Non-covalent interaction between nucleocapsid protein of Tula hantavirus and small ubiquitin-related modifier-1, SUMO-1

Interplay between RNA Viruses and Promyelocytic Leukemia Nuclear Bodies

Promyelocytic leukemia nuclear bodies (PML NBs) are nuclear membrane-less sub structures that play a critical role in diverse cellular pathways including cell proliferation, DNA damage, apoptosis, transcriptional regulation, stem cell renewal, alternative lengthening of telomeres, chromatin organization, epigenetic regulation, protein turnover, autophagy, intrinsic and innate antiviral immunity. While intrinsic and innate immune functions of PML NBs or PML NB core proteins are well defined in the context of nuclear replicating DNA viruses, several studies also confirm their substantial roles in the context of RNA viruses. In the present review, antiviral activities of PML NBs or its core proteins on diverse RNA viruses that replicate in cytoplasm or the nucleus were discussed. In addition, viral counter mechanisms that reorganize PML NBs, and specifically how viruses usurp PML NB functions in order to create a cellular environment favorable for replication and pathogenesis, are also discussed.

The RNA Replication Site of Tula Orthohantavirus Resides within a Remodelled Golgi Network

The family Hantaviridae within the Bunyavirales order comprises tri-segmented negative sense RNA viruses, many of which are rodent-borne emerging pathogens associated with fatal human disease. In contrast, hantavirus infection of corresponding rodent hosts results in inapparent or latent infections, which can be recapitulated in cultured cells that become persistently infected. In this study, we used Tula virus (TULV) to investigate the location of hantavirus replication during early, peak and persistent phases of infection, over a 30-day time course. Using immunofluorescent (IF) microscopy, we showed that the TULV nucleocapsid protein (NP) is distributed within both punctate and filamentous structures, with the latter increasing in size as the infection progresses. Transmission electron microscopy of TULV-infected cell sections revealed these filamentous structures comprised aligned clusters of filament bundles. The filamentous NP-associated structures increasingly co-localized with the Golgi and with the stress granule marker TIA-1 over the infection time course, suggesting a redistribution of these cellular organelles. The analysis of the intracellular distribution of TULV RNAs using fluorescent in-situ hybridization revealed that both genomic and mRNAs co-localized with Golgi-associated filamentous compartments that were positive for TIA. These results show that TULV induces a dramatic reorganization of the intracellular environment, including the establishment of TULV RNA synthesis factories in re-modelled Golgi compartments.

HTNV Sensitizes Host Toward TRAIL-Mediated Apoptosis-A Pivotal Anti-hantaviral Role of TRAIL

Hantaviruses can cause hemorrhagic fever with renal syndrome (HFRS) in Eurasia and have led to public health threat in China. The pathogenesis of HFRS is complex and involves capillary leakage due to the infection of vascular endothelial cells. Accumulating evidence has demonstrated that hantavirus can induce apoptosis in many cells, but the mechanism remains unclear. Our studies showed that Hantaan virus (HTNV) infection could induce TNF-related apoptosis-inducing ligand (TRAIL) expression in primary human umbilical vein endothelial cells (HUVECs) and sensitize host cells toward TRAIL-mediated apoptosis. Furthermore, TRAIL interference could inhibit apoptosis and enhance the production of HTNV as well as reduce IFN-β production, while exogenous TRAIL treatment showed reverse outcome: enhanced apoptosis and IFN-β production as well as a lower level of viral replication. We also observed that nucleocapsid protein (NP) and glycoprotein (GP) of HTNV could promote the transcriptions of TRAIL and its receptors. Thus, TRAIL was upregulated by HTNV infection and then exhibited significant antiviral activities in vitro, and it was further confirmed in the HTNV-infected suckling mice model that TRAIL treatment significantly reduced viral load, alleviated virus-induced tissue lesions, increased apoptotic cells, and decreased the mortality. In conclusion, these results demonstrate that TRAIL-dependent apoptosis and IFN-β production could suppress HTNV replication and TRAIL treatment might be a novel therapeutic target for HTNV infection.

SUMO and Cytoplasmic RNA Viruses: From Enemies to Best Friends

SUMO is a ubiquitin-like protein that covalently binds to lysine residues of target proteins and regulates many biological processes such as protein subcellular localization or stability, transcription, DNA repair, innate immunity, or antiviral defense. SUMO has a critical role in the signaling pathway governing type I interferon (IFN) production, and among the SUMOylation substrates are many IFN-induced proteins. The overall effect of IFN is increasing global SUMOylation, pointing to SUMO as part of the antiviral stress response. Viral agents have developed different mechanisms to counteract the antiviral activities exerted by SUMO, and some viruses have evolved to exploit the host SUMOylation machinery to modify their own proteins. The exploitation of SUMO has been mainly linked to nuclear replicating viruses due to the predominant nuclear localization of SUMO proteins and enzymes involved in SUMOylation. However, SUMOylation of numerous viral proteins encoded by RNA viruses replicating at the cytoplasm has been lately described. Whether nuclear localization of these viral proteins is required for their SUMOylation is unclear. Here, we summarize the studies on exploitation of SUMOylation by cytoplasmic RNA viruses and discuss about the requirement for nuclear localization of their proteins.

The nucleocapsid protein of hantaviruses: much more than a genome-wrapping protein

The nucleocapsid (N) protein of hantaviruses represents an impressive example of a viral multifunctional protein. It encompasses properties as diverse as genome packaging, RNA chaperoning, intracellular protein transport, DNA degradation, intervention in host translation, and restricting host immune responses. These functions all rely on the capability of N to interact with RNA and other viral and cellular proteins. We have compiled data on the N protein of different hantavirus species together with information of the recently published three-dimensional structural data of the protein. The array of diverse functional activities accommodated in the hantaviral N protein goes far beyond to be a static structural protein and makes it an interesting target in the development of antiviral therapeutics.

