Nucleolar-cytoplasmic shuttling of PRRSV nucleocapsid protein: a simple case of molecular mimicry or the complex regulation by nuclear import, nucleolar localization and nuclear export signal sequences

Control of nuclear localization of the nucleocapsid protein of SARS-CoV-2

The nucleocapsid (N) protein of coronaviruses is a structural protein that binds viral RNA for assembly into the mature virion, a process that occurs in the cytoplasm. Several coronavirus N proteins also localize to the nucleus. Herein, we identify that two sequences (NLSs) are required for nuclear localization of the SARS-CoV-2 N protein. Deletion or mutation of these two sequences creates an N protein that does not localize to the nucleus in HEK293T cells. Overexpression of both wild-type and NLS-mutated N proteins dysregulate a largely overlapping set of mRNAs in HEK293T cells, suggesting that these N proteins do not have direct nuclear effects on transcription. Consistent with that hypothesis, both N proteins induce nuclear localization of NF-κB p65 and dysregulate a set of previously identified NF-κB-dependent genes. The effects of N on nuclear properties are proposed to alter host cell functions that contribute to viral pathogenesis or replication.

Research progress on the N protein of porcine reproductive and respiratory syndrome virus

Porcine reproductive and respiratory syndrome (PRRS) is a highly contagious disease caused by the porcine reproductive and respiratory syndrome virus (PRRSV). PRRSV exhibits genetic diversity and complexity in terms of immune responses, posing challenges for eradication. The nucleocapsid (N) protein of PRRSV, an alkaline phosphoprotein, is important for various biological functions. This review summarizes the structural characteristics, genetic evolution, impact on PRRSV replication and virulence, interactions between viral and host proteins, modulation of host immunity, detection techniques targeting the N protein, and progress in vaccine development. The discussion provides a theoretical foundation for understanding the pathogenic mechanisms underlying PRRSV virulence, developing diagnostic techniques, and designing effective vaccines.

Suppression of TRIM19 by arterivirus nonstructural protein 1 promotes viral replication

Tripartite motif (TRIM)-containing proteins are a family of regulatory proteins that can participate in the induction of antiviral cytokines and antagonize viral replication. Promyelocytic leukemia (PML) protein is known as TRIM19 and is a major scaffold protein organizing the PML nuclear bodies (NBs). PML NBs are membrane-less organelles in the nucleus and play a diverse role in maintaining cellular homeostasis including antiviral response. Porcine reproductive and respiratory syndrome virus (PRRSV), a member virus of the family Arteriviridae, inhibits type I interferon (IFN) response during infection, and nonstructural protein 1 (nsp1) of the virus has been identified as a potent IFN antagonist. We report that the numbers of PML NBs per nucleus were significantly downregulated during infection of PRRSV. The overexpression of all six isoforms of PML suppressed the PRRSV replication, and conversely, the silencing of PML gene expression enhanced the PRRSV replication. The suppression of PML NBs by the nsp1 protein was common in other member viruses of the family, represented by equine arteritis virus, lactate dehydrogenase elevating virus of mice, and simian hemorrhagic fever virus. Our study unveils a conserved viral strategy in arteriviruses for innate immune evasion.

The Disruption of a Nuclear Export Signal in the C-Terminus of the Herpes Simplex Virus 1 Determinant of Pathogenicity UL24 Protein Leads to a Syncytial Plaque Phenotype

UL24 of herpes simplex virus 1 (HSV-1) has been shown to be a determinant of pathogenesis in mouse models of infection. The N-terminus of UL24 localizes to the nucleus and drives the redistribution of nucleolin and B23. In contrast, when expressed alone, the C-terminal domain of UL24 accumulates in the Golgi apparatus; its importance during infection is unknown. We generated a series of mammalian expression vectors encoding UL24 with nested deletions in the C-terminal domain. Interestingly, enhanced nuclear staining was observed for several UL24-deleted forms in transient transfection assays. The substitution of a threonine phosphorylation site had no effect on UL24 localization or viral titers in cell culture. In contrast, mutations targeting a predicted nuclear export signal (NES) significantly enhanced nuclear localization, indicating that UL24 is able to shuttle between the nucleus and the cytoplasm. Recombinant viruses that encode UL24-harboring substitutions in the NES led to the accumulation of UL24 in the nucleus. Treatment with the CRM-1-specific inhibitor leptomycin B blocked the nuclear export of UL24 in transfected cells but not in the context of infection. Viruses encoding UL24 with NES mutations resulted in a syncytial phenotype, but viral yield was unaffected. These results are consistent with a role for HSV-1 UL24 in late cytoplasmic events in HSV-1 replication.

PCNA promotes PRRSV replication by increasing the synthesis of viral genome

Porcine reproductive and respiratory syndrome virus (PRRSV) is an RNA virus belonging to the Arteriviridae family. Currently, the strain has undergone numerous mutations, bringing massive losses to the swine industry worldwide. Despite several studies had been conducted on PRRSV, the molecular mechanisms by which it causes infection remain unclear. Proliferating cell nuclear antigen (PCNA) is a sign of DNA damage and it participates in DNA replication and repair. Therefore, in this study, we investigated the potential role of PCNA in PRRSV infection. We observed that PCNA expression was stable after PRRSV infection in vitro; however, PCNA was translocated from the nucleus to the cytoplasm. Notably, we found the redistribution of PCNA from the nucleus to the cytoplasm in cells transfected with the N protein. PCNA silencing inhibited PRRSV replication and the synthesis of PRRSV shorter subgenomic RNA (sgmRNA) and genomic RNA (gRNA), while PCNA overexpression promoted virus replication and PRRSV shorter sgmRNA and gRNA synthesis. By performing immunoprecipitation and immunofluorescence colocalization, we confirmed that PCNA interacted with replication-related proteins, namely NSP9, NSP12, and N, but not with NSP10 and NSP11. Domain III of the N protein (41-72 aa) interacted with the IDCL domain of PCNA (118-135 aa). Therefore, we propose cytoplasmic transport of PCNA and its subsequent influence on PRRSV RNA synthesis could be a viral strategy for manipulating cell function, thus PCNA is a potential target to prevent and control PRRSV infection.

Secondary Structures of MERS-CoV, SARS-CoV, and SARS-CoV-2 Spike Proteins Revealed by Infrared Vibrational Spectroscopy

All coronaviruses are characterized by spike glycoproteins whose S1 subunits contain the receptor binding domain (RBD). The RBD anchors the virus to the host cellular membrane to regulate the virus transmissibility and infectious process. Although the protein/receptor interaction mainly depends on the spike's conformation, particularly on its S1 unit, their secondary structures are poorly known. In this paper, the S1 conformation was investigated for MERS-CoV, SARS-CoV, and SARS-CoV-2 at serological pH by measuring their Amide I infrared absorption bands. The SARS-CoV-2 S1 secondary structure revealed a strong difference compared to those of MERS-CoV and SARS-CoV, with a significant presence of extended β-sheets. Furthermore, the conformation of the SARS-CoV-2 S1 showed a significant change by moving from serological pH to mild acidic and alkaline pH conditions. Both results suggest the capability of infrared spectroscopy to follow the secondary structure adaptation of the SARS-CoV-2 S1 to different environments.

Signaling Lymphocytic Activation Molecule Family Member 1 Inhibits Porcine Reproductive and Respiratory Syndrome Virus Replication

The porcine reproductive and respiratory syndrome virus (PRRSV) causes a highly contagious disease in domestic swine. Signaling lymphocytic activation molecule family member 1 (SLAMF1) is a costimulatory factor that is involved in innate immunity, inflammation, and infection. Here, we demonstrate that overexpression of the SLAMF1 gene inhibited PRRSV replication significantly and reduced the levels of key signaling pathways, including MyD88, RIG-I, TLR2, TRIF, and inflammatory factors IL-6, IL-1β, IL-8, TNF-β, TNF-α, and IFN-α in vitro. However, the knockdown of the SLAMF1 gene could enhance replication of the PRRSV and the levels of key signaling pathways and inflammatory factors. Overall, our results identify a new, to our knowledge, antagonist of the PRRSV, as well as a novel antagonistic mechanism evolved by inhibiting innate immunity and inflammation, providing a new reference and direction for PRRSV disease resistance breeding.

