro substrate specificity. Mutational analysis and biochemistry provide a mechanism for SENP2 substrate preferences that explains why SENP2 catalyzes SUMO deconjugation more efficiently than processing.

Functional interaction between human herpesvirus 6 immediate-early 2 protein and ubiquitin-conjugating enzyme 9 in the absence of sumoylation

The immediate-early 2 (IE2) protein of human herpesvirus 6 is a potent transactivator of cellular and viral promoters. To better understand the biology of IE2, we generated a LexA-IE2 fusion protein and screened, using the yeast two-hybrid system, a Jurkat T-cell cDNA library for proteins that could interact with IE2. The most frequently isolated IE2-interacting protein was the human ubiquitin-conjugating enzyme 9 (Ubc9), a protein involved in the small ubiquitin-like modifier (SUMO) conjugation pathway. Using deletion mutants of IE2, we mapped the IE2-Ubc9-interacting region to residues 989 to 1037 of IE2. The interaction was found to be of functional significance to IE2, as Ubc9 overexpression significantly repressed promoter activation by IE2. The C93S Ubc9 mutant exhibited a similar effect on IE2, indicating that the E2 SUMO-conjugating function of Ubc9 is not required for its repressive action on IE2. No consensus sumoylation sites or evidence of IE2 conjugation to SUMO could be demonstrated under in vivo or in vitro conditions. Moreover, expression levels and nuclear localization of IE2 were not altered by Ubc9 overexpression, suggesting that Ubc9's repressive function likely occurs at the transcriptional complex level. Overall, our results indicate that Ubc9 influences IE2's function and provide new information on the complex interactions that occur between herpesviruses and the sumoylation pathway.

FAT10/diubiquitin-like protein-deficient mice exhibit minimal phenotypic differences

The FAT10 gene encodes a diubiquitin-like protein containing two tandem head-to-tail ubiquitin-like domains. There is a high degree of similarity between murine and human FAT10 sequences at both the mRNA and protein levels. In various cell lines, FAT10 expression was shown to be induced by gamma interferon or by tumor necrosis factor alpha. In addition, FAT10 expression was found to be up-regulated in some Epstein-Barr virus-infected B-cell lines, in activated dendritic cells, and in several epithelial tumors. However, forced expression of FAT10 in cultured cells was also found to produce apoptotic cell death. Overall, these findings suggest that FAT10 may modulate cellular growth or cellular viability. Here we describe the steps to generate, by genetic targeting, a FAT10 gene knockout mouse model. The FAT10 knockout homozygous mice are viable and fertile. No gross lesions or obvious histological differences were found in these mutated mice. Examination of lymphocyte populations from spleen, thymus, and bone marrow did not reveal any abnormalities. However, flow cytometry analysis demonstrated that the lymphocytes of FAT10 knockout mice were, on average, more prone to spontaneous apoptotic death. Physiologically, these mice demonstrated a high level of sensitivity toward endotoxin challenge. These findings indicate that FAT10 may function as a survival factor.

Involvement of SUMO modification in MBD1- and MCAF1-mediated heterochromatin formation

Small ubiquitin-related modifiers, SUMO-2/3 and SUMO-1, are involved in gene regulation and nuclear structures. However, little is known about the roles of SUMO, in heterochromatin formation of mammalian cells. Here we demonstrate that SUMOs directly interact with human MCAF1, which forms complexes with either the methyl-CpG-binding protein MBD1 or SETDB1, which trimethylates histone H3 at lysine 9 (H3-K9) in the presence of MCAF1. Modification of MBD1 with either SUMO-2/3 or SUMO-1 facilitated the interaction between MBD1 and MCAF1, suggesting that SUMOylation links the methylation of DNA and histones. In a cultured human cell line, SUMOs were localized in MBD1- and MCAF1-containing heterochromatin regions that were enriched in trimethyl-H3-K9 and the heterochromatin proteins HP1beta and HP1gamma. Specific knockdown of either SUMO-2/3 or SUMO-1 induced dissociation of MCAF1, trimethyl-H3-K9, and the HP1 proteins from the MBD1-containing heterochromatin foci, suggesting a requirement for SUMOs for heterochromatin assembly. These findings provide insights into the roles of SUMOylation in the regulation of heterochromatin formation and gene silencing.

