Characterization of ORF2 and its encoded protein of the Helicoverpa armigera nucleopolyhedrovirus

Identification of a novel regulatory sequence of actin nucleation promoting factor encoded by Autographa californica multiple nucleopolyhedrovirus

Actin polymerization induced by nucleation promoting factors (NPFs) is one of the most fundamental biological processes in eukaryotic cells. NPFs contain a conserved output domain (VCA domain) near the C terminus, which interacts with and activates the cellular actin-related protein 2/3 complex (Arp2/3) to induce actin polymerization and a diverse regulatory domain near the N terminus. Autographa californica multiple nucleopolyhedrovirus (AcMNPV) nucleocapsid protein P78/83 is a virus-encoded NPF that contains a C-terminal VCA domain and induces actin polymerization in virus-infected cells. However, there is no similarity between the N terminus of P78/83 and that of other identified NPFs, suggesting that P78/83 may possess a unique regulatory mechanism. In this study, we identified a multifunctional regulatory sequence (MRS) located near the N terminus of P78/83 and determined that one of its functions is to serve as a degron to mediate P78/83 degradation in a proteasome-dependent manner. In AcMNPV-infected cells, the MRS also binds to another nucleocapsid protein, BV/ODV-C42, which stabilizes P78/83 and modulates the P78/83-Arp2/3 interaction to orchestrate actin polymerization. In addition, the MRS is also essential for the incorporation of P78/83 into the nucleocapsid, ensuring virion mobility powered by P78/83-induced actin polymerization. The triple functions of the MRS enable P78/83 to serve as an essential viral protein in the AcMNPV replication cycle, and the possible roles of the MRS in orchestrating the virus-induced actin polymerization and viral genome decapsidation are discussed.

Comparative proteomics reveal fundamental structural and functional differences between the two progeny phenotypes of a baculovirus

The replication of lepidopteran baculoviruses is characterized by the production of two progeny phenotypes: the occlusion-derived virus (ODV), which establishes infection in midgut cells, and the budded virus (BV), which disseminates infection to different tissues within a susceptible host. To understand the structural, and hence functional, differences between BV and ODV, we employed multiple proteomic methods to reveal the protein compositions and posttranslational modifications of the two phenotypes of Helicoverpa armigera nucleopolyhedrovirus. In addition, Western blotting and quantitative mass spectrometry were used to identify the localization of proteins in the envelope or nucleocapsid fractions. Comparative protein portfolios of BV and ODV showing the distribution of 54 proteins, encompassing the 21 proteins shared by BV and ODV, the 12 BV-specific proteins, and the 21 ODV-specific proteins, were obtained. Among the 11 ODV-specific envelope proteins, 8 either are essential for or contribute to oral infection. Twenty-three phosphorylated and 6 N-glycosylated viral proteins were also identified. While the proteins that are shared by the two phenotypes appear to be important for nucleocapsid assembly and trafficking, the structural and functional differences between the two phenotypes are evidently characterized by the envelope proteins and posttranslational modifications. This comparative proteomics study provides new insight into how BV and ODV are formed and why they function differently.

Helicoverpa armaigera nucleopolyhedrovirus ORF50 is an early gene not essential for virus propagation in vitro and in vivo

