The surface morphology of Atractylodes macrocephala polysaccharide and its inhibitory effect on PCV2 replication

BACKGROUND

Porcine infection with Porcine circovirus type 2 (PCV2) causes immunosuppression, which is easy to cause concurrent or secondary infection, making the disease complicated and difficult to treat, and causing huge economic losses to the pig industry. Total polysaccharide from the rhizoma of Atractylodes macrocephala Koidz. (PAMK) is outstanding in enhancing non-specific immunity and cellular immunity, and effectively improving the body's disease resistance, indicating its potential role in antiviral immunotherapy.

RESULTS

PAMK had the characteristics of compact, polyporous and agglomerated morphology, but does not have triple helix conformation. PCV2 infection led to the increase in LC3-II, degradation of p62 and the increase of viral Cap protein expression and viral copy number. PAMK treatment significantly alleviated PCV2-induced autophagy and inhibited PCV2 replication. Moreover, PAMK treatment significantly attenuated the increase of PINK1 protein expression and the decrease of TOMM20 protein expression caused by PCV2 infection, alleviated Parkin recruitment from cytoplasm to mitochondria and intracellular reactive oxygen species accumulation, restored mitochondrial membrane charge, alleviated viral Cap protein expression.

CONCLUSION

PAMK alleviates PCV2-induced mitophagy to suppress PCV2 replication by inhibiting the Pink 1/Parkin pathway. These findings may provide new insights into the prevention and treatment of PCV2. © 2023 Society of Chemical Industry.

Porcine circovirus type 2 infection promotes the SUMOylation of nucleophosmin-1 to facilitate the viral circular single-stranded DNA replication

The mechanism of genome DNA replication in circular single-stranded DNA viruses is currently a mystery, except for the fact that it undergoes rolling-circle replication. Herein, we identified SUMOylated porcine nucleophosmin-1 (pNPM1), which is previously reported to be an interacting protein of the viral capsid protein, as a key regulator that promotes the genome DNA replication of porcine single-stranded DNA circovirus. Upon porcine circovirus type 2 (PCV2) infection, SUMO2/3 were recruited and conjugated with the K263 site of pNPM1's C-terminal domain to SUMOylate pNPM1, subsequently, the SUMOylated pNPM1 were translocated in nucleoli to promote the replication of PCV2 genome DNA. The mutation of the K263 site reduced the SUMOylation levels of pNPM1 and the nucleolar localization of pNPM1, resulting in a decrease in the level of PCV2 DNA replication. Meanwhile, the mutation of the K263 site prevented the interaction of pNPM1 with PCV2 DNA, but not the interaction of pNPM1 with PCV2 Cap. Mechanistically, PCV2 infection increased the expression levels of Ubc9, the only E2 enzyme involved in SUMOylation, through the Cap-mediated activation of ERK signaling. The upregulation of Ubc9 promoted the interaction between pNPM1 and TRIM24, a potential E3 ligase for SUMOylation, thereby facilitating the SUMOylation of pNPM1. The inhibition of ERK activation could significantly reduce the SUMOylation levels and the nucleolar localization of pNPM1, as well as the PCV2 DNA replication levels. These results provide new insights into the mechanism of circular single-stranded DNA virus replication and highlight NPM1 as a potential target for inhibiting PCV2 replication.

Ubiquitination-dependent degradation of nucleolin mediated by porcine circovirus type 3 capsid protein

IMPORTANCE

Porcine circovirus type 3 (PCV3) is an emerging pathogen that causes multisystem disease in pigs and poses a severe threat to the swine industry. However, the mechanisms of how PCV3 uses host proteins to regulate its own life cycle are not well understood. In this study, we found that PCV3 capsid protein interacts with nucleolin and degrades it. Degradation of nucleolin by the PCV3 capsid protein requires recruitment of the enzyme RNF34, which is transported to the nucleolus from the cytoplasm in the presence of the PCV3 capsid protein. Nucleolin also decreases PCV3 replication by promoting the release of interferon β. These findings clarify the mechanism by which nucleolin modulates PCV3 replication in cells, thereby facilitating to provide an important strategy for preventing and controlling PCV3 infection.

Development of a Gold Nanoparticle-Based Immunochromatographic Strip for Rapid Detection of Porcine Circovirus Type 2

Porcine circovirus type 2 (PCV2) is an important swine infectious pathogen that seriously threatens the global swine industry. PCV2 Cap protein is the only structural and the main immunogenic protein constituting the viral capsid. In this study, a gold nanoparticle-based immunochromatographic strip with high sensitivity and specificity was developed which could be used for rapid detection of PCV2 virions or Cap protein in research. The visual detection limit of the strip was 103.18 50% tissue culture infective does (TCID50)/mL for PCV2, and 2.03 μg/mL for PCV2 Cap protein. No cross-reactivity was observed with the PCV1 and PCV3 Cap proteins and other common swine pathogens such as porcine reproductive and respiratory syndrome virus, classical swine fever virus, pseudorabies virus, porcine epidemic diarrhea virus, porcine parvovirus, and swine influenza virus. The repeatability of the strip was good. The stability of the strip was perfect for 12 months in a dry state at room temperature. Visual results could be obtained within 5 min by simply inserting the strip into the diluted sample. The strip is a time-saving, labor-saving, and reliable tool for testing of PCV2 virions or Cap protein in research. The idea of this study might open a new perspective for the application of the strip. IMPORTANCE Porcine circovirus type 2 (PCV2) Cap protein is the only structural and the main immunogenic protein constituting the viral capsid. Although many methods can be used to identify PCV2 or PCV2 Cap protein in vaccine research, they usually require high workload and time. The developed strip can specifically detect PCV2 virions or Cap protein, and visual qualitative results can be obtained within 5 min by simply diluting the sample and inserting the strip into the sample. The final value of the strip is providing a simple and time-saving method for real-time monitoring of PCV2 antigen in vaccine research with reliable results, such as the different stages of PCV2 Cap protein expression and purification, as well as the different stages of PCV2 reproduction and purification.