A Method of Mapping Protein Sumoylation Sites by Mass Spectrometry Using a Modified Small Ubiquitin-like Modifier 1 (SUMO-1) and a Computational Program

Post-translational modification by small ubiquitin-like modifier 1 (SUMO-1) is a highly conserved process from yeast to humans and plays important regulatory roles in many cellular processes. Sumoylation occurs at certain internal lysine residues of target proteins via an isopeptide bond linkage. Unlike ubiquitin whose carboxyl-terminal sequence is RGG, the tripeptide at the carboxyl terminus of SUMO is TGG. The presence of the arginine residue at the carboxyl terminus of ubiquitin allows tryptic digestion of ubiquitin conjugates to yield a signature peptide containing a diglycine remnant attached to the target lysine residue and rapid identification of the ubiquitination site by mass spectrometry. The absence of lysine or arginine residues in the carboxyl terminus of mammalian SUMO makes it difficult to apply this approach to mapping sumoylation sites. We performed Arg scanning mutagenesis by systematically substituting amino acid residues surrounding the diglycine motif and found that a SUMO variant terminated with RGG can be conjugated efficiently to its target protein under normal sumoylation conditions. We developed a Programmed Data Acquisition (PDA) mass spectrometric approach to map target sumoylation sites using this SUMO variant. A web-based computational program designed for efficient identification of the modified peptides is described.

PIAS proteins are involved in the SUMO-1 modification, intracellular translocation and transcriptional repressive activity of RET finger protein

Ret finger protein (RFP) is a nuclear protein that is highly expressed in testis and in various tumor cell lines. RFP functions as a transcriptional repressor and associates with Enhancer of Polycomb 1 (EPC1), a member of the Polycomb group proteins, and Mi-2beta, a main component of the nucleosome remodeling and deacetylase (NuRD) complex. We show that RFP binds with PIAS (protein inhibitor of activated STAT) proteins, PIAS1, PIAS3, PIASxalpha and PIASy at their carboxyl-terminal region and is covalently modified by SUMO-1 (sumoylation). PIAS proteins enhance the sumoylation of RFP in a dose-dependent manner and induce the translocation of RFP into nuclear bodies reminiscent of the PML bodies. In addition, co-expression of PIAS proteins or SUMO-1 strengthened the transcriptional repressive activity of RFP. Finally, our immunohistochemical results show that RFP, SUMO-1 and PIASy localize in a characteristic nuclear structure juxtaposed with the inner nuclear membrane (XY body) of primary spermatocytes in mouse testis. These results demonstrate that the intracellular location and the transcriptional activity of RFP are modified by PIAS proteins which possess SUMO E3 ligase activities and suggest that they may play a co-operative role in spermatogenesis.

A basis for SUMO protease specificity provided by analysis of human Senp2 and a Senp2-SUMO complex

Modification of cellular proteins by the ubiquitin-like protein SUMO is essential for nuclear metabolism and cell cycle progression in yeast. X-ray structures of the human Senp2 catalytic protease domain and of a covalent thiohemiacetal transition-state complex obtained between the Senp2 catalytic domain and SUMO-1 revealed details of the respective protease and substrate surfaces utilized in interactions between these two proteins. Comparative biochemical and structural analysis between Senp2 and the yeast SUMO protease Ulp1 revealed differential abilities to process SUMO-1, SUMO-2, and SUMO-3 in maturation and deconjugation reactions. Further biochemical characterization of the three SUMO isoforms into which an additional Gly-Gly di-peptide was inserted, or whereby the respective SUMO tails from the three isoforms were swapped, suggests a strict dependence for SUMO isopeptidase activity on residues C-terminal to the conserved Gly-Gly motif and preferred cleavage site for SUMO proteases.

