Covalent modification of the transactivator protein IE2-p86 of human cytomegalovirus by conjugation to the ubiquitin-homologous proteins SUMO-1 and hSMT3b

OTUB1 contributes to the stability and function of Influenza A virus NS2

The influenza A virus (IAV) consists of 8 single-stranded, negative-sense viral RNA (vRNA) segments. After infection, vRNA is transcribed, replicated, and wrapped by viral nucleoprotein (NP) to form viral ribonucleoprotein (vRNP). The transcription, replication, and nuclear export of the viral genome are regulated by the IAV protein, NS2, which is translated from spliced mRNA transcribed from viral NS vRNA. This splicing is inefficient, explaining why NS2 is present in low abundance after IAV infection. The levels of NS2 and its subsequent accumulation are thought to influence viral RNA replication and vRNP nuclear export. Here we show that NS2 is ubiquitinated at the K64 and K88 residues by K48-linked and K63-linked polyubiquitin (polyUb) chains, leading to the degradation of NS2 by the proteasome. Additionally, we show that a host deubiquitinase, OTUB1, can remove polyUb chains conjugated to NS2, thereby stabilizing NS2. Accordingly, knock down of OTUB1 by siRNA reduces the nuclear export of vRNP, and reduces the overall production of IAV. These results collectively demonstrate that the levels of NS2 in IAV-infected cells are regulated by a ubiquitination-deubiquitination system involving OTUB1 that is necessary for optimal IAV replication.

A UL26-PIAS1 complex antagonizes anti-viral gene expression during Human Cytomegalovirus infection

Viral disruption of innate immune signaling is a critical determinant of productive infection. The Human Cytomegalovirus (HCMV) UL26 protein prevents anti-viral gene expression during infection, yet the mechanisms involved are unclear. We used TurboID-driven proximity proteomics to identify putative UL26 interacting proteins during infection to address this issue. We find that UL26 forms a complex with several immuno-regulatory proteins, including several STAT family members and various PIAS proteins, a family of E3 SUMO ligases. Our results indicate that UL26 prevents STAT phosphorylation during infection and antagonizes transcriptional activation induced by either interferon α (IFNA) or tumor necrosis factor α (TNFα). Additionally, we find that the inactivation of PIAS1 sensitizes cells to inflammatory stimulation, resulting in an anti-viral transcriptional environment similar to ΔUL26 infection. Further, PIAS1 is important for HCMV cell-to-cell spread, which depends on the presence of UL26, suggesting that the UL26-PIAS1 interaction is vital for modulating intrinsic anti-viral defense.

The role of SUMO specific peptidase 3 in secondary inflammation of ischemic stroke in mice

Ischemic stroke is the main cause of death and disability, and microglia play a crucial role in the pathophysiology of hypoxic ischemic brain injury. We found that SENP3 is highly expressed in the early stages of ischemic stroke in both in vivo and in vitro mouse models, and may be related to the deSUMOylation of the key kinase MKK7 in the TLR4/p-JNK signaling pathway. Knocking down SENP3 can inhibit the deSUMOylation of MKK7, thereby inhibiting the activation of the TLR4/p-JNK signaling pathway in an in vitro stroke model. Proteomic analysis showed that SENP3 undergoes phosphorylation at the T429 site after ischemic stroke. Computer simulation predictions show a significant enhancement of the interaction between pT429-SENP3 and MKK7, which has been confirmed through experiments on the interaction of biological macromolecules (SPR). The mitochondrial metabolic abnormalities caused by energy abnormalities in the early stages of stroke provide a good explanation for the phosphorylation of SENP3. Therefore, we used the mitochondrial complex inhibitor TTFA to reverse demonstrate that the phosphorylation of SENP3 comes from the large amount of adenosine triphosphate produced by mitochondrial abnormal metabolism caused by early oxygen glucose deficiency. Finally, proteomic analysis indicates that a significant amount of oxidative phosphorylation does occur in the early stages of stroke. In summary, targeted regulation of SENP3 phosphorylation to affect the deSUMOylation of MKK7 may inhibit secondary inflammation in ischemic stroke.

Discovery of a Novel Antiviral Effect of the Restriction Factor SPOC1 against Human Cytomegalovirus

The chromatin-remodeler SPOC1 (PHF13) is a transcriptional co-regulator and has been identified as a restriction factor against various viruses, including human cytomegalovirus (HCMV). For HCMV, SPOC1 was shown to block the onset of immediate-early (IE) gene expression under low multiplicities of infection (MOI). Here, we demonstrate that SPOC1-mediated restriction of IE expression is neutralized by increasing viral titers. Interestingly, our study reveals that SPOC1 exerts an additional antiviral function beyond the IE phase of HCMV replication. Expression of SPOC1 under conditions of high MOI resulted in severely impaired viral DNA replication and viral particle release, which may be attributed to inefficient viral transcription. With the use of click chemistry, the localization of viral DNA was investigated at late time points after infection. Intriguingly, we detected a co-localization of SPOC1, RNA polymerase II S5P and polycomb repressor complex 2 (PRC2) components in close proximity to viral DNA in areas that are hypothesized to harbor viral transcription sites. We further identified the N-terminal domain of SPOC1 to be responsible for interaction with EZH2, a subunit of the PRC2 complex. With this study, we report a novel and potent antiviral function of SPOC1 against HCMV that is efficient even with unrestricted IE gene expression.

IE1 of Human Cytomegalovirus Inhibits Necroptotic Cell Death via Direct and Indirect Modulation of the Necrosome Complex

Programmed necrosis is an integral part of intrinsic immunity, serving to combat invading pathogens and restricting viral dissemination. The orchestration of necroptosis relies on a precise interplay within the necrosome complex, which consists of RIPK1, RIPK3 and MLKL. Human cytomegalovirus (HCMV) has been found to counteract the execution of necroptosis during infection. In this study, we identify the immediate-early 1 (IE1) protein as a key antagonist of necroptosis during HCMV infection. Infection data obtained in a necroptosis-sensitive cell culture system revealed a robust regulation of post-translational modifications (PTMs) of the necrosome complex as well as the importance of IE1 expression for an effective counteraction of necroptosis. Interaction analyses unveiled an association of IE1 and RIPK3, which occurs in an RHIM-domain independent manner. We propose that this interaction manipulates the PTMs of RIPK3 by promoting its ubiquitination. Furthermore, IE1 was found to exert an indirect activity by modulating the levels of MLKL via antagonizing its interferon-mediated upregulation. Overall, we claim that IE1 performs a broad modulation of innate immune signaling to impede the execution of necroptotic cell death, thereby generating a favorable environment for efficient viral replication.

The SUMOylation of Human Cytomegalovirus Capsid Assembly Protein Precursor (UL80.5) Affects Its Interaction with Major Capsid Protein (UL86) and Viral Replication

Human Cytomegalovirus Capsid Assembly Protein Precursor (pAP, UL80.5) plays a key role in capsid assembly by forming an internal protein scaffold with Major Capsid Protein (MCP, UL86) and other capsid subunits. In this study, we revealed UL80.5 as a novel SUMOylated viral protein. We confirmed that UL80.5 interacted with the SUMO E2 ligase UBC9 (58-93aa) and could be covalently modified by SUMO1/SUMO2/SUMO3 proteins. ³⁷¹Lysine located within a ψKxE consensus motif on UL80.5 carboxy-terminal was the major SUMOylation site. Interestingly, the SUMOylation of UL80.5 restrained its interaction with UL86 but had no effects on translocating UL86 into the nucleus. Furthermore, we showed that the removal of the ³⁷¹lysine SUMOylation site of UL80.5 inhibited viral replication. In conclusion, our data demonstrates that SUMOylation plays an important role in regulating UL80.5 functions and viral replication.

Cross-Species Analysis of Innate Immune Antagonism by Cytomegalovirus IE1 Protein

The human cytomegalovirus (CMV) immediate early 1 (IE1) protein has evolved as a multifunctional antagonist of intrinsic and innate immune mechanisms. In addition, this protein serves as a transactivator and potential genome maintenance protein. Recently, the crystal structures of the human and rat CMV IE1 (hIE1, rIE1) core domain were solved. Despite low sequence identity, the respective structures display a highly similar, all alpha-helical fold with distinct variations. To elucidate which activities of IE1 are either species-specific or conserved, this study aimed at a comparative analysis of hIE1 and rIE1 functions. To facilitate the quantitative evaluation of interactions between IE1 and cellular proteins, a sensitive NanoBRET assay was established. This confirmed the species-specific interaction of IE1 with the cellular restriction factor promyelocytic leukemia protein (PML) and with the DNA replication factor flap endonuclease 1 (FEN1). To characterize the respective binding surfaces, helix exchange mutants were generated by swapping hIE1 helices with the corresponding rIE1 helices. Interestingly, while all mutants were defective for PML binding, loss of FEN1 interaction was confined to the exchange of helices 1 and 2, suggesting that FEN1 binds to the stalk region of IE1. Furthermore, our data reveal that both hIE1 and rIE1 antagonize human STAT2; however, distinct regions of the respective viral proteins mediated the interaction. Finally, while PML, FEN1, and STAT2 binding were conserved between primate and rodent proteins, we detected that rIE1 lacks a chromatin tethering function suggesting that this activity is dispensable for rat CMV. In conclusion, our study revealed conserved and distinct functions of primate and rodent IE1 proteins, further supporting the concept that IE1 proteins underwent a narrow co-evolution with their respective hosts to maximize their efficacy in antagonizing innate immune mechanisms and supporting viral replication.

