A baculovirus vector can express intron-containing genes

Exploiting the Intron-splicing Mechanism of Insect Cells to Produce Viral Vectors Harboring Toxic Genes for Suicide Gene Therapy

Two mammalian introns, the human growth hormone intron and the Simian virus 40 large T antigen intron, were inserted into the coding sequences of diphtheria toxin fragment A (DT-A) and barnase (Bar), respectively, to disrupt their open-reading frames (ORFs). Expression of these two toxic proteins were totally abolished, which enabled the production of normal levels of recombinant baculoviral and adeno-associated viral (AAV) vectors in insect cells. When these viral vectors were introduced into mammalian cells, the introns were spliced out and the toxic proteins were expressed, which resulted in apoptosis in mammalian cells. This is the first report to show that viral vectors harboring toxin genes can be produced at normal levels by exploiting the intron-splicing mechanism of insect cells. Furthermore, viral vectors carrying the DT-A gene under control of tumor-specific promoters were able to exert tumor-specific cell killing. This novel method to produce viral vectors harboring toxic genes under control of tumor-specific promoter offers a powerful tool for further research, as well as for the development of toxin-based suicide gene therapy drugs.Molecular Therapy - Nucleic Acids (2012) 1, e57; doi:10.1038/mtna.2012.48; published online 27 November 2012.

Factors influencing recombinant protein yields in an insect cell-bacuiovirus expression system: multiplicity of infection and intracellular protein degradation

The insect cell (Sf9)-baculovirus (AcNPV) expression system was employed for the synthesis of beta-galactosidase, a model heterologous protein. In the recombinant virus studied, the lacZ gene is fused to a portion of the polyhedrin structural gene and is under the control of the polyhedrin promoter. The effect of the multiplicity of infection (MOI) on product titer was determined by infecting cells with MOI values ranging from 0 to 100 and monitoring the production of beta-galactosidase with time. The relationship between final product titer and MOI was dependent on the growth phase of the cells prior to infection. The final product titer from cells infected in the early exponential phase was relatively independent of MOI. For cells infected in late-exponential phase there was a logarithmic relationship between the final beta-galactosidase titer and the MOI used, with the highest MOI studied resulting in greatest protein synthesis. The synthesis and degradation rates of beta-galactosidase were investigated by a pulse-chase technique using L-[(35)S]-methionine. At 24 h postinfection, the degradation rate is of the same order of magnitude as the synthesis rate. However, the synthesis rate of beta-galactosidase increases dramatically at 96 h postinfection. During this later period, the degradation rate is negligible. Although degradation of recombinant protein occurs in this system, degradation activity declines as infection proceeds and is insignificant late in intention when recombinant protein expression is intense.

Human endogenous retrovirus HERV-K113 is capable of producing intact viral particles

Of all human endogenous retroviruses known today, HERV-K is the only one that has been shown to produce viral particles. While the first of the approximately 30 HERV-K sequences integrated into the human genome more than 40 million years ago, evidence is accumulating that HERV-K was active more recently, provirus HERV-K113 being the youngest sequence found. However, it is unclear which HERV-K sequences code for the viral particles that are produced by human germ-cell tumours or melanomas. Here, we show that the provirus HERV-K113, cloned into a baculovirus expression vector, is capable of producing intact particles of retroviral morphology, exhibiting the typical structure of those particles that were characterized in cell lines derived from human germ-cell tumours. Thus, the HERV-K113 sequence is a candidate for particle production in vivo and for an active human endogenous retrovirus of today.

Intron splicing-mediated expression of AAV Rep and Cap genes and production of AAV vectors in insect cells

An artificial intron containing the insect cell polyhedrin (polh) promoter was designed, constructed, and inserted into the adeno-associated virus (AAV) Rep and Cap coding sequences to express the Rep and Cap proteins, respectively. The artificial intron was spliced out and full-length Rep78 or VP1 proteins were expressed from the insect promoters located upstream of their respective AUG start codons. The polh promoter located inside the artificial intron was functional, expressed the Rep52 or VP2/VP3 proteins located downstream of the artificial intron, and overlapped with the Rep78 or VP1 proteins. This is the first report that an artificial intron containing an insect cell promoter can be inserted into a coding sequence to express genes with overlapping open-reading frames (ORFs). A method was also established for AAV vector production in insect cells with these intron-containing Rep and Cap coding sequences, and the vectors produced thereby were infectious. These intron-containing AAV Rep and Cap coding sequences were very stable in recombinant baculoviruses and showed no apparent loss of protein expression even after five consecutive amplifications of the plaque-purified recombinant baculoviruses. This newly established AAV production method should prove to be a useful tool for large-scale AAV vector production.