Coordinated functions of WSS1, PSY2 and TOF1 in the DNA damage response

The stabilization and processing of stalled replication forks is required to maintain genome integrity in all organisms. In an effort to identify novel proteins that might be involved in stabilizing stalled replication forks, Saccharomyces cerevisiae mutant wss1Delta was isolated from a high-throughput screening of approximately 5000 deletion strains for genes involved in the response to continuous, low-intensity UV irradiation. Disruption of WSS1 resulted in synergistic increases in UV sensitivity with null mutants of genes involved in recombination (RAD52) and cell cycle control (RAD9 and RAD24). WSS1 was also found to interact genetically with SGS1, TOP3, SRS2 and CTF4, which are involved in recombination, repair of replication forks and the establishment of sister chromatid cohesion. A yeast two-hybrid screen identified a potential physical interaction between Wss1 and both Psy2 and Tof1. Genetic interactions were also detected between PSY2 and TOF1, as well as between each gene and RAD52 and SRS2, and between WSS1 and TOF1. Tof1 is known to be involved in stabilizing stalled replication forks and our data suggest that Wss1 and Psy2 similarly function to stabilize or process stalled or collapsed replication forks.

Ubc9 is required for damage-tolerance and damage-induced interchromosomal homologous recombination in S. cerevisiae

Ubc9 is an enzyme involved in the conjugation of small ubiquitin related modifier (SUMO) to target proteins. A Saccharomyces cerevisiae ubc9 temperature sensitive (ts) mutant showed higher sensitivity to various DNA damaging agents such as methylmethanesulfonate (MMS) and UV at a semi-permissive temperature than wild-type cells. The sensitivity of ubc9ts cells was not suppressed by the introduction of a mutated UBC9 gene, UBC9-C93S, whose product is unable to covalently bind to SUMO and consequently fails to conjugate SUMO to target proteins. Diploid ubc9ts cells were more sensitive to various DNA damaging agents than haploid ubc9ts cells suggesting the involvement of homologous recombination in the sensitivity of ubc9ts cells. The frequency of interchromosomal recombination between heteroalleles, his1-1/his1-7 loci, in wild-type cells was remarkably increased upon exposure to MMS or UV. Although the frequency of spontaneous interchromosomal recombination between the heteroalleles in ubc9ts cells was almost the same as that of wild-type cells, no induction of interchromosomal recombination was observed in ubc9ts cells upon exposure to MMS or UV.

Increase of SUMO-1 expression in response to hypoxia: direct interaction with HIF-1alpha in adult mouse brain and heart in vivo

The present study investigates the regulation of small ubiquitin-related modifier-1 (SUMO-1) expression in response to hypoxia in adult mouse brain and heart. We observed a significant increase in SUMO-1 mRNAs and proteins after hypoxic stimulation in vivo. Because SUMO-1 interacts with various transcription factors, including hypoxia-inducible factor-1beta (HIF-1beta) in vitro, we not only demonstrated that the HIF-1alpha expression is increased by hypoxia in brain and heart, but also provided evidence that SUMO-1 co-localizes in vivo with HIF-1alpha in response to hypoxia by demonstrating the co-expression of these two proteins in neurons and cardiomyocytes. The specific interaction between SUMO-1 and HIF-1alpha was additionally demonstrated with co-immunoprecipitation. These results indicate that the increased levels of SUMO-1 participate in the modulation of HIF-1alpha function through sumoylation in brain and heart.

SENP1 enhances androgen receptor-dependent transcription through desumoylation of histone deacetylase 1

SUMO (also called Sentrin) is a ubiquitin-like protein that plays an important role in regulating protein function and localization. It is known that several nuclear receptors are modified by SUMO; however, the effect of desumoylation in regulating nuclear receptor function has not been elucidated. Here we show that androgen receptor (AR)-mediated transcription is markedly enhanced by SENP1, a member of SUMO-specific protease family. SENP1's ability to enhance AR-dependent transcription is not mediated through desumoylation of AR, but rather through its ability to deconjugate histone deacetylase 1 (HDAC1), thereby reducing its deacetylase activity. HDAC1's repressive effect on AR-dependent transcription could be reversed by SENP1 and by deletion of its sumoylation sites. RNA interference depletion of endogenous HDAC1 also reduced SENP1's effect. Thus, SENP1 could regulate AR-dependent transcription through desumoylation of HDAC1. These studies provide insights on the potential role of desumoylation in the regulation of nuclear receptor activity.