Dual signaling via interferon and DNA damage response elicits entrapment by giant PML nuclear bodies

PML nuclear bodies (PML-NBs) are dynamic interchromosomal macromolecular complexes implicated in epigenetic regulation as well as antiviral defense. During herpesvirus infection, PML-NBs induce epigenetic silencing of viral genomes, however, this defense is antagonized by viral regulatory proteins such as IE1 of human cytomegalovirus (HCMV). Here, we show that PML-NBs undergo a drastic rearrangement into highly enlarged PML cages upon infection with IE1-deficient HCMV. Importantly, our results demonstrate that dual signaling by interferon and DNA damage response is required to elicit giant PML-NBs. DNA labeling revealed that invading HCMV genomes are entrapped inside PML-NBs and remain stably associated with PML cages in a transcriptionally repressed state. Intriguingly, by correlative light and transmission electron microscopy (EM), we observed that PML cages also entrap newly assembled viral capsids demonstrating a second defense layer in cells with incomplete first line response. Further characterization by 3D EM showed that hundreds of viral capsids are tightly packed into several layers of fibrous PML. Overall, our data indicate that giant PML-NBs arise via combined interferon and DNA damage signaling which triggers entrapment of both nucleic acids and proteinaceous components. This represents a multilayered defense strategy to act in a cytoprotective manner and to combat viral infections.

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.

Virus-host protein interactions as footprints of human cytomegalovirus replication

Human cytomegalovirus (HCMV) is a pervasive β-herpesvirus that causes lifelong infection. The lytic replication cycle of HCMV is characterized by global organelle remodeling and dynamic virus-host interactions, both of which are necessary for productive HCMV replication. With the advent of new technologies for investigating protein-protein and protein-nucleic acid interactions, numerous critical interfaces between HCMV and host cells have been identified. Here, we review temporal and spatial virus-host interactions that support different stages of the HCMV replication cycle. Understanding how HCMV interacts with host cells during entry, replication, and assembly, as well as how it interfaces with host cell metabolism and immune responses promises to illuminate processes that underlie the biology of infection and the resulting pathologies.

Cytomegalovirus immediate-early 1 proteins form a structurally distinct protein class with adaptations determining cross-species barriers

Restriction factors are potent antiviral proteins that constitute a first line of intracellular defense by blocking viral replication and spread. During co-evolution, however, viruses have developed antagonistic proteins to modulate or degrade the restriction factors of their host. To ensure the success of lytic replication, the herpesvirus human cytomegalovirus (HCMV) expresses the immediate-early protein IE1, which acts as an antagonist of antiviral, subnuclear structures termed PML nuclear bodies (PML-NBs). IE1 interacts directly with PML, the key protein of PML-NBs, through its core domain and disrupts the dot-like multiprotein complexes thereby abrogating the antiviral effects. Here we present the crystal structures of the human and rat cytomegalovirus core domain (IE1_CORE). We found that IE1_CORE domains, also including the previously characterized IE1_CORE of rhesus CMV, form a distinct class of proteins that are characterized by a highly similar and unique tertiary fold and quaternary assembly. This contrasts to a marked amino acid sequence diversity suggesting that strong positive selection evolved a conserved fold, while immune selection pressure may have fostered sequence divergence of IE1. At the same time, we detected specific differences in the helix arrangements of primate versus rodent IE1_CORE structures. Functional characterization revealed a conserved mechanism of PML-NB disruption, however, primate and rodent IE1 proteins were only effective in cells of the natural host species but not during cross-species infection. Remarkably, we observed that expression of HCMV IE1 allows rat cytomegalovirus replication in human cells. We conclude that cytomegaloviruses have evolved a distinct protein tertiary structure of IE1 to effectively bind and inactivate an important cellular restriction factor. Furthermore, our data show that the IE1 fold has been adapted to maximize the efficacy of PML targeting in a species-specific manner and support the concept that the PML-NBs-based intrinsic defense constitutes a barrier to cross-species transmission of HCMV.

Cytomegaloviruses have evolved in very close association with their hosts resulting in a highly species-specific replication. Cell-intrinsic proteins, known as restriction factors, constitute important barriers for cross-species infection of viruses. All cytomegaloviruses characterized so far express an abundant immediate-early protein, termed IE1, that binds to the cellular restriction factor promyelocytic leukemia protein (PML) and antagonizes its repressive activity on viral gene expression. Here, we present the crystal structures of the PML-binding domains of rat and human cytomegalovirus IE1. Despite low amino-acid sequence identity both proteins share a highly similar and unique fold forming a distinct protein class. Functional characterization revealed a common mechanism of PML antagonization. However, we also detected that the respective IE1 proteins only interact with PML proteins of the natural host species. Interestingly, expression of HCMV IE1 allows rat cytomegalovirus infection in human cells. This indicates that the cellular restriction factor PML forms an important barrier for cross-species infection of cytomegaloviruses that might be overcome by adaptation of IE1 protein function. Our data suggest that the cytomegalovirus IE1 structure represents an evolutionary optimized protein fold targeting PML proteins via coiled-coil interactions.

Sumoylation of the Carboxy-Terminal of Human Cytomegalovirus DNA Polymerase Processivity Factor UL44 Attenuates Viral DNA Replication

Controlled regulation of genomic DNA synthesis is a universally conserved process for all herpesviruses, including human cytomegalovirus (HCMV), and plays a key role in viral pathogenesis, such as persistent infections. HCMV DNA polymerase processivity factor UL44 plays an essential role in viral DNA replication. To better understand the biology of UL44, we performed a yeast two-hybrid screen for host proteins that could interact with UL44. The most frequently isolated result was the SUMO-conjugating enzyme UBC9, a protein involved in the sumoylation pathway. The UBC9-UL44 interaction was confirmed by in vitro His-tag pull-down and in vivo co-immunoprecipitation assays. Using deletion mutants of UL44, we mapped two small regions of UL44, aa 11–16, and 260–269, which might be critical for the interaction with UBC9. We then demonstrated that UL44 was a target for sumoylation by in vitro and in vivo sumoylation assays, as well as in HCMV-infected cells. We further confirmed that ⁴¹⁰lysine located within a ψKxE consensus motif on UL44 carboxy-terminal was the major sumoylation site of UL44. Interestingly, although ⁴¹⁰lysine had no effects on subcellular localization or protein stability of UL44, the removal of ⁴¹⁰lysine sumoylation site enhanced both viral DNA synthesis in transfection-replication assays and viral progeny production in infected cells for HCMV, suggesting sumoylation can attenuate HCMV replication through targeting UL44. Our results suggest that sumoylation plays a key role in regulating UL44 functions and viral replication, and reveal the crucial role of the carboxy-terminal of UL44, for which little function has been known before.

Functional regulation of the structure-specific endonuclease FEN1 by the human cytomegalovirus protein IE1 suggests a role for the re-initiation of stalled viral replication forks

Flap endonuclease 1 (FEN1) is a member of the family of structure-specific endonucleases implicated in regulation of DNA damage response and DNA replication. So far, knowledge on the role of FEN1 during viral infections is limited. Previous publications indicated that poxviruses encode a conserved protein that acts in a manner similar to FEN1 to stimulate homologous recombination, double-strand break (DSB) repair and full-size genome formation. Only recently, cellular FEN1 has been identified as a key component for hepatitis B virus cccDNA formation. Here, we report on a novel functional interaction between Flap endonuclease 1 (FEN1) and the human cytomegalovirus (HCMV) immediate early protein 1 (IE1). Our results provide evidence that IE1 manipulates FEN1 in an unprecedented manner: we observed that direct IE1 binding does not only enhance FEN1 protein stability but also phosphorylation at serine 187. This correlates with nucleolar exclusion of FEN1 stimulating its DSB-generating gap endonuclease activity. Depletion of FEN1 and inhibition of its enzymatic activity during HCMV infection significantly reduced nascent viral DNA synthesis demonstrating a supportive role for efficient HCMV DNA replication. Furthermore, our results indicate that FEN1 is required for the formation of DSBs during HCMV infection suggesting that IE1 acts as viral activator of FEN1 in order to re-initiate stalled replication forks. In summary, we propose a novel mechanism of viral FEN1 activation to overcome replication fork barriers at difficult-to-replicate sites in viral genomes.