An unusual member of the human growth hormone/placental lactogen (GH/PL) family, the testicular alternative splicing variant hPL-A2: recombinant expression revealed a membrane-associated growth factor molecule

The human growth hormone/placental lactogen (GH/PL) gene cluster consists of five highly-related genes (GH-N, GH-V, PL-L, PL-A, PL-B). This evolutionarily young gene cluster codes for an array of mRNAs and proteins, such as the major 22 k forms (hGH-N/V, identical PL-A and B), 20 k and 17.5 k hGH-N and the recently described 25 k hGH-Delta4, a presumably chimeric molecule. In addition, two longer alternatively spliced, (intron D retaining) mRNAs isoforms, termed PL-A2 and GH-V2, have been described in placenta and testis. To elucidate the role of hPL-A2 in male reproduction and pregnancy, testicular PL-A2 cDNA was cloned in a complementary overlapping 2-way RT-PCR approach to analyze translation, localization and structure/function of this unusual member of the GH/PL growth factor family. Analysis of insect mRNA revealed that intron D-retaining PL-A2 cDNA was expressed without splicing in the baculovirus expression system. Thus, PL-A2 mRNA does not represent a nuclear intermediate splicing product simply co-isolated with the mature RNA, but is a stable mRNA isoform generated by placental/testis-specific splicing factors. Recombinant protein was present in whole cell extracts, and no secreted protein was detected in the supernatant. Immunologically, the N-terminus of the 230 amino acid protein is similar to 22 k hPL-A/B, as determined by hPL-specific monoclonal antibodies. In contrast, the C-terminus shares a hydrophobic region presumably responsible for membrane insertion. By the use of confocal microscopy recombinant hPL-A2 was localized in the cell membrane. Thus, hPL-A2 might exert its function by modulating GH/PL actions or act as an independent growth-regulatory molecule itself and its functions in male reproduction and embryonic development remain to be investigated.

Human herpesvirus 8 glycoprotein K8.1: expression, post-translational modification and localization analyzed by monoclonal antibody

BACKGROUND

The genome of human herpesvirus 8 (HHV-8) contains at least 84 open reading frames, including the highly immunogenic K8.1. Other studies have determined that K8.1 gene generates at least two spliced transcripts in the HHV-8 infected BCBL-1 cells, termed as glycoprotein (gp)K8.1A and gpK8.1B.

OBJECTIVE

To analyze the expression, post-translational modification and localization of HHV-8 gpK8.1 by monoclonal antibody (mAb).

STUDY DESIGN

Mabs to HHV-8 produced by conventional hybridization and several clones identified. A mAb was used by various immunological assays to analyze HHV-8 K8.1 proteins in BCBL-1- and Sf9 insect cells.

RESULTS

MAb clone 19B4 identified a 0.75-kb insert from the lambdaZAP cDNA expression library of 12-O-tetradecanoylphorbol-13-acetate (TPA)-induced BCBL-1 cells. Sequence analysis revealed that the cDNA insert corresponds to the published spliced ORF K8.1 mRNA of HHV-8. By immunofluorescence assay, the mAb stained the cell membrane, cytoplasm and perinuclear region of TPA induced BCBL-1 cells and showed no cross-reactivity with other herpesviruses. By immunoblotting assay, mAb 19B4 reacted with two species polypeptides giving a diffuse band with rMW from 42 to 64 kDa (gpK8.1A) and two closely migrating polypeptides of rMW 35/37 kDa (gpK8.1B). Both species were labeled by [14C]glucosamine, indicating that they are glycosylated and only gpK8.1A was detected in the virions. Expression of the full length K8.1 derived from cDNA in baculovirus system confirmed that these two glycoproteins are encoded by K8.1 gene. Enzymatic deglycosylation with endoF/peptide-N-glycosidase F, led to the reduction of rMW of both polypeptides whereas deglycosylation with O-glycosidase led only the reduction of rMW of K8.1A.