Promyelocytic leukemia (PML) nuclear bodies are disorganized in colorectal tumors with total loss of major histocompatibility complex class I expression and LMP7 downregulation

The promyelocytic leukemia (PML) protein is the product of the PML gene that fuses with the retinoic acid receptor-alpha (RARalpha) gene in acute promyelocytic leukemia (APL) and produces disruption of PML bodies. Wild-type PML localizes in the nucleus with a typical speckled pattern. PML bodies accumulate several proteins involved in multiple cellular pathways such as apoptosis, transcriptional regulation, and proteasomal degradation of ubiquitinated proteins. The ubiquitin-proteasome pathway at PML bodies is dependent on proteasome component recruitment. Proteasome components such as low-molecular weight proteins (LMPs) are frequently downregulated in different tumor tissues that present impaired major histocompatibility complex (MHC) class I expression. We have recently documented LMP7 downregulation in colorectal tumors with total loss of MHC class I antigen. An immunohistochemical study of PML protein in these tumors revealed a disrupted pattern of PML bodies in a nuclear diffuse form, as observed in APL cells. Therefore, the disruption of the PML bodies was clearly associated with LMP7 downregulation.

Association of the nucleocapsid protein of the Seoul and Hantaan hantaviruses with small ubiquitin-like modifier-1-related molecules

We performed yeast two-hybrid screening of a human kidney cell cDNA library to study the biological role of the hantavirus nucleocapsid protein (NP). We found that Seoul virus (SEOV) and Hantaan virus (HTNV) NPs were associated with small ubiquitin-like modifier (SUMO)-1-interacting proteins PIAS1, PIASxbeta, HIPK2, CHD3, and TTRAP, which interacted with the SUMO-1 conjugating enzyme (Ubc-9) and SUMO-1 in the yeast two-hybrid assay. Interactions between the HIPK2, CHD3, and TTRAP proteins and SEOV NP were also shown in a mammalian two-hybrid assay. However, there was no interaction between PIAS proteins and NP, which was probably due to the inhibitory effect of PIAS on transcription in the mammalian two-hybrid assay. Nevertheless, a co-expression experiment suggested the existence of a PIAS-NP interaction in the cytoplasm. The region spanning amino acids 100-125 of SEOV NP, which represents a critical region for NP-NP polymerization, was found to be responsible for the interaction with SUMO-1-related molecules in both the yeast and mammalian two-hybrid assays. These results add to the information on interactions of hantavirus NP and host cellular proteins.

Replication checkpoint kinase Cds1 regulates recombinational repair protein Rad60

Genome integrity is protected by Cds1 (Chk2), a checkpoint kinase that stabilizes arrested replication forks. How Cds1 accomplishes this task is unknown. We report that Cds1 interacts with Rad60, a protein required for recombinational repair in fission yeast. Cds1 activation triggers Rad60 phosphorylation and nuclear delocalization. A Rad60 mutant that inhibits regulation by Cds1 renders cells specifically sensitive to replication fork arrest. Genetic and biochemical studies indicate that Rad60 functions codependently with Smc5 and Smc6, subunits of an SMC (structural maintenance of chromosomes) complex required for recombinational repair. These studies indicate that regulation of Rad60 is an important part of the replication checkpoint response controlled by Cds1. We propose that control of Rad60 regulates recombination events at stalled forks.