Heme Oxygenase-1 and Its Metabolites Carbon Monoxide and Biliverdin, but Not Iron, Exert Antiviral Activity against Porcine Circovirus Type 3

Porcine circovirus type 3 (PCV3) is a newly discovered pathogen that causes porcine dermatitis and nephropathy syndrome (PDNS)-like clinical signs, multisystemic inflammation, and reproductive failure. Heme oxygenase-1 (HO-1), a stress-inducible enzyme, exerts protective functions by converting heme into carbon monoxide (CO), biliverdin (BV), and iron. However, the effects of HO-1 and its metabolites on PCV3 replication remain unknown. In this study, experiments involving specific inhibitors, lentivirus transduction, and small interfering RNA (siRNA) transfection revealed that active PCV3 infection reduced HO-1 expression and that the expression of HO-1 negatively regulated virus replication in cultured cells, depending on its enzymatic activity. Subsequently, the effects of the HO-1 metabolites (CO, BV, and iron) on PCV3 infection were investigated. The CO inducers (cobalt protoporphyrin IX [CoPP] or tricarbonyl dichloro ruthenium [II] dimer [CORM-2]) mediate PCV3 inhibition by generating CO, and this inhibition is reversed by hemoglobin (Hb; a CO scavenger). The inhibition of PCV3 replication by BV depended on BV-mediated reactive oxygen species (ROS) reduction, as N-acetyl-l-cysteine affected PCV3 replication while reducing ROS production. The reduction product of BV, bilirubin (BR), specifically promoted nitric oxide (NO) generation and further activated the cyclic GMP/protein kinase G (cGMP/PKG) pathway to attenuate PCV3 infection. Both the iron provided by FeCl3 and the iron chelated by deferoxamine (DFO) with CoPP treatment failed to affect PCV3 replication. Our data demonstrate that the HO-1-CO-cGMP/PKG, HO-1-BV-ROS, and HO-1-BV-BR-NO-cGMP/PKG pathways contribute crucially to the inhibition of PCV3 replication. These results provide important insights regarding preventing and controlling PCV3 infection. IMPORTANCE The regulation of host protein expression by virus infection is the key to facilitating self-replication. As an important emerging pathogen of swine, clarification of the interaction between PCV3 infection and the host enables us to understand the viral life cycle and pathogenesis better. Heme oxygenase-1 (HO-1) and its metabolites carbon monoxide (CO), biliverdin (BV), and iron have been demonstrated to involve a wealth of viral replications. Here, we, for the first time, demonstrated that HO-1 expression decreases in PCV3-infected cells and negatively regulates PCV3 replication and that the HO-1 metabolic products CO and BV inhibit PCV3 replication by the CO- or BV/BR/NO-dependent cGMP/PKG pathway or BV-mediated ROS reduction, but the iron (the third metabolic product) does not. Specifically, PCV3 infection maintains normal proliferation by downregulating HO-1 expression. These findings clarify the mechanism by which HO-1 modulates PCV3 replication in cells and provide important targets for preventing and controlling PCV3 infection.

Precise Assembly of Multiple Antigens on Nanoparticles with Specially Designed Affinity Peptides

Antigen proteins, assembled on nanoparticles, can be recognized by antigen-presenting cells effectively to enhance antigen immunogenicity. The ability to simultaneously display multiantigens on the same nanoparticle could have numerous applications but remained technical challenges. Here, we described a method for precise assembly of multiple antigens on nanoparticles with specially designed affinity peptides. First, we designed and screened affinity peptides with high affinity and specificity, which could respectively target the key amino acid residues of classical swine fever virus (CSFV) E2 protein or porcine circovirus type 2 capsid protein (PCV2 Cap) accurately. Then, we conjugated the antigen proteins to poly(lactic acid-glycolic acid) copolymer (PLGA) and Gram-positive enhancer matrix (GEM) nanoparticles through the peptides and perfectly assembled two kinds of multiantigen display nanoparticles with different particle sizes. Subsequently, the immunological properties of the assembled nanoparticles were tested. The results showed that the antigen display nanoparticles could promote the maturation, phagocytosis, and proinflammatory effects of antigen-presenting cells (APCs). Besides, compared with the antigen proteins, multiantigen display nanoparticles could induce much higher levels of antibodies and neutralizing antibodies in mice. This strategy may provide a technical support for the study of protein structure and the research and development of polyvalent vaccines.

PCV2 targets cGAS to inhibit type I interferon induction to promote other DNA virus infection

Viruses use diverse strategies to impair the antiviral immunity of host in order to promote infection and pathogenesis. Herein, we found that PCV2 infection promotes the infection of DNA viruses through inhibiting IFN-β induction in vivo and in vitro. In the early phase of infection, PCV2 promotes the phosphorylation of cGAS at S278 via activation of PI3K/Akt signaling, which directly silences the catalytic activity of cGAS. Subsequently, phosphorylation of cGAS at S278 can facilitate the K48-linked poly-ubiquitination of cGAS at K389, which can been served as a signal for recognizing by the ubiquitin-binding domain of histone deacetylase 6 (HDAC6), to promote the translocation of K48-ubiquitinated-cGAS from cytosol to autolysosome depending on the deacetylase activity of HDAC6, thereby eventually resulting in a markedly increased cGAS degradation in PCV2 infection-induced autophagic cells relative to Earle’s Balanced Salt Solution (EBSS)-induced autophagic cells (a typical starving autophagy). Importantly, we found that PCV2 Cap and its binding protein gC1qR act as predominant regulators to promote porcine cGAS phosphorylation and HDAC6 activation through mediating PI3K/AKT signaling and PKCδ signaling activation. Based on this finding, gC1qR-binding activity deficient PCV2 mutant (PCV2RmA) indeed shows a weakened inhibitory effect on IFN-β induction and a weaker boost effect for other DNA viruses infection compared to wild-type PCV2. Collectively, our findings illuminate a systematic regulation mechanism by which porcine circovirus counteracts the cGAS-STING signaling pathway to inhibit the type I interferon induction and promote DNA virus infection, and identify gC1qR as an important regulator for the immunosuppression induced by PCV2.

PCV2 is well known for its ability to induce immunosuppression in pigs. However, how PCV2 infection interferes cGAS-STING signaling is still poorly understood. Herein, we demonstrate that PCV2 infection can phosphorylate porcine cGAS via gC1qR-mediated PI3K/AKT signaling to silence the catalytic activity of cGAS, while activates PKCδ signaling to promote histone deacetylase 6 (HDAC6) activation depending on the assistance of gC1qR. Subsequently, phosphorylation of cGAS facilitates the poly-ubiquitination of cGAS, then ubiquitinated-cGAS proteins are recruited and transported to autolysosome by activated HDAC6 depending on its deacetylase activity and ubiquitin-binding function, thereby eventually resulting in the autophagic degradation of cGAS in PCV2-infected cells. This study reveals that PCV2 can inhibit the activation of cGAS signaling pathway through two different mechanisms at different stages of infection and clarifies the internal relationship and cooperation model between these two mechanisms.