Homologs of Helicoverpa armigera nucleopolyhedrovirus ORF50 (HA50) are found in most alphabaculoviruses, but their functions remain unknown. Here, we characterized whether Ha50 is indispensable for virus progration. Ha50 transcript was first detected at 3 h post-infection from HearNPV-infected HzAM1 cells. 3'RACE analysis showed that Ha50 transcript was polyadenlylated. 5'RACE analysis revealed two transcription initiation sites, one of which was mapped to the canonical baculovirus early transcription initiator motif CAGT. HA50 protein could be detected from infected cells harvested at 12 h post-infection. Transient expression assays showed that GFP-fused HA50 localized in the cytoplasm and nucleus of HzAM1 cells with or without superinfection. To further examine the role of Ha50 in the virus life cycle, a Ha50 knockout bacmid and a repair bacmid carrying Ha50 under the control of its native promoter elements were constructed using bacmid technology. One-step growth curve analysis showed that the kinetics of infectious budded virus production of Ha50 knockout virus was similar to that of the parental virus or the repair virus. Analysis of the expression of viral early protein IE-1, late protein VP39 and very late protein suggested that viral protein expression was not affected by Ha50 inactivation. Electron microscopy revealed that HaBacΔ50-PH-G occluded viruses (ODVs) and occlusion bodies were indistinguishable from those of the wild-type virus. Similarly, bioassays showed no significant difference in the LC(50) values between Ha50 deletion virus and wild-type virus. Our results together demonstrate that Ha50 is an early gene dispensable for virus propagation in vitro and in vivo.

Putative phosphorylation sites on WCA domain of HA2 is essential for Helicoverpa armigera single nucleopolyhedrovirus replication

Protein phosphorylation is one of the most common post-translational modification processes that play an essential role in regulating protein functionality. The Helicoverpa armigera single nucleopolyhedrovirus (HearNPV) orf2-encoded nucleocapsid protein HA2 participates in orchestration of virus-induced actin polymerization through its WCA domain, in which phosphorylation status are supposed to be critical in respect to actin polymerization. In the present study, two putative phosphorylation sites ((232)Thr and (250)Ser) and a highly conserved Serine ((245)Ser) on the WCA domain of HA2 were mutated, and their phenotypes were characterized by reintroducing the mutated HA2 into the HearNPV genome. Viral infectivity assays demonstrated that only the recombinant HearNPV bearing HA2 mutation at (245)Ser can produce infectious virions, both (232)Thr and (250)Ser mutations were lethal to the virus. However, actin polymerization assay demonstrated that all the three viruses bearing HA2 mutations were still capable of initiating actin polymerization in the host nucleus, which indicated the putative phosphorylation sites on HA2 may contribute to HearNPV replication through another unidentified pathway.

ODV-associated proteins of the Pieris rapae granulovirus

Alphabaculovirus (lepidopteran-specific nucleopolyhedroviruses, NPV) and Betabaculovirus (granuloviruses, GV) are two main genera of the family Baculoviridae. The virion proteomes of Alphabaculovirus have been well studied; however, the Betabaculovirus virion compositions remain unclear. Pieris rapae granulovirus (PrGV) can kill larvae of P. rapae, a worldwide and important pest of mustard family crops. In this study, the occlusion-derived virus (ODV)-associated proteins of PrGV were identified using three mass spectrometry (MS) approaches. The MS analyses demonstrated that 47 proteins were present in PrGV-ODV. Of the 47 PrGV-ODV proteins, 33 have homologues identified previously in other baculovirus ODV/BVs, whereas 14 (P10, Pr21, Pr29, Pr35, Pr42, Pr54, P45/48, Pr83, Pr84, Pr89, Pr92, Pr111, Pr114 and FGF3) were newly identified ODV proteins. Seven of the 14 newly identified ODV proteins are specific to Betabaculovirus, including Pr35, Pr42, Pr54, Pr83, Pr84, Pr111 and Pr114. Furthermore, the data derived from these MS approaches were validated by immunoblotting analysis using antisera prepared from 11 randomly selected recombinant PrGV-ODV proteins (including 5 Betabaculovirus-unique proteins). Comparison analyses revealed the similar and different compositions between Betabaculovirus and Alphabaculovirus virions, which deepen our understanding of the baculovirus virion structure and provide helpful information on Betabaculovirus--host interaction studies.