CONCLUSION

The mAb 19B4 reacts specifically with BCBL-1 and Sf9 cells infected with recombinant baculovirus containing HHV-8 K8.1 gene. In several assays the mAb reacts with gpK8.1A and gpK8.1B. Only the mature spliced gpK8.1A is incorporated into virion. Enzymatic deglacosylation determined that gpK8.1A is N- and O-glycosylated, whereas gpK8.1B may lack O-glycosylation.

Expression of Aleutian mink disease parvovirus proteins in a baculovirus vector system

We have previously published a detailed transcription map of Aleutian mink disease parvovirus (ADV) and proposed a model for the translation of the two virion structural proteins (VP1 and VP2) and three nonstructural proteins (NS-1, NS-2, and NS-3) (S. Alexandersen, M. E. Bloom, and S. Perryman, J. Virol. 62:3684-3994, 1988). To verify and further characterize this model, we cloned the predicted open reading frames for NS-1, NS-2, NS-3, VP1-VP2, and VP2 alone into a recombinant baculovirus and expressed them in Sf9 insect cells. Expression of VP1-VP2 or VP2 alone in cDNA and in the genomic form was achieved. The expressed proteins had molecular weights similar to those of the corresponding proteins of wild-type ADV-G, although the ratio of VP1 to VP2 was altered. The recombinant baculovirus-expressed ADV VP1 and VP2 showed nuclear localization in Sf9 cells and were able to form particles indistinguishable, by electron microscopy, from wild-type virus. The large nonstructural protein, NS-1, showed predominantly nuclear localization in Sf9 cells when analyzed by immunofluorescence and had a molecular weight similar to that of wild-type ADV NS-1. Moreover, expression of NS-1 in Sf9 cells caused a change in morphology of the cells and resulted in 10-times-lower titers of recombinant baculovirus during infection, suggesting a cytostatic or cytotoxic action of this protein. The smaller NS-2 gene product seems to be located in the cytoplasm. When analyzed by Western immunoblotting, NS-2 comigrated with an approximately 16-kDa band seen in lysates of ADV-infected feline kidney cells. The putative NS-3 gene product exhibited a diffuse distribution in Sf9 cells and had a molecular weight of approximately 10,000. All of the expressed ADV-encoded proteins were recognized by sera from ADV-infected mink. Thus, expression of ADV cDNAs allowed assignment of the different mRNAs to the viral proteins observed during ADV infection in cell culture and supported our previously proposed ADV transcriptional and translational scheme. Moreover, the production of structural proteins from a full-length NS-2 mRNA may add to the repertoire of parvovirus gene expression.

Post-translational processing of barley beta-glucan endohydrolases in the baculovirus-insect cell expression system

Two cDNAs encoding barley (1-->3,1-->4)-beta-glucanase (EC 3.2.1.73) isoenzymes EI and EII have been expressed in Spodoptera frugiperda (Sf9) cell cultures using the baculovirus AcNPV vector. Modifications to both the 5' and 3' ends of the cDNAs were required before satisfactory levels of expression were obtained. The modified cDNAs directed high levels of (1-->3,1-->4)-beta-glucanase expression in the Sf9 insect cell cultures, with yields of approximately 10 mg/liter of isoenzyme EI (expEI) and 15 mg/liter of isoenzyme EII (expEII). Amino acid sequence analyses showed that the expressed enzymes were processed correctly at their amino termini. However, affinity chromatography of the isoenzyme expEII on concanavalin-A (conA)-Sepharose indicated that, although the enzyme is glycosylated, the structures of the carbohydrate chains differ from those of the native enzyme. When a cDNA encoding the homologous barley (1-->3)-beta-glucanase (EC 3.2.1.39) isoenzyme GII was expressed in insect cells, aberrant amino-terminal processing of the nascent polypeptide was sometimes observed. The forms with incompletely removed signal peptides retained their substrate specificity, but exhibited slightly reduced catalytic efficiency, altered chromatographic behavior, and reduced stability at elevated temperatures. The results show that high levels of expression of recombinant plant proteins can be obtained in insect cells, but they emphasize the need to characterize thoroughly the products that are expressed in the heterologous insect cell system before comparisons are made with the native enzyme or with engineered enzyme mutants.