Apoptosis during arenavirus infection: mechanisms and evasion strategies

In recent years there has been a greatly increased interest in the interactions of arenaviruses with the apoptotic machinery, and particularly the extent to which these interactions may be an important contributor to pathogenesis. Here we summarize the current state of our knowledge on this subject and address the potential for interplay with other immunological mechanisms known to be regulated by these viruses. We also compare and contrast what is known for arenavirus-induced apoptosis with observations from other segmented hemorrhagic fever viruses.

Phosphorylation of the nucleocapsid protein of Hantaan virus by casein kinase II

Hantaanvirus (HTNV) is the prototype of the genus Hantavirus, which belongs to the family Bunyaviridae. Hantaviruses are carried and transmitted by rodents and are known to cause two serious disease syndromes in humans i.e., hemorrhagic fever with renal syndrome (HFRS) and the hantavirus pulmonary syndrome (HPS). HTNV is an enveloped virus that contains a tripartite genome consisting of three negative-sense RNA segments (L, M, S), and the S and M segment of HTNV, respectively, encode the viral nucleocapsid protein (NP) and envelope glycoproteins. Possible phosphorylation motifs of casein kinase II (CKII) and protein kinase C (PKC) were identified in HTNV NP through bioinformatics searches. Sucrose gradient SDS-PAGE analysis indicated that dephosphorylated HTNV NP migrated faster than non-dephosphorylated NP, suggesting that HTNV NP is phosphorylated in infected Vero E6 cells. Immunoblot anaylsis of HTNV particles with anti-phosphoserine antibody and anti-phosphothreonine antibody after immunoprecipitation showed that viral particles are readily phosphorylated at threonine residues. In vitro kinase assay further showed that HTNV NP is phosphorylated by CK II, but not by PKC. Full length or truncated HTNV NPs expressed in E. coli were phosphorylated in vitro by CKII suggesting that phosphorylation may occur in vivo at multiple sites. Site specific mutagenesis studies suggest that HTNV NP phosphorylation might occur at unknown sites excluding the site-directly mutagenized locations. Taken together, HTNV NP can be phosphorylated mainly at threonine residues in vivo by CK II treatment.

Antigenic properties of N protein of hantavirus

Hantavirus causes two important rodent-borne viral zoonoses, hemorrhagic fever with renal syndrome (HFRS) in Eurasia and hantavirus pulmonary syndrome (HPS) in North and South America. Twenty-four species that represent sero- and genotypes have been registered within the genus Hantavirus by the International Committee on Taxonomy of Viruses (ICTV). Among the viral proteins, nucleocapsid (N) protein possesses an immunodominant antigen. The antigenicitiy of N protein is conserved compared with that of envelope glycoproteins. Therefore, N protein has been used for serological diagnoses and seroepidemiological studies. An understanding of the antigenic properties of N protein is important for the interpretation of results from serological tests using N antigen. N protein consists of about 430 amino acids and possesses various epitopes. The N-terminal quarter of N protein bears linear and immunodominant epitopes. However, a serotype-specific and multimerization-dependent antigenic site was found in the C-terminal half of N protein. In this paper, the structure, function, and antigenicity of N protein are reviewed.

Proteomic analysis of differentially expressed proteins in Fenneropenaeus chinensis hemocytes upon white spot syndrome virus infection

To elucidate molecular responses of shrimp hemocytes to white spot syndrome virus (WSSV) infection, two-dimensional gel electrophoresis was applied to investigate differentially expressed proteins in hemocytes of Chinese shrimp (Fenneropenaeus chinensis) at 24 h post infection (hpi). Approximately 580 protein spots were detected in hemocytes of healthy and WSSV-infected shrimps. Quantitative intensity analysis revealed 26 protein spots were significantly up-regulated, and 19 spots were significantly down-regulated. By mass spectrometry, small ubiquitin-like modifier (SUMO) 1, cytosolic MnSOD, triosephosphate isomerase, tubulin alpha-1 chain, microtubule-actin cross-linking factor 1, nuclear receptor E75 protein, vacuolar ATP synthase subunit B L form, inositol 1,4,5-trisphosphate receptor, arginine kinase, etc., amounting to 33 differentially modulated proteins were identified successfully. According to Gene Ontology annotation, the identified proteins were classified into nine categories, consisting of immune related proteins, stimulus response proteins, proteins involved in glucose metabolic process, cytoskeleton proteins, DNA or protein binding proteins, proteins involved in steroid hormone mediated signal pathway, ATP synthases, proteins involved in transmembrane transport and ungrouped proteins. Meanwhile, the expression profiles of three up-regulated proteins (SUMO, heat shock protein 70, and arginine kinase) and one down-regulated protein (prophenoloxidase) were further analyzed by real-time RT-PCR at the transcription level after WSSV infection. The results showed that SUMO and heat shock protein 70 were significantly up-regulated at each sampling time point, while arginine kinase was significantly up-regulated at 12 and 24 hpi. In contrast, prophenoloxidase was significantly down-regulated at each sampling time point. The results of this work provided preliminary data on proteins in shrimp hemocytes involved in WSSV infection.

Ubiquitin-like protein modifiers and their potential for antiviral and anti-HCV therapy

All viral infections subvert the host immune response. Targeting the host mechanisms that are modulated by viral infection offers new avenues for antiviral drug development. Host ubiquitin and multiple ubiquitin-like modifiers (Ubls) are commonly altered by, or important for, viral infection. Protein modification by ubiquitin or Ubls contributes to numerous cellular processes, such as protein degradation, signal transduction, protein relocalization and pathogen-host interactions. This post-translational modification plays an essential role for viral life cycles and host antiviral mechanisms. Some Ubls, such as ISG15 and SUMO, have been shown to modulate virus infections and are potential targets for therapeutic manipulation. Hepatitis C virus (HCV) is a positive-stranded RNA virus that predominantly infects hepatocytes. Recent data suggest that ISG15 might be a potential drug target for anti-HCV therapy. Inhibition of ISG15 expression and/or ISG15 conjugation (ISGylation) provides a rationale for the design of new anti-HCV drugs.