Construction of DNA-displaying nanoparticles by enzymatic conjugation of DNA and elastin-like polypeptides using a replication initiation protein

DNA-displaying nanoparticles comprised of conjugates of single-stranded DNA (ssDNA) and elastin-like polypeptide (ELP) were developed. ssDNA was enzymatically conjugated to ELPs via a catalytic domain of Porcine Circovirus type 2 replication initiation protein (pRep) fused to ELPs. Nanoparticles were formed upon heating to temperatures above the phase transition temperature due to the hydrophobicity of ELPs and the hydrophilicity of conjugated ssDNA. We demonstrated the applicability of the resultant nanoparticles as drug carriers with tumor-targeting properties by conjugating a DNA aptamer, which is known to bind to Mucin 1 (MUC1), to ELPs. DNA aptamer-displaying nanoparticles encapsulating the anti-cancer drug paclitaxel were able to bind to cells overexpressing MUC1 and induce cell death.

Cellular p32 Is a Critical Regulator of Porcine Circovirus Type 2 Nuclear Egress

Circoviruses are the smallest DNA viruses known to infect mammalian and avian species. Although circoviruses are known to be associated with a range of clinical diseases, the details of circovirus DNA release still remain unknown. Here, we identified p32 as a key regulator for porcine circoviral nuclear egress. Upon porcine circovirus type 2 (PCV2) infection, p32 was recruited into the nucleus by the viral capsid (Cap) protein; simultaneously, protein kinase C isoform δ (PKC-δ) was phosphorylated at threonine 505 by phospholipase C (PLC)-mediated signaling at the early stage of infection, which was further amplified by Jun N-terminal protein kinase (JNK) and extracellular signal-regulated kinase (ERK) signaling at the late infection phase. p32 functioned as an adaptor to recruit phosphorylated PKC-δ and Cap to the nuclear membrane to phosphorylate lamin A/C, resulting in a rearrangement of nuclear lamina and thus facilitating viral nuclear egress. Consistent with these findings, knockout (KO) of p32 in PCV2-infected cells markedly reduced the phosphorylation of PKC-δ and impeded the recruitment of p-PKC-δ and Cap to the nuclear membrane, hence abolishing the phosphorylation of lamin A/C and the rearrangement of nuclear lamina. As a result, p32 depletion profoundly impaired the production of cell-free viruses during PCV2 infection. We further identified the N-terminal 24RRR26 of Cap to be crucial for binding to p32, and mutation of these three arginine residues significantly weakened the replication and pathogenesis of PCV2 in vivo In summary, our findings highlight a critical role of p32 in the activation and recruitment of PKC-δ to phosphorylate lamin A/C and facilitate porcine circoviral nuclear egress, and they certainly help understanding of the mechanism of PCV2 replication.IMPORTANCE Circovirus infections are highly prevalent in mammalian and avian species. Circoviral capsid protein is the only structural protein of the virion that plays an essential role in viral assembly. However, the machinery of circovirus nuclear egress is currently unknown. In this work, we identified p32 as a key regulator of porcine circovirus type 2 (PCV2) nuclear egress that forms a complex with the viral capsid (Cap) protein to enhance protein kinase C isoform δ (PKC-δ) activity; this resulted in a recruitment of phosphorylated PKC-δ to the nuclear membrane, which further phosphorylates lamin A/C to promote the rearrangement of nuclear lamina and facilitate viral nuclear egress. Notably, we found that the N-terminal 24RRR26 of Cap, a highly conserved motif among circovirus species, was required for interacting with p32, and that mutation of this motif markedly impeded PCV2 nuclear egress. These data indicate that p32 is a critical regulator of PCV2 nuclear egress and reveal the importance of this finding in circovirus replication.

ITRAQ-based quantitative proteomics reveals the first proteome profiles of piglets infected with porcine circovirus type 3

Porcine circovirus type 3 (PCV3) infection induces porcine dermatitis and nephropathy syndrome, reproductive failure, and multisystemic inflammatory lesions in piglets and sows. To better understand the host responses to PCV3 infection, isobaric tags for relative and absolute quantification (iTRAQ) labeling combined with LC-MS/MS analysis was used for quantitative determination of differentially regulated cellular proteins in the lungs of specific-pathogen-free piglets after 4 weeks of PCV3 infection. Totally, 3429 proteins were detected in three independent mass spectrometry analyses, of which 242 differential cellular proteins were significantly regulated, consisting of 100 upregulated proteins and 142 downregulated proteins in PCV3-infected group relative to control group. Bioinformatics analysis revealed that these higher or lower abundant proteins involved primarily metabolic processes, innate immune response, MHC-I and MHC-II components, and phagosome pathways. Ten genes encoding differentially regulated proteins were selected for investigation via real-time RT-PCR. The expression levels of six representative proteins, OAS1, Mx1, ISG15, IFIT3, SOD2, and HSP60, were further confirmed by Western blotting and immunohistochemistry. This study attempted for the first time to investigate the protein profile of PCV3-infected piglets using iTRAQ technology; our findings provide valuable information to better understand the mechanisms underlying the host responses to PCV3 infection in piglets. SIGNIFICANCE: Our study identified differentially abundant proteins related to a variety of potential signaling pathways in the lungs of PCV3-infected piglets. These findings provide valuable information to better understand the mechanisms of host responses to PCV3 infection.

Identification of Nuclear Localization Signals in the ORF2 Protein of Porcine Circovirus Type 3

Porcine circovirus type 3 (PCV3) contains two major open reading frames (ORFs) and the ORF2 gene encodes the major structural capsid protein. In this study, nuclear localization of ORF2 was demonstrated by fluorescence observation and subcellular fractionation assays in ORF2-transfected PK-15 cells. The subcellular localization of truncated ORF2 indicated that the 38 N-terminal amino acids were responsible for the nuclear localization of ORF2. The truncated and site-directed mutagenesis of this domain were constructed, and the results demonstrated that the basic amino acid residues at positions 8-32 were essential for the strict nuclear localization. The basic motifs ₈RRR-R-RRR₁₆ and ₁₆RRRHRRR₂₂ were further shown to be the key functional nucleolar localization signals that guide PCV3 ORF2 into nucleoli. Furthermore, sequence analysis showed that the amino acids of PCV3 nuclear localization signals were highly conserved. Overall, this study provides insight into the biological and functional characteristics of the PCV3 ORF2 protein.