Expression, purification, and functional characterization of adenovirus 5 and 12 E1A proteins produced in insect cells

The 12 S and 13 S E1A cDNAs from both the Adenovirus (Ad) nononcogenic type 5 and the oncogenic type 12 were overexpressed in an insect cell/baculovirus system. Upon infection of Spodoptera frugiperda cells, the production of E1A proteins reached a level of about 15 micrograms/10(6) cells. The E1A proteins are highly soluble and apparently are processed authentically. They are readily recognized by various antibodies and display phosphorylation patterns similar to those of E1A proteins synthesized in mammalian cells. Single-step immunoaffinity chromatography was used to purify the Ad5 E1A proteins to near homogeneity under nondenaturing conditions. The Ad5 and Ad12 E1A proteins are able to form complexes with the retinoblastoma susceptibility gene product (Rb) and other cellular proteins. Interestingly, the presence of a cellular extract seems to be a prerequisite for association between highly purified E1A and Rb polypeptides.

Expression of bovine immunodeficiency-like virus envelope glycoproteins by a recombinant baculovirus in insect cells

The bovine immunodeficiency-like virus (BIV) env open reading frame (ORF) contains both sequences encoding env and sequences for exon 1 of the putative rev gene. Recombinant baculoviruses incorporating BIV env ORF sequences were constructed to characterize the expression, processing, and immunogenicity of products of the BIV env ORF in insect cells and to develop reagents to study native BIV Env glycoproteins. A recombinant baculovirus containing the entire env ORF synthesized a nonglycosylated, 20-kDa, BIV-specific protein, apparently unrelated to native BIV Env proteins. In contrast, a recombinant baculovirus containing a truncated env ORF in which the coding sequences for rev exon 1 were deleted synthesized three size classes of glycosylated proteins in insect cells related to the BIV Env precursor (gp145), surface (gp100), and transmembrane (gp45) glycoproteins observed in BIV-infected mammalian cells. Oligomers of recombinant BIV Env proteins also formed in these baculovirus-infected insect cells. Immunofluorescence staining of intact insect cells infected by the baculovirus expressing BIV Env with BIV-specific serum demonstrated that the recombinant Env glycoproteins were expressed on the cell surface. Antisera raised to recombinant Env glycoproteins immunoprecipitated native gp145, gp100, and gp45 in BIV-infected bovine cells similar to sera from animals naturally or experimentally infected with BIV.

Identification of spliced baculovirus RNAs expressed late in infection

Previous to this study, the Autographa californica multicapsid nuclear polyhedrosis virus (AcMNPV) was known to express only one spliced RNA (spliced IE1 or IE0). We have conducted an analysis of RNA expressed during infection of Spodoptera frugiperda cells with AcMNPV and have identified a set of five additional spliced RNAs expressed late in infection. A reverse transcription-polymerase chain reaction analysis was used to confirm the identification of the LS (late, spliced) RNAs. S1 nuclease and primer extension analyses were used to map the transcription initiation sites of LS RNAs. LS1 and LS2 initiated at positions -138 and -117, respectively (relative to the IE0 +1 transcription start site). Both LS1 and LS2 contain an additional cistron potentially encoding a small, highly basic polypeptide. LS3 (-79), LS4 (-22), and LS5 (+51/52) RNAs encode only the predicted downstream IE0 ORF. Although several baculovirus late gene consensus transcription initiation sites (ATAAG) were identified within this region, only LS5 initiated at one of these conserved motifs. An S1 nuclease analysis was done to determine whether unspliced precursors of LS RNAs could be detected. Early in infection, a greater proportion of IE0 RNA was detected in the spliced form; however, during the late phase of infection a significantly greater amount of unspliced precursor forms of LS RNAs was observed.