Cell Fusion Induced by a Fusion-Active Form of Human Cytomegalovirus Glycoprotein B (gB) Is Inhibited by Antibodies Directed at Antigenic Domain 5 in the Ectodomain of gB

Human cytomegalovirus (HCMV) is a ubiquitous pathogen that can cause severe clinical disease in allograft recipients and infants infected in utero Virus-neutralizing antibodies defined in vitro have been proposed to confer protection against HCMV infection, and the virion envelope glycoprotein B (gB) serves as a major target of neutralizing antibodies. The viral fusion protein gB is nonfusogenic on its own and requires glycoproteins H (gH) and L (gL) for membrane fusion, which is in contrast to requirements of related class III fusion proteins, including vesicular stomatitis virus glycoprotein G (VSV-G) or baculovirus gp64. To explore requirements for gB's fusion activity, we generated a set of chimeras composed of gB and VSV-G or gp64, respectively. These gB chimeras were intrinsically fusion active and led to the formation of multinucleated cell syncytia when expressed in the absence of other viral proteins. Utilizing a panel of virus-neutralizing gB-specific monoclonal antibodies (MAbs), we could demonstrate that syncytium formation of the fusogenic gB/VSV-G chimera can be significantly inhibited by only a subset of neutralizing MAbs which target antigenic domain 5 (AD-5) of gB. This observation argues for differential modes of action of neutralizing anti-gB MAbs and suggests that blocking the membrane fusion function of gB could be one mechanism of antibody-mediated virus neutralization. In addition, our data have important implications for the further understanding of the conformation of gB that promotes membrane fusion as well as the identification of structures in AD-5 that could be targeted by antibodies to block this early step in HCMV infection.IMPORTANCE HCMV is a major global health concern, and antiviral chemotherapy remains problematic due to toxicity of available compounds and the emergence of drug-resistant viruses. Thus, an HCMV vaccine represents a priority for both governmental and pharmaceutical research programs. A major obstacle for the development of a vaccine is a lack of knowledge of the nature and specificities of protective immune responses that should be induced by such a vaccine. Glycoprotein B of HCMV is an important target for neutralizing antibodies and, hence, is often included as a component of intervention strategies. By generation of fusion-active gB chimeras, we were able to identify target structures of neutralizing antibodies that potently block gB-induced membrane fusion. This experimental system provides an approach to screen for antibodies that interfere with gB's fusogenic activity. In summary, our data will likely contribute to both rational vaccine design and the development of antibody-based therapies against HCMV.

A quantitative nuclear egress assay to investigate the nucleocytoplasmic capsid release of human cytomegalovirus

Nuclear egress is a rate-limiting step of herpesviral replication, restricting the nucleocytoplasmic transport of viral capsids. The process is regulated by two viral nuclear egress proteins (core NEC pUL50-pUL53), which recruit additional cellular and viral proteins. The multicomponent NEC mediates disassembly of the nuclear lamina barrier and the docking of nuclear capsids. The quantitation of nuclear egress has been accomplished by electron microscopic analysis, but is generally hampered by the low number of detectable cytoplasmic capsids. A newly established method for the quantitation of viral nuclear egress improves the characterization of viral mutants, host cell permissiveness and antiviral drug efficacy. In this study, various strains of human cytomegalovirus (HCMV) were used to measure the replication efficiencies in primary human fibroblasts, applying methods of cell fractionation, DNase digestion, sucrose cushions and quantitative PCR. Several stages of optimization led to a reliable quantitative assay that allowed the characterization of viral nuclear egress efficacy. Using this assay, recovery of the nuclear egress of a NEC-defective HCMV mutant was quantitatively assessed by applying an inducible NEC-expressing fibroblast culture for trans-complementation. This novel assay system can be further used to accurately quantitate and characterize the functionality of nuclear egress of HCMV or other herpesviruses.

Phosphosite Analysis of the Cytomegaloviral mRNA Export Factor pUL69 Reveals Serines with Critical Importance for Recruitment of Cellular Proteins Pin1 and UAP56/URH49

Human cytomegalovirus (HCMV) encodes the viral mRNA export factor pUL69, which facilitates the cytoplasmic accumulation of mRNA via interaction with the cellular RNA helicase UAP56 or URH49. We reported previously that pUL69 is phosphorylated by cellular CDKs and the viral CDK-like kinase pUL97. Here, we set out to identify phosphorylation sites within pUL69 and to characterize their importance. Mass spectrometry-based phosphosite mapping of pUL69 identified 10 serine/threonine residues as phosphoacceptors. Surprisingly, only a few of these sites localized to the N terminus of pUL69, which could be due to the presence of additional posttranslational modifications, like arginine methylation. As an alternative approach, pUL69 mutants with substitutions of putative phosphosites were analyzed by Phos-tag SDS-PAGE. This demonstrated that serines S46 and S49 serve as targets for phosphorylation by pUL97. Furthermore, we provide evidence that phosphorylation of these serines mediates cis/trans isomerization by the prolyl isomerase Pin1, thus forming a functional Pin1 binding motif. Surprisingly, while abrogation of the Pin1 motif did not affect the replication of recombinant cytomegaloviruses, mutation of serines next to the interaction site for UAP56/URH49 strongly decreased viral replication. This was correlated with a loss of UAP56/URH49 recruitment. Intriguingly, the critical serines S13 and S15 were located within a sequence resembling the UAP56 binding motif (UBM) of cellular mRNA adaptor proteins like REF and UIF. We propose that betaherpesviral mRNA export factors have evolved an extended UAP56/URH49 recognition sequence harboring phosphorylation sites to increase their binding affinities. This may serve as a strategy to successfully compete with cellular mRNA adaptor proteins for binding to UAP56/URH49.IMPORTANCE The multifunctional regulatory protein pUL69 of human cytomegalovirus acts as a viral RNA export factor with a critical role in efficient replication. Here, we identify serine/threonine phosphorylation sites for cellular and viral kinases within pUL69. We demonstrate that the pUL97/CDK phosphosites within alpha-helix 2 of pUL69 are crucial for its cis/trans isomerization by the cellular protein Pin1. Thus, we identified pUL69 as the first HCMV-encoded protein that is phosphorylated by cellular and viral serine/threonine kinases in order to serve as a substrate for Pin1. Furthermore, our study revealed that betaherpesviral mRNA export proteins contain extended binding motifs for the cellular mRNA adaptor proteins UAP56/URH49 harboring phosphorylated serines that are critical for efficient viral replication. Knowledge of the phosphorylation sites of pUL69 and the processes regulated by these posttranslational modifications is important in order to develop antiviral strategies based on a specific interference with pUL69 phosphorylation.

Bright and Early: Inhibiting Human Cytomegalovirus by Targeting Major Immediate-Early Gene Expression or Protein Function

The human cytomegalovirus (HCMV), one of eight human herpesviruses, establishes lifelong latent infections in most people worldwide. Primary or reactivated HCMV infections cause severe disease in immunosuppressed patients and congenital defects in children. There is no vaccine for HCMV, and the currently approved antivirals come with major limitations. Most approved HCMV antivirals target late molecular processes in the viral replication cycle including DNA replication and packaging. “Bright and early” events in HCMV infection have not been exploited for systemic prevention or treatment of disease. Initiation of HCMV replication depends on transcription from the viral major immediate-early (IE) gene. Alternative transcripts produced from this gene give rise to the IE1 and IE2 families of viral proteins, which localize to the host cell nucleus. The IE1 and IE2 proteins are believed to control all subsequent early and late events in HCMV replication, including reactivation from latency, in part by antagonizing intrinsic and innate immune responses. Here we provide an update on the regulation of major IE gene expression and the functions of IE1 and IE2 proteins. We will relate this insight to experimental approaches that target IE gene expression or protein function via molecular gene silencing and editing or small chemical inhibitors.