The Arginines in the N-Terminus of the Porcine Circovirus 2 Virus-like Particles Are Responsible for Disrupting the Membranes at Neutral and Acidic pH

Non-enveloped viruses that are endocytosed employ numerous mechanisms to disrupt endosomal membranes for escape into the cellular cytoplasm. These include the use of amphipathic helices or sheets, hydrophobic loops, myristoylated peptides, and proteins with phospholipase activity. Some mechanisms result in immediate deterioration of the endosome, while others form pores in the membrane causing osmolysis to disrupt the endosome and allow viral escape. We describe an additional mechanism by a non-enveloped virus to disrupt endosomal membranes. Porcine circovirus 2 (PCV2) possesses a 41-amino acid arginine-rich motif (ARM) at the N-terminus of its capsid protein that appears to be in the interior of the virus-like particle (VLP). Using in vitro membrane disruption assays, we demonstrate that PCV2 VLP, unassembled capsid, and ARM peptide possess the ability to disrupt endosomal-like membranes, whereas VLP lacking the ARM sequence does not possess this capability. Membrane disruption by VLP is insensitive to pH, but unassembled capsid protein and ARM peptide exhibit diminished activity at low pH. Our liposome disruption assays, circular dichroism, and intrinsic tryptophan fluorescence assays allow us to propose a model for PCV2-endosomal membrane interaction wherein the ARM peptide externalizes from the capsid, its C-terminus (amino acids 28-40) anchors into the membrane, and the arginine-rich N-terminus (amino acids 1-27) drives membrane disruption. To our knowledge, this is the first example of a non-enveloped virus using the arginines of an ARM to disrupt membranes. Also, this is the first example of such study for the Circoviridae family of viruses.

Suppression of lymphocyte apoptosis in spleen by CXCL13 after porcine circovirus type 2 infection and regulatory mechanism of CXCL13 expression in pigs

Porcine circovirus-associated disease (PCVAD) is one of the most serious infectious diseases in pigs worldwide. The primary causative agent of PCVAD is porcine circovirus type 2 (PCV2), which can cause lymphoid depletion and immunosuppression in pigs. Our previous study demonstrated that Laiwu (LW) pigs, a Chinese indigenous pig breed, have stronger resistance to PCV2 infection than Yorkshire × Landrace (YL) pigs. In this study, we found that the YL pigs showed more severe lymphocyte apoptosis and higher viral load in the spleen tissue than LW pigs. To illustrate the differential gene expression between healthy and infected spleens, transcriptome profiling of spleen tissues from PCV2-infected and control YL pigs was compared by RNA sequencing. A total of 90 differentially expressed genes (DEGs) was identified, including CD207, RSAD2, OAS1, OAS2, MX2, ADRB3, CXCL13, CCR1, and ADRA2C, which were significantly enriched in gene ontology (GO) terms related to the defense response to virus and cell-cell signaling, and another nine DEGs, KLF11, HGF, PTGES3, MAP3K11, XDH, CYCS, ACTC1, HSPH1, and RYR2, which were enriched in GO terms related to regulation of cell proliferation or apoptosis. Among these DEGs, the CXCL13 gene, which can suppress lymphocyte apoptosis during PCV2 infection, was significantly down-regulated in response to PCV2 infection in YL but not in LW pigs. By analysis of the regulatory elements in the promoter and 3'-untranslated region (3'-UTR) of porcine CXCL13, we found that the single nucleotide polymorphism (SNP) -1014 G (LW) > A (YL) and the Sus scrofa microRNA-296-5p (ssc-miR-296-5p) participated in regulating CXCL13 expression during the response to PCV2 infection.

Efficient production of porcine circovirus virus-like particles using the nonconventional yeast Kluyveromyces marxianus

Porcine circovirus type 2 (PCV2) is a ubiquitous virus with high pathogenicity closely associated with the postweaning multisystemic wasting syndrome (PMWS) and porcine circovirus diseases (PCVDs), which caused significant economic losses in the swine industry worldwide every year. The PCV2 virus-like particles (VLPs) are a powerful subunit vaccine that can elicit high immune response due to its native PCV2 virus morphology. The baculovirus expression system is the widely used platform for producing commercial PCV2 VLP vaccines, but its yield and cost limited the development of low-cost vaccines for veterinary applications. Here, we applied a nonconventional yeast Kluyveromyces marxianus to enhance the production of PCV2 VLPs. After codon optimization, the PCV2 Cap protein was expressed in K. marxianus and assemble spontaneously into VLPs. Using a chemically defined medium, we achieved approximately 1.91 g/L of PCV2 VLP antigen in a 5-L bioreactor after high cell density fermentation for 72 h. That yield greatly exceeded to recently reported PCV2 VLPs obtained by baculovirus-insect cell, Escherichia coli and Pichia pastoris. By the means of two-step chromatography, 652.8 mg of PCV2 VLP antigen was obtained from 1 L of the recombinant K. marxianus cell culture. The PCV2 VLPs induced high level of anti-PCV2 IgG antibody in mice serums and decreased the virus titers in both livers and spleens of the challenged mice. These results illustrated that K. marxianus is a powerful yeast for cost-effective production of PCV2 VLP vaccines.

High-level expression and immunogenicity of porcine circovirus type 2b capsid protein without nuclear localization signal expressed in Hansenula polymorpha

Currently, porcine circovirus type 2b (PCV2b) is the dominant PCV2 genotype causing postweaning multisystemic wasting disease (PMWS) in pigs worldwide. Efforts have been made to develop various recombinant capsid proteins of PCV2b used in vaccines against PCV2b. However, the nuclear localization signal (NLS) of PCV2b capsid protein (CP) was found to inhibit the expression of the whole length capsid protein in E.coli. Here, we expressed a NLS-deleted capsid protein (ΔCP) of PCV2b in Hansenula polymorpha based on the capsid protein of PCV2b strain Y-7 isolated in China. Comparatively, the ΔCP was expressed at a higher level than the CP. The purified ΔCP could self-assemble into virus like particles (VLPs) with similar morphology of the VLPs formed by CP. The purified ΔCP could be recognized by the anti-sera derived from the mice immunized by inactivated PCV2b particles. Furthermore, it induced higher levels of PCV2b specific antibodies than the purified CP in mice. These results showed that the ΔCP, a recombinant PCV2b capsid protein without nuclear localization signal sequence, could be efficiently expressed in Hansenula polymorpha, and used as a candidate antigen for the development of PCV2b vaccines.

In silico analysis of surface structure variation of PCV2 capsid resulting from loop mutations of its capsid protein (Cap)

Outbreaks of porcine circovirus (PCV) type 2 (PCV2)-associated diseases have caused substantial economic losses worldwide in the last 20 years. The PCV capsid protein (Cap) is the sole structural protein and main antigenic determinant of this virus. In this study, not only were phylogenetic trees reconstructed, but variations of surface structure of the PCV capsid were analysed in the course of evolution. Unique surface patterns of the icosahedral fivefold axes of the PCV2 capsid were identified and characterized, all of which were absent in PCV type 1 (PCV1). Icosahedral fivefold axes, decorated with Loops BC, HI and DE, were distinctly different between PCV2 and PCV1. Loops BC, determining the outermost surface around the fivefold axes of PCV capsids, had limited homology between Caps of PCV1 and PCV2. A conserved tyrosine phosphorylation motif in Loop HI that might be recognized by non-receptor tyrosine kinase(s) in vivo was present only in PCV2. Particularly, the concurrent presence of 60 pairs of the conserved tyrosine and a canonical PXXP motif on the PCV2 capsid surface could be a mechanism for PXXP motif binding to and activation of an SH3-domain-containing tyrosine kinase in host cells. Additionally, a conserved cysteine in Loop DE of the PCV2 Cap was substituted by an arginine in PCV1, indicating potentially distinct assembly mechanisms of the capsid in vitro between PCV1 and PCV2. Therefore, these unique patterns on the PCV2 capsid surface, absent in PCV1 isolates, might be related to cell entry, virus function and pathogenesis.