The putative pocket protein binding site of Autographa californica nucleopolyhedrovirus BV/ODV-C42 is required for virus-induced nuclear actin polymerization

Nuclear filamentous actin (F-actin) is essential for nucleocapsid morphogenesis of lepidopteran nucleopolyhedroviruses. Previously, we had demonstrated that Autographa californica multiple nucleopolyhedrovirus (AcMNPV) BV/ODV-C42 (C42) is involved in nuclear actin polymerization by recruiting P78/83, an AcMNPV orf9-encoded N-WASP homology protein that is capable of activating an actin-related-protein 2/3 (Arp2/3) complex to initiate actin polymerization, to the nucleus. To further investigate the role of C42 in virus-induced actin polymerization, the recombinant bacmid vAc(p78/83nls-gfp), with a c42 knockout, p78/83 tagged with a nuclear localization signal coding sequence, and egfp as a reporter gene under the control of the Pp10 promoter, was constructed and transfected to Sf9 cells. In the nuclei of vAc(p78/83nls-gfp)-transfected cells, polymerized F-actin filaments were absent, whereas other actin polymerization elements (i.e., P78/83, G-actin, and Arp2/3 complex) were present. This in vivo evidence indicated that C42 actively participates in the nuclear actin polymerization process as a key element, besides its role in recruiting P78/83 to the nucleus. In order to collect in vitro evidence for the participation of C42 in actin polymerization, an anti-C42 antibody was used to neutralize the viral nucleocapsid, which is capable of initiating actin polymerization in vitro. Both the kinetics of pyrene-actin polymerization and F-actin-specific staining by phalloidin indicated that anti-C42 can significantly attenuate the efficiency of F-actin formation compared to that with control antibodies. Furthermore, we have identified the putative pocket protein binding sequence (PPBS) on C42 that is essential for C42 to exert its function in nuclear actin polymerization.

Characterization of a Bombyx mori nucleopolyhedrovirus with Bmvp80 disruption

A BmNPV Bacmid with the Bmvp80 gene disrupted was constructed using the ET-recombination system in Escherichia coli to investigate the role of Bmvp80 during the baculovirus life cycle. Disruption of Bmvp80 resulted in single cell infection phenotype, whereas a rescue BmBacmid restored budded virus titers to wild type levels; however, the homologous gene Ac104 (Acvp80) from AcMNPV could not complement the BmBacmid lacking a functional Bmvp80 gene. Electron microscopy of cells transfected with BmNPV lacking functional Bmvp80 revealed that the number of nucleocapsids was markedly lower. These results suggest that Bmvp80 is essential for normal budded virus production and nucleocapsid maturation, and is functionally divergent between baculovirus species.

Identification of a hydrophobic domain of HA2 essential to morphogenesis of Helicoverpa armigera nucleopolyhedrovirus

The HA2 protein of the Helicoverpa armigera single-nucleocapsid nucleopolyhedrovirus (HearNPV) is a WASP homology protein capable of nucleating branched actin filaments in the presence of the Arp2/3 complex in vitro. To determine the role of ha2 in the HearNPV life cycle, ha2 knockout and ha2 repair bacmids were constructed. Transfection and infection analysis demonstrated that the ha2 null bacmid was unable to produce infectious budded virus (BV), while the repair bacmid rescued the defect. In vitro analysis demonstrated that the WCA domain of HA2 accelerates Arp2/3-mediated actin assembly and is indispensable to the function of HA2. However, analysis of the repaired recombinant with a series of truncated ha2 mutants demonstrated that the WCA domain was essential but not enough to yield infectious virions, and a hydrophobic domain (H domain) consisting of amino acids (aa) 167 to 193 played a pivotal role in the production of BV. Subcellular localization analysis with enhanced green fluorescent protein fusions showed that the H domain functioned as a nuclear localization signal. In addition, deletion of the C terminus of the ha2 product, a phosphatidylinositol 4-kinase homolog, dramatically decreased the viral titer, while deletion of 128 aa from the N terminus did not affect HA2 function.