Casein kinase-2-mediated phosphorylation increases the SUMO-dependent activity of the cytomegalovirus transactivator IE2

Many viral factors manipulate the host post-translational modification (PTM) machinery for efficient viral replication. In particular, phosphorylation and SUMOylation can distinctly regulate the activity of the human cytomegalovirus (HCMV) transactivator immediate early 2 (IE2). However, the molecular mechanism of this process is unknown. Using various structural, biochemical, and cell-based approaches, here we uncovered that IE2 exploits a cross-talk between phosphorylation and SUMOylation. A scan for small ubiquitin-like modifier (SUMO)-interacting motifs (SIMs) revealed two SIMs in IE2, and a real-time SUMOylation assay indicated that the N-terminal SIM (IE2-SIM1) enhances IE2 SUMOylation up to 4-fold. Kinetic analysis and structural studies disclosed that IE2 is a SUMO cis-E3 ligase. We also found that two putative casein kinase 2 (CK2) sites adjacent to IE2-SIM1 are phosphorylated in vitro and in cells. The phosphorylation drastically increased IE2-SUMO affinity, IE2 SUMOylation, and cis-E3 activity of IE2. Additional salt bridges between the phosphoserines and SUMO accounted for the increased IE2-SUMO affinity. Phosphorylation also enhanced the SUMO-dependent transactivation activity and auto-repression activity of IE2. Together, our findings highlight a novel mechanism whereby SUMOylation and phosphorylation of the viral cis-E3 ligase and transactivator protein IE2 work in tandem to enable transcriptional regulation of viral gene.

A conserved and buried edge-to-face aromatic interaction in small ubiquitin-like modifier (SUMO) has a role in SUMO stability and function

Aromatic amino acids buried at a protein's core are often involved in mutual paired interactions. Ab initio energy calculations have highlighted that the conformational orientations and the effects of substitutions are important for stable aromatic interactions among aromatic rings, but studies in the context of a protein's fold and function are elusive. Small ubiquitin-like modifier (SUMO) is a common post-translational modifier that affects diverse cellular processes. Here, we report that a highly conserved aromatic triad of three amino acids, Phe36-Tyr51-Phe64, is a unique SUMO signature that is absent in other ubiquitin-like homologous folds. We found that a specific edge-to-face conformation between the Tyr51-Phe64 pair of interacting aromatics is vital to the fold and stability of SUMO. Moreover, the noncovalent binding of SUMO-interacting motif (SIM) at the SUMO surface was critically dependent on the paired aromatic interactions buried at the core. NMR structural studies revealed that perturbation of the Tyr51-Phe64 conformation disrupts several long-range tertiary contacts in SUMO, leading to a heterogeneous and dynamic protein with attenuated SUMOylation both in vitro and in cells. A subtle perturbation of the edge-to-face conformation by a Tyr to Phe substitution significantly decreased stability, SUMO/SIM affinity, and the rate of SUMOylation. Our results highlight that absolute co-conservation of specific aromatic pairs inside the SUMO protein core has a role in its stability and function.

Chromatin-Remodeling Factor SPOC1 Acts as a Cellular Restriction Factor against Human Cytomegalovirus by Repressing the Major Immediate Early Promoter

The cellular protein SPOC1 (survival time-associated PHD [plant homeodomain] finger protein in ovarian cancer 1) acts as a regulator of chromatin structure and the DNA damage response. It binds H3K4me2/3-containing chromatin and promotes DNA condensation by recruiting corepressors such as KAP-1 and H3K9 methyltransferases. Previous studies identified SPOC1 as a restriction factor against human adenovirus (HAdV) infection that is antagonized by E1B-55K/E4-orf6-dependent proteasomal degradation. Here, we demonstrate that, in contrast to HAdV-infected cells, SPOC1 is transiently upregulated during the early phase of human cytomegalovirus (HCMV) replication. We show that the expression of immediate early protein 1 (IE1) is sufficient and necessary to induce SPOC1. Additionally, we discovered that during later stages of infection, SPOC1 is downregulated in a glycogen synthase kinase 3β (GSK-3β)-dependent manner. We provide evidence that SPOC1 overexpression severely impairs HCMV replication by repressing the initiation of viral immediate early (IE) gene expression. Consistently, we observed that SPOC1-depleted primary human fibroblasts displayed an augmented initiation of viral IE gene expression. This occurs in a multiplicity of infection (MOI)-dependent manner, a defining hallmark of intrinsic immunity. Interestingly, repression requires the presence of high SPOC1 levels at the start of infection, while later upregulation had no negative impact, suggesting distinct temporal roles of SPOC1 during the HCMV replicative cycle. Mechanistically, we observed a highly specific association of SPOC1 with the major immediate early promoter (MIEP), strongly suggesting that SPOC1 inhibits HCMV replication by MIEP binding and the subsequent recruitment of heterochromatin-building factors. Thus, our data add SPOC1 as a novel factor to the endowment of a host cell to restrict cytomegalovirus infections.IMPORTANCE Accumulating evidence indicates that during millennia of coevolution, host cells have developed a sophisticated compilation of cellular factors to restrict cytomegalovirus infections. Defining this equipment is important to understand cellular barriers against viral infection and to develop strategies to utilize these factors for antiviral approaches. So far, constituents of PML nuclear bodies and interferon gamma-inducible protein 16 (IFI16) were known to mediate intrinsic immunity against HCMV. In this study, we identify the chromatin modulator SPOC1 as a novel restriction factor against HCMV. We show that preexisting high SPOC1 protein levels mediate a silencing of HCMV gene expression via a specific association with an important viral cis-regulatory element, the major immediate early promoter. Since SPOC1 expression varies between cell types, this factor may play an important role in tissue-specific defense against HCMV.

SUMOylation of IE2p86 is required for efficient autorepression of the human cytomegalovirus major immediate-early promoter

The human cytomegalovirus (HCMV) IE2p86 protein is pivotal for coordinated regulation of viral gene expression. Besides functioning as a promiscuous transactivator, IE2p86 is also known to negatively regulate its own transcription. This occurs via direct binding of IE2p86 to a 14-bp palindromic DNA element located between the TATA box and the transcription start site of the major immediate-early promoter (MIEP), which is referred to as the cis repression signal (CRS). However, the exact mechanism of IE2p86-based autorepression is still unclear. By testing a series of IE2p86 mutants in transient expression assays, we found that not only did a DNA binding-deficient mutant of IE2p86 fail to repress the MIEP, but SUMOylation-negative mutants also failed to repress it. This finding was further supported by infection studies with primary fibroblasts harbouring a MIEP-driven transgene as a reporter. Here, we observed that a recombinant HCMV expressing SUMOylation-negative IE2p86 was defective in transgene downregulation, in contrast to wild-type HCMV. Interestingly, however, a double-mutant virus in which both the SUMO acceptor sites and the SUMO interaction motif (SIM) of IE2p86 were inactivated regained the ability to silence the MIEP. This correlated with increased expression levels of the IE2 isoforms IE2p40 and IE2p60, suggesting that these late proteins may contribute to MIEP suppression, thus compensating for the loss of IE2p86 SUMOylation. In summary, our results show that autorepression of the MIEP is not only regulated by late isoforms of IE2, but also depends on posttranslational SUMO modification, revealing a novel mechanism to fine-tune the expression of this important viral gene region.

Addition of an EGFP-tag to the N-terminal of influenza virus M1 protein impairs its ability to accumulate in ND10

A previous report demonstrated that influenza virus infection induces accumulation of EGFP-tagged M1 protein (EGFP-M1) in the sub-nuclear domain ND10. Here, we show that the transfection of four viral protein (NP, PB2, PB1, PA) expression vectors and eight RNA segment expression vectors induced the formation of nuclear dots of EGFP-M1 as seen in virus infections. Omission of the segment 7 RNA expression vector, however, abolished the nuclear dots of EGFP-M1. This result suggests an essential role for authentic M1 protein and/or M2 protein, both of which are encoded in segment 7, in the formation of nuclear dots of EGFP-M1. Co-expression of M1 protein but not M2 protein with EGFP-M1 induced the formation of nuclear dots of EGFP-M1. The dots co-localized with PML protein, which is an indicator of ND10. When only M1 protein was expressed, immunostaining of M1 protein clearly revealed the nuclear dots and their colocalization with PML protein. These results demonstrate that the accumulation in ND10 is an intrinsic characteristic of M1 protein and EGFP addition abolishes this characteristic. The addition of EGFP to M1 protein induced a defect in M1 protein.

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.