Nuclear dynamics: Formation of bodies and trafficking in plant nuclei

The existence of the nucleus distinguishes prokaryotes and eukaryotes. Apart from containing most of the genetic material, the nucleus possesses several nuclear bodies composed of protein and RNA molecules. The nucleus is separated from the cytoplasm by a double membrane, regulating the trafficking of molecules in- and outwards. Here, we investigate the composition and function of the different plant nuclear bodies and molecular clues involved in nuclear trafficking. The behavior of the nucleolus, Cajal bodies, dicing bodies, nuclear speckles, cyclophilin-containing bodies, photobodies and DNA damage foci is analyzed in response to different abiotic stresses. Furthermore, we research the literature to collect the different protein localization signals that rule nucleocytoplasmic trafficking. These signals include the different types of nuclear localization signals (NLSs) for nuclear import, and the nuclear export signals (NESs) for nuclear export. In contrast to these unidirectional-movement signals, the existence of nucleocytoplasmic shuttling signals (NSSs) allows bidirectional movement through the nuclear envelope. Likewise, nucleolar signals are also described, which mainly include the nucleolar localization signals (NoLSs) controlling nucleolar import. In contrast, few examples of nucleolar export signals, called nucleoplasmic localization signals (NpLSs) or nucleolar export signals (NoESs), have been reported. The existence of consensus sequences for these localization signals led to the generation of prediction tools, allowing the detection of these signals from an amino acid sequence. Additionally, the effect of high temperatures as well as different post-translational modifications in nuclear and nucleolar import and export is discussed.

Research Progress in Porcine Reproductive and Respiratory Syndrome Virus–Host Protein Interactions

Porcine reproductive and respiratory syndrome (PRRS) is a highly contagious disease caused by porcine reproductive and respiratory syndrome virus (PRRSV), which has been regarded as a persistent challenge for the pig industry in many countries. PRRSV is internalized into host cells by the interaction between PRRSV proteins and cellular receptors. When the virus invades the cells, the host antiviral immune system is quickly activated to suppress the replication of the viruses. To retain fitness and host adaptation, various viruses have evolved multiple elegant strategies to manipulate the host machine and circumvent against the host antiviral responses. Therefore, identification of virus–host interactions is critical for understanding the host defense against viral infections and the pathogenesis of the viral infectious diseases. Most viruses, including PRRSV, interact with host proteins during infection. On the one hand, such interaction promotes the virus from escaping the host immune system to complete its replication. On the other hand, the interactions regulate the host cell immune response to inhibit viral infections. As common antiviral drugs become increasingly inefficient under the pressure of viral selectivity, therapeutic agents targeting the intrinsic immune factors of the host protein are more promising because the host protein has a lower probability of mutation under drug-mediated selective pressure. This review elaborates on the virus–host interactions during PRRSV infection to summarize the pathogenic mechanisms of PRRSV, and we hope this can provide insights for designing effective vaccines or drugs to prevent and control the spread of PRRS.

Fundamental and Advanced Therapies, Vaccine Development against SARS-CoV-2

Coronavirus disease (COVID-19) caused by the SARS-CoV-2 virus has been affecting the world since the end of 2019. The severity of the disease can range from an asymptomatic or mild course to acute respiratory distress syndrome (ARDS) with respiratory failure, which may lead to death. Since the outbreak of the pandemic, scientists around the world have been studying the genome and molecular mechanisms of SARS-CoV-2 infection to develop effective therapies and prevention. In this review, we summarize the progressive development of various treatments and vaccines as they have emerged, a year after the outbreak of the pandemic. Initially for COVID-19, patients were recommended drugs with presumed antiviral, anti-inflammatory, and antimicrobial effects that were previously used to treat other diseases. Thereafter, therapeutic interventions were supplemented with promising approaches based on antibodies, peptides, and stem cells. However, licensed COVID-19 vaccines remain the most effective weapon in combating the pandemic. While there is an enormous effort to enhance the vaccination rate to increase the entire population immunity, the production and delivery of vaccines is becoming limited in several countries. In this regard, there are new challenges needing to be addressed by combining non-pharmacological intervention with effective therapies until vaccination is accessible to all.

COVID-19 infection and nanomedicine applications for development of vaccines and therapeutics: An overview and future perspectives based on polymersomes

The severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2), which emerged in December 2019 and caused the coronavirus disease 2019 (COVID-19) pandemic, took the world by surprise with an unprecedented public health emergency. Since this pandemic began, extraordinary efforts have been made by scientists to understand the pathogenesis of COVID-19, and to fight the infection by providing various preventive, diagnostic and treatment opportunities based on either novel hypotheses or past experiences. Despite all the achievements, COVID-19 continues to be an accelerating health threat with no specifically approved vaccine or therapy. This review highlights the recent advances in COVID-19 infection, with a particular emphasis on nanomedicine applications that can help in the development of effective vaccines or therapeutics against COVID-19. A novel future perspective has been proposed in this review based on utilizing polymersome nano-objects for effectively suppressing the cytokine storm, which may reduce the severity of COVID-19 infection.

Characterization of Localization and Export Signals of Bovine Torovirus Nucleocapsid Protein Responsible for Extensive Nuclear and Nucleolar Accumulation and Their Importance for Virus Growth

Torovirus (ToV) has recently been classified into the new family Tobaniviridae, although historically, it belonged to the Coronavirus (CoV) family. The nucleocapsid (N) proteins of CoVs are predominantly localized in the cytoplasm, where the viruses replicate, but in some cases the proteins are partially located in the nucleolus. Many studies have investigated the subcellular localization and nucleocytoplasmic trafficking signals of the CoV N proteins, but little is known about ToV N proteins. Here, we studied the subcellular localization of the bovine ToV (BToV) N protein (BToN) and characterized its nucleocytoplasmic trafficking signals. Unlike other CoVs, BToN in infected cells was transported mainly to the nucleolus during early infection but was distributed predominantly in the nucleoplasm rather than in the nucleolus during late infection. Interestingly, a small quantity of BToN was detected in the cytoplasm during infection. Examination of a comprehensive set of substitution or deletion mutants of BToN fused with enhanced green fluorescent protein (EGFP) revealed that clusters of arginine (R) residues comprise nuclear/nucleolar localization signals (NLS/NoLS), and the C-terminal region served as a chromosomal maintenance 1 (CRM1)-independent nuclear export signal (NES). Moreover, recombinant viruses with mutations in the NLS/NoLS, but retaining nuclear accumulation, were successfully rescued and showed slightly reduced growth ability, while the virus that lost the NLS/NoLS-mediated nuclear accumulation of BToN was not rescued. These results indicate that BToN uniquely accumulates mainly in nuclear compartments during infection, regulated by an R-rich NLS/NoLS and a CRM1-independent NES, and that the BToN accumulation in the nuclear compartment driven by NLS/NoLS is important for virus growth.IMPORTANCE ToVs are diarrhea-causing pathogens detected in many species, including humans. BToV has spread worldwide, leading to economic loss, and there is currently no treatment or vaccine available. Positive-stranded RNA viruses, including ToVs, replicate in the cytoplasm, and their structural proteins generally accumulate in the cytoplasm. Interestingly, BToN accumulated predominantly in the nucleus/nucleolus during all infectious processes, with only a small fraction accumulating in the cytoplasm despite being a major structural protein. Furthermore, we identified unique nucleocytoplasmic trafficking signals and demonstrated the importance of NLS/NoLS for virus growth. This study is the first to undertake an in-depth investigation of the subcellular localization and intracellular trafficking signals of BToN. Our findings additionally suggest that the NLS/NoLS-mediated nuclear accumulation of BToN is important for virus replication. An understanding of the unique features of BToV may provide novel insights into the assembly mechanisms of not only ToVs but also other positive-stranded RNA viruses.