Death-domain associated protein-6 (DAXX) mediated apoptosis in hantavirus infection is counter-balanced by activation of interferon-stimulated nuclear transcription factors

Hantaviruses are negative strand RNA species that replicate predominantly in the cytoplasm. They also activate numerous cellular responses, but their involvement in nuclear processes is yet to be established. Using human umbilical vein endothelial cells (HUVECs), this study investigates the molecular finger-print of nuclear transcription factors during hantavirus infection. The viral-replication-dependent activation of pro-myelocytic leukemia protein (PML) was followed by subsequent localization in nuclear bodies (NBs). PML was also found in close proximity to activated Sp100 nuclear antigen and interferon-stimulated gene 20 kDa protein (ISG-20), but co-localization with death-domain associated protein-6 (DAXX) was not observed. These data demonstrate that hantavirus triggers PML activation and localization in NBs in the absence of DAXX-PLM-NB co-localization. The results suggest that viral infection interferes with DAXX-mediated apoptosis, and expression of interferon-activated Sp100 and ISG-20 proteins may indicate intracellular intrinsic antiviral attempts.

Hantaan virus nucleocapsid protein stimulates MDM2-dependent p53 degradation

Apoptosis has been shown to be induced and downregulated by the Hantaan virus (HTNV) nucleocapsid (N) protein. To address these conflicting data, expression of the p53 protein, one of the key molecules involved in apoptosis, was assessed in the presence of the N protein in A549 and HeLa cells. The amount of p53, increased by drug treatment, was reduced when cells were infected with HTNV or transfected with an expression vector of the HTNV N protein. When cells were treated with a proteasome inhibitor (MG132) or an MDM2 antagonist (Nutlin-3), p53 expression was not reduced in N protein-overexpressed cells. We concluded that the HTNV N protein ubiquitinates and degrades p53 MDM2-dependently. Here we report downregulation of p53 expression through a post-translational mechanism: MDM2-dependent ubiquitination and degradation by the HTNV N protein. These results indicate that N protein-dependent p53 degradation through the ubiquitin proteasome system is one of the anti-apoptotic mechanisms employed by HTNV.

The murine model for Hantaan virus-induced lethal disease shows two distinct paths in viral evolutionary trajectory with and without ribavirin treatment

In vitro, ribavirin acts as a lethal mutagen in Hantaan virus (HTNV)-infected Vero E6 cells, resulting in an increased mutation load and viral population extinction. In this study, we asked whether ribavirin treatment in the lethal, suckling mouse model of HTNV infection would act similarly. The HTNV genomic RNA (vRNA) copy number and infectious virus were measured in lungs of untreated and ribavirin-treated mice. In untreated, HTNV-infected mice, the vRNA copy number increased for 10 days postinfection (dpi) and thereafter remained constant through 26 dpi. Surprisingly, in ribavirin-treated, HTNV-infected mice, vRNA levels were similar to those in untreated mice between 10 and 26 dpi. Infectious virus levels, however, were different: in ribavirin-treated mice, the amount of infectious HTNV was significantly decreased relative to that in untreated mice, suggesting that ribavirin reduced the specific infectivity of the virus (amount of infectious virus produced per vRNA copy). Mutational analysis revealed a ribavirin-associated elevation in mutation frequency in HTNV vRNA similar to that previously reported in vitro. Codon-based analyses of rates of nonsynonymous (dN) and synonymous (dS) substitutions in the S segment revealed a positive selection for codons within the HTNV N protein gene in the ribavirin-treated vRNA population. In contrast, the vRNA population in untreated, HTNV-infected mice showed a lower level of diversity, reflecting purifying selection for the wild-type genome. In summary, these experiments show two different evolutionary paths that Hantavirus may take during infection in a lethal murine model of disease, as well as the importance of the in vivo host environment in the evolution of the virus, which was not apparent in our prior in vitro model system.

SUMO downregulates GLP-1-stimulated cAMP generation and insulin secretion

Glucagon-like peptide-1 (GLP-1)-based incretin therapy is becoming central to the treatment of type 2 diabetes. Activation of incretin hormone receptors results in rapid elevation of cAMP followed by enhanced insulin secretion. However, the incretin effect may be significantly impaired in diabetes. The objective of this study is to investigate downregulation of GLP-1 signaling by small ubiquitin-related modifier protein (SUMO). Mouse islets exposed to high glucose showed increased expression of endogenous SUMO transcripts and its conjugating enzyme Ubc-9. Overexpression of SUMO-1 in mouse insulinoma 6 (MIN6) cells and primary mouse β-cells resulted in reduced static and real-time estimates of intracellular cAMP upon receptor stimulation with exendin-4, a GLP-1 receptor (GLP-1R) agonist. GLP1-R was covalently modified by SUMO. Overexpression of SUMO-1 attenuated cell surface trafficking of GLP-1R, which resulted in significantly reduced insulin secretion when stimulated by exendin-4. Partial knock down of SUMO-conjugating enzyme Ubc-9 resulted in enhanced exendin-4-stimulated insulin secretion in mouse islets exposed to high glucose. Thus, SUMO modification of the GLP-1R could be a contributing factor to reduced incretin responsiveness. Elucidating mechanisms of GLP-1R regulation by sumoylation will help improve our understanding of incretin biology and of GLP-1-based treatment of type 2 diabetes.

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.