[Construction and identification of a recombinant PRRSV expressing ORF2 of porcine circovirus type 2]

Porcine reproductive and respiratory syndrome virus (PRRSV) and porcine circovirus type 2 (PCV2) are very two important pathogens that have coursed huge economic losses in swine production in worldwide. In this study,a vector pCMV-TJM containing the full-length cDNA clone of PRRSV attenuated strain TJM-F92 was firstly constructed by PCR method. Then a gene sequence containing Afl II/Mlu I e restriction enzyme sites and a transcription regulatory sequence for ORF6 (TRS6) was inserted be- tween ORF7 and 3'UTR, yielding a expression vector pCMV-TJM-TRS. Subsequently, a plasmid pCMV-TJM-Cap was constructed by cloning of PCV2 ORF2 gene into the unique sites Afl II /Mlu I of pCMV- TJM-TRS plasmid DNA. Then three recombinant PRRSV, rTJM, rTJM/TRS and rTJM/Cap, were rescued by transfection of pCMV-TJM, pCMV-TJM-TRS and pCMV-TJM-Cap into Marc-145 cells, respectively,and confirmed by the genome sequence, restriction enzyme digestion, Western Blot and IFA. They all had the molecular markers which was different from the parent virus. The growth characteristics of the rescued viruses were similar to that of parent virus. rTJM/Cap could also express efficiently PCV2 Cap protein in Marc-145 cells. At passage 8, it still had PCV2 ORF2 gene which examined by RT-PCR. It indicated that the full-length cDNA clone of PRRSV attenuated strain TJM-F92 and recombinant PRRSV rTJM/Cap expressing PCV2 Cap protein were successfully constructed. It made an important foundation for studying on the pathogenic mechanisms of PRRSV and PRRSV-PCV2 vaccine in the future.

Porcine circovirus type 2 induces autophagy via the AMPK/ERK/TSC2/mTOR signaling pathway in PK-15 cells

Porcine circovirus type 2 (PCV2) uses autophagy machinery to enhance its replication in PK-15 cells. However, the underlying mechanisms are unknown. By the use of specific inhibitors, RNA interference, and coimmunoprecipitation, we show that PCV2 induces autophagy in PK-15 cells through a pathway involving the kinases AMP-activated protein kinase (AMPK) and extracellular signal-regulated kinase 1/2 (ERK1/2), the tumor suppressor protein TSC2, and the mammalian target of rapamycin (mTOR). AMPK and ERK1/2 positively regulate autophagy through negative control of the mTOR pathway by phosphorylating TSC2 in PCV2-infected PK-15 cells. Thus, PCV2 might induce autophagy via the AMPK/ERK/TSC2/mTOR signaling pathway in the host cells, representing a pivotal mechanism for PCV2 pathogenesis.

Expression and antigenicity characterization for truncated capsid protein of porcine circovirus type 2

Three pairs of specific primers were designed to amplify F2-1, F2-2, and XF2-2 truncated capsid protein genes of porcine circovirus type 2 (PCV-2). Amplified sequences were subcloned to pET-32a(+) vectors and expressed in Rosetta (DE3) Escherichia coli by induction of isopropy-β-D-thiogalactoside (IPTG). All of the fusion proteins had positive reactions to PCV-2 antiserum and His-XF2-2 showed the best reactivity. Proteins were used to immunize BALB/c mice to produce monoclonal antibodies (mAbs), and 7 mAbs were selected. Capsid protein N-terminal parts 55 to 96 amino acid (aa), 97 to 141 aa, and 143 to 211 aa were confirmed as binding regions of the 7 mAbs. Reactivity between His-XF2-2 and the 7 mAbs was detected, FmAb-8 showed the best reactivity. The dominant B-cell epitope was located at 97 to 141 aa. The PEPSCAN indicated that the P122-136 peptide contained the dominant B-cell epitope.

Characterization of monoclonal antibody against replication-associated protein of porcine circovirus

The replication-associated (Rep) protein of porcine circovirus (PCV) was suggested to play an essential role in the replication and translation of viral DNA. In this study, one monoclonal antibody (mAb) specific for Rep protein of porcine circovirus type 1 (PCV1), two mAbs against Rep protein of porcine circovirus type 2 (PCV2), and five mAbs to both Rep protein of PCV1 and PCV2 were generated using, respectively, Rep protein of PCV1 and PCV2 expressed in Escherichia coli as an immunogen. Western blot analysis showed that native Rep protein of PCV2 virions appeared in two forms with different molecular weight in PCV2-infected cells. Laser confocal analyses further exhibited that Rep protein distributed mainly in the cellular nucleoplasm at the early stage of PCV2 infection, and moved to the nuclear periphery and the cytoplasm at the last stage of PCV2 infection. The results from this study confirmed that Rep protein of PCV2 distributed in both nucleus and cytoplasm, and provided an mAb tool to further analyze replications of PCV1 and PCV2 in vitro and in vivo.

[Construction and immunogenicity of recombinant adenoviruses expressing Cap protein of PCV2 and GP5 protein of PRRSV in mice]

Porcine circovirus 2 (PCV2) has been implicated as the etiological agent of postweaning multisystemic wasting syndrome (PMWS). Co-infection of PCV2 and porcine reproductive and respiratory syndrome virus (PRRSV) can result in severe economic losses to the swine industry. In this study, we constructed the recombinant adenovirus rAd-Cap-GP5 expressing Cap of PCV2 and GP5 of PRRSV. And the expression of Cap and GP5 protein in the HEK-293 cells inoculated with rAd-Cap-GP5 were confirmed by immunoperoxidase monolayer assay (IPMA), indirect immunofluorescence assay (IFA) and Western blotting, respectively. The immunogenicity of recombinant adenoviruses rAd-Cap-GP5 was examined in mice by vaccination with the recombinant adenovirus. The results showed that the mice could produce anti-PCV2 and PRRSV antibodies detected by indirect ELISA and virus neutralization assay. It indicated that rAd-Cap-GP5 could provide humoral immunity responses in mice. The recombinant adenovirus rAd-Cap-GP5 might be an attractive candidate vaccine for preventing the disease associated with PCV2 and PRRSV infection.