Comprehensive Structural and Molecular Comparison of Spike Proteins of SARS-CoV-2, SARS-CoV and MERS-CoV, and Their Interactions with ACE2

The severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) has recently emerged in China and caused a disease called coronavirus disease 2019 (COVID-19). The virus quickly spread around the world, causing a sustained global outbreak. Although SARS-CoV-2, and other coronaviruses, SARS-CoV and Middle East respiratory syndrome CoV (MERS-CoV) are highly similar genetically and at the protein production level, there are significant differences between them. Research has shown that the structural spike (S) protein plays an important role in the evolution and transmission of SARS-CoV-2. So far, studies have shown that various genes encoding primarily for elements of S protein undergo frequent mutation. We have performed an in-depth review of the literature covering the structural and mutational aspects of S protein in the context of SARS-CoV-2, and compared them with those of SARS-CoV and MERS-CoV. Our analytical approach consisted in an initial genome and transcriptome analysis, followed by primary, secondary and tertiary protein structure analysis. Additionally, we investigated the potential effects of these differences on the S protein binding and interactions to angiotensin-converting enzyme 2 (ACE2), and we established, after extensive analysis of previous research articles, that SARS-CoV-2 and SARS-CoV use different ends/regions in S protein receptor-binding motif (RBM) and different types of interactions for their chief binding with ACE2. These differences may have significant implications on pathogenesis, entry and ability to infect intermediate hosts for these coronaviruses. This review comprehensively addresses in detail the variations in S protein, its receptor-binding characteristics and detailed structural interactions, the process of cleavage involved in priming, as well as other differences between coronaviruses.

Porcine Reproductive and Respiratory Syndrome Virus Enhances Self-Replication via AP-1-Dependent Induction of SOCS1

Porcine reproductive and respiratory syndrome virus (PRRSV) has caused tremendous economic losses in the swine industry since its emergence in the late 1980s. PRRSV exploits various strategies to evade immune responses and establish chronic persistent infections. Suppressor of cytokine signaling (SOCS) 1, a member of the SOCS family, is a crucial intracellular negative regulator of innate immunity. In this study, it was shown that SOCS1 can be co-opted by PRRSV to evade host immune responses, facilitating viral replication. It was observed that PRRSV induced SOCS1 production in porcine alveolar macrophages, monkey-derived Marc-145 cells, and porcine-derived CRL2843-CD163 cells. SOCS1 inhibited the expression of IFN-β and IFN-stimulated genes, thereby markedly enhancing PRRSV replication. It was observed that the PRRSV N protein has the ability to upregulate SOCS1 production and that nuclear localization signal-2 (NLS-2) is essential for SOCS1 induction. Moreover, SOCS1 upregulation was dependent on p38/AP-1 and JNK/AP-1 signaling pathways rather than classical type I IFN signaling pathways. In summary, to our knowledge, the findings of this study uncovered the molecular mechanism that underlay SOCS1 induction during PRRSV infection, providing new insights into viral immune evasion and persistent infection.

Identification of peptide domains involved in the subcellular localization of the feline coronavirus 3b protein

Feline coronavirus (FCoV) has been identified as the aetiological agent of feline infectious peritonitis (FIP), a highly fatal systemic disease in cats. FCoV open reading frame 3 (ORF3) encodes accessory proteins 3a, 3b and 3 c. The FCoV 3b accessory protein consists of 72 amino acid residues and localizes to nucleoli and mitochondria. The present work focused on peptide domains within FCoV 3b that drive its intracellular trafficking. Transfection of different cell types with FCoV 3b fused to enhanced green fluorescent protein (EGFP) or 3×FLAG confirmed localization of FCoV 3b in the mitochondria and nucleoli. Using serial truncated mutants, we showed that nucleolar accumulation is controlled by a joint nucleolar and nuclear localization signal (NoLS/NLS) in which the identified overlapping pat4 motifs (residues 53–57) play a critical role. Mutational analysis also revealed that mitochondrial translocation is mediated by N-terminal residues 10–35, in which a Tom20 recognition motif (residues 13–17) and two other overlapping hexamers (residues 24–30) associated with mitochondrial targeting were identified. In addition, a second Tom20 recognition motif was identified further downstream (residues 61–65), although the mitochondrial translocation evoked by these residues seemed less efficient as a diffuse cytoplasmic distribution was also observed. Assessing the spatiotemporal distribution of FCoV 3b did not provide convincing evidence of dynamic shuttling behaviour between the nucleoli and the mitochondria.

Interaction of PIAS1 with PRRS virus nucleocapsid protein mediates NF-κB activation and triggers proinflammatory mediators during viral infection

Porcine reproductive and respiratory syndrome virus (PRRSV) activates NF-κB during infection. We examined the ability of all 22 PRRSV genes for NF-κB regulation and determined the nucleocapsid (N) protein as the NF-κB activator. Protein inhibitor of activated STAT1 (signal transducer and activator of transcription 1) (PIAS1) was identified as a cellular protein binding to N. PIAS1 is known to bind to p65 (RelA) in the nucleus and blocks its DNA binding, thus functions as a repressor of NF-κB. Binding of N to PIAS1 released p65 for NF-κB activation. The N-terminal half of PIAS1 was mapped as the N-binding domain, and this region overlapped its p65-binding domain. For N, the region between 37 and 72 aa was identified as the binding domain to PIAS1, and this domain alone was able to activate NF-κB. A nuclear localization signal (NLS) knock-out mutant N did not activate NF-κB, and this is mostly likely due to the lack of its interaction with PIAS1 in the nucleus, demonstrating the positive correlation between the binding of N to PIAS1 and the NF-κB activation. Our study reveals a role of N in the nucleus for NF-κB activation and proinflammatory cytokine production during infection.

Origins, pathophysiology, diagnosis, vaccination and prevention of Chikungunya virus

Chikungunya virus is an Alphavirus that possesses characteristics similar to that of an arthropod-borne virus. Chikungunya virus has been one of the major concerns for the last few decades due to its nature of explosive spreading throughout the world. This article is intended to give detailed information about Chikungunya virus, and includes its pathogenesis, origins, diagnosis, treatment and prevention. Although, recent researches suggests various approaches to treating Chikungunya virus, extensive literature search on Chikungunya virus has revealed that, currently, there is no effective treatment available and the virus is greatly dependent on its vectors. Patients affected by Chikungunya virus mainly show symptoms of fever, arthralgia, joint pain and skin rash. Since there is no effective treatment available, public awareness is the most significant factor for potential prevention against Chikungunya virus.

Karyopherin Alpha 6 Is Required for Replication of Porcine Reproductive and Respiratory Syndrome Virus and Zika Virus

Movement of macromolecules between the cytoplasm and the nucleus occurs through the nuclear pore complex (NPC). Karyopherins comprise a family of soluble transport factors facilitating the nucleocytoplasmic translocation of proteins through the NPC. In this study, we found that karyopherin α6 (KPNA6; also known as importin α7) was required for the optimal replication of porcine reproductive and respiratory syndrome virus (PRRSV) and Zika virus (ZIKV), which are positive-sense, single-stranded RNA viruses replicating in the cytoplasm. The KPNA6 protein level in virus-infected cells was much higher than that in mock-infected controls, whereas the KPNA6 transcript remains stable. Viral infection blocked the ubiquitin-proteasomal degradation of KPNA6, which led to an extension of the KPNA6 half-life and the elevation of the KPNA6 level in comparison to mock-infected cells. PRRSV nsp12 protein induced KPNA6 stabilization. KPNA6 silencing was detrimental to the replication of PRRSV, and KPNA6 knockout impaired ZIKV replication. Moreover, KPNA6 knockout blocked the nuclear translocation of PRRSV nsp1β but had a minimal effect on two other PRRSV proteins with nuclear localization. Exogenous restitution of KPNA6 expression in the KPNA6-knockout cells results in restoration of the nuclear translocation of PRRSV nsp1β and the replication of ZIKV. These results indicate that KPNA6 is an important cellular factor for the replication of PRRSV and ZIKV.IMPORTANCE Positive-sense, single-stranded RNA (+ssRNA) viruses replicate in the cytoplasm of infected cells. The roles of transport factors in the nucleocytoplasmic trafficking system for the replication of +ssRNA viruses are not known. In this study, we discovered that PRRSV and ZIKV viruses needed karyopherin α6 (KPNA6), one of the transport factors, to enhance the virus replication. Our data showed that viral infection induced an elevation of the KPNA6 protein level due to an extension of the KPNA6 half-life via viral interference of the ubiquitin-proteasomal degradation of KPNA6. Notably, KPNA6 silencing or knockout dramatically reduced the replication of PRRSV and ZIKV. PRRSV nsp1β depended on KPNA6 to translocate into the nucleus. In addition, exogenous restitution of KPNA6 expression in KPNA6-knockout cells led to the restoration of nsp1β nuclear translocation and ZIKV replication. These results reveal a new aspect in the virus-cell interaction and may facilitate the development of novel antiviral therapeutics.