Hantaviruses in the americas and their role as emerging pathogens

The continued emergence and re-emergence of pathogens represent an ongoing, sometimes major, threat to populations. Hantaviruses (family Bunyaviridae) and their associated human diseases were considered to be confined to Eurasia, but the occurrence of an outbreak in 1993–94 in the southwestern United States led to a great increase in their study among virologists worldwide. Well over 40 hantaviral genotypes have been described, the large majority since 1993, and nearly half of them pathogenic for humans. Hantaviruses cause persistent infections in their reservoir hosts, and in the Americas, human disease is manifest as a cardiopulmonary compromise, hantavirus cardiopulmonary syndrome (HCPS), with case-fatality ratios, for the most common viral serotypes, between 30% and 40%. Habitat disturbance and larger-scale ecological disturbances, perhaps including climate change, are among the factors that may have increased the human caseload of HCPS between 1993 and the present. We consider here the features that influence the structure of host population dynamics that may lead to viral outbreaks, as well as the macromolecular determinants of hantaviruses that have been regarded as having potential contribution to pathogenicity.

Antagonism of type I interferon responses by new world hantaviruses

Evasion of interferon (IFN)-mediated antiviral signaling is a common defense strategy for pathogenic RNA viruses. To date, research on IFN antagonism by hantaviruses is limited and has focused on only a subset of the numerous recognized hantavirus species. The host IFN response has two phases, an initiation phase, resulting in the induction of alpha/beta IFN (IFN-α/β), and an amplification phase, whereby IFN-α/β signals through the Jak/STAT pathway, resulting in the establishment of the cellular antiviral state. We examined interactions between these critical host responses and the New World hantaviruses. We observed delayed cellular responses in both Andes virus (ANDV)- and Sin Nombre virus (SNV)-infected A549 and Huh7-TLR3 cells. We found that IFN-β induction is inhibited by coexpression of ANDV nucleocapsid protein (NP) and glycoprotein precursor (GPC) and is robustly inhibited by SNV GPC alone. Downstream amplification by Jak/STAT signaling is also inhibited by SNV GPC and by either NP or GPC of ANDV. Therefore, ANDV- and SNV-encoded proteins have the potential for inhibiting both IFN-β induction and signaling, with SNV exhibiting the more potent antagonism ability. Herein we identify ANDV NP, a previously unrecognized inhibitor of Jak/STAT signaling, and show that IFN antagonism by ANDV relies on expression of both the glycoproteins and NP, whereas the glycoproteins appear to be sufficient for antagonism by SNV. These data suggest that IFN antagonism strategies by hantaviruses are quite variable, even between species with similar disease phenotypes, and may help to better elucidate species-specific pathogenesis.

Interaction between hantaviral nucleocapsid protein and the cytoplasmic tail of surface glycoprotein Gn

Hantaviral N and Gn proteins were shown to interact, thus providing the long-awaited evidence for one of the crucial steps in the virus replication at which RNPs are directed to the site of the virus assembly. Using pull-down assay and point mutagenesis it was demonstrated that intact, properly folded zinc fingers in the Gn protein cytoplasmic tail as well as the middle domain of the N protein (that includes aa residues 80-248) are essential for the interaction.

Modulation of apoptosis and immune signaling pathways by the Hantaan virus nucleocapsid protein

Herein, we show a direct relationship between the Hantaan virus (HTNV) nucleocapsid (N) protein and the modulation of apoptosis. We observed an increase in caspase-7 and -8, but not -9 in cells expressing HTNV N protein mutants lacking amino acids 270-330. Similar results were observed for the New World hantavirus, Andes virus. Nuclear factor kappa B (NF-kappaB) was sequestered in the cytoplasm after tumor necrosis factor receptor (TNFR) stimulation in cells expressing HTNV N protein. Further, TNFR stimulated cells expressing HTNV N protein inhibited caspase activation. In contrast, cells expressing N protein truncations lacking the region from amino acids 270-330 were unable to inhibit nuclear import of NF-kappaB and the mutants also triggered caspase activity. These results suggest that the HTNV circumvents host antiviral signaling and apoptotic response mediated by the TNFR pathway through host interactions with the N protein.

Production of FMDV virus-like particles by a SUMO fusion protein approach in Escherichia coli

Virus-like particles (VLPs) are formed by the self-assembly of envelope and/or capsid proteins from many viruses. Some VLPs have been proven successful as vaccines, and others have recently found applications as carriers for foreign antigens or as scaffolds in nanoparticle biotechnology. However, production of VLP was usually impeded due to low water-solubility of recombinant virus capsid proteins. Previous studies revealed that virus capsid and envelope proteins were often posttranslationally modified by SUMO in vivo, leading into a hypothesis that SUMO modification might be a common mechanism for virus proteins to retain water-solubility or prevent improper self-aggregation before virus assembly. We then propose a simple approach to produce VLPs of viruses, e.g., foot-and-mouth disease virus (FMDV). An improved SUMO fusion protein system we developed recently was applied to the simultaneous expression of three capsid proteins of FMDV in E. coli. The three SUMO fusion proteins formed a stable heterotrimeric complex. Proteolytic removal of SUMO moieties from the ternary complexes resulted in VLPs with size and shape resembling the authentic FMDV. The method described here can also apply to produce capsid/envelope protein complexes or VLPs of other disease-causing viruses.

Ubiquitination, ubiquitin-like modifiers, and deubiquitination in viral infection

Ubiquitin is important for nearly every aspect of cellular physiology. All viruses rely extensively on host machinery for replication; therefore, it is not surprising that viruses connect to the ubiquitin pathway at many levels. Viral involvement with ubiquitin occurs either adventitiously because of the unavoidable usurpation of cellular processes, or for some specific purpose selected for by the virus to enhance viral replication. Here, we review current knowledge of how the ubiquitin pathway alters viral replication and how viruses influence the ubiquitin pathway to enhance their own replication.

