Construction and characterization of the infectious clone of porcine parvovirus carrying genetic marker

Nucleocytoplasmic Shuttling of Porcine Parvovirus NS1 Protein Mediated by the CRM1 Nuclear Export Pathway and the Importin α/β Nuclear Import Pathway

Porcine parvovirus (PPV) NS1, the major nonstructural protein of this virus, plays an important role in PPV replication. We show, for the first time, that NS1 dynamically shuttles between the nucleus and cytoplasm, although its subcellular localization is predominantly nuclear. NS1 contains two nuclear export signals (NESs) at amino acids 283 to 291 (designated NES2) and amino acids 602 to 608 (designated NES1). NES1 and NES2 are both functional and transferable NESs, and their nuclear export activity is blocked by leptomycin B (LMB), suggesting that the export of NS1 from the nucleus is dependent upon the chromosome region maintenance 1 (CRM1) pathway. Deletion and site-directed mutational analyses showed that NS1 contains a bipartite nuclear localization signal (NLS) at amino acids 256 to 274. Coimmunoprecipitation assays showed that NS1 interacts with importins α5 and α7 through its NLS. The overexpression of CRM1 and importins α5 and α7 significantly promoted PPV replication, whereas the inhibition of CRM1- and importin α/β-mediated transport by specific inhibitors (LMB, importazole, and ivermectin) clearly blocked PPV replication. The mutant viruses with deletions of the NESs or NLS motif of NS1 by using reverse genetics could not be rescued, suggesting that the NESs and NLS are essential for PPV replication. Collectively, these findings suggest that NS1 shuttles between the nucleus and cytoplasm, mediated by its functional NESs and NLS, via the CRM1-dependent nuclear export pathway and the importin α/β-mediated nuclear import pathway, and PPV proliferation was inhibited by blocking NS1 nuclear import or export. IMPORTANCE PPV replicates in the nucleus, and the nuclear envelope is a barrier to its entry into and egress from the nucleus. PPV NS1 is a nucleus-targeting protein that is important for viral DNA replication. Because the NS1 molecule is large (>50 kDa), it cannot pass through the nuclear pore complex by diffusion alone and requires specific transport receptors to permit its nucleocytoplasmic shuttling. In this study, the two functional NESs in the NS1 protein were identified, and their dependence on the CRM1 pathway for nuclear export was demonstrated. The nuclear import of NS1 utilizes importins α5 and α7 in the importin α/β nuclear import pathway.

T598 and T601 phosphorylation sites of canine parvovirus NS1 are crucial for viral replication and pathogenicity

Canine parvovirus-2 (CPV-2) is an important pathogen causing severe diseases in dogs and other wild carnivores. Phosphorylation of NS1 may be related to CPV-2 pathogenicity, but the exact mechanism is unclear. Here, we conducted parvovirus disease surveillance in Shaanxi Province of China and 51 fecal swabs were detected to be infected with CPV-2. The 7 CPV-2 strains were identified, all of which belonged to CPV-2c. The complete genome sequence of one of the strains (CPV-2c XY) was cloned into pKQLL plasmid to construct a full-length infectious clone plasmid pX-CPV-2c, which carried a genetic marker. The plasmid pX-CPV-2c was transfected into F81 cells for virus rescue. And the rescued virus, which was designed as X-CPV-2c, showed the similar biological property to parental CPV-2c XY in vitro and in vivo. We further constructed four NS1 phosphorylation site mutant strains (X-CPV-2c^T584A, X-CPV-2c^S592A, X-CPV-2c^T598A/T601A and X-CPV-2c^T617A) on the basis of X-CPV-2c. After the analysis and comparison of biological characteristics, the low pathogenic strain X-CPV-2c^T598A/T601A was further screened out, which emphasized the importance of phosphorylation sites 598 T/601 T for the pathogenicity of CPV-2. Overall, our data indicated that T598 and T601, the C-terminal phosphorylation site of CPV-2 NS1, play important roles in viral pathogenicity and laid the foundation for the development of new attenuated live vaccine vectors.

Coatomer protein COPƐ, a novel NS1-interacting protein, promotes the replication of Porcine Parvovirus via attenuation of the production of type I interferon

Porcine Parvovirus (PPV) is a pathogen causing porcine reproductive disorders. Non-structural protein NS1 appears diverse functions acting as a predominant regulator in promoting PPV replication. In this study, we identified a PPV NS1 binding protein coatomer subunit epsilon (COPƐ), and found that COPƐ is a critical regulator during PPV replication. In NS1 transfected or PPV infected cells, COPƐ was interacted with NS1 and translocated into nucleus together with NS1. Knockout of COPƐ could inhibit PPV production by increasing the expression levels of IFN-β, while overexpression of COPƐ enhanced PPV production by reducing the expression levels of IFN-β. Furthermore, the domain mapping assay showed that the N-terminal amino acids domain of NS1 (25-EAFSYVF-31) were required for the interaction of COPƐ with NS1. Sequence alignment result displays that parvovirus NS1 (EAFSYVF) amino acids domain is highly conservative among PPV, CPV, FPV and MEV, and down-regulation of COPƐ could also significantly reduce the replication of these viruses. Notably, we found that the interaction of COPƐ with NS1 play an important role in promoting the production of type I interferon during PPV or CPV infection, which affect the replication of these viruses. Taken together, the results presented here show a novel function of NS1 interaction with COPƐ that regulates the parvovirus replication through modulating the type I interferons signaling pathway, provided a potential target for the control of parvovirus-associated diseases.

A novel porcine parvovirus DNA-launched infectious clone carrying stable double labels as an effective genetic platform

Porcine parvovirus (PPV) is one of the major pathogens causing reproductive failure of swine. However, its specific pathogenesis has not been fully elucidated. Infectious clone is a powerful tool for further studying the pathogenic mechanism of PPV. In the present study, a PPV infectious clone was constructed, and the clone carries His-tag and Flag-tag double-genetic marker at the end of the ns1 gene 3' terminal and vp1 gene 5' terminal, respectively. The PPV DNA fragment F1 (1-182) in 5' end and the other PPV DNA fragment F2 (4788-5074) in 3' end were synthesized and assembled to the lower copy plasmid to construct pKQLL(F1 + F2), while the PPV DNA genome as a template to amplify carrying tags sequence PPV middle DNA fragment F3 and F4 by introducing Flag and His tags sequence in primers. Subsequently, the fused fragment F3/F4 were cloned into the Stu I/Sna B I sites of pKQLL(F1 + F2) plasmid to assemble the complete full-length PPV DNA recombinant plasmids, named as pD-PPV. The pD-PPV was transfected into PK-15 cells to gain rescued PPV virus, designed as D-PPV. Moreover, D-PPV showed similar replicate capability and pathogenicity comparing to the wild-type parental PPV through in vitro and in vivo studies, and the double labels can effectively indicate the expression and localization of viral proteins. Finally, the rescued D-PPV was found to be a convenient tool for antiviral drug screening. These data indicated that the newly established reverse genetic system for PPV would be a useful tool for further studying the pathogenesis mechanisms of PPV, developing labeled vaccine and screening antiviral drug.