The ND10 Component Promyelocytic Leukemia Protein Acts as an E3 Ligase for SUMOylation of the Major Immediate Early Protein IE1 of Human Cytomegalovirus

Previous studies identified the nuclear domain 10 (ND10) components promyelocytic leukemia protein (PML), hDaxx, and Sp100 as factors of an intrinsic immune response against human cytomegalovirus (HCMV). This antiviral function of ND10, however, is antagonized by viral effector proteins like IE1p72, which induces dispersal of ND10. Furthermore, we have shown that both major immediate early proteins of HCMV, IE1p72 and IE2p86, transiently colocalize with ND10 subnuclear structures and undergo modification by the covalent attachment of SUMO. Since recent reports indicate that PML acts as a SUMO E3 ligase, we asked whether the SUMOylation of IE1p72 and IE2p86 is regulated by PML. To address this, PML-depleted fibroblasts, as well as cells overexpressing individual PML isoforms, were infected with HCMV. Western blot experiments revealed a clear correlation between the degree of IE1p72 SUMO conjugation and the abundance of PML. On the other hand, the SUMOylation of IE2p86 was not affected by PML. By performing in vitro SUMOylation assays, we were able to provide direct evidence that IE1p72 is a substrate for PML-mediated SUMOylation. Interestingly, disruption of the RING finger domain of PML, which is proposed to confer SUMO E3 ligase activity, abolished PML-induced SUMOylation of IE1p72. In contrast, IE1p72 was still efficiently SUMO modified by a SUMOylation-defective PML mutant, indicating that intact ND10 bodies are not necessary for this effect. Thus, this is the first report that the E3 ligase PML is capable of stimulating the SUMOylation of a viral protein which is supposed to serve as a cellular mechanism to compromise specific functions of IE1p72.IMPORTANCE The major immediate early proteins of human cytomegalovirus, termed IE1p72 and IE2p86, have previously been shown to undergo posttranslational modification by covalent coupling to SUMO moieties at specific lysine residues. However, the enzymatic activities that are responsible for this modification have not been identified. Here, we demonstrate that the PML protein, which mediates an intrinsic immune response against HCMV, specifically serves as an E3 ligase for SUMO modification of IE1p72. Since SUMO modification of IE1p72 has previously been shown to interfere with STAT factor binding, thus compromising the interferon-antagonistic function of this viral effector protein, our finding highlights an additional mechanism through which PML is able to restrict viral infections.

The Epstein-Barr virus miR-BHRF1-1 targets RNF4 during productive infection to promote the accumulation of SUMO conjugates and the release of infectious virus

Post-translational modification by the Small Ubiquitin-like Modifier (SUMO) regulates a variety of cellular functions, and is hijacked by viruses to remodel the host cell during latent and productive infection. Here we have monitored the activity of the SUMO conjugation machinery in cells productively infected with Epstein-Barr virus (EBV). We found that SUMO2/3 conjugates accumulate during the late phase of the productive virus cycle, and identified several viral proteins as bone fide SUMOylation substrates. Analysis of the mechanism involved in the accumulation of SUMOylated proteins revealed upregulation of several components of the SUMO-conjugation machinery and post-transcriptional downregulation of the SUMO-targeted ubiquitin ligase RNF4. The latter effect was mediated by selective inhibition of RNF4 protein expression by the viral miR-BHRF1-1. Reconstitution of RNF4 in cells expressing an inducible miR-BHRF1-1 sponge or a miR-BHRF1-1 resistant RNF4 was associated with reduced levels of early and late viral proteins and impaired virus release. These findings illustrate a novel strategy for viral interference with the SUMO pathway, and identify the EBV miR-BHRF1-1 and the cellular RNF4 as regulators of the productive virus cycle.

We have investigated the activity of the SUMOylation machinery in cells infected with Epstein-Barr virus (EBV), a human herpesvirus that infects B-lymphocytes and is associated with malignancies. We found that activation of the productive virus cycle is accompanied by accumulation of SUMO conjugates, upregulation of components of the SUMO conjugation machinery, and downregulation of the SUMO-targeted ubiquitin ligase RNF4. The decrease of RNF4 is due to post-transcriptional downregulation by miR-BHRF1-1, a member of the BHRF1 microRNA cluster that is upregulated during productive infection. The effect of miR-BHRF1-1 was confirmed in luciferase reported assays, by mutation of the RNF4 3’UTR seed site, by transfection of a synthetic miR-BHRF1-1 mimic, by ectopic expression of miR-BHRF1-1 and by the reversal of RNF4 downregulation in cells expressing a miR-BHRF1-1 sponge. We also found that several early and late viral proteins are bona fide SUMOylation substrates. Reconstitution of RNF4 in productively infected cells was accompanied by proteasome-dependent degradation of the SUMOylated viral protein and by a significantly reduced virus yield. These findings illustrate a new strategy for viral interference with the SUMO pathway, an unexpected contribution of miR-BHRF1-1 to the productive cycle of EBV and a previously unrecognized role of the RNF4 ligase in the regulation of virus production.

Characterization of a thienylcarboxamide derivative that inhibits the transactivation functions of cytomegalovirus IE2 and varicella zoster virus IE62

Previously we established reporter cell lines for human cytomegalovirus (HCMV) and varicella zoster virus (VZV) and identified several antiviral compounds against these viruses using the reporter cells. In this study, we found that one of the identified anti-HCMV compounds, a thienylcarboxamide derivative (coded as 133G4), was effective against not only HCMV but also VZV. The following findings indicate that 133G4 inhibits the activation of early gene promoters by HCMV IE2 and VZV IE62: i) 133G4 decreased the expression of HCMV early and late genes but not that of HCMV IE1/IE2 in HCMV-infected cells, ii) 133G4 inhibited the activation of several HCMV early gene promoters of transiently-transfected plasmids in HCMV-infected cells, and iii) in transient transfection assays, 133G4 decreased the activation of HCMV (or VZV) early gene promoters by HCMV IE2 (or VZV IE62) in the absence of other viral protein expression. The inhibition of early gene activation was observed in the human and African green monkey cell lines but not in the rodent cell lines, and the compound was not effective against murine CMV. In addition, VZV IE62 activated HCMV early promoters, and 133G4 still inhibited such promoter activation. Therefore, we hypothesized that 133G4 targets a cellular factor used commonly in activation of human herpesvirus promoters and examined whether 133G4 affects the functions of cellular proteins USF1, TBP, Med25 and EAP, the involvement of which in VZV IE62-dependent viral gene activation has been well characterized. Our experimental results using one-hybrid and bimolecular fluorescence complementation assays demonstrated that 133G4 did not inhibit the recruitment of USF1 or TBP to their binding sites, nor inhibited the direct interactions of VZV IE62 with Med25 and EAP. Thus, 133G4 is a unique anti-VZV and -HCMV compound, which warrants further studies to find out its inhibitory mechanism.

The Human Cytomegalovirus IE1 Protein Antagonizes PML Nuclear Body-Mediated Intrinsic Immunity via the Inhibition of PML De Novo SUMOylation

PML nuclear bodies (NBs) are accumulations of cellular proteins embedded in a scaffold-like structure built by SUMO-modified PML/TRIM19. PML and other NB proteins act as cellular restriction factors against human cytomegalovirus (HCMV); however, this intrinsic defense is counteracted by the immediate early protein 1 (IE1) of HCMV. IE1 directly interacts with the PML coiled-coil domain via its globular core region and disrupts NB foci by inducing a loss of PML SUMOylation. Here, we demonstrate that IE1 acts via abrogating the de novo SUMOylation of PML. In order to overcome reversible SUMOylation dynamics, we made use of a cell-based assay that combines inducible IE1 expression with a SUMO mutant resistant to SUMO proteases. Interestingly, we observed that IE1 expression did not affect preSUMOylated PML; however, it clearly prevented de novo SUMO conjugation. Consistent results were obtained by in vitro SUMOylation assays, demonstrating that IE1 alone is sufficient for this effect. Furthermore, IE1 acts in a selective manner, since K160 was identified as the main target lysine. This is strengthened by the fact that IE1 also prevents As₂O₃-mediated hyperSUMOylation of K160, thereby blocking PML degradation. Since IE1 did not interfere with coiled-coil-mediated PML dimerization, we propose that IE1 affects PML autoSUMOylation either by directly abrogating PML E3 ligase function or by preventing access to SUMO sites. Thus, our data suggest a novel mechanism for how a viral protein counteracts a cellular restriction factor by selectively preventing the de novo SUMOylation at specific lysine residues without affecting global protein SUMOylation.

IMPORTANCE

The human cytomegalovirus IE1 protein acts as an important antagonist of a cellular restriction mechanism that is mediated by subnuclear structures termed PML nuclear bodies. This function of IE1 is required for efficient viral replication and thus constitutes a potential target for antiviral strategies. In this paper, we further elucidate the molecular mechanism for how IE1 antagonizes PML NBs. We show that tight binding of IE1 to PML interferes with the de novo SUMOylation of a distinct lysine residue that is also the target of stress-mediated hyperSUMOylation of PML. This is of importance since it represents a novel mechanism used by a viral antagonist of intrinsic immunity. Furthermore, it highlights the possibility of developing small molecules that specifically abrogate this PML-antagonistic activity of IE1 and thus inhibit viral replication.