Hantaan virus nucleocapsid protein binds to importin alpha proteins and inhibits tumor necrosis factor alpha-induced activation of nuclear factor kappa B

Hantaviruses such as Hantaan virus (HTNV) and Andes virus cause two human diseases, hemorrhagic fever with renal syndrome and hantavirus pulmonary syndrome, respectively. For both, disease pathogenesis is thought to be immunologically mediated and there have been numerous reports of patients with elevated levels of proinflammatory and inflammatory cytokines, including tumor necrosis factor alpha (TNF-alpha), in their sera. Multiple viruses have developed evasion strategies to circumvent the host cell inflammatory process, with one of the most prevalent being the disruption of nuclear factor kappa B (NF-kappaB) activation. We hypothesized that hantaviruses might also moderate host inflammation by interfering with this pathway. We report here that the nucleocapsid (N) protein of HTNV was able to inhibit TNF-alpha-induced activation of NF-kappaB, as measured by a reporter assay, and the activation of endogenous p65, an NF-kappaB subunit. Surprisingly, there was no defect in the degradation of the inhibitor of NF-kappaB (IkappaB) protein, nor was there any alteration in the level of p65 expression in HTNV N-expressing cells. However, immunofluorescence antibody staining demonstrated that cells expressing HTNV N protein and a green fluorescent protein-p65 fusion had limited p65 nuclear translocation. Furthermore, we were able to detect an interaction between HTNV N protein and importin alpha, a nuclear import molecule responsible for shuttling NF-kappaB to the nucleus. Collectively, our data suggest that HTNV N protein can sequester NF-kappaB in the cytoplasm, thus inhibiting NF-kappaB activity. These findings, which were obtained using cells transfected with cDNA representing the HTNV N gene, were confirmed using HTNV-infected cells.

Oligomerization of hantaviral nucleocapsid protein: charged residues in the N-terminal coiled-coil domain contribute to intermolecular interactions

The nucleocapsid (N) protein of hantaviruses (family Bunyaviridae) is the most abundant component of the virion; it encapsidates genomic RNA segments and participates in viral genome transcription and replication, as well as in virus assembly. During RNA encapsidation, the N protein forms intermediate trimers and then oligomers via 'head-to-head, tail-to-tail' interactions. In previous work, using Tula hantavirus (TULV) N protein as a model, it was demonstrated that an intact coiled-coil structure of the N terminus is crucial for the oligomerization capacity of the N protein and that the hydrophobic 'a' residues from the second alpha-helix are especially important. Here, the importance of charged amino acid residues located within the coiled-coil for trimer formation and oligomerization was analysed. To predict the interacting surfaces of the monomers, the previous in silico model of TULV coiled-coils was first upgraded, taking advantage of the recently published crystal structure of the N-terminal coiled-coil of the Sin Nombre virus N protein. The results obtained using a mammalian two-hybrid assay suggested that conserved, charged amino acid residues within the coiled-coil make a substantial contribution to N protein oligomerization. This contribution probably involves (i) the formation of interacting surfaces of the N monomers (residues D35 and D38, located at the tip of the coiled-coil loop, and R63 appear particularly important) and (ii) stabilization of the coiled-coil via intramolecular ionic bridging (with E55 as a key player). It is hypothesized that the tips of the coiled-coils are the first to come into direct contact and thus to initiate tight packing of the three structures.

NMR structure of the N-terminal coiled coil domain of the Andes hantavirus nucleocapsid protein

The hantaviruses are emerging infectious viruses that in humans can cause a cardiopulmonary syndrome or a hemorrhagic fever with renal syndrome. The nucleocapsid (N) is the most abundant viral protein, and during viral assembly, the N protein forms trimers and packages the viral RNA genome. Here, we report the NMR structure of the N-terminal domain (residues 1-74, called N1-74) of the Andes hantavirus N protein. N1-74 forms two long helices (alpha1 and alpha2) that intertwine into a coiled coil domain. The conserved hydrophobic residues at the helix alpha1-alpha2 interface stabilize the coiled coil; however, there are many conserved surface residues whose function is not known. Site-directed mutagenesis, CD spectroscopy, and immunocytochemistry reveal that a point mutation in the conserved basic surface formed by Arg22 or Lys26 lead to antibody recognition based on the subcellular localization of the N protein. Thus, Arg22 and Lys26 are likely involved in a conformational change or molecular recognition when the N protein is trafficked from the cytoplasm to the Golgi, the site of viral assembly and maturation.

An improved SUMO fusion protein system for effective production of native proteins

Expression of recombinant proteins as fusions with SUMO (small ubiquitin-related modifier) protein has significantly increased the yield of difficult-to-express proteins in Escherichia coli. The benefit of this technique is further enhanced by the availability of naturally occurring SUMO proteases, which remove SUMO from the fusion protein. Here we have improved the exiting SUMO fusion protein approach for effective production of native proteins. First, a sticky-end PCR strategy was applied to design a new SUMO fusion protein vector that allows directional cloning of any target gene using two universal cloning sites (Sfo1 at the 5'-end and XhoI at the 3'-end). No restriction digestion is required for the target gene PCR product, even the insert target gene contains a SfoI or XhoI restriction site. This vector produces a fusion protein (denoted as His(6)-Smt3-X) in which the protein of interest (X) is fused to a hexahistidine (His(6))-tagged Smt3. Smt3 is the yeast SUMO protein. His(6)-Smt3-X was purified by Ni(2+) resin. Removal of His(6)-Smt3 was performed on the Ni(2+) resin by an engineered SUMO protease, His(6)-Ulp1(403-621)-His(6). Because of its dual His(6) tags, His(6)-Ulp1(403-621)-His(6) exhibits a high affinity for Ni(2) resin and associates with Ni(2+) resin after cleavage reaction. One can carry out both fusion protein purification and SUMO protease cleavage using one Ni(2+)-resin column. The eluant contains only the native target protein. Such a one-column protocol is useful in developing a better high-throughput platform. Finally, this new system was shown to be effective for cloning, expression, and rapid purification of several difficult-to-produce authentic proteins.

