Structural and Intracellular Proteins of the Nonoccluded Baculovirus HZ-1

A non-coding RNA of insect HzNV-1 virus establishes latent viral infection through microRNA

Heliothis zea nudivirus-1 (HzNV-1) is an insect virus previously known as Hz-1 baculovirus. One of its major early genes, hhi1, is responsible for the establishment of productive viral infection; another gene, pag1, which expresses a non-coding RNA, is the only viral transcript detectable during viral latency. Here we showed that this non-coding RNA was further processed into at least two distinct miRNAs, which targeted and degraded hhi1 transcript. This is a result strikingly similar to a recent report that herpes simplex virus produces tightly-regulated latent specific miRNAs to silence its own key early transcripts. Nevertheless, proof for the establishment of viral latency by miRNA is still lacking. We further showed that HzNV-1 latency could be directly induced by pag1-derived miRNAs in cells infected with a pag1-deleted, latency-deficient virus. This result suggests the existence of a novel mechanism, where miRNAs can be functional for the establishment of viral latency.

Heliothis zea nudivirus 1 gene hhi1 induces apoptosis which is blocked by the Hz-iap2 gene and a noncoding gene, pag1

Heliothis zea nudivirus 1 (HzNV-1 or Hz-1 virus), previously regarded as a nonoccluded baculovirus, recently has been placed in the Nudivirus genus. This virus generates HzNV-1 HindIII-I 1 (hhi1) and many other transcripts during productive viral infection; during latent viral infection, however, persistency-associated gene 1 (pag1) is the only gene expressed. In this report, we used transient expression assays to show that hhi1 can trigger strong apoptosis in transfected cells, which can be blocked, at least partially, by the inhibitor of apoptosis genes Autographa californica iap2 (Ac-iap2) and H. zea iap2 (Hz-iap2). In addition to these two genes, unexpectedly, pag1, which encodes a noncoding RNA with no detectable protein product, was found to efficiently suppress hhi1-induced apoptosis. The assay of pro-Sf-caspase-1 processing by hhi1 transfection did not detect the small P12 subunit at any of the time intervals tested, suggesting that hhi1 of HzNV-1 induces apoptosis through alternative caspase pathways.

The early gene hhi1 reactivates Heliothis zea nudivirus 1 in latently infected cells

Heliothis zea nudivirus 1 (HzNV-1), previously known as Hz-1 virus, is an insect virus able to establish both productive and latent infections in several lepidopteran insect cells. Here, we have cloned and characterized one of the HzNV-1 early genes, hhi1, which maps to the HindIII-I fragment of the viral genome. During the productive viral infection, a 6.2-kb hhi1 transcript was detectable as early as 0.5 h postinfection (hpi). The level of transcript reached a maximum at 2 hpi and gradually decreased after 4 hpi. The transcript was not detectable during the latent phase of viral infection. Upon cycloheximide treatment, much higher levels of hhi1 transcript were detected throughout the productive viral infection cycle, suggesting that newly synthesized proteins are not needed for the expression of hhi1. Nevertheless, viral coinfection can further stimulate the expression of transfected hhi1 promoter in a plasmid. Transient hhi1 expression in latently infected cells resulted in a significant increase in virus titer and viral DNA propagation, suggesting that hhi1 plays a critical role in viral reactivation. Additional experiments showed that six early genes, which possibly function in transcription or DNA replication, were activated in the latent cells upon hhi1 transfection. Among these six genes, orf90 and orf121 expression could be induced by hhi1 alone without the need for other viral genes. Our discovery should be useful for future mechanistic study of the switches of latent/productive HzNV-1 viral infections.

Nudiviruses and other large, double-stranded circular DNA viruses of invertebrates: new insights on an old topic

Nudiviruses (NVs) are a highly diverse group of large, circular dsDNA viruses pathogenic for invertebrates. They have rod-shaped and enveloped nucleocapsids, replicate in the nucleus of infected host cells, and possess interesting biological and molecular properties. The unassigned viral genus Nudivirus has been proposed for classification of nudiviruses. Currently, the nudiviruses comprise five different viruses: the palm rhinoceros beetle virus (Oryctes rhinoceros NV, OrNV), the Hz-1 virus (Heliothis zea NV-1, HzNV-1), the cricket virus (Gryllus bimaculatus NV, GbNV), the corn earworm moth Hz-2 virus (HzNV-2), and the occluded shrimp Monodon Baculovirus reassigned as Penaeus monodon NV (PmNV). Thus far, the genomes of OrNV, GbNV, HzNV-1 and HzNV-2 have been completely sequenced. They vary between 97 and 230kbp in size and encode between 98 and 160 open reading frames (ORFs). All sequenced nudiviruses have 33 ORFs in common. Strikingly, 20 of them are homologous to baculovirus core genes involved in RNA transcription, DNA replication, virion structural components and other functions. Another nine conserved ORFs are likely associated with DNA replication, repair and recombination, and nucleotide metabolism; one is homologous to baculovirus iap-3 gene; two are nudivirus-specific ORFs of unknown function. Interestingly, one nudivirus ORF is similar to polh/gran gene, encoding occlusion body protein matrix and being conserved in Alpha- Beta- and Gammabaculoviruses. Members of nudiviruses are closely related and form a monophyletic group consisting of two sister clades of OrNV/GbNV and HzNVs/PmNV. It is proposed that nudiviruses and baculoviruses derived from a common ancestor and are evolutionarily related to other large DNA viruses such as the insect-specific salivary gland hypertrophy virus (SGHV) and the marine white spot syndrome virus (WSSV).