The N-terminal hydrophobic sequence of Autographa californica nucleopolyhedrovirus PIF-3 is essential for oral infection

The Autographa californica nucleopolyhedrovirus (AcMNPV) open reading frame 115 has been identified as a per os infection factor (pif-3) and is essential for oral infection. Here, we have characterized the pif-3 of AcMNPV in more detail. The pif-3 transcripts were detected from 12 to 96 h post-infection (hpi) in Sf9 cells infected with AcMNPV. Polyclonal antiserum first recognized a 25-kDa protein at 36 hpi. Western blot analysis indicated that PIF-3 is a component of occlusion-derived virus but not of budded virus. The subcellular localization demonstrated that the 21-amino-acid (aa) N-terminal hydrophobic domain of PIF-3, which is conserved in PIF-1, PIF2 and PIF-3, acts as a nuclear location signal and is essential for trafficking the protein to the nucleus. Deletion of either pif-3 or the 21-aa N-terminal hydrophobic domain of pif-3 from AcMNPV abolished per os infectivity but had no effect on the infectivity of the budded virus phenotype.

HA2 from the Helicoverpa armigera nucleopolyhedrovirus: a WASP-related protein that activates Arp2/3-induced actin filament formation

Filamentous actin is required for the productive replication of lepidopteran nucleopolyhedroviruses. We have demonstrated that nucleocapsids of the Helicoverpa armigera nucleopolyhedrovirus (HearNPV) are capable of nucleating actin polymerization in vitro in a dose-dependent manner. Actin polymerization is the main mechanism used in cell locomotion and is also utilized by the Listeria bacteria and by vaccinia virus for intracellular and intercellular movements. The WASP family of proteins has been shown to stimulate the assembly of branched actin filaments by the Arp2/3 complex. The process is conserved in eukaryotic cells. HearNPV ORF 2 (HA2), a WASP homologue, could nucleate branched actin filaments in the presence of Arp2/3 complex in vitro. We also demonstrate that HA2 co-localizes with Arp2/3 complex in the nucleus of infected cells, suggesting that HA2 and Arp2/3 complex are involved in nuclear actin polymerization. In summary, HA2 activates Arp2/3-induced actin filament network formation in vitro and in vivo.

Helicoverpa armigera nucleopolyhedrovirus ORF80 encodes a late, nonstructural protein

The Helicoverpa armigera nucleopolyhedrovirus (HearNPV) ORF80 (ha80) has 765 bp encoding a protein with approximately 254 amino acids and a predicted molecular weight of 30.8 kDa. Homologues of ha80 are found in most baculovirus sequences, including those from lepidopteran NPVs, lepidopteran granuloviruses (GVs), hymenopteran baculoviruses, and one dipteran baculovirus, yet their functions remain unclear. In this study we characterized ha80, and showed that it was transcribed late in infected host cells (HzAM1). The product of ha80 was a 31 kDa protein that was not a structural protein of budded virus (BV) or occlusion-derived virus (ODV) particles. Ha80 was first detected in the cytoplasm of infected HzAM1 cells at 12 h p.i., and was observed in the nucleus at later stages of infection, suggesting that it may be involved in transporting viral proteins into the host cell nucleus or play its roles in the nucleus.

Dynamic Nuclear Actin Assembly by Arp2/3 Complex and a Baculovirus WASP-Like Protein

Diverse bacterial and viral pathogens induce actin polymerization in the cytoplasm of host cells to facilitate infection. Here, we describe a pathogenic mechanism for promoting dynamic actin assembly in the nucleus to enable viral replication. The baculovirus Autographa californica multiple nucleopolyhedrovirus induced nuclear actin polymerization by translocating the host actin-nucleating Arp2/3 complex into the nucleus, where it was activated by p78/83, a viral Wiskott-Aldrich syndrome protein (WASP)-like protein. Nuclear actin assembly by p78/83 and Arp2/3 complex was essential for viral progeny production. Recompartmentalizing dynamic host actin may represent a conserved mode of pathogenesis and reflect viral manipulation of normal functions of nuclear actin.