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

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

IMPORTANCE

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

Characterization of Recombinant Human Cytomegaloviruses Encoding IE1 Mutants L174P and 1-382 Reveals that Viral Targeting of PML Bodies Perturbs both Intrinsic and Innate Immune Responses

PML is the organizer of cellular structures termed nuclear domain 10 (ND10) or PML-nuclear bodies (PML-NBs) that act as key mediators of intrinsic immunity against human cytomegalovirus (HCMV) and other viruses. The antiviral function of ND10 is antagonized by viral regulatory proteins such as the immediate early protein IE1 of HCMV. IE1 interacts with PML through its globular core domain (IE1CORE) and induces ND10 disruption in order to initiate lytic HCMV infection. Here, we investigate the consequences of a point mutation (L174P) in IE1CORE, which was shown to abrogate the interaction with PML, for lytic HCMV infection. We found that a recombinant HCMV encoding IE1-L174P displays a severe growth defect similar to that of an IE1 deletion virus. Bioinformatic modeling based on the crystal structure of IE1CORE suggested that insertion of proline into the highly alpha-helical domain severely affects its structural integrity. Consistently, L174P mutation abrogates the functionality of IE1CORE and results in degradation of the IE1 protein during infection. In addition, our data provide evidence that IE1CORE as expressed by a recombinant HCMV encoding IE1 1-382 not only is required to antagonize PML-mediated intrinsic immunity but also affects a recently described function of PML in innate immune signaling. We demonstrate a coregulatory role of PML in type I and type II interferon-induced gene expression and provide evidence that upregulation of interferon-induced genes is inhibited by IE1CORE. In conclusion, our data suggest that targeting PML by viral regulatory proteins represents a strategy to antagonize both intrinsic and innate immune mechanisms.

IMPORTANCE

PML nuclear bodies (PML-NBs), which represent nuclear multiprotein complexes consisting of PML and additional proteins, represent important cellular structures that mediate intrinsic resistance against many viruses, including human cytomegalovirus (HCMV). During HCMV infection, the major immediate early protein IE1 binds to PML via a central globular domain (IE1CORE), and we have shown previously that this is sufficient to antagonize intrinsic immunity. Here, we demonstrate that modification of PML by IE1CORE not only abrogates intrinsic defense mechanisms but also attenuates the interferon response during infection. Our data show that PML plays a novel coregulatory role in type I as well as type II interferon-induced gene expression, which is antagonized by IE1CORE. Importantly, our finding supports the view that targeting of PML-NBs by viral regulatory proteins has evolved as a strategy to inhibit both intrinsic and innate immune defense mechanisms.

pUL69 of Human Cytomegalovirus Recruits the Cellular Protein Arginine Methyltransferase 6 via a Domain That Is Crucial for mRNA Export and Efficient Viral Replication

The regulatory protein pUL69 of human cytomegalovirus acts as a viral mRNA export factor, facilitating the cytoplasmic accumulation of unspliced RNA via interaction with the cellular mRNA export factor UAP56. Here we provide evidence for a posttranslational modification of pUL69 via arginine methylation within the functionally important N terminus. First, we demonstrated a specific immunoprecipitation of full-length pUL69 as well as pUL69aa1-146 by a mono/dimethylarginine-specific antibody. Second, we observed a specific electrophoretic mobility shift upon overexpression of the catalytically active protein arginine methyltransferase 6 (PRMT6). Third, a direct interaction of pUL69 and PRMT6 was confirmed by yeast two-hybrid and coimmunoprecipitation analyses. We mapped the PRMT6 interaction motif to the pUL69 N terminus and identified critical amino acids within the arginine-rich R1 box of pUL69 that were crucial for PRMT6 and/or UAP56 recruitment. In order to test the impact of putative methylation substrates on the functions of pUL69, we constructed various pUL69 derivatives harboring arginine-to-alanine substitutions and tested them for RNA export activity. Thus, we were able to discriminate between arginines within the R1 box of pUL69 that were crucial for UAP56/PRMT6-interaction and/or mRNA export activity. Remarkably, nuclear magnetic resonance (NMR) analyses revealed the same α-helical structures for pUL69 sequences encoding either the wild type R1/R2 boxes or a UAP56/PRMT6 binding-deficient derivative, thereby excluding the possibility that R/A amino acid substitutions within R1 affected the secondary structure of pUL69. We therefore conclude that the pUL69 N terminus is methylated by PRMT6 and that this critically affects the functions of pUL69 for efficient mRNA export and replication of human cytomegalovirus.

IMPORTANCE

The UL69 protein of human cytomegalovirus is a multifunctional regulatory protein that acts as a viral RNA export factor with a critical role for efficient replication. Here, we demonstrate that pUL69 is posttranslationally modified via arginine methylation and that the protein methyltransferase PRMT6 mediates this modification. Furthermore, arginine residues with a crucial function for RNA export and for binding of the cellular RNA export factor UAP56 as well as PRMT6 were mapped within the arginine-rich R1 motif of pUL69. Importantly, we demonstrated that mutation of those arginines did not alter the secondary structure of R1, suggesting that they may serve as critical methylation substrates. In summary, our study reveals a novel posttranslational modification of pUL69 which has a significant impact on the function of this important viral regulatory protein. Since PRMTs appear to be amenable to selective inhibition by small molecules, this may constitute a novel target for antiviral therapy.

Contribution of the Major ND10 Proteins PML, hDaxx and Sp100 to the Regulation of Human Cytomegalovirus Latency and Lytic Replication in the Monocytic Cell Line THP-1

Promyelocytic leukemia nuclear bodies, also termed nuclear domain 10 (ND10), have emerged as nuclear protein accumulations mediating an intrinsic cellular defense against viral infections via chromatin-based mechanisms, however, their contribution to the control of herpesviral latency is still controversial. In this study, we utilized the monocytic cell line THP-1 as an in vitro latency model for human cytomegalovirus infection (HCMV). Characterization of THP-1 cells by immunofluorescence andWestern blot analysis confirmed the expression of all major ND10 components. THP-1 cells with a stable, individual knockdown of PML, hDaxx or Sp100 were generated. Importantly, depletion of the major ND10 proteins did not prevent the terminal cellular differentiation of THP-1 monocytes. After construction of a recombinant, endotheliotropic human cytomegalovirus expressing IE2-EYFP, we investigated whether the depletion of ND10 proteins affects the onset of viral IE gene expression. While after infection of differentiated, THP-1-derived macrophages as well as during differentiation-induced reactivation from latency an increase in the number of IE-expressing cells was readily detectable in the absence of the major ND10 proteins, no effect was observed in non-differentiated monocytes. We conclude that PML, hDaxx and Sp100 primarily act as cellular restriction factors during lytic HCMV replication and during the dynamic process of reactivation but do not serve as key determinants for the establishment of HCMV latency.

In vivo characterization of the properties of SUMO1-specific monobodies

Monobodies are small recombinant proteins designed to bind with high affinity to target proteins. Monobodies have been generated to mimic the SIM [SUMO (small ubiquitin-like modifier)-interacting motif] present in many SUMO target proteins, but their properties have not been determined in cells. In the present study we characterize the properties of two SUMO1-specific monobodies (hS1MB4 and hS1MB5) in HEK (human embyronic kidney)-293 and HeLa cells and examine their ability to purify SUMO substrates from cell lines and rat brain. Both hS1MB4 and hS1MB5 compared favourably with commercially available antibodies and were highly selective for binding to SUMO1 over SUMO2/3 in pull-down assays against endogenous and overexpressed SUMO and SUMOylated proteins. Monobodies expressed in HeLa cells displayed a nuclear and cytosolic distribution that overlaps with SUMO1. Expression of the monobodies effectively inhibited protein SUMOylation by SUMO1 and, surprisingly, by SUMO2/3, but were not cytotoxic for at least 36 h. We attribute the effects on SUMO2/3 to the role of SUMO1 in chain termination and/or monobody inhibition of the SUMO-conjugating E1 enzyme complex. Taken together, these data provide the first demonstration that monobodies represent useful new tools both to isolate SUMO conjugates and to probe cell SUMOylation pathways in vivo.

Crystal structure of cytomegalovirus IE1 protein reveals targeting of TRIM family member PML via coiled-coil interactions

PML nuclear bodies (PML-NBs) are enigmatic structures of the cell nucleus that act as key mediators of intrinsic immunity against viral pathogens. PML itself is a member of the E3-ligase TRIM family of proteins that regulates a variety of innate immune signaling pathways. Consequently, viruses have evolved effector proteins to modify PML-NBs; however, little is known concerning structure-function relationships of viral antagonists. The herpesvirus human cytomegalovirus (HCMV) expresses the abundant immediate-early protein IE1 that colocalizes with PML-NBs and induces their dispersal, which correlates with the antagonization of NB-mediated intrinsic immunity. Here, we delineate the molecular basis for this antagonization by presenting the first crystal structure for the evolutionary conserved primate cytomegalovirus IE1 proteins. We show that IE1 consists of a globular core (IE1_CORE) flanked by intrinsically disordered regions. The 2.3 Å crystal structure of IE1_CORE displays an all α-helical, femur-shaped fold, which lacks overall fold similarity with known protein structures, but shares secondary structure features recently observed in the coiled-coil domain of TRIM proteins. Yeast two-hybrid and coimmunoprecipitation experiments demonstrate that IE1_CORE binds efficiently to the TRIM family member PML, and is able to induce PML deSUMOylation. Intriguingly, this results in the release of NB-associated proteins into the nucleoplasm, but not of PML itself. Importantly, we show that PML deSUMOylation by IE1_CORE is sufficient to antagonize PML-NB-instituted intrinsic immunity. Moreover, co-immunoprecipitation experiments demonstrate that IE1_CORE binds via the coiled-coil domain to PML and also interacts with TRIM5α We propose that IE1_CORE sequesters PML and possibly other TRIM family members via structural mimicry using an extended binding surface formed by the coiled-coil region. This mode of interaction might render the antagonizing activity less susceptible to mutational escape.