Interaction of porcine reproductive and respiratory syndrome virus proteins with SUMO-conjugating enzyme reveals the SUMOylation of nucleocapsid protein

SUMOylation is a reversible post-translational modification that regulates the function of target protein. In this study, we first predicted by software that the multiple proteins of porcine reproductive and respiratory syndrome virus (PRRSV) could be sumoylated. Next, we confirmed that Nsp1β, Nsp4, Nsp9, Nsp10 and nucleocapsid (N) protein of PRRSV could interact with the sole SUMO E2 conjugating enzyme Ubc9, and Ubc9 could be co-localized with Nsp1β, Nsp4, Nsp9 and Nsp10 in the cytoplasm, while with N protein in both the cytoplasm and nucleus. Finally, we demonstrated that N protein could be sumoylated by either SUMO1 or SUMO2/3. In addition, the overexpression of Ubc9 could inhibit viral genomic replication at early period of PRRSV infection and the knockdown of Ubc9 by siRNA could promote the virus replication. These findings reveal the SUMOylation property of PRRSV N protein and the involvement of Ubc9 in PRRSV replication through interaction with multiple proteins of PRRSV. To our knowledge, this is the first study indicating the interplay between SUMO modification system and PRRSV.

Perturbation of Wound Healing, Cytoskeletal Organization and Cellular Protein Networks during Hazara Virus Infection

Normal epithelial and endothelial renewal and healing after bacterial and viral challenges are essential for homeostasis along the intestine and the blood and lymphatic vessels. We thus investigated whether and how virus affects migration of human epithelial cells and specifically how the nucleocapsid protein (N) modulates the cellular proteome and interactome using human Caco-2 cells in a wound-healing assay with Hazara virus as a model. Here, Hazara virus blocked cell migration in a dose- and time-dependent manner, disrupted the actin cytoskeleton and specifically reduced the expression of the IQ-motif-containing GTPase-activating protein 1 (IQGAP1) and water channel aquaporin 6 (AQP6) that regulate cytoskeletal organization, water homeostasis and vesicle communication. Moreover, in the Caco-2 cell proteome, we identified several distinct groups of molecules associating with N upon Hazara virus infection, being involved in the ensemble of important cellular processes, e.g., chaperone activity, metabolism, cellular defense against infections, cell morphology, and migration. These events do not only facilitate the virus life cycle, but they are also crucial for membrane and cytoskeleton dynamics, cellular self-renewal and wound healing, being so essential for body integrity and homeostasis.

The viral innate immune antagonism and an alternative vaccine design for PRRS virus

•

PRRS virus has evolved to suppress the antiviral innate immunity during infection.

•

Type I interferons are potent antiviral cytokines and function to stimulate the adaptive immune responses.

•

Six viral proteins have been identified as interferon antagonists and characterized for their molecular actions.

•

Interferon antagonism-negative viruses are attenuated and have been proven induce protective immunity.

•

Interferon suppression-negative PRRS virus may serve as an alternative vaccine for PRRS.

Porcine reproductive and respiratory syndrome (PRRS) remains one of the most economically significant diseases in the swine industry worldwide. The current vaccines are less satisfactory to confer protections from heterologous infections and long-term persistence, and the need for better vaccines are urgent. The immunological hallmarks in PRRSV-infected pigs include the unusually poor production of type I interferons (IFNs-α/β) and the aberrant and delayed adaptive immune responses, indicating that PRRSV has the ability to suppress both innate and adaptive immune responses in the host. Type I IFNs are the potent antiviral cytokines and recent studies reveal their pleiotropic functions in the priming of expansion and maturation of adaptive immunity. Thus, IFN antagonism-negative PRRSV is hypothesized to be attenuated and to build effective and broad- spectrum innate and adaptive immune responses in pigs. Such vaccines are promising alternatives to traditional vaccines for PRRSV.

Nuclear localization signal sequence is required for VACM-1/CUL5-dependent regulation of cellular growth

VACM-1/CUL5 is a member of the cullin family of proteins involved in the E3 ligase-dependent degradation of diverse proteins that regulate cellular proliferation. The ability of VACM-1/CUL5 to inhibit cellular growth is affected by its posttranslational modifications and its localization to the nucleus. Since the mechanism of VACM-1/CUL5 translocation to the nucleus is not clear, the goal of this project was to determine the role that the putative nuclear localization signal (NLS) we identified in the VACM-1/CUL5 (⁶⁴⁰PKLKRQ⁶⁴⁶) plays in the cellular localization of VACM-1/CUL5 and its effect on cellular growth. We used site-directed mutagenesis to change Lys642 and Lys644 to Gly and the mutated cDNA constructs were transfected into COS-1 cells. Mutation of the NLS in VACM-1/CUL5 significantly reduced its localization to the nucleus and compromised its effect on cellular growth. We have shown previously that the antiproliferative effect of VACM-1/CUL5 could be reversed by mutation of PKA-specific phosphorylation sequence (^S730AVACM-1/CUL5), which was associated with its increased nuclear localization and modification by NEDD8. Thus, we examined whether these properties can be controlled by the NLS. The mutation of NLS in ^S730AVACM-1/CUL5 cDNA compromised its proliferative effect and reduced its localization to the nucleus. The immunocytochemistry results showed that, in cells transfected with the mutant cDNAs, the nuclear NEDD8 signal was decreased. Western blot analysis of total cell lysates, however, showed that VACM-1/CUL5 neddylation was not affected. Together, these results suggest that the presence of the NLS, both in VACM-1/CUL5 and in ^S730AVACM-1/CUL5 sequences, is critical for their control of cell proliferation.

Evaluation of the Cross-Protective Efficacy of a Chimeric Porcine Reproductive and Respiratory Syndrome Virus Constructed Based on Two Field Strains

One of the major hurdles to porcine reproductive and respiratory syndrome (PRRS) vaccinology is the limited or no cross-protection conferred by current vaccines. To overcome this challenge, a PRRS chimeric virus (CV) was constructed using an FL12-based cDNA infectious clone in which open reading frames (ORFs) 3-4 and ORFs 5-6 were replaced with the two Korean field isolates K08-1054 and K07-2273,respectively. This virus was evaluated as a vaccine candidate to provide simultaneous protection against two genetically distinct PRRS virus (PRRSV) strains. Thirty PRRS-negative three-week-old pigs were divided into five groups and vaccinated with CV, K08-1054, K07-2273, VR-2332, or a mock inoculum. At 25 days post-vaccination (dpv), the pigs in each group were divided further into two groups and challenged with either K08-1054 or K07-2273. All of the pigs were observed until 42 dpv and were euthanized for pathological evaluation. Overall, the CV-vaccinated group exhibited higher levels of tumor necrosis factor-alpha (TNF-α), interferon-gamma (IFN-γ), and interleukin-12 (IL-12) expression and of serum virus-neutralizing antibodies compared with the other groups after vaccination and also demonstrated better protection levels against both viruses compared with the challenge control group. Based on these results, it was concluded that CV might be an effective vaccine model that can confer a broader range of cross-protection to various PRRSV strains.

Molecular identification and genetic diversity of open reading frame 7 field isolated porcine reproductive and respiratory syndrome in North Sumatera, Indonesia, in the period of 2008-2014

AIM

Molecular identification and genetic diversity of open reading frame 7 (ORF7) of field isolated porcine reproductive and respiratory syndrome virus (PRRSV) in North Sumatera, Indonesia, in the period of 2008-2014.

MATERIALS AND METHODS

A total of 47 PRRSV samples were collected from the death case of pigs. The samples were collected from different districts in the period of 2008-2014 from North Sumatera province. Two pairs of primer were designed to amplify ORF7 of Type 1 and 2 PRRSV based on the sequence of reference viruses VR2332 and Lelystad. Viral RNAs were extracted from samples using PureLink™ micro-to-Midi total RNA purification system (Invitrogen). To amplify the ORF7 of PRRSV, the synthesis cDNA and DNA amplification were performed by reverse transcription polymerase chain reaction (RT-PCR) and nested PCR method. Then the DNA sequencing of PCR products and phylogenetic analysis were accomplished by molecular evolutionary genetics analysis version 6.0 software program.