Adeno-associated virus rep proteins produced in insect and mammalian expression systems: wild-type and dominant-negative mutant proteins bind to the viral replication origin

The adeno-associated virus (AAV) rep gene proteins, Rep78 and Rep68, are required for replication of AAV DNA and bind to the AAV replication origin. An AAV genome having a Lys340 to His (K340H) mutation in the consensus purine nucleotide binding site of the rep gene protein exhibited a dominant-negative phenotype for DNA replication. We synthesized both wild-type and the K340H mutant Rep78 protein in a baculovirus expression system. Nuclear extracts of Sf9 cells containing these proteins were examined in gel mobility-shift assays with radiolabeled AAV terminal repeat DNA. Each protein bound specifically to the hairpin configuration of the AAV terminal repeat DNA to yield three shifted components. However the mobility of these components observed with the mutant Rep protein was slightly decreased compared to that with the wild-type Rep78. The addition of an antibody made against an oligopeptide from the carboxyl terminal region of the Rep78 protein generated novel shifted bands in the presence of either extract. Similar results were observed when the wild-type and mutant Rep proteins were expressed from an inducible expression system employing the human immunodeficiency virus type 1 transcription promoter in human 293 cells. These results suggest that the dominant-negative phenotype of the K340H mutation may be mediated by binding of the mutant protein to the AAV replication origin.

High-level expression of a mitochondrial enzyme, ornithine transcarbamylase from rat liver, in a baculovirus expression system

The mitochondrial enzyme, ornithine transcarbamylase (OTC) from rat liver was expressed in Spodoptera frugiperda (Sf) insect cells using a baculovirus vector. When insect cells were infected with recombinant Autographica californica nuclear polyhedrosis virus (AcNPV) containing a cDNA encoding the precursor form of OTC (pOTC) inserted into the polyhedrin gene, they expressed catalytically active enzyme at levels of approximately 2.5 micrograms/10(6) cells. About 25% of the active enzyme was a novel, partially processed product of pOTC containing four extra amino acids at the amino terminus of OTC. The most abundant protein found in mitochondria from infected insect cells was the normal processing intermediate iOTC, which contains 8 extra amino acids at the amino terminus of OTC. Whereas this species, present at 20 micrograms/10(6) cells, was not active and did not bind the transition-state analog inhibitor of OTC, delta-PALO, the novel processing product did bind and was affinity-purified, along with mature OTC, on a PALO-affinity column. The OTC expressed in insect cells was located in the same compartment of the mitochondrion as in rat liver. The incomplete processing occurred in vitro in both noninfected and infected insect cells. The high level of expression of iOTC using the baculoviral expression system provides a means of overproducing an obligatory intermediate in the mitochondrial import process.

Bioreactor development for production of viral pesticides or heterologous proteins in insect cell cultures

The insect cell-baculovirus expression system has significant potential for producing proteins requiring some degree of posttranslational modification. T. ni cells appear to be as good a host as S. frugiperda cells for heterologous protein production as demonstrated by production of beta-galactosidase. Attachment-dependent cells of T. ni can be effectively cultured in a packed-bed reactor using glass beads. When cell in such a reactor were infected, they produced 35% of the total protein as beta-galactosidase. No cell detachment was observed even 70 h postinfection. A model of viral entry has been proposed and tested.

Tissue-specific expression of silkmoth chorion genes in vivo using Bombyx mori nuclear polyhedrosis virus as a transducing vector