Mapping of the nuclear localization signals in open reading frame 2 protein from porcine circovirus type 1

Porcine circovirus type 1 (PCV1) contains two major open reading frames encoding the replication-associated proteins and the major structural capsid (Cap) protein. PCV1 Cap has an N-terminus carrying several potential monopartite or bipartite nuclear localization signals (NLS). The contribution of these partially overlapping motifs to nuclear importing was identified by expression of mutated PCV1 Cap versions fused to enhanced green fluorescent protein (EGFP). The C-terminus truncated PCV1 Cap-EGFP was localized in nuclei of PK-15 cells similar to the wild-type PCV1 Cap-EGFP, whereas truncation of the N-terminus rendered the fusion protein distributed into cytoplasm, indicating that the nuclear import of PCV1 Cap was efficiently mediated by its N-terminal region. Substitutions of basic residues in stretches 9RRRR12 or the right part of 25RRPYLAHPAFRNRYRWRRK43 resulted in a diffused distribution of the fusion protein in both nuclei and cytoplasm, indicating that the two NLSs were responsible for restricted nuclear targeting of PCV1 Cap.

The involvement of Fas/FasL interaction in porcine circovirus type 2 and porcine reproductive and respiratory syndrome virus co-inoculation-associated lymphocyte apoptosis in vitro

Lymphoid depletion of various lymphoid organs is one of the major lesions in pigs suffering from postweaning multisystemic wasting syndrome (PMWS). The co-existence of porcine circovirus type 2 (PCV2) and porcine reproductive and respiratory syndrome virus (PRRSV) in PMWS-affected pigs along with the more severe and wider range of lymphocyte depletion of lymphoid organs in PCV2 and PRRSV dually-inoculated pigs imply that PCV2 and PRRSV may interact in the pathogenesis of PMWS. The mechanism for the development of lymphoid depletion in PMWS-affected pigs remains controversial. The objective of the present study was to evaluate and compare the effects of inoculation of both viruses, singularly or in combination, on swine splenic macrophages (SMs) and co-cultured splenic (SLs) and peripheral blood (PBLs) lymphocytes in vitro. A significant reduction in the survival rate and increase in the apoptotic rate of the co-cultured SLs and PBLs and concurrent increase in the expression levels of Fas ligand (FasL) in SMs and Fas in co-cultured SLs and PBLs were demonstrated in PRRSV alone- and PCV2 and PRRSV dually-inoculated groups with the latter more prominent. The increased FasL was proven capable of inducing Fas/FasL-mediated apoptosis in co-cultured FasL-sensitive Jurkat T cells. The de novo expression and production of FasL in PCV2 and PRRSV dually-inoculated SMs and concurrently increased surface expression of Fas in co-cultured lymphocytes may contribute, at least partially, to lymphoid depletion in PMWS-affected pigs with PCV2 and PRRSV dual infection.

Expression of porcine circovirus 2 ORF2 gene requires codon optimized E. coli cells

Expression and purification of whole and nuclear localization signal (NLS) deleted ORF2 capsid protein of porcine circovirus 2 (PCV2) is demonstrated in the present study. Gene coding for both protein forms were cloned into pDest17 vector and expressed in BL21 (DE3)AI cells and in BL21-CodonPlus (DE3)-RIPL E. coli cells. The later cells were used to overcome difficulties with the heterologous expression of viral proteins in prokaryotic systems. Whole 30 kDa recombinant ORF2 protein was successfully expressed in BL21-CodonPlus (DE3)-RIPL cells only, 3 mg of pure protein was consistently obtained per liter of bacterial culture. NLS deleted ORF2 protein was expressed in both cell types. Resulting proteins reacted with PCV2 positive swine serum in immunofluorescent test and immunoblot.

Porcine circovirus type 2 replicase binds the capsid protein and an intermediate filament-like protein

Porcine circovirus type 2 (PCV2) is an important porcine pathogen that establishes persistent subclinical infections but may, on activation, contribute to the development of post-weaning multisystemic wasting syndrome (PMWS). This disease is characterized by weight loss, respiratory or digestive disorders and enlarged lymph nodes with lymphocyte depletion. The molecular mechanisms behind the development of the disease are completely unknown. In order to clarify functions of the different viral proteins and, if possible, to connect these new findings to molecular mechanisms behind the pathogenesis or the viral life cycle, a bacterial two-hybrid screening of a porcine expression library from PK-15A cells was conducted. Using viral proteins corresponding to ORFs 1, 2, 3 and 4 as bait, a number of interactions were identified and two of them were chosen for further characterization. GST pull-down assays confirmed that viral replicase (Rep) interacted with an intermediate filament protein, similar to human syncoilin, and with the transcriptional regulator c-myc. Furthermore, interactions of the viral proteins to each other revealed an interaction between PCV2 Rep and the capsid (Cap) protein and Cap to itself.

Rolling-circle replication of an animal circovirus genome in a theta-replicating bacterial plasmid in Escherichia coli

A bacterial plasmid containing 1.75 copies of double-stranded porcine circovirus (PCV) DNA in tandem (0.8 copy of PCV type 1 [PCV1], 0.95 copy of PCV2) with two origins of DNA replication (Ori) yielded three different DNA species when transformed into Escherichia coli: the input construct, a unit-length chimeric PCV1(Rep)/PCV2(Cap) genome with a composite Ori but lacking the plasmid vector, and a molecule consisting of the remaining 0.75 copy PCV1(Cap)/PCV2(Rep) genome with a different composite Ori together with the bacterial plasmid. Replication of the input construct was presumably via the theta replication mechanism utilizing the ColE(1) Ori, while characteristics of the other two DNA species, including a requirement of two PCV Oris and the virus-encoded replication initiator Rep protein, suggest they were generated via the rolling-circle copy-release mechanism. Interestingly, the PCV-encoded Rep' protein essential for PCV DNA replication in mammalian cells was not required in bacteria. The fact that the Rep' protein function(s) can be compensated by the bacterial replication machinery to support the PCV DNA replication process echoes previous suggestions that circular single-stranded DNA animal circoviruses, plant geminiviruses, and nanoviruses may have evolved from prokaryotic episomal replicons.