RESULTS

RT-: PCR and nested PCR used in this study had successfully detected of 18 samples positive PRRS virus with the amplification products at 703bp and 508bp, respectively. Sequencing of the ORF7 shows that 18 PRRS viruses isolated from North Sumatera belonged to North American (NA). JXA1 Like and classic NA type viruses. Several mutations were detected, particularly in the area of nuclear localization signal (NLS1) and in NLS2. In the local viruses, which were related closed to JXA1 virus; there are two differences in amino acids in position 12 and 43 of ORF7. Our tested viruses showed that the amino acid positions 12 and 43 are Asparagine and Arginine, while the reference virus (VR2332, Lelystad, and JXA1) occupied both by Lysine. Based on differences in two amino acids at position 12 and 43 showed that viruses from North Sumatera has its own uniqueness and related closed to highly pathogenic PRRS (HP-PRRS) virus (JXA1).

CONCLUSION

The results demonstrated that North Sumatera type PRRS virus has caused PRRS outbreaks in pig in North Sumatera between 2008 and 2014. The JAX1 like viruses had unique amino acid residue in position 12 and 43 of asparagine and lysine, and these were genetic determinants of North Sumatera viruses compared to other PRRS viruses.

The Nucleolus: Structure and Function

The nucleolus is the largest nuclear organelle and is the primary site of ribosome subunit biogenesis in eukaryotic cells. It is assembled around arrays of ribosomal DNA genes, forming specific chromosomal features known as nucleolar organizing regions (NORs) which are the sites of ribosomal DNA transcription. While the nucleolus main activity involve different steps of ribosome biogenesis, the presence of proteins with no obvious relationship with ribosome subunit production suggests additional functions for the nucleolus, such as regulation of mitosis, cell cycle progression, stress response and biogenesis of multiple ribonucleoprotein complexes. The many novel factors and separate classes of proteins identified within the nucleolus support this view that the nucleolus may perform additional functions beyond its known role in ribosome subunit biogenesis. Here we review our knowledge of the nucleolar functions and will provide a detailed picture of how the nucleolus is involved in many cellular pathways.

Nucleocytoplasmic transport of nucleocapsid proteins of enveloped RNA viruses

Most viruses with non-segmented single stranded RNA genomes complete their life cycle in the cytoplasm of infected cells. However, despite undergoing replication in the cytoplasm, the structural proteins of some of these RNA viruses localize to the nucleus at specific times in the virus life cycle, primarily early in infection. Limited evidence suggests that this enhances successful viral replication by interfering with or inhibiting the host antiviral response. Nucleocapsid proteins of RNA viruses have a well-established, essential cytoplasmic role in virus replication and assembly. Intriguingly, nucleocapsid proteins of some RNA viruses also localize to the nucleus/nucleolus of infected cells. Their nuclear function is less well understood although significant advances have been made in recent years. This review will focus on the nucleocapsid protein of cytoplasmic enveloped RNA viruses, including their localization to the nucleus/nucleolus and function therein. A greater understanding of the nuclear localization of nucleocapsid proteins has the potential to enhance therapeutic strategies as it can be a target for the development of live-attenuated vaccines or antiviral drugs.

Characterization of RanBPM molecular determinants that control its subcellular localization

RanBPM/RanBP9 is a ubiquitous, nucleocytoplasmic protein that is part of an evolutionary conserved E3 ubiquitin ligase complex whose function and targets in mammals are still unknown. RanBPM itself has been implicated in various cellular processes that involve both nuclear and cytoplasmic functions. However, to date, little is known about how RanBPM subcellular localization is regulated. We have conducted a systematic analysis of RanBPM regions that control its subcellular localization using RanBPM shRNA cells to examine ectopic RanBPM mutant subcellular localization without interference from the endogenously expressed protein. We show that several domains and motifs regulate RanBPM nuclear and cytoplasmic localization. In particular, RanBPM comprises two motifs that can confer nuclear localization, one proline/glutamine-rich motif in the extreme N-terminus which has a dominant effect on RanBPM localization, and a second motif in the C-terminus which minimally contributes to RanBPM nuclear targeting. We also identified a nuclear export signal (NES) which mutation prevented RanBPM accumulation in the cytoplasm. Likewise, deletion of the central RanBPM conserved domains (SPRY and LisH/CTLH) resulted in the relocalization of RanBPM to the nucleus, suggesting that RanBPM cytoplasmic localization is also conferred by protein-protein interactions that promote its cytoplasmic retention. Indeed we found that in the cytoplasm, RanBPM partially colocalizes with microtubules and associates with α-tubulin. Finally, in the nucleus, a significant fraction of RanBPM is associated with chromatin. Altogether, these analyses reveal that RanBPM subcellular localization results from the combined effects of several elements that either confer direct transport through the nucleocytoplasmic transport machinery or regulate it indirectly, likely through interactions with other proteins and by intramolecular folding.

The N-N non-covalent domain of the nucleocapsid protein of type 2 porcine reproductive and respiratory syndrome virus enhances induction of IL-10 expression

Porcine reproductive and respiratory syndrome virus (PRRSV) usually establishes a prolonged infection and causes an immunosuppressive state. It has been proposed that IL-10 plays an important role in PRRSV-induced immunosuppression. However, this mechanism has not been completely elucidated. In this study, we found that transfection of 3D4/2 macrophages with the N protein gene of type 2 PRRSV significantly upregulated IL-10 expression at the transcriptional level. Moreover, alanine substitution mutation analysis revealed that the N protein residues 33-37, 65-68 and 112-123 were related to the upregulation of IL-10 promoter activity. Recombinant PRRSV with mutations at residues 33-37 in the N protein (rQ33-5A and rS36A) recovered from corresponding infectious cDNA clones and induced significantly lower levels of IL-10 production in infected monocyte-derived dendritic cells, as compared with their revertants rQ33-5A(R) and rS36A(R), and the wild-type recombinant PRRSV strain rNT/wt. These data indicate that the type 2 PRRSV N protein plays an important role in IL-10 induction and the N-N non-covalent domain is associated with this activity.

Engineering the PRRS virus genome: updates and perspectives

•

We review PRRSV infectious clones and their applications.

•

14 infectious clones are available so far for genotypes I and II.

•

Genomic mutations, insertions, deletions, and replacements are successful.

•

We discuss advances and utilization of PRRSV reverse genetics and future potential.

Porcine reproductive and respiratory syndrome virus (PRRSV) is endemic in most pig producing countries worldwide and causes enormous economic losses to the pork industry. Infectious clones for PRRSV have been constructed, and so far at least 14 different infectious clones are available representing both genotypes I and II. Two strategies have been taken for progeny reconstitution: RNA transfection and DNA transfection. Mutations, insertions, deletions, and replacements of the viral genome have been employed to study the structure function relationship, foreign gene expression, functional complementation, and virulence determinants. Essential regions and non-essential regions for viral replication have been identified in both the coding regions and non-encoding regions. Foreign sequences have successfully been inserted into the nsp2 and N regions and in the space between ORF1b and ORF2a. Chimeras between member viruses in the family Arteriviridae have also been constructed and utilized to study cell tropism and functional complementation. This review discusses the advances and utilization of PRRSV reverse genetics and its potential for future research.

A new porcine reproductive and respiratory syndrome virus strain with highly conserved molecular characteristics in its parental and attenuated strains

Porcine reproductive and respiratory syndrome virus (PRRSV) causes substantial economic losses to many swine-producing regions. In this study, PRRSV strain NT0801-F80 was derived from its parental isolate NT0801 by 80 passages in Marc-145 cells. Experimental infection of piglets clearly demonstrated that strain NT0801-F80 is less virulent than NT0801. However, whole genome sequencing showed that the genomes of the parental and attenuated strains are highly conserved compared with those of four other pairs of virulent parental/attenuated vaccine strains (VR2332 and RespPRRS MLV, JA142 and Ingelvac(®) ATP MLV, CH-1a and CH-1R, and JXA1 and JXAR). The attenuated strain NT0801-F80 has only 21 nucleotide changes, producing only 14 amino acid changes in NSP2, GP2, GP3, and GP5, compared with those aa sequences of the virulent parental strain. These mutated aa in the attenuated virus may be involved in virulence. These data provide valuable information on the attenuation mechanism of PRRSV that should be useful in future research.