A pair of silkmoth chorion chromosomal genes, HcA.12-HcB.12, was inserted into a baculovirus transfer vector, pBmp2, derived from the nuclear polyhedrosis virus of Bombyx mori. This vector, which permits the insertion of foreign genetic material in the vicinity of a mutationally inactivated polyhedrin gene, was used to acquire the corresponding recombinant virus. Injection of mutant silkmoth pupae that lack all Hc chorion genes with the recombinant virus resulted in the infection of all internal organs including follicular tissue. Analysis of RNA from infected tissues has demonstrated that the two chorion genes present in the viral genome are correctly transcribed under the control of their own promoter in follicular cells, the tissue in which chorion genes are normally expressed. The chorion primary transcripts are also correctly processed in the infected follicular cells and yield mature mRNAs indistinguishable from authentic chorion mRNAs present in wild-type follicles. These results demonstrate that recombinant nuclear polyhedrosis viruses can be used as transducing vectors for introducing genetic material of host origin into the cells of the organism and that the transduced genes are transiently expressed in a tissue-specific manner under the control of their resident regulatory sequences. Thus we show the in vivo expression of cloned genes under cellular promoter control in an insect other than Drosophila melanogaster. The approach should be applicable to all insect systems that are subject to nuclear polyhedrosis virus infection.

Recombinant baculoviruses as vectors for identifying proteins encoded by intron-containing members of complex multigene families

Using a transfer vector derived from Bombyx mori nuclear polyhedrosis virus (BmNPV), we have constructed recombinant baculoviruses that contain complete silk moth chorion chromosomal genes encoding high-cysteine proteins under the control of the polyhedrin promoter. Silk moth tissue culture cells infected with these recombinant viruses were found to contain abundant RNA sequences of sizes similar to those of the authentic chorion mRNAs. Chorion transcripts present in infected cells were initiated almost exclusively at the cap site of the polyhedrin start site. Primer extension and RNase protection experiments revealed that a considerable proportion of the resultant transcripts were spliced at the same sites as those utilized in follicular cells for the production of functional chorion mRNA. Electrophoretic analysis and immunoprecipitation of the proteins of host cells infected with the recombinant viruses revealed the presence of the corresponding chorion proteins. We conclude that baculovirus vectors can be used for expressing efficiently not only cDNAs or simple genes devoid of intervening sequences but also intron-containing chromosomal genes. Thus, recombinant baculoviruses offer a powerful alternative to hybrid-selected translation, particularly when the identification of proteins encoded by members of complex multigene families is required.

Synthesis of functional bovine leukemia virus (BLV) p34tax protein by recombinant baculoviruses

The bovine leukemia virus (BLV) p34tax (also called tat, p34, XLOR gene product) is a 34-kDa polypeptide encoded in the 3'-terminal region of the virus. This protein is responsible for positive transcriptional trans-activation of promoter elements located within the BLV long-terminal repeat. We introduced the protein-coding region of BLV p34tax into the genome of the baculovirus Autographa californica nuclear polyhedrosis virus. After infection of the insect Spodoptera frugiperda (SF9) cell line, this recombinant strain of baculovirus produced approximately 100 to 150 mg of p34tax per 2 X 10(9) cells. This protein, when introduced into mammalian fibroblasts by using a cell-to-cell fusion technique, functionally trans-activated the BLV long-terminal repeat. Analysis of 32P-labeled proteins of SF9 cells expressing BLV tax by two-dimensional gel electrophoresis indicated that the BLV p34tax was phosphorylated.

Expression of simian virus 40 T antigen in insect cells using a baculovirus expression vector

Simian virus 40 (SV40) large T and small t antigens were synthesized in insect cells using the baculovirus Autographa californica as an expression vector. A recombinant virus containing a genomic copy of the SV40 early region expressed high levels of small t antigen but only low levels of large T antigen. However, very high levels of T antigen synthesis were observed when viruses were constructed with a cDNA copy of the large T antigen mRNA. Insect cells were capable of modifying T antigen by phosphorylation, palmitylation, glycosylation, and oligomerization. Functional assays demonstrated that the origin-specific DNA binding, ATPase, and helicase activities of insect cell-derived T antigen were comparable to T antigen synthesized in mammalian cells. Use of the baculovirus vector system to produce T antigen should facilitate future investigations requiring large quantities of T antigen.