New Genetic Lineage of Tula Hantavirus in Microtus arvalis obscurus in Eastern Kazakhstan

Genomic sequences of Tula (TULV) hantavirus were recovered from tissue samples of European common voles Microtus arvalis (subspecies obscurus) captured in Kazakhstan, Central Asia. Phylogenetic analysis of the S genomic segment of Kazakh TULV strains showed that they form distinct, well supported genetic lineage and share a more ancient common ancestor with two Russian lineages of TULV. The deduced sequence of the nucleocapsid (N) protein of Kazakh TULV strains carried specific amino acid signature: T274Q276T281. The Microtus arvalis group includes several sibling species and/or subspecies in Eurasia, indicating recent and ongoing evolutionary radiation. Our data on TULV lineages in Central Asia, the region not studied for hantaviruses earlier, highlight the diversity of both Microtus host and the virus and also their co-evolution.

Dynein-dependent transport of the hantaan virus nucleocapsid protein to the endoplasmic reticulum-Golgi intermediate compartment

In contrast to most negative-stranded RNA viruses, hantaviruses and other viruses in the family Bunyaviridae mature intracellularly, deriving the virion envelope from the endoplasmic reticulum (ER) or Golgi compartment. While it is generally accepted that Old World hantaviruses assemble and bud into the Golgi compartment, some studies with New World hantaviruses have raised the possibility of maturation at the plasma membrane as well. Overall, the steps leading to virion assembly remain largely undetermined for hantaviruses. Because hantaviruses do not have matrix proteins, the nucleocapsid protein (N) has been proposed to play a key role in assembly. Herein, we examine the intracellular trafficking and morphogenesis of the prototype Old World hantavirus, Hantaan virus (HTNV). Using confocal microscopy, we show that N colocalized with the ER-Golgi intermediate compartment (ERGIC) in HTNV-infected Vero E6 cells, not with the ER, Golgi compartment, or early endosomes. Brefeldin A, which effectively disperses the ER, the ERGIC, and Golgi membranes, redistributed N with the ERGIC, implicating membrane association; however, subcellular fractionation experiments showed the majority of N in particulate fractions. Confocal microscopy revealed that N was juxtaposed to and distributed along microtubules and, over time, became surrounded by vimentin cages. To probe cytoskeletal association further, we probed trafficking of N in cells treated with nocodazole and cytochalasin D, which depolymerize microtubules and actin, respectively. We show that nocodazole, but not cytochalasin D, affected the distribution of N and reduced levels of intracellular viral RNA. These results suggested the involvement of microtubules in trafficking of N, whose movement could occur via molecular motors such as dynein. Overexpression of dynamitin, which is associated with dynein-mediated transport, creates a dominant-negative phenotype blocking transport on microtubules. Overexpression of dynamitin reduced N accumulation in the perinuclear region, which further supports microtubule components in N trafficking. The combined results of these experiments support targeting of N to the ERGIC prior to its movement to the Golgi compartment and the requirement of an intact ERGIC for viral replication and, thus, the possibility of virus factories in this region.

Tying SUMO modifications to dynamic behaviors of chromosomes during meiotic prophase of Saccharomyces cerevisiae

In budding yeast Saccharomyces cerevisiae, centromeres and telomeres are tethered to the nuclear envelope during premeiotic interphase. Immediately after cells enter meiotic prophase, chromosomes undergo global reorganization, including bouquet formation (telomere clustering), non-homologous centromere coupling, homologous pairing, and assembly/disassembly of synaptonemal complexes. These chromosome dynamics have been implicated in promoting pairing, synapsis, crossover DNA recombination and segregation between homologous chromosomes. This review discusses recent studies related to the role of small ubiquitin-like modifier (SUMO) modification in controlling the overall budding yeast chromosome dynamics during meiotic prophase.

Genetic Studies of the beta-hairpin loop of Rous sarcoma virus capsid protein

The first few residues of the Rous sarcoma virus (RSV) CA protein comprise a structurally dynamic region that forms part of a Gag-Gag interface in immature virus particles. Dissociation of this interaction during maturation allows refolding and formation of a beta-hairpin structure important for assembly of CA monomers into the mature capsid shell. A consensus binding site for the cellular Ubc9 protein was previously identified within this region, suggesting that binding of Ubc9 and subsequent small ubiquitin-like modifier protein 1 (SUMO-1) modification of CA may play a role either in regulating the assembly activity of CA in immature particles or mature cores or in controlling postentry function(s) during the establishment of infection. In the present study, mutations designed to eliminate the consensus binding site were used to dissect the potentially overlapping functions of these residues. The resulting replication defects could not be traced to a failure to form particles of normal composition but, rather, to a deficit in genome replication. Genetic suppressors of two detrimental beta-hairpin mutations improved infectivity without restoring the consensus site or creating a novel one elsewhere. Optimal restoration of infectivity to a Lys-to-Arg mutant required a combination of secondary changes, one on the surface of each domain of CA. Rather than arguing for a critical role of Ubc9 and SUMO in RSV replication, these findings provide strong support for a structural role of the N-terminal residues and a particularly striking example of long-range interactions between regions of CA in achieving a functional core competent for genome replication.