Molecular characterizations of subcellular localization signals in the nucleocapsid protein of porcine epidemic diarrhea virus

The nucleolus is a dynamic subnuclear structure, which is crucial to the normal operation of the eukaryotic cell. The porcine epidemic diarrhea virus (PEDV), coronavirus nucleocapsid (N) protein, plays important roles in the process of virus replication and cellular infection. Virus infection and transfection showed that N protein was predominately localized in the cytoplasm, but also found in the nucleolus in Vero E6 cells. Furthermore, by utilizing fusion proteins with green fluorescent protein (GFP), deletion mutations or site-directed mutagenesis of PEDV N protein, coupled with live cell imaging and confocal microscopy, it was revealed that, a region spanning amino acids (aa), 71–90 in region 1 of the N protein was sufficient for nucleolar localization and R87 and R89 were critical for its function. We also identified two nuclear export signals (NES, aa221–236, and 325–364), however, only the nuclear export signal (aa325–364) was found to be functional in the context of the full-length N protein. Finally, the activity of this nuclear export signal (NES) was inhibited by the antibiotic Lepomycin B, suggesting that N is exported by a chromosome region maintenance 1-related export pathway.

Viruses and the nucleolus: the fatal attraction

Viruses are small obligatory parasites and as a consequence, they have developed sophisticated strategies to exploit the host cell's functions to create an environment that favors their own replication. A common feature of most – if not all – families of human and non-human viruses concerns their interaction with the nucleolus. The nucleolus is a multifunctional nuclear domain, which, in addition to its well-known role in ribosome biogenesis, plays several crucial other functions. Viral infection induces important nucleolar alterations. Indeed, during viral infection numerous viral components localize in nucleoli, while various host nucleolar proteins are redistributed in other cell compartments or are modified, and non-nucleolar cellular proteins reach the nucleolus. This review highlights the interactions reported between the nucleolus and some human or animal viral families able to establish a latent or productive infection, selected on the basis of their known interactions with the nucleolus and the nucleolar activities, and their links with virus replication and/or pathogenesis. This article is part of a Special Issue entitled: Role of the Nucleolus in Human Disease.

•

Most viruses interact with the nucleolus that plays a major role in virus life cycle.

•

Virus/nucleolus interaction is crucial for virus replication and pathogenesis.

•

Role of nucleoli in the infection with selected RNA viruses and herpes viruses

Overview: Replication of porcine reproductive and respiratory syndrome virus

Porcine reproductive and respiratory syndrome virus (PRRSV), an arterivirus that causes significant losses in the pig industry, is one of the most important animal pathogens of global significance. Since the discovery of the virus, significant progress has been made in understanding its epidemiology and transmission, but no adequate control measures are yet available to eliminate infection with this pathogen. The genome replication of PRRSV is required to reproduce, within a few hours of infection, the millions of progeny virions that establish, disseminate, and maintain infection. Replication of the viral RNA genome is a multistep process involving a replication complex that is formed not only from components of viral and cellular origin but also from the viral genomic RNA template; this replication complex is embedded within particular virus-induced membrane vesicles. PRRSV RNA replication is directed by at least 14 replicase proteins that have both common enzymatic activities, including viral RNA polymerase, and also unusual and poorly understood RNA-processing functions. In this review, we summarize our current understanding of PRRSV replication, which is important for developing a successful strategy for the prevention and control of this pathogen.

Characterization of signal sequences determining the nuclear export of Newcastle disease virus matrix protein

The Newcastle disease virus (NDV) matrix (M) protein has been demonstrated to be a nuclear-cytoplasmic trafficking protein. Previous studies have shown that the M protein localizes in the nucleus through a bipartite nuclear localization signal. Here, we report that the ability of the M protein to shuttle to the cytoplasm is mediated by three nuclear export signal sequences (NESs). Using leptomycin B (LMB), a specific inhibitor of CRM1, we found that the nuclear export of the three NESs was LMB insensitive and thus was CRM1 independent. In addition, inactivation of these NESs led to nuclear accumulation of the M protein. Our results highlight the significance of these NESs to the nuclear export of the NDV M protein.

The cellular interactome of the coronavirus infectious bronchitis virus nucleocapsid protein and functional implications for virus biology

The coronavirus nucleocapsid (N) protein plays a multifunctional role in the virus life cycle, from regulation of replication and transcription and genome packaging to modulation of host cell processes. These functions are likely to be facilitated by interactions with host cell proteins. The potential interactome of the infectious bronchitis virus (IBV) N protein was mapped using stable isotope labeling with amino acids in cell culture (SILAC) coupled to a green fluorescent protein-nanotrap pulldown methodology and liquid chromatography-tandem mass spectrometry. The addition of the SILAC label allowed discrimination of proteins that were likely to specifically bind to the N protein over background binding. Overall, 142 cellular proteins were selected as potentially binding to the N protein, many as part of larger possible complexes. These included ribosomal proteins, nucleolar proteins, translation initiation factors, helicases, and hnRNPs. The association of selected cellular proteins with IBV N protein was confirmed by immunoblotting, cosedimentation, and confocal microscopy. Further, the localization of selected proteins in IBV-infected cells as well as their activity during virus infection was assessed by small interfering RNA-mediated depletion, demonstrating the functional importance of cellular proteins in the biology of IBV. This interactome not only confirms previous observations made with other coronavirus and IBV N proteins with both overexpressed proteins and infectious virus but also provides novel data that can be exploited to understand the interaction between the virus and the host cell.

Molecular evolution of PRRSV in Europe: current state of play

Porcine reproductive and respiratory syndrome virus (PRRSV) is a major threat to European swine production. The existence of extensive genetic variation in endemic strains and the presence of highly virulent strains in other geographic regions pose the threat of devastating epidemic outbreaks. Here we describe the current knowledge of genetic variation in European PRRSV isolates, the implications for PRRSV evolution, and the presence of multiple genetic lineages of Type 2 (North American genotype) isolates in Europe. In Type 1 (European genotype) PRRSV, three genetic subtypes are recognized and a fourth subtype appears to be present. Type 2 PRRSV was considered to be genetically homogenous in Europe due to a unique presence of an introduced vaccine strain, but independent introductions of virulent Type 2 field viruses are now evident. In Type 1 PRRSV, only subtype 1 (Lelystad virus-like) circulates in Central and Western Europe and globally. In Eastern Europe, all subtypes are present. The subtypes of Type 1 PRRSV also exhibit length differences in the nucleocapsid protein, ranging in size from 124 to 132 amino acids depending on subtype. This size heterogeneity is unparalleled in the nucleocapsid proteins of Type 2 PRRSV or other viruses. Surprisingly, it affects the C-terminus, otherwise thought to be under strong structural constraints. Finally, divergent subtypes of Type 1 PRRSV have produced high rates of false-negative RT-PCR results in diagnostic tests, and may also degrade the reliability of serodiagnostic assays using the nucleocapsid protein antigen. In summary, the extensive genetic diversity of Type 1 PRRSV is of relevance for understanding nucleocapsid protein structure/function relationships. Further, the extensive genetic diversity of Type 1 PRRSV in Europe, and the presence of diverse Type 2 PRRSV strains, together emphasize the importance of relevant validation of PRRSV diagnostics. More extensive and systematic molecular phylogeny studies are needed to fully understand PRRSV diversity in Europe, to provide swine producers with reliable diagnostics, and to better assess the potential consequences of endemic spread and exotic introductions.