Demonstration of nicking/joining activity at the origin of DNA replication associated with the rep and rep' proteins of porcine circovirus type 1

The replication of porcine circovirus type 1 (PCV1) is thought to occur by rolling-circle replication (RCR), whereby the introduction of a single-strand break generates a free 3'-hydroxyl group serving as a primer for subsequent DNA synthesis. The covalently closed, single-stranded genome of PCV1 replicates via a double-stranded replicative intermediate, and the two virus-encoded replication-associated proteins Rep and Rep' have been demonstrated to be necessary for virus replication. However, although postulated to be involved in RCR-based virus replication, the mechanism of action of Rep and Rep' is as yet unknown. In this study, the ability of PCV1 Rep and Rep' to "nick" and "join" strand discontinuities within synthetic oligonucleotides corresponding to the origin of replication of PCV1 was investigated in vitro. Both proteins were demonstrated to be able to cleave the viral strand between nucleotides 7 and 8 within the conserved nonanucleotide motif (5'-TAGTATTAC-3') located at the apex of a putative stem-loop structure. In addition, the Rep and Rep' proteins of PCV1 were demonstrated to be capable of joining viral single-stranded DNA fragments, suggesting that these proteins also play roles in the termination of virus DNA replication. This joining activity was demonstrated to be strictly dependent on preceding substrate cleavage and the close proximity of origin fragments accomplished by base pairing in the stem-loop structure. The dual "nicking/joining" activities associated with PCV1 Rep and Rep' are pivotal events underlying the RCR-based replication of porcine circoviruses in mammalian cells.

Porcine circovirus 2 uses heparan sulfate and chondroitin sulfate B glycosaminoglycans as receptors for its attachment to host cells

Monocyte/macrophage lineage cells are target cells in vivo for porcine circovirus 2 (PCV2) replication. The porcine monocytic cell line 3D4/31 supports PCV2 replication in vitro, and attachment and internalization kinetics of PCV2 have been established in these cells. However, PCV2 receptors remain unknown. Glycosaminoglycans (GAG) are used by several viruses as receptors. The present study examined the role of GAG in attachment and infection of PCV2. Heparin, heparan sulfate (HS), chondroitin sulfate B (CS-B), but not CS-A, and keratan sulfate reduced PCV2 infection when these GAG were incubated with PCV2 prior to and during inoculation of 3D4/31 cells. Enzymatic removal of HS and CS-B prior to PCV2 inoculation of 3D4/31 cells significantly reduced PCV2 infection. Similarly, when PCV2 virus-like particles (VLP) were allowed to bind onto 3D4/31 cells in the presence of heparin and CS-B, attachment was strongly reduced. Titration of field isolates and low- and high-passage laboratory strains of PCV2 in the presence of heparin significantly reduced PCV2 titers, showing that the capacity of PCV2 to bind GAG was not acquired during in vitro cultivation but is an intrinsic feature of wild-type virus. When Chinese hamster ovary (CHO) cells were inoculated with PCV2, relative percentages of PCV2-infected cells were 27% +/- 8% for HS-deficient and 12% +/- 10% for GAG-deficient cells compared to wild-type cells (100%). Furthermore, it was shown using heparin-Sepharose chromatography that both PCV2 and PCV2 VLP directly interacted with heparin. Together, these results show that HS and CS-B are attachment receptors for PCV2.

[The expression of porcine circovirus type 2 ORF2 gene in insect cells and its character]

To produce the recombinant baculovirus transfer plasmid pFast-ORF2, the ORF2 gene of Porcine Circovirus type 2 (PCV2) was subcloned into baculovirus transfer vector (pFastBac(TM1) ) using Bac-to-Bac baculovirus expression system. E. coli DH10Bac (Gibco BRL) containing baculovirus shutter vector (bacmid) and helper vector was transformed with recombinant plasmid pFast-ORF2. Within E. coli DH10Bac, the ORF2 gene was transposed into the bacmid. The colonies of E. coli containing recombinant bacmid (Bac. ORF2) were collected by blue/white selection. The Bac. ORF2 was transfected into sf9 cells to yield AcNPV carrying the PCV2 ORF2 gene, referred to as Ac. ORF2. Expression of the ORF2 gene of PCV2 was confirmed by indirect immunofluorescent assay (IIFA), SDS-PAGE and Western-blotting. The expressed ORF2 gene product had a molecular mass of 28kD and could be recognized by the positive serum of PCV2. The results indicated the ORF2 gene was properly expressed in sf9 cell. It was noteworthy that many self-assembled virus-like particles (VLPs) were found in purified and phosphotungstic acid (PTA) stained PCV2 ORF2 protein by electron microscope. The particles were of similar morphology to the PCV2 virion and some self-assembled virus-like particles had darkly stained centers that made them appear to be empty capsids. Both PCV2 particles and self-assembled particles were approximately 17 nm in diameter.

Binding and entry characteristics of porcine circovirus 2 in cells of the porcine monocytic line 3D4/31

Porcine circovirus 2 (PCV2) is associated with post-weaning multisystemic wasting syndrome and reproductive problems in pigs. Cells of the monocyte/macrophage lineage are important target cells in PCV2-infected pigs, but the method of binding and entry of PCV2 into these cells is unknown. Therefore, binding and entry of PCV2 to the porcine monocytic cell line 3D4/31 were studied by visualization of binding and internalization of PCV2 virus-like particles (VLPs) by confocal microscopy and chemical inhibition of endocytic pathways (clathrin- and caveolae-mediated endocytosis and macropinocytosis), followed by evaluation of the level of PCV2 infection. It was shown that PCV2 VLPs bound to all cells, with maximal binding starting from 30 min post-incubation. Bound PCV2 VLPs were internalized in 47+/-5.0 % of cells. Internalization was continuous, with 70.5+/-9.7 % of bound PCV2 VLPs internalized at 360 min post-incubation. Internalizing PCV2 VLPs co-localized with clathrin. PCV2 infection was decreased significantly by chemical inhibitors that specifically blocked (i) actin-dependent processes, including cytochalasin D (75.5+/-7.0 % reduction) and latrunculin B (71.0+/-3.0 % reduction), and (ii) clathrin-mediated endocytosis, including potassium depletion combined with hypotonic shock (50.2+/-6.3 % reduction), hypertonic medium (56.4+/-5.7 % reduction), cytosol acidification (59.1+/-7.1 % reduction) and amantadine (52.6+/-6.7 % reduction). Inhibiting macropinocytosis with amiloride and caveolae-dependent endocytosis with nystatin did not decrease PCV2 infection significantly. PCV2 infection was reduced by the lysosomotropic weak bases ammonium chloride (47.0+/-7.9 % reduction) and chloroquine diphosphate (49.0+/-5.6 % reduction). Together, these data demonstrate that PCV2 enters 3D4/31 cells predominantly via clathrin-mediated endocytosis and requires an acidic environment for infection.