The type 2 dengue virus envelope protein interacts with small ubiquitin-like modifier-1 (SUMO-1) conjugating enzyme 9 (Ubc9)

Dengue viruses are mosquito-borne flaviviruses and may cause the life-threatening dengue hemorrhagic fever and dengue shock syndrome. Its envelope protein is responsible mainly for the virus attachment and entry to host cells. To identify the human cellular proteins interacting with the envelope protein of dengue virus serotype 2 inside host cells, we have performed a screening with the yeast-two-hybrid-based "Functional Yeast Array". Interestingly, the small ubiquitin-like modifier-1 conjugating enzyme 9 protein, modulating cellular processes such as those regulating signal transduction and cell growth, was one of the candidates interacting with the dengue virus envelope protein. With co-precipitation assay, we have demonstrated that it indeed could interact directly with the Ubc9 protein. Site-directed mutagenesis has demonstrated that Ubc9 might interact with the E protein via amino acid residues K51 and K241. Furthermore, immunofluorescence microscopy has shown that the DV2E-EGFP proteins tended to progress toward the nuclear membrane and co-localized with Flag-Ubc9 proteins around the nuclear membrane in the cytoplasmic side, and DV2E-EGFP also shifted the distribution of Flag-Ubc9 from evenly in the nucleus toward concentrating around the nuclear membrane in the nucleic side. In addition, over-expression of Ubc9 could reduce the plaque formation of the dengue virus in mammalian cells. This is the first report that DV envelope proteins can interact with the protein of sumoylation system and Ubc9 may involve in the host defense system to prevent virus propagation.

Interaction of moloney murine leukemia virus capsid with Ubc9 and PIASy mediates SUMO-1 addition required early in infection

Yeast two-hybrid screens led to the identification of Ubc9 and PIASy, the E2 and E3 small ubiquitin-like modifier (SUMO)-conjugating enzymes, as proteins interacting with the capsid (CA) protein of the Moloney murine leukemia virus. The binding site in CA for Ubc9 was mapped by deletion and alanine-scanning mutagenesis to a consensus motif for SUMOylation at residues 202 to 220, and the binding site for PIASy was mapped to residues 114 to 176, directly centered on the major homology region. Expression of CA and a tagged SUMO-1 protein resulted in covalent transfer of SUMO-1 to CA in vivo. Mutations of lysine residues to arginines near the Ubc9 binding site and mutations at the PIASy binding site reduced or eliminated CA SUMOylation. Introduction of these mutations into the complete viral genome blocked virus replication. The mutants exhibited no defects in the late stages of viral gene expression or virion assembly. Upon infection, the mutant viruses were able to carry out reverse transcription to synthesize normal levels of linear viral DNA but were unable to produce the circular viral DNAs or integrated provirus normally found in the nucleus. The results suggest that the SUMOylation of CA mediated by an interaction with Ubc9 and PIASy is required for early events of infection, after reverse transcription and before nuclear entry and viral DNA integration.

Functional modulation of parkin through physical interaction with SUMO-1

Parkinson disease (PD) is the second most common neurodegenerative disorder and is characterized by the extensive and progressive loss of dopaminergic neurons in the CNS substantia nigra pars compacta region. Mutations in the parkin gene, which encodes for E3 ubiquitin ligase, have been implicated in autosomal recessive juvenile parkinsonism, an early-onset and common familial form of PD. Although several parkin substrates have already been identified, the molecular mechanism underlying the regulation of enzymatic activity of parkin has yet to be clarified. In a previous study, we demonstrated that RanBP2 becomes a new target for parkin E3 ubiquitin ligase and is processed via parkin-mediated ubiquitination and subsequent proteasomal degradation. RanBP2, which is localized in the cytoplasmic filament of the nuclear pore complex, belongs to the small ubiquitin-related modifier (SUMO) E3 ligase family. Here we show that parkin appears to bind selectively to the SUMO-1 in vivo and in vitro. Moreover, the physical association of SUMO-1 with parkin results in an increase in the nuclear transport of parkin as well as its self-ubiquitination. Our findings suggest that the E3 ubiquitin ligase activity of parkin and its intracellular localization may be modulated through the SUMO-1 association.

Regions of importance for interaction of puumala virus nucleocapsid subunits

Puumala virus (PUUV) is a hantavirus that causes a mild form of hemorrhagic fever with renal syndrome in northern and central Europe, and in large parts of Russia. The nucleocapsid (N) protein encoded by hantaviruses plays an important role in the life-cycle of these viruses, and one important function for the N-protein is to oligomerize, surround and protect the viral RNAs. We have identified amino- and carboxy-terminal regions involved in PUUV N-N interactions, which comprise amino acids 100-120 and 330-405. Our findings strengthen the hypothesis that the amino-terminus of the N-protein of hantaviruses holds a more regulatory function regarding N-N interactions, while conserved residues in the carboxy-terminal region, F335 together with F336 and W392, in concert with Y388 and/or F400 seems to play a more critical role in the PUUV N-N formation. This study provides evidence that the amino-terminal regions involved in the N-N interaction of Puumala virus are similar to those reported for Seoul virus (SEOV) and to some extent Hantaan virus (HTNV), even though the identity between PUUV N and SEOV/HTNV N is markedly lower than between PUUV N and Tula virus (TULV) N or Sin Nombre virus (SNV) N.

Oligomerization of hantavirus nucleocapsid protein: analysis of the N-terminal coiled-coil domain

Hantaviruses constitute a genus in the family Bunyaviridae. They are enveloped negative-strand RNA viruses with a tripartite genome encoding the nucleocapsid (N) protein, the two surface glycoproteins Gn and Gc, and an RNA-dependent RNA polymerase. The N protein is the most abundant component of the virion; it encapsidates genomic RNA segments forming ribonucleoproteins and participates in genome transcription and replication as well as virus assembly. In the course of RNA encapsidation, N protein forms intermediate trimers via head-to-head and tail-to-tail interactions. We analyzed the amino-terminal trimerization domain (amino acid residues 1 to 77) of Tula hantavirus using computer modeling, mammalian two-hybrid assay, and immunofluorescence assay. The results obtained were consistent with the existence of an antiparallel coiled-coil stabilized by interactions between hydrophobic residues. Residues L44, V51, and L58 were important for the N-N interaction; other residues, e.g., L25 and V32, also made a contribution, albeit a modest one. Our alignments of the N-terminal domain of the hantaviral N proteins suggest the coiled-coil structure, and hence the mode of N-protein oligomerization, is conserved among hantaviruses.