Significance of genetic variation of PRRSV ORF5 in virus neutralization and molecular determinants corresponding to cross neutralization among PRRS viruses

A high rate of genetic and antigenic variability among porcine reproductive and respiratory syndrome viruses (PRRSVs) hampers effective prevention and control of the disease caused by PRRSV. The major envelope protein (GP5) encoded by the ORF5 of PRRSV has a critical role in inducing virus neutralizing (VN) antibody and cross protection among different strains of PRRSV. This study was conducted to identify sequence elements related to cross neutralization by comparing the ORF5 sequences of 69 field isolates in conjunction with their susceptibility to VN antibody raised against the VR2332 strain in vitro and in vivo. Five common variable sites (amino acid position 32-34, 38-39, 57-59, 137 and 151) were identified between susceptible and resistant viral isolates. Mutants whose ORF5 amino acid sequences were substituted with the sequences corresponding to the 5 identified common variable sites individually or concurrently were generated from a VR2332-backboned infectious clone by site mutagenesis. The change in the susceptibility of the mutants to VN antibodies specific for VR2332 or a heterologous PRRSV was assessed to determine the association of those 5 identified sites with cross neutralization. Among the five sites, the changes of amino acid sequences at three sites (32-34, 38-39, and 57-59) located in the N-terminal ectodomain of ORF5 significantly influenced the susceptibility of the mutant viruses to VN antibody, suggesting that sequence homology at these sites can be utilized as genetic markers to predict the degree of cross neutralization among different PRRSVs.

Porcine reproductive and respiratory syndrome virus induces interleukin-15 through the NF-κB signaling pathway

Porcine reproductive and respiratory syndrome virus (PRRSV) mainly infects macrophages/dendritic cells and modulates cytokine expression in these cells. Interleukin-15 (IL-15) is a pleiotropic cytokine involved in wide range of biological activities. It has been shown to be essential for the generation, activation, and proliferation of NK and NKT cells and for the survival and activation of CD8(+) effector and memory T cells. In this study, we discovered that PRRSV infection upregulated IL-15 production at both the mRNA and protein levels in porcine alveolar macrophages (PAMs), blood monocyte-derived macrophages (BMo), and monocyte-derived dendritic cells (DCs). We subsequently demonstrated that the NF-κB signaling pathway was essential for PRRSV infection-induced IL-15 production. First, addition of an NF-κB inhibitor drastically reduced PRRSV infection-induced IL-15 production. We then found that NF-κB was indeed activated upon PRRSV infection, as evidenced by IκB phosphorylation and degradation. Moreover, we revealed an NF-κB binding motif in the cloned porcine IL-15 (pIL-15) promoter, deletion of which abrogated the pIL-15 promoter activity in PRRSV-infected alveolar macrophages. In addition, we demonstrated that PRRSV nucleocapsid (N) protein had the ability to induce IL-15 production in porcine alveolar macrophage cell line CRL2843 by transient transfection, which was mediated by its multiple motifs, and it also activated NF-κB. These data indicated that PRRSV infection-induced IL-15 production was likely through PRRSV N protein-mediated NF-κB activation. Our findings provide new insights into the molecular mechanisms underling the IL-15 production induced by PRRSV infection.

Expression of the nucleocapsid protein of porcine reproductive and respiratory syndrome virus in soybean seed yields an immunogenic antigenic protein

Porcine reproductive and respiratory syndrome (PRRS), caused by the PRRS virus (PRRSV), is a serious disease of swine and contributes to severe worldwide economic losses in swine production. Current vaccines against PRRS rely on the use of an attenuated-live virus; however, these are unreliable. Thus, alternative effective vaccines against PRRS are needed. Plant-based subunit vaccines offer viable, safe, and environmentally friendly alternatives to conventional vaccines. In this study, efforts have been undertaken to develop a soybean-based vaccine against PRRSV. A construct carrying a synthesized PRRSV-ORF7 antigen, nucleocapsid N protein of PRRSV, has been introduced into soybean, Glycine max (L.) Merrill. cvs. Jack and Kunitz, using Agrobacterium-mediated transformation. Transgenic plants carrying the sORF7 transgene have been successfully generated. Molecular analyses of T(0) plants confirmed integration of the transgene and transcription of the PRRSV-ORF7. Presence of a 15-kDa protein in seeds of T(1) transgenic lines was confirmed by Western blot analysis using PRRSV-ORF7 antisera. The amount of the antigenic protein accumulating in seeds of these transgenic lines was up to 0.65% of the total soluble protein (TSP). A significant induction of a specific immune response, both humoral and mucosal, against PRRSV-ORF7 was observed following intragastric immunization of BALB/c female mice with transgenic soybean seeds. These findings provide a 'proof of concept', and serve as a critical step in the development of a subunit plant-based vaccine against PRRS.

Disulfide linkages mediating nucleocapsid protein dimerization are not required for porcine arterivirus infectivity

The nucleocapsid (N) proteins of the North American (type II) and European (type I) genotypes of porcine reproductive and respiratory syndrome virus (PRRSV) share only approximately 60% genetic identity, and the functionality of N in both genotypes, especially its role in virion assembly, is still poorly understood. In this study, we demonstrated that the ORF7 3' untranslated region or ORF7 of type I is functional in the type II PRRSV background. Based on these results, we postulated that the cysteine at position 90 (Cys90) of the type II N protein, which corresponds to an alanine in the type I protein, is nonessential for virus infectivity. The replacement of Cys90 with alanine confirmed this hypothesis. We then hypothesized that all of the cysteines in the N protein could be replaced by alanines. Mutational analysis revealed that, in contradiction to previously reported findings, the replacement of all of the cysteines, either singly or in combination, did not impair the growth of either type II or type I PRRSV. Treatment with the alkylating agent N-ethylmaleimide inhibited cysteine-mediated N dimerization in living cells but not in released virions. Additionally, bimolecular fluorescence complementation assays revealed noncovalent interactions in living cells among the N and C termini and between the N-terminal and C-terminal regions of the N proteins of both genotypes of PRRSV. These results demonstrate that the disulfide linkages mediating the N dimerization are not required for PRRSV viability and help to promote our understanding of the mechanism underlying arterivirus particle assembly.

Glycosyl-phosphatidylinositol (GPI)-anchored membrane association of the porcine reproductive and respiratory syndrome virus GP4 glycoprotein and its co-localization with CD163 in lipid rafts

The porcine reproductive and respiratory syndrome virus (PRRSV) glycoprotein 4 (GP4) resembles a typical type I membrane protein in its structure but lacks a hydrophilic tail at the C-terminus, suggesting that GP4 may be a lipid-anchored membrane protein. Using the human decay-accelerating factor (DAF; CD55), a known glycosyl-phosphatidylinositol (GPI) lipid-anchored protein, chimeric constructs were made to substitute the GPI-anchor domain of DAF with the putative lipid-anchor domain of GP4, and their membrane association and lipase cleavage were determined in cells. The DAF-GP4 fusion protein was transported to the plasma membrane and was cleaved by phosphatidylinositol-specific phospholipase C (PI-PLC), indicating that the C-terminal domain of GP4 functions as a GPI anchor. Mutational studies for residues adjacent to the GPI modification site and characterization of respective mutant viruses generated from infectious cDNA clones show that the ability of GP4 for membrane association corresponded to virus viability and growth characteristics. The residues T158 (ω − 2, where ω is the GPI moiety at E160), P159 (ω − 1), and M162 (ω + 2) of GP4 were determined to be important for virus replication, with M162 being of particular importance for virus infectivity. The complete removal of the peptide–anchor domain in GP4 resulted in a complete loss of virus infectivity. The depletion of cholesterol from the plasma membrane of cells reduced the virus production, suggesting a role of lipid rafts in PRRSV infection. Remarkably, GP4 was found to co-localize with CD163 in the lipid rafts on the plasma membrane. Since CD163 has been reported as a cellular receptor for PRRSV and GP4 has been shown to interact with this receptor, our data implicates an important role of lipid rafts during entry of the virus.

Identification of a functional, CRM-1-dependent nuclear export signal in hepatitis C virus core protein

Hepatitis C virus (HCV) infection is a major cause of chronic liver disease worldwide. HCV core protein is involved in nucleocapsid formation, but it also interacts with multiple cytoplasmic and nuclear molecules and plays a crucial role in the development of liver disease and hepatocarcinogenesis. The core protein is found mostly in the cytoplasm during HCV infection, but also in the nucleus in patients with hepatocarcinoma and in core-transgenic mice. HCV core contains nuclear localization signals (NLS), but no nuclear export signal (NES) has yet been identified.We show here that the aa(109-133) region directs the translocation of core from the nucleus to the cytoplasm by the CRM-1-mediated nuclear export pathway. Mutagenesis of the three hydrophobic residues (L119, I123 and L126) in the identified NES or in the sequence encoding the mature core aa(1-173) significantly enhanced the nuclear localisation of the corresponding proteins in transfected Huh7 cells. Both the NES and the adjacent hydrophobic sequence in domain II of core were required to maintain the core protein or its fragments in the cytoplasmic compartment. Electron microscopy studies of the JFH1 replication model demonstrated that core was translocated into the nucleus a few minutes after the virus entered the cell. The blockade of nucleocytoplasmic export by leptomycin B treatment early in infection led to the detection of core protein in the nucleus by confocal microscopy and coincided with a decrease in virus replication.Our data suggest that the functional NLS and NES direct HCV core protein shuttling between the cytoplasmic and nuclear compartments, with at least some core protein transported to the nucleus. These new properties of HCV core may be essential for virus multiplication and interaction with nuclear molecules, influence cell signaling and the pathogenesis of HCV infection.