Development of a reverse transcription-PCR assay to detect porcine circovirus type 2 transcription as a measure of replication

Porcine circovirus type 2 (PCV2) is a non-enveloped, single-stranded, circular DNA virus. In situ hybridization and PCR assays have detected PCV2 DNA in multiple organs and cell types from infected pigs; however, it is not clear if this represents replicating virus or virion DNA. We describe the development of a single-tube RT-PCR assay to differentiate PCV2 replication products and virus DNA. Primers targeted to the open-reading frame 2 (ORF2) of PCV2 were designed to amplify both virus DNA (984 bp) and the spliced Cap mRNA (594 bp). The 984 bp fragment, but not the 594 bp fragment, was amplified from PCV2 stock, confirming that the spliced Cap mRNA was not present in the PCV2 stock. The 594 bp fragment was amplified from DNase-treated RNA extracted from PCV2-infected PK-15 cells, and was detected as early as 14 h post-infection. No products were amplified from either the PCV1 stock or PCV1-infected PK-15 cells, or from cells infected with UV-inactivated PCV2. Therefore, the presence of the 594 bp fragment is specific for PCV2 replication. This assay will be useful in assessing cell populations that support PCV2 replication in vivo or in vitro and advance the understanding of PCV2 replication and pathogenesis.

Palindrome regeneration by template strand-switching mechanism at the origin of DNA replication of porcine circovirus via the rolling-circle melting-pot replication model

Palindromic sequences (inverted repeats) flanking the origin of DNA replication with the potential of forming single-stranded stem-loop cruciform structures have been reported to be essential for replication of the circular genomes of many prokaryotic and eukaryotic systems. In this study, mutant genomes of porcine circovirus with deletions in the origin-flanking palindrome and incapable of forming any cruciform structures invariably yielded progeny viruses containing longer and more stable palindromes. These results suggest that origin-flanking palindromes are essential for termination but not for initiation of DNA replication. Detection of template strand switching in the middle of an inverted repeat strand among the progeny viruses demonstrated that both the minus genome and a corresponding palindromic strand served as templates simultaneously during DNA biosynthesis and supports the recently proposed rolling-circle "melting-pot" replication model. The genome configuration presented by this model, a four-stranded tertiary structure, provides insights into the mechanisms of DNA replication, inverted repeat correction (or conversion), and illegitimate recombination of any circular DNA molecule with an origin-flanking palindrome.

The essential and nonessential transcription units for viral protein synthesis and DNA replication of porcine circovirus type 2

During porcine circovirus (PCV) replication in PK15 cells, nine PCV type 2 (PCV2)-specific RNAs are synthesized. They include the capsid RNA (CR), five Rep-associated RNAs (Rep, Rep', Rep3a, Rep3b, and Rep3c), and three NS-associated RNAs (NS515, NS672, and NS0). In this work, mutational analyses were conducted to investigate the involvement of each PCV2 transcription unit in viral protein synthesis and DNA replication. The results demonstrated that a stop codon introduced at the very 5'-end of CR did not affect Rep-associated antigens or viral DNA synthesis. Altering the consensus dinucleotides at the splice junctions of the minor RNAs (Rep3a, Rep3b, Rep3c, NS515, and NS672) or introducing a stop codon in the abundant NS0 RNA also did not have any effect on viral protein synthesis or DNA replication. However, mutations that resulted in truncated Rep or Rep' proteins caused greater than 99% reduction of viral protein synthesis and complete shut down of viral DNA replication. These results demonstrated that both Rep and Rep' are absolutely essential for PCV2 replication.

Rep and Rep' protein of porcine circovirus type 1 bind to the origin of replication in vitro

Genome replication of Porcine circovirus type 1 (PCV1) relies upon expression of the full-length protein Rep and a spliced isoform (Rep'), and the presence of a 111-bp genomic fragment comprising the origin of replication. Using an electrophoretic mobility shift assay (EMSA), the capability of both Rep proteins to bind to partial fragments of the origin of replication of PCV1 was investigated in vitro. Both proteins formed complexes with double-stranded DNA origin fragments containing a stem-loop structure with a conserved nonamer and four hexamer repeats (5'-CGGCAG; H1 to H4). Use of truncated EMSA substrates identified minimal binding sites (MBS) for Rep and Rep' protein: The Rep binding site was mapped to the right leg of the stem-loop and the two inner hexamer repeats H1/H2, while binding of Rep' required only the presence of two hexamer repeats. Two differentially retarded complexes were observed with Rep protein, which presumably result from alternative binding to the MBS or to H3/4.

Nuclear localization of the ORF2 protein encoded by porcine circovirus type 2

Infectious porcine circovirus type 2 (PCV2) was generated following transfection of a porcine retina cell line (VIDO R1) with cloned circovirus DNA. Expression of open reading frame 2 (ORF2) was detected at 24 h postinfection and onwards increasingly throughout the infection by Western blot analysis using ORF2 specific polyclonal antibody. Moreover, the ORF2 protein was also detected in purified PCV2 virus, indicating that ORF2 is a structural component of PCV2 viral capsid. Nuclear localization of PCV2 ORF2 was demonstrated by immunofluorescence assay in PCV2-infected cells. An analysis of the subcellular localization of a series of truncation mutants of ORF2 fused with the green fluorescent protein indicated that the nuclear localization signal of ORF2 was conferred by the N-terminal 41 amino acids. This domain was further analyzed through site-directed mutagenesis, suggesting that the presence of basic amino acid residues at positions 12 to 18 and 34 to 41 are important for the strict nuclear targeting of PCV2 ORF2.

Two mRNAs are transcribed from banana bunchy top virus DNA-1

We have mapped the mRNA transcripts of banana bunchy top virus (BBTV) DNA-1. Northern hybridization and 3' RACE analysis identified two poly-adenylated RNAs associated with BBTV DNA-1. Previously, one major ORF in the virion sense of DNA-1 had been identified, which encoded a putative replication protein (Rep). An mRNA was identified in BBTV infected bananas that was clearly transcribed from this Rep ORF. Further, a second transcript was identified which mapped to an ORF completely within the Rep ORF. This encoded a putative 5 kDa protein of unknown function. Both these transcripts were also identified in a tobacco plant that had been transformed with Agrobacterium tumefaciens harbouring a binary construct containing the Rep ORF from BBTV DNA-1. This Rep ORF was inserted 3' of a cauliflower mosaic virus 35S promoter and 5' of a vegetable storage protein terminator. The transcripts mapped from these tobacco plants were identical at the 3' end to the transcripts from BBTV infected banana plants. The site of polyadenylation for the Rep ORF was at base 963 immediately 3' of the translational stop codon confirming that the polyadenylation signals for this transcript were all within the ORF. However, the internal ORF had a large untranslated region of 272 bases with its site of polyadenylation at nucleotide 803 and a polyadenylation signal 3' of the translational stop codon. A possible upstream termination signal (A/TTGTAA) was identified and was conserved within BBTV DNA-1 sequences from different international isolates.