A study of the glycoproteins of Autographa californica nuclear polyhedrosis virus (AcNPV)

Pulse labeling with tritiated mannose was used to follow the time course of Autographa californica nuclear polyhedrosis virus (AcNPV) glycoprotein synthesis in Spodoptera frugiperda IPLB-21 cells. Nine viral-induced intracellular glycoproteins were first detected from as early as 2 hr postinoculation (67K, early phase) to as late as 14 hr (36K and 19K glycoproteins, intermediate phase). Glycosylation of these proteins was observed to continue to the end of the experiment (28 hr postinoculation). Seven of these intracellular glycoproteins could also be detected in infected Trichoplusia ni TN-368 cells 24 hr postinoculation. When the glycosylation inhibitor tunicamycin was present (from 0 hr postinoculation) there was no detectable glycosylation of any of these viral-induced glycoproteins. Metabolic labeling of the nonoccluded virus budded from IPLB-21 and TN-368 with tritiated mannose or N-acetylglucosamine identified 11 structural glycoproteins, 8 of which were identical in both virus preparations. All of these structural glycoproteins were sensitive to the inhibitory action of tunicamycin. A single 42K structural glycoprotein was detected (with acetylglucosamine only) in the occluded form of AcNPV. Glycosylation of this structural protein appeared to be insensitive to tunicamycin. Lactoperoxidase-catalyzed radioiodination was used to determine which of the virus structural glycoproteins are exposed on the virion surface.

Location, nucleotide sequence, and regulation of the p51 late gene of the hz-1 insect virus: identification of a putative late regulatory element

An Hz-1 insect virus (Hz-1V) late gene encoding, a predicted polypeptide of 51 kDa was isolated from a cDNA library and mapped to the HindIII-J region (40-44.6 map units) of the viral genome. The p51 gene was characterized by DNA sequence, Northern blot, and primer extension analyses. The 1,152 bp open reading frame (ORF) is transcribed as a 1.8 kb RNA between 8 and 18 h post-infection (hpi) with maximum expression at 12 hpi. Homology was not detected between the nucleotide sequence upstream of the p51 ORF and the baculovirus conserved late promoter element NTAAG. Primer extension analysis detected one major late transcription initiation site at -205 nucleotides relative to the start of the p51 ORF and seven minor late initiation sites at positions upstream of this primary site. Comparison of the upstream regulatory regions of the p51 gene and the Hz- 1V p34 late gene revealed a region of significant homology comprised of the 9 bp sequence TTATAGTAT. The primary p51 transcription initiation site and all p34 transcription initiation sites were mapped to different nucleotides within this nonanucleotide sequence. This 9 bp motif was not observed in the ORFs of these genes, and no significant homology was detected between this motif and the 5' regulatory regions of any other characterized genes. The results of our study suggest that this conserved sequence may serve an important role in the regulation of Hz-1V late genes.

Replication of the gonad-specific virus Hz-2V in Ld652Y cells mimics replication in vivo

A newly discovered, nonoccluded insect virus, known as gonad-specific virus or Hz-2V, was found to replicate differently in two insect cell lines derived from ovarian tissues (Tn-368 cells from Trichoplusia ni and Ld652Y from Lymantria dispar). Differences between these two cell lines were observed in virus plaque forming ability, rate of viral DNA replication, time course of infectious virus production, and the mechanism of virus release from infected cells. Replication of Hz-2V in Ld652Y cells was more productive and more closely resembled in vivo virus replication.

Superinfection-induced apoptosis and its correlation with the reduction of viral progeny in cells persistently infected with Hz-1 baculovirus

Differential induction of necrosis or apoptosis was found upon challenge of cells of the insect Spodoptera frugiperda productively or persistently infected with Hz-1 baculovirus, respectively. Unlike parental SF9 cells, which were essentially all killed by virally induced necrosis, persistently infected cells underwent a process of massive cell death by apoptosis; cells which were not killed by apoptosis then reestablished a cell monolayer. Upon viral challenge, the yield of viral progeny was reduced greatly in persistently virus-infected cells but not in parental cells. Immunolabelling of individual cells revealed that upon viral challenge, production of viral progeny was detectable only in necrotic cells and not in apoptotic cells. These results indicated that induction of apoptosis greatly reduces the yield of viral progeny in cells persistently infected with Hz-1 baculovirus. This is the first report of apoptosis induction in persistently infected cells upon viral superinfection.

Differential expression of Hz-1 baculovirus genes during productive and persistent viral infections

Hz-1 viral RNA transcription was studied during productive and persistent infections. The RNAs were localized to 10- to 30-kb regions within the viral genome, and the timing of their expression was determined. During productive infections, we detected 101 virus-specific transcripts that could be grouped into three categories by time of appearance. At 2 h postinoculation (p.i.), a total of 34 virus-specific transcripts were detected. An additional 51 and 16 virus-specific transcripts appeared between 4 and 6 h p.i. and at 8 h p.i., respectively. After 8 h, no new transcripts were found. Under conditions of persistent infection, we detected only one viral persistency-associated transcript (PAT1). The region of the viral DNA which encodes PAT1 was cloned. During productive infections, three transcripts were derived from this region. Each had the same polarity as PAT1. One of them was of the same size as PAT1 and had similar, if not identical, 3' and 5' ends. This report provides detailed and very useful information concerning sequentially expressed transcripts of the Hz-1 baculovirus.