Research of the last few years has revealed that microbial infections are not only controlled by innate and adaptive immune mechanisms, but also by cellular restriction factors, which give cells the capacity to resist pathogens. PML nuclear bodies (PML-NBs) are dot-like nuclear structures representing multiprotein complexes that consist of the PML protein, a member of the TRIM family of proteins, as well as a multitude of additional regulatory factors. PML-NB components act as a barrier against many viral infections; however, viral antagonistic proteins have evolved to modify PML-NBs, thus abrogating this cellular defense. Here, we delineate the molecular basis for antagonization by the immediate-early protein IE1 of the herpesvirus human cytomegalovirus. We present the first crystal structure for the evolutionary conserved core domain (IE1_CORE) of primate cytomegalovirus IE1, which exhibits a novel, unusual fold. IE1_CORE modifies PML-NBs by releasing other PML-NB proteins into the nucleoplasm which is sufficient to antagonize intrinsic immunity. Importantly, IE1_CORE shares secondary structure features with the coiled-coil domain (CC) of TRIM factors, and we demonstrate strong binding of IE1 to the PML-CC. We propose that IE1_CORE sequesters PML and possibly other TRIM family members via an extended binding surface formed by the coiled-coil domain.

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 human cytomegalovirus IE1 protein: past and present developments

ABSTRACT: 

Human cytomegalovirus (HCMV), a member of the β-herpesvirus subfamily, is an important pathogen that infects the majority of the human population. The evolutionary success of HCMV largely depends on its ability to evade host defense systems and establish a lifelong persistence after primary infection. In fact, HCMV has dedicated a considerable part of its gene products to manipulate or disable immune effector processes. This review focuses on the major immediate–early protein IE1 – a multifunctional key regulator that has the capacity to counteract the first host defense activities. We summarize the known structural and mechanistic features by which IE1 modulates innate immune mechanisms as well as other cellular processes, and discuss how the individual functions of IE1 contribute to the success of a lytic HCMV infection.

A unique SUMO-2-interacting motif within LANA is essential for KSHV latency

Kaposi's sarcoma-associated herpesvirus (KSHV) stabilizes hypoxia-inducible factor α (HIF-1α) during latent infection, and HIF-1α reactivates lytic replication under hypoxic stress. However, the mechanism utilized by KSHV to block lytic reactivation with the accumulation of HIF-1α in latency remains unclear. Here, we report that LANA encoded by KSHV contains a unique SUMO-interacting motif (LANA^SIM) which is specific for interaction with SUMO-2 and facilitates LANA SUMOylation at lysine 1140. Proteomic and co-immunoprecipitation analysis further reveal that the SUMO-2 modified transcription repressor KAP1 is a critical factor recruited by LANA^SIM. Deletion of LANA^SIM led to functional loss of both LANA-mediated viral episome maintenance and lytic gene silencing. Moreover, hypoxia reduced KAP1 SUMOylation and resulted in dissociation of both KAP1 and Sin3A repressors from LANA^SIM-associated complex. Therefore, the LANA^SIM motif plays an essential role in KSHV latency and is a potential drug target against KSHV-associated cancers.

Hypoxia stress is a common feature of tumor microenvironment and is widely associated with pathogenesis linked to many oncogenic viruses. Kaposi's sarcoma-associated herpesvirus (KSHV), the etiological cause of Kaposi's sarcoma and primary effusion lymphoma, has been reported to encode several proteins that usurp hypoxia signaling during infection. One encoded protein LANA is a latent protein important for regulation of KSHV life cycle and cell transformation. The molecular mechanisms of how KSHV controls life cycle in normoxia and hypoxia is not fully understood. In this study, we show that LANA contains a unique SUMO-interacting motif (LANA^SIM) which is specific for SUMO-2 binding. Importantly, SUMO-2 modified KAP1, a chromatin remodeling factor recruited by LANA^SIM is hypoxia sensitive, and plays a critical role in silencing viral gene expression. This discovery not only adds to our understanding of hypoxia-mediated remodeling of the viral episome in a SUMO dependent manner, but also provides a new dimension to development of therapeutic strategies against KSHV-associated cancers.

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.

SCE1, the SUMO-conjugating enzyme in plants that interacts with NIb, the RNA-dependent RNA polymerase of Turnip mosaic virus, is required for viral infection

SUMOylation, which is catalyzed by small ubiquitin-like modifier (SUMO) enzymes, is a transient, reversible posttranslational protein modification that regulates diverse cellular processes. Potyviruses, the largest group of known plant viruses, comprise many agriculturally important viruses, such as Turnip mosaic virus (TuMV). The potyviral genome encodes 11 mature proteins. To investigate if SUMOylation plays a role in potyvirus infection, a yeast two-hybrid screen was performed to examine possible interactions of each of the 11 viral proteins of TuMV with AtSCE1, the only SUMO-conjugating enzyme in Arabidopsis thaliana homologous to the key SUMO-conjugating enzyme E2 in mammalian cells or Ubc9 in yeast. A positive reaction was found between AtSCE1 and NIb, the potyviral RNA-dependent RNA polymerase. Further bimolecular fluorescence complementation (BiFC) and fluorescence resonance energy transfer (FRET) assays revealed that the NIb and AtSCE1 interaction occurred in both the cytoplasm and nuclei of epidermal cells of Nicotiana benthamiana. The interaction motif was mapped to a region encompassing NIb amino acids 171 to 300 which contains a potential negatively charged amino acid-dependent SUMOylation motif (NDSM). An Escherichia coli SUMOylation assay showed that NIb can be SUMOylated and that the lysine residue (K172) in the motif is a potent SUMOylation site. A TuMV infectious clone with an arginine (R) substitution mutation at K172 compromised TuMV infectivity in plants. In comparison with wild-type Arabidopsis plants, sce1 knockdown plants exhibited increased resistance to TuMV as well as a nonrelated RNA virus. To the best of our knowledge, this is the first report showing that the host SUMO modification system plays an essential role in infection by plant RNA viruses.

An endogenous accelerator for viral gene expression confers a fitness advantage

Many signaling circuits face a fundamental tradeoff between accelerating their response speed while maintaining final levels below a cytotoxic threshold. Here, we describe a transcriptional circuitry that dynamically converts signaling inputs into faster rates without amplifying final equilibrium levels. Using time-lapse microscopy, we find that transcriptional activators accelerate human cytomegalovirus (CMV) gene expression in single cells without amplifying steady-state expression levels, and this acceleration generates a significant replication advantage. We map the accelerator to a highly self-cooperative transcriptional negative-feedback loop (Hill coefficient ∼7) generated by homomultimerization of the virus's essential transactivator protein IE2 at nuclear PML bodies. Eliminating the IE2-accelerator circuit reduces transcriptional strength through mislocalization of incoming viral genomes away from PML bodies and carries a heavy fitness cost. In general, accelerators may provide a mechanism for signal-transduction circuits to respond quickly to external signals without increasing steady-state levels of potentially cytotoxic molecules.