Sumoylation of Smad4, the common Smad mediator of transforming growth factor-beta family signaling

Transforming growth factor-beta (TGF-beta) and TGF-beta-related factors regulate cell growth, differentiation, and apoptosis, and play key roles in normal development and tumorigenesis. TGF-beta family-induced changes in gene expression are mediated by serine/threonine kinase receptors at the cell surface and Smads as intracellular effectors. Receptor-activated Smads combine with a common Smad4 to translocate into the nucleus where they cooperate with other transcription factors to activate or repress transcription. The activities of the receptor-activated Smads are controlled by post-translational modifications such as phosphorylation and ubiquitylation. Here we show that Smad4 is modified by sumoylation. Sumoylation of Smad4 was enhanced by the conjugating enzyme Ubc9 and members of the PIAS family of SUMO ligases. A major sumoylation site in Smad4 was localized to Lys-159 in its linker segment with an additional site at Lys-113 in the MH-1 domain. Increased sumoylation in the presence of the PIASy E3 ligase correlated with targeting of Smad4 to subnuclear speckles that contain SUMO-1 and PIASy. Replacement of lysines 159 and 113 by arginines or increased sumoylation enhanced the stability of Smad4, and transcription in mammalian cells and Xenopus embryos. These observations suggest a role for Smad4 sumoylation in the regulation of TGF-beta signaling through Smads.

Design, synthesis, and evaluation of novel boronic-chalcone derivatives as antitumor agents

A series of boronic-chalcone derivatives were synthesized and tested for antitumor activity against human breast cancer cell lines. The results show the boronic-chalcones are more toxic to breast cancer cells compared to normal breast cells than other known chalcones.

Sterol regulatory element-binding proteins are negatively regulated through SUMO-1 modification independent of the ubiquitin/26 S proteasome pathway

Sterol regulatory element-binding proteins (SREBPs) are major transcription factors that activate the genes involved in cholesterol and fatty acid biosynthesis. We here report that the nuclear forms of SREBPs are modified by the small ubiquitin-related modifier (SUMO)-1. Mutational analyses identified two major sumoylation sites (Lys(123) and Lys(418)) in SREBP-1a and a single site (Lys(464)) in SREBP-2. Mutant SREBPs lacking one or two sumoylation sites exhibited increased transactivation capacity on an SREBP-responsive promoter. Overexpression of SUMO-1 reduced whereas its dominant negative form increased mRNA levels of SREBP-responsive genes. Nuclear SREBPs interacted with the SUMO-1-conjugating enzyme Ubc9, and overexpression of a dominant negative form of Ubc9 increased the mRNA levels of SREBP-responsive genes. Pulse-chase experiments revealed that sumoylation did not affect the degradation of SREBPs through the ubiquitin-proteasome pathway. In vitro ubiquitylation assay showed no competition between ubiquitin and SUMO-1 for the same lysine. Considered together, our results indicate that SUMO-1 modification suppresses the transactivation capacity of nuclear SREBPs in a manner different from the negative regulatory mechanism mediated by proteolysis.

SUMO-1 co-localized with mutant atrophin-1 with expanded polyglutamines accelerates intranuclear aggregation and cell death

To investigate the implication of small ubiquitin-related modifier-1 (SUMO-1) in the formation of neuronal intranuclear inclusions in polyglutamine diseases, we examined the localization of SUMO-1 in dentatorubral-pallidoluysian atrophy (DRPLA) brain tissues and PC12 cells expressing truncated atrophin-1 with expanded poly-glutamine stretches. SUMO-1 was co-localized with neuronal intranuclear inclusions in DRPLA brain and the DRPLA model cells, which showed that the aggregates formed by expanded polyglutamine stretches were highly SUMOlylated. In addition, to examine the role of SUMO-1 in nuclear aggregate formation and cell death, either SUMO-1 or DeltaSUMO-1, which is a SUMOlylation defective mutant lacking the C-terminal motif, was co-transfected with atrophin-1 with expanded polyglutamine stretches. Co-transfection of DeltaSUMO-1 decreased number of the cells with nuclear aggregates and consequent apoptosis of PC12 cells, both of which were markedly enhanced by co-transfection of SUMO-1 with atrophin-1 with expanded polyglutamine stretches. These results suggest that SUMO-1 is implicated in the pathogenesis of DRPLA and accelerates aggregate formation and cell death.