Molecular determinants responsible for the subcellular localization of HSV-1 UL4 protein

The function of the herpes simplex virus type 1 (HSV-1) UL4 protein is still elusive. Our objective is to investigate the subcellular transport mechanism of the UL4 protein. In this study, fluorescence microscopy was employed to investigate the subcellular localization of UL4 and characterize the transport mechanism in living cells. By constructing a series of deletion mutants fused with enhanced yellow fluorescent protein (EYFP), the nuclear export signals (NES) of UL4 were for the first time mapped to amino acid residues 178 to 186. In addition, the N-terminal 19 amino acids are identified to be required for the granule-like cytoplasmic pattern of UL4. Furthermore, the UL4 protein was demonstrated to be exported to the cytoplasm through the NES in a chromosomal region maintenance 1 (CRM1)-dependent manner involving RanGTP hydrolysis.

Packaging of porcine reproductive and respiratory syndrome virus replicon RNA by a stable cell line expressing its nucleocapsid protein

Porcine reproductive and respiratory syndrome virus (PRRSV), a member of the Arteriviridae family, is one of the most common and economically important swine pathogens. Although both live-attenuated and killed-inactivated vaccines against the virus have been available for a decade, PRRSV is still a major problem in the swine industry worldwide. To explore the possibility of producing single-round infectious PRRSV replicon particles as a potential vaccine strategy, we have now generated two necessary components: 1) a stable cell line (BHK/Sinrepl9/PRRSV-N) that constitutively expresses the viral nucleocapsid (N) protein localized to the cytoplasm and the nucleolus and 2) a PRRSV replicon vector (pBAC/PRRSV/Replicon-AN) with a 177-nucleotide deletion, removing the 3′-half portion of ORF7 in the viral genome, from which the self-replicating propagation-defective replicon RNAs were synthesized in vitro by SP6 polymerase run-off transcription. Transfection of this replicon RNA into N protein-expressing BHK-21 cells led to the secretion of infectious particles that packaged the replicon RNA, albeit with a low production efficiency of 0.4 × 10² to 1.1 × 10² infectious units/ml; the produced particles had only single-round infectivity with no cell-to-cell spread. This trans-complementation system for PRRSV provides a useful platform for studies to define the packaging signals and motifs present within the viral genome and N protein, respectively, and to develop viral replicon-based antiviral vaccines that will stop the infection and spread of this pathogen.

Nucleolar proteomics and viral infection

Recent advances in proteomics have been combined with traditional methods for isolation of nucleoli from mammalian and plant cells. This approach has confirmed the growing body of data showing a wide role for the nucleolus in eukaryotic cell biology beyond ribosome generation into many areas of cell function from regulation of the cell cycle, modulation of the cell stress response to innate immune responses. This has been reflected in the growing body of evidence that viruses specifically target the nucleolus by sequestering cellular nucleolar proteins or by targeting viral proteins to the nucleolus in order to maximise viral replication. This review covers those key areas and looks at the latest approaches using high‐throughput quantitative proteomics of the nucleolus in virus infected cells to gain an insight into the role of this fascinating compartment in viral infection.

Polymorphic genetic characterization of the ORF7 gene of porcine reproductive and respiratory syndrome virus (PRRSV) in China

Background

Porcine reproductive and respiratory syndrome virus (PRRSV) exhibits extensive genetic variation. The outbreak of a highly pathogenic PRRS in 2006 led us to investigate the extent of PRRSV genetic diversity in China. To this end, we analyzed the Nsp2 and ORF7 gene sequences of 98 Chinese PRRSV isolates.

Results

Preliminary analysis indicated that highly pathogenic PRRSV strains with a 30-amino acid deletion in the Nsp2 protein are the dominant viruses circulating in China. Further analysis based on ORF7 sequences revealed that all Chinese isolates were divided into 5 subgroups, and that the highly pathogenic PRRSVs were distantly related to the MLV or CH-1R vaccine, raising doubts about the efficacy of these vaccines. The ORF7 sequence data also showed no apparent associations between geographic or temporal origin and heterogeneity of PRRSV in China.

Conclusion

These findings enhance our knowledge of the genetic characteristics of Chinese PRRSV isolates, and may facilitate the development of effective strategies for monitoring and controlling PRRSV in China.

Nonstructural protein 1α subunit-based inhibition of NF-κB activation and suppression of interferon-β production by porcine reproductive and respiratory syndrome virus

Induction of type I interferon (IFN-α/β) is an early antiviral response of the host, and porcine reproductive and respiratory syndrome virus (PRRSV) has been reported to downregulate the IFN response during infection in cells and pigs. We report that the PRRSV nonstructural protein 1α (Nsp1α) subunit of Nsp1 is a nuclear-cytoplasmic protein distributed to the nucleus and contains a strong suppressive activity for IFN-β production that is mediated through the retinoic acid-inducible gene I (RIG-I) signaling pathway. Nsp1α suppressed the activation of nuclear factor (NF)-κB when stimulated with dsRNA or tumor necrosis factor (TNF)-α, and NF-κB suppression was RIG-I-dependent. The suppression of NF-κB activation was associated with the poor production of IFN-β during PRRSV infection. The C-terminal 14 amino acids of the Nsp1α subunit were critical in maintaining immunosuppressive activity of Nsp1α for both IFN-β and NF-κB, suggesting that the newly identified zinc finger configuration comprising of Met180 may be crucial for inhibitory activities. Nsp1α inhibited IκB phosphorylation and as a consequence NF-κB translocation to the nucleus was blocked, leading to the inhibition of NF-κB stimulated gene expression. Our results suggest that PRRSV Nsp1α is a multifunctional nuclear protein participating in the modulation of the host IFN system.

Nucleolar localization/retention signal is responsible for transient accumulation of histone H2B in the nucleolus through electrostatic interactions

The majority of known nuclear proteins are highly mobile. The molecular mechanisms by which they accumulate inside stable compartments that are not separated from the nucleoplasm by membranes are obscure. The compartmental retention of some proteins is associated with their biological function; however, some protein interactions within distinct nuclear structures may be non-specific. The non-specific retention may lead to the accumulation of proteins in distinct structural domains, even if the protein does not function inside this domain. In this study, we have shown that histone H2B-EGFP initially accumulated in the nucleolus after ectopic expression, and then gradually incorporated into the chromatin to leave only a small amount of nucleolus-bound histone that was revealed by removing chromatin-bound proteins with DNase I treatment. Nucleolar histone H2B had several characteristics: (i) it preferentially bound to granular component of the nucleolus and interacted with RNA or RNA-containing nucleolar components; (ii) it freely exchanged between the nucleolus and nucleoplasm; (iii) it associated with the nuclear matrix; and (iv) it bound to interphase prenuclear bodies that formed after hypotonic treatment. The region in histone H2B that acts as a nucleolar localization/retention signal (NoRS) was identified. This signal overlapped with a nuclear localization signal (NLS), which appears to be the primary function of this region. The NoRS activity of this region was non-specific, but the molecular mechanism was probably similar to the NoRSs of other nucleolar proteins. All known NoRSs are enriched with basic amino acids, and we demonstrated that positively charged motifs (nona-arginine (R9) and nona-lysine (K9)) were sufficient for the nucleolar accumulation of EGFP. Also, the correlation between measured NoRS activity and the predicted charge was observed. Thus, NoRSs appear to achieve their function through electrostatic interactions with the negatively charged components of the nucleolus. Though these interactions are non-specific, the functionally unrelated retention of a protein can increase the probability of its interaction with specific and functionally related binding sites.