Small ubiquitin-related modifier (SUMO) pathway-mediated enhancement of human cytomegalovirus replication correlates with a recruitment of SUMO-1/3 proteins to viral replication compartments

Recent studies have suggested that the small ubiquitin-related modifier (SUMO) conjugation pathway may play an important role in intrinsic antiviral resistance and thus for repression of herpesviral infections. In particular, it was shown that the herpes simplex virus type-1 regulatory protein ICP0 acts as a SUMO-targeted ubiquitin ligase (STUbL), inducing the widespread degradation of SUMO-conjugated proteins during infection. As the IE1 protein of human cytomegalovirus (HCMV) is known to mediate a de-SUMOylation of PML, we investigated whether HCMV uses a similar mechanism to counteract intrinsic antiviral resistance. We generated primary human fibroblasts stably expressing FLAG-SUMO-1 or FLAG-SUMO-3 and analysed the SUMOylation pattern after HCMV infection or isolated IE1 expression. However, Western blot experiments did not reveal a global loss of SUMO conjugates, either in HCMV-infected or in IE1-expressing cells, arguing against a function of IE1 as an STUbL. Interestingly, we observed that FLAG-SUMO-1 and FLAG-SUMO-3, subsequent to IE1-mediated promyelocytic leukemia protein (PML) de-SUMOylation and the consequent disruption of PML nuclear bodies, were recruited into viral replication compartments. This raised the question of whether FLAG-SUMO-1/3 might promote HCMV replication. Intriguingly, overexpression of FLAG-SUMO-1/3 enhanced accumulation of viral DNA, which correlated with an increase in viral replication and in virus particle release. Together, these data indicate that HCMV, in contrast to other herpesviruses, has evolved subtle mechanisms enabling it to utilize the SUMO conjugation pathway for its own benefit, resulting in an overall positive effect of SUMO conjugation for HCMV replication.

Coordination of late gene transcription of human cytomegalovirus with viral DNA synthesis: recombinant viruses as potential therapeutic vaccine candidates

INTRODUCTION

During productive infection, human cytomegalovirus (HCMV) genes are expressed in a temporal cascade, with temporal phases designated as immediate-early (IE), early, and late. The major IE (MIE) genes, UL123 and UL122 (IE1/IE2), play a critical role in subsequent viral gene expression and the efficiency of viral replication. The early viral genes encode proteins necessary for viral DNA replication. Following viral DNA replication, delayed-early and late viral genes are expressed which encode structural proteins for the virion. The late genes can be divided into two broad classes. At early times the gamma-1 or leaky-late class are expressed at low levels after infection and are dramatically upregulated at late times. In contrast, the gamma-2 or 'true' late genes are expressed exclusively after viral DNA replication. Expression of true late (gamma-2 class) viral genes is completely prevented by inhibition of viral DNA synthesis.

AREAS COVERED

This review addresses the viral genes required for HCMV late gene transcription. Recombinant viruses that are defective for late gene transcription allow for early viral gene expression and viral DNA synthesis, but not infectious virus production. Since current HCMV prophylaxis is limited by several shortcomings, the use of defective recombinant viruses to induce HCMV cell-mediated and humoral immunity is discussed.

EXPERT OPINION

HCMV DNA replication and late gene transcription are not completely linked. Viral-encoded trans-acting factors are required. Recombinant viruses proficient in MIE and early viral gene expression and defective in late gene expression may be an alternative therapeutic vaccine candidates for the induction of cell-mediated and humoral immunity.

Human cytomegalovirus infection leads to elevated levels of transplant arteriosclerosis in a humanized mouse aortic xenograft model

Recent findings emphasized an important role of human cytomegalovirus (HCMV) infection in the development of transplant arteriosclerosis. Therefore, the aim of this study was to develop a human peripheral blood lymphocyte (hu-PBL)/Rag-2(-/-) γc(-/-) mouse-xenograft-model to investigate both immunological as well as viral effector mechanisms in the progression of transplant arteriosclerosis. For this, sidebranches from the internal mammary artery were recovered during coronary artery bypass graft surgery, tissue-typed and infected with HCMV. Then, size-matched sidebranches were implanted into the infrarenal aorta of Rag-2(-/-) γc(-/-) mice. The animals were reconstituted with human peripheral blood mononuclear cells (PBMCs) 7 days after transplantation. HCMV-infection was confirmed by Taqman-PCR and immunofluorescence analyses. Arterial grafts were analyzed by histology on day 40 after transplantation. PBMC-reconstituted Rag-2(-/-) γc(-/-) animals showed splenic chimerism levels ranging from 1-16% human cells. After reconstitution, Rag-2(-/-) γc(-/-) mice developed human leukocyte infiltrates in their grafts and vascular lesions that were significantly elevated after infection. Cellular infiltration revealed significantly increased ICAM-1 and PDGF-R-β expression after HCMV-infection of the graft. Arterial grafts from unreconstituted Rag-2(-/-) γc(-/-) recipients showed no vascular lesions. These data demonstrate a causative relationship between HCMV-infection as an isolated risk factor and the development of transplant-arteriosclerosis in a humanized mouse arterial-transplant-model possibly by elevated ICAM-1 and PDGF-R-β expression.

Transfer of the UAP56 interaction motif of human cytomegalovirus pUL69 to its murine cytomegalovirus homolog converts the protein into a functional mRNA export factor that can substitute for pUL69 during viral infection

Nucleocytoplasmic shuttling and interaction with the cellular mRNA export factor UAP56 are prerequisites for the mRNA export activity of human cytomegalovirus (HCMV) pUL69. Although the murine cytomegalovirus homolog pM69 shuttles, it fails to export mRNAs due to its inability to recruit UAP56. However, chimeric proteins comprising pM69 fused to N-terminal pUL69 fragments, including its UAP56 interaction motif, acquire mRNA export activity. Importantly, growth curves of recombinant HCMVs illustrate that such a chimeric protein, but not pM69, substitutes for pUL69 during HCMV infection.

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.

Regulation of the nucleocytoplasmic trafficking of viral and cellular proteins by ubiquitin and small ubiquitin-related modifiers

Nucleocytoplasmic trafficking of many cellular proteins is regulated by nuclear import/export signals as well as post-translational modifications such as covalent conjugation of ubiquitin and small ubiquitin-related modifiers (SUMOs). Ubiquitination and SUMOylation are rapid and reversible ways to modulate the intracellular localisation and function of substrate proteins. These pathways have been co-opted by some viruses, which depend on the host cell machinery to transport their proteins in and out of the nucleus. In this review, we will summarise our current knowledge on the ubiquitin/SUMO-regulated nuclear/subnuclear trafficking of cellular proteins and describe examples of viral exploitation of these pathways.

Regulation of vaccinia virus E3 protein by small ubiquitin-like modifier proteins

The vaccinia virus (VACV) E3 protein is essential for virulence and has antiapoptotic activity and the ability to impair the host innate immune response. Here we demonstrate that E3 interacts with SUMO1 through a small ubiquitin-like modifier (SUMO)-interacting motif (SIM). SIM integrity is required for maintaining the stability of the viral protein and for the covalent conjugation of E3 to SUMO1 or SUMO2, a modification that has a negative effect on the E3 transcriptional transactivation of the p53-upregulated modulator of apoptosis (PUMA) and APAF-1 genes. We also demonstrate that E3 is ubiquitinated, a modification that does not destabilize the wild-type protein but triggers the degradation of an E3-ΔSIM mutant. This report constitutes the first demonstration of the important roles that both SUMO and ubiquitin play in the regulation of the VACV protein E3.

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.

Changing the ubiquitin landscape during viral manipulation of the DNA damage response

Viruses often induce signaling through the same cellular cascades that are activated by damage to the cellular genome. Signaling triggered by viral proteins or exogenous DNA delivered by viruses can be beneficial or detrimental to viral infection. Viruses have therefore evolved to dissect the cellular DNA damage response pathway during infection, often marking key cellular regulators with ubiquitin to induce their degradation or change their function. Signaling controlled by ubiquitin or ubiquitin-like proteins has recently emerged as key regulator of the cellular DNA damage response. Situated at the interface between DNA damage signaling and the ubiquitin system, viruses can reveal key convergence points in this important cellular pathway. In this review, we examine how viruses harness the diversity of the cellular ubiquitin system to modulate the DNA damage signaling pathway. We discuss the implications of viral infiltration of this pathway for both the transcriptional program of the virus and for the cellular response to DNA damage.

Sumoylation of the P protein at K254 plays an important role in growth of parainfluenza virus 5

The P protein of parainfluenza virus 5 (PIV5) is an essential cofactor of the viral RNA-dependent RNA polymerase. Phosphorylation of the P protein can positively or negatively regulate viral gene expression, depending on the precise phosphorylation sites. Sumoylation, a process of adding small ubiquitin-like modifier (SUMO) to proteins posttranslationally, plays an important role in regulating protein function. In this study, we have found that the P protein of PIV5 was sumoylated with SUMO1 in both transfected and infected cells. The K254 residue of the P protein is within a consensus sumoylation motif. Mutation of the P protein at K254 to arginine (P-K254R) reduced PIV5 minigenome activity, as well as the sumoylation level of the P protein. Incorporation of K254R into a recombinant PIV5 (rPIV5-P-K254R) resulted in a virus that grew to a lower titer and had lower levels of viral RNA synthesis and protein expression than wild-type PIV5, suggesting that sumoylation of the P protein at K254 is important for PIV5 growth. Biochemical studies did not reveal any defect of P-K254R in its interactions with viral proteins NP and L or formation of homotetramers. We propose that sumoylation of the P protein at K254 regulates PIV5 gene expression through a host protein.