SUMO-1 conjugation to intact DNA topoisomerase I amplifies cleavable complex formation induced by camptothecin

DNA topoisomerase I (Topo1) manages the topological state of DNA. Cleavable complexes, the covalent Topo1-DNA intermediates, become DNA damaged when the catalytic cycles are inhibited by the anti-tumor drug camptothecin (CPT). Intriguingly, Topo1 is modified rapidly and extensively with SUMO-1, a ubiquitin-like protein, in response to CPT. This study shows that the sumoylation enhances the cleavable complex formation and apoptosis induced by CPT. Indeed, substitutions of Lys117 and Lys153, identified as Topo1 sumoylation sites, reduced the CPT-induced cleavable complexes without influencing its in vitro catalytic activity. Consistent with this observation, CPT-induced cleavable complexes of wild-type Topo1 increased in a sumoylation-dependent manner. We also found that Topo1 sumoylation occurred independently of CPT when Topo1 was inactivated by mutation of the catalytic Tyr723. These findings suggested that Topo1 inactivation by CPT treatment can trigger Topo1 sumoylation, leading to enhanced cleavable complex formation.

Proteins interacting with Lin 1p, a putative link between chromosome segregation, mRNA splicing and DNA replication in Saccharomyces cerevisiae

Proteins involved in chromosome segregation during mitosis are likely to participate in other cell cycle-coordinated processes. Using a two-hybrid screen we identified a novel nuclear protein, Lin1, interacting with Irr1p/Scc3p, a component of the cohesin complex. The second round of two-hybrid assay with Lin1p as the bait resulted in the identification of six proteins: Prp8, Slx5, Siz2, Wss1, Rfc1 and YIL149w. These proteins have previously been shown to participate in mRNA splicing, DNA replication, chromosome condensation, chromatid separation and alternative cohesion. We propose that Lin1p may constitute a link among these processes.

Ubc9 is essential for viability of higher eukaryotic cells

Ubc9 is an enzyme involved in the conjugation of SUMO-1 (small ubiquitin related modifier 1) to target proteins. The SUMO-1 conjugation system is well conserved from yeasts to higher eukaryotes, but many SUMO-1 target proteins reported recently in higher eukaryotic cells, including IkappaBalpha, MDM2, p53, and PML, are not present in yeasts. To determine the physiological roles of SUMO-1 conjugation in higher eukaryotic cells, we constructed a conditional UBC9 mutant of chicken DT40 cells containing the UBC9 transgene under control of a tetracycline-repressible promoter and characterized their loss of function phenotypes. Ubc9 disappeared 3 days after the addition of tetracycline and the increase in viable cell number stopped 4 days after the addition of drug. In contrast to the cases of ubc9 mutants of budding and fission yeasts, which show defects in progression of G2 or early M phase and in chromosome segregation, respectively, we did not observe accumulation of cells in G2/M phase or a considerable increase in the frequency of chromosome missegregation upon depletion of Ubc9 but we did observe an increase in the number of cells containing multiple nuclei, indicating defects in cytokinesis. A considerable portion of the Ubc9-depleted cell population was committed to apoptosis without accumulating in a specific phase of the cell cycle, suggesting that chromosome damages are accumulated in Ubc9-depleted cells, and apoptosis is triggered without activating checkpoint mechanisms under conditions of SUMO-1 conjugation system impairment.

Versatile protein tag, SUMO: its enzymology and biological function

Small ubiquitin-related modifier (SUMO) is a member of a ubiquitin-like protein family that regulates cellular function of a variety of target proteins. SUMO and ubiquitin are synthesized as precursors that need to be processed prior to conjugation to target proteins, and their mature forms have a similar tertiary structure. The mechanism for SUMO conjugation is also analogous to that of the ubiquitin system, such as the utilization of E1, E2, and E3 cascade enzymes. However, the biological consequence of SUMO modification is quite different from that of the ubiquitin system. Whereas ubiquitination of most proteins is for the degradative pathway, SUMO modification of target proteins is involved in nuclear protein targeting, formation of subnuclear structures, regulation of transcriptional activities or DNA binding abilities of transcription factors, and control of protein stability. This review will summarize the recent progress made in the enzymology of SUMO and its biological significance.