Proteome analysis of plant-virus interactome: comprehensive data for virus multiplication inside their hosts

Known host-parasite molecular interactions are widespread among parasite families, but these interactions have to be particularly large considering that viruses generally encode few proteins. Although some particular virus-host interactions are well described, no global study has yet shown multiple and simultaneous interactions in a host-parasite biological system. To prove that these multiple interactions occur in biological conditions, the complexes formed by a plant virus (rice yellow mottle virus) and the proteins of its natural host (rice) were extracted and purified from infected tissue sample. Remarkably mass spectrometry permitted the identification of a large number of proteins from the complexes that are involved in different functions not encoded by the virus but probably essential for its biological life cycle. This recruiting of proteins was strongly confirmed by the repetition of experiments using different pairs of virus-host and the use of high salt concentration to extract the complexes. We mainly identified proteins involved in plant defense, metabolism, translation, and protein synthesis and some proteins involved in transport. This study demonstrates that viruses are able to recruit many proteins from their hosts to ensure their development. Among different pairs of virus-host, similar protein functions were identified suggesting a particular importance of these proteins for viruses. The identification of particular paralog proteins among multigenic families suggests the high specificity of the recruiting for some protein functions.

Hantavirus nucleocapsid protein: a multifunctional molecule with both housekeeping and ambassadorial duties

In recent years important progress has been made studying the nucleocapsid (N) protein of hantaviruses. The N protein presents a good example of a multifunctional viral macromolecule. It is a major structural component of a virion that encapsidates viral RNA (vRNA). It also interacts with the virus polymerase (L protein) and one of the glycoproteins. On top of these "house keeping" duties, the N protein performs interactive "ambassadorial" functions interfering with important regulatory pathways in the infected cells.

Oligomerization of Hantavirus N protein: C-terminal alpha-helices interact to form a shared hydrophobic space

The structure of the nucleocapsid protein of bunyaviruses has not been defined. Earlier we have shown that Tula hantavirus N protein oligomerization is dependent on the C-terminal domains. Of them, the helix-loop-helix motif was found to be an essential structure. Computer modeling predicted that oligomerization occurs via helix protrusions, and the shared hydrophobic space formed by amino acids residues 380-IILLF-384 in the first helix and 413-LI-414 in the second helix is responsible for stabilizing the interaction. The model was validated by two approaches. First, analysis of the oligomerization capacity of the N protein mutants performed with the mammalian two-hybrid system showed that both preservation of the helix structure and formation of the shared hydrophobic space are crucial for the interaction. Second, oligomerization was shown to be a prerequisite for the granular pattern of transiently expressed N protein in transfected cells. N protein trimerization was supported by three-dimensional reconstruction of the N protein by electron microscopy after negative staining. Finally, we discuss how N protein trimerization could occur.

Dynamin interacts with members of the sumoylation machinery

Dynamin is a GTP-binding protein whose oligomerization-dependent assembly around the necks of lipid vesicles mediates their scission from parent membranes. Dynamin is thus directly involved in the regulation of endocytosis. Sumoylation is a post-translational protein modification whereby the ubiquitin-like modifier Sumo is covalently attached to lysine residues on target proteins by a process requiring the concerted action of an activating enzyme (ubiquitin-activating enzyme), a conjugating enzyme (ubiquitin carrier protein), and a ligating enzyme (ubiquitin-protein isopeptide ligase). Here, we show that dynamin interacts with Sumo-1, Ubc9, and PIAS-1, all of which are members of the sumoylation machinery. Ubc9 and PIAS-1 are known ubiquitin carrier protein and ubiquitin-protein isopeptide ligase enzymes, respectively, for the process of sumoylation. We have identified the coiled-coil GTPase effector domain (GED) of dynamin as the site on dynamin that interacts with Sumo-1, Ubc9, and PIAS-1. Although we saw no evidence of covalent Sumo-1 attachment to dynamin, Sumo-1 and Ubc9 are shown here to inhibit the lipid-dependent oligomerization of dynamin. Expression of Sumo-1 and Ubc9 in mammalian cells down-regulated the dynamin-mediated endocytosis of transferrin, whereas dynamin-independent fluid-phase uptake was not affected. Furthermore, using high resolution NMR spectroscopy, we have identified amino acid residues on Sumo-1 that directly interact with the GED of dynamin. The results suggest that the GED of dynamin may serve as a scaffold that concentrates the sumoylation machinery in the vicinity of potential acceptor proteins.

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.

Mapping of the regions involved in homotypic interactions of Tula hantavirus N protein

Hantavirus nucleocapsid (N) protein has been suggested to form homodimers and homotrimers that are further integrated into the nucleocapsid filaments around the viral RNA. Here we report detailed mapping of the regions involved in the homotypic N protein interactions in Tula hantavirus (TULV). Peptide scan screening was used to define the interaction regions, and the mammalian two-hybrid assay was used for the functional analysis of N protein mutants. To study linear regions responsible for N protein interaction(s), we used peptide scanning in which N peptides synthesized on membranes recognize recombinant TULV N protein. The data showed that the N protein bound to membrane-bound peptides comprising amino acids 13 to 30 and 41 to 57 in the N-terminal part and 340 to 379, 391 to 407, and 410 to 419 in the C-terminal part of the molecule. Further mapping of the interaction regions by alanine scanning indicated the importance of basic amino acids along the N protein and especially asparagine-394, histidine-395, and phenyalanine-396 in forming the binding interface. Analysis of truncated mutants in the mammalian two-hybrid assay showed that N-terminal amino acids 1 to 43 are involved in and C-terminal amino acids 393 to 398 (VNHFHL) are absolutely crucial for the homotypic interactions. Furthermore, our data suggested a tail-to-tail and head-to-head binding scheme for the N proteins.