A protein encoded by the herpes simplex virus (HSV) type 1 2-kilobase latency-associated transcript is phosphorylated, localized to the nucleus, and overcomes the repression of expression from exogenous promoters when inserted into the quiescent HSV genome

Herpes simplex virus (HSV) is characterized by its ability to establish a latent infection in sensory neurons, from which it can periodically reactivate. The mechanisms of latency, however, remain unclear. The HSV genome is quiescent during latency except for the expression of the latency-associated transcripts (LATs). Although the exact function of the LATs remains obscure, current evidence suggests they are multifunctional and are involved in both establishment of latency and reactivation from latency. The LATs contain several open reading frames (ORFs). One or more of the functions of the LATs could therefore be protein mediated. We have previously reported that deregulated expression of the largest of the HSV type 1 (HSV-1) LAT ORFs ( approximately 274 amino acids) greatly enhances virus growth in cell types that are normally relatively nonpermissive for HSV replication and also that it complements mutations to the immediate-early (IE) gene ICP0 (S. K. Thomas, G. Gough, D. S. Latchman, and R. S. Coffin, J. Virol. 73:6618-6625, 1999). Here we show that LAT ORF expression overcomes the repression of expression from exogenous promoters introduced into the HSV-1 genome which normally occurs in the absence of IE gene expression. To further explore LAT ORF function, we have generated an epitope-tagged LAT ORF, LATmycHis, which forms punctate structures in the infected-cell nucleus reminiscent of the structures formed by ICP0. These are associated with the appearance of a phosphorylated form of the protein and are formed adjacent to, or around the edges of, viral replication compartments. These results provide further evidence that the HSV-1 LAT ORF protein is biologically functional and that the tightly regulated expression of this protein may be important in the wild-type latency phenotype in vivo.

Structural basis for E2-mediated SUMO conjugation revealed by a complex between ubiquitin-conjugating enzyme Ubc9 and RanGAP1

E2 enzymes catalyze attachment of ubiquitin and ubiquitin-like proteins to lysine residues directly or through E3-mediated reactions. The small ubiquitin-like modifier SUMO regulates nuclear transport, stress response, and signal transduction in eukaryotes and is essential for cell-cycle progression in yeast. In contrast to most ubiquitin conjugation, the SUMO E2 enzyme Ubc9 is sufficient for substrate recognition and lysine modification of known SUMO targets. Crystallographic analysis of a complex between mammalian Ubc9 and a C-terminal domain of RanGAP1 at 2.5 A reveals structural determinants for recognition of consensus SUMO modification sequences found within SUMO-conjugated proteins. Structure-based mutagenesis and biochemical analysis of Ubc9 and RanGAP1 reveal distinct motifs required for substrate binding and SUMO modification of p53, IkappaBalpha, and RanGAP1.

Viral interaction with the host cell sumoylation system

A novel host cell post-translational modification system termed sumoylation was discovered recently. Sumoylation is an enzymatic process that is biochemically analogous to, but functionally distinct from ubiquitinylation. As in ubiquitinylation, sumoylation involves the attachment of a small protein moiety, SUMO, to substrate proteins. Conjugation of SUMO does not typically lead to degradation of the substrate and instead causes functional alterations or changes in intracellular localization. While the majority of identified SUMO targets are cellular proteins, both herpesvirus and papillomavirus proteins have also been identified as authentic substrates for this modification. The exact effect of sumoylation on viral proteins appears to be substrate specific, but does have functional consequences that are likely to be important for the viral life cycle. In addition to viral proteins being targets for sumoylation, there is both direct and indirect evidence that viruses can alter the sumoylation status of host cell proteins. Such modulation of critical host proteins may be important for inhibiting cellular defense mechanisms or for promoting an intracellular state that is supportive of viral reproduction. This review highlights the enzymology of sumoylation and discusses the known examples of how viruses impact and are impacted by sumoylation.