Regulation of host RNA levels during baculovirus infection

During infection of the permissive host insect cell line Spodoptera frugiperda IPLB-SF-21 by the baculovirus Autographa californica nuclear polyhedrosis virus (AcMNPV), the levels of host actin, histone, and heat shock 70 (hsp70) RNAs are reduced substantially. Reduction of the host RNA levels occurs primarily during a narrow window of the replication process, from approximately 12 to 18 hr postinfection (p.i.), corresponding to the phase in which the extracellular form of the virus buds into the media. A late viral protein appears to be required for this reduction since cycloheximide, an inhibitor of cytosolic protein synthesis, and aphidicolin, an inhibitor of host and viral DNA polymerases, inhibit the reduction of actin and histone RNA levels. A cDNA corresponding to the carboxyl half of the S. frugiperda mitochondrial cytochrome c oxidase subunit III (COIII) gene was isolated, sequenced, and characterized. Two differentially regulated mitochondrial transcripts of this gene are observed. The level of the larger of these transcripts, which is dependent on active cytosolic protein synthesis, is reduced during virus infection in a fashion similar to that of the nuclear host genes. The smaller COIII transcript is stable until at least 24 hr p.i. but the level of this RNA eventually declines by 48 hr p.i.

Synthesis of functional human immunodeficiency virus tat protein in baculovirus as determined by a cell-cell fusion assay

The human immunodeficiency virus tat protein is a strong trans-activator of the expression of mRNAs originating from the viral long terminal repeat. We have expressed the first 72 amino acids (coding exon 1) of this protein in eucaryotic Spodoptera frugiperda SF9 cells by using a baculovirus vector, Autographa californica nuclear polyhedrosis virus. We show that the baculovirus vector stably produced the 72-amino-acid form of the tat protein but was unable to stably synthesize a larger 101-amino-acid full-length version of the same polypeptide. The 72-amino-acid tat protein, when introduced into mammalian fibroblasts by using a cell-cell fusion technique, functionally trans-activated the expression of the human immunodeficiency virus long terminal repeat.

Expression and complex formation of simian virus 40 large T antigen and mouse p53 in insect cells

Recombinant baculoviruses were constructed which express simian virus 40 large T antigen (SVT-Ag) or murine p53 to high levels in infected insect cells. Characterization of the expressed proteins revealed that they display many properties of the corresponding mammalian-derived proteins. Both proteins are of wild-type size, localize to the nucleus, are recognized by several SVT-Ag- or p53-specific monoclonal antibodies, and are phosphorylated in this system. Complexes are formed between baculovirus-derived SVT-Ag and p53 after coinfection of insect cells with both recombinant viruses. After infection of insect cells with either virus individually, each protein can self-associate to form a variety of oligomeric species. Pulse-chase experiments indicated that both SVT-Ag and p53 are highly stable in insect cells, even in the absence of complex formation.

Multiple early transcripts and splicing of the Autographa californica nuclear polyhedrosis virus IE-1 gene

The immediate-early IE-1 gene of Autographa californica nuclear polyhedrosis virus was cloned, and its nucleotide sequence was determined. Sequence analysis indicated that this gene would encode a protein of 582 amino acids with a predicted molecular weight of 66,822. Analysis of IE-1 gene expression during baculovirus infection identified two transcripts. One, 1.9 kilobases (kb), was expressed at constant steady-state levels throughout infection, whereas the other, 2.1 kb, was expressed only early in infection. Analysis of IE-1 cDNA clones demonstrated that the 2.1-kb transcript contained the entire 1.9-kb transcript (exon 1) plus an additional 5' end (exon 0). Genomic Southern analysis placed the exon 0 sequences on the EcoRI B fragment, 4 kilobase pairs upstream of exon 1. Sequencing of the upstream region identified an open reading frame whose 5' end was identical to the exon 0 sequences in the cDNAs. Examination of the genomic DNA sequences around the exon-exon junction revealed sequences similar to published consensus splice acceptor and donor sequences. This is the first example of splicing of any viral transcript during baculovirus infection.

Purification and functional properties of simian virus 40 large and small T antigens overproduced in insect cells

The insect baculovirus Autographa californica nuclear polyhedrosis virus was used as an expression vector for the simian virus 40 (SV40) small t (t) and large T (T) antigens. Spodoptera frugiperda (SF9) cells infected with recombinant viruses encoding these proteins produced approximately 1 to 2 micrograms of t and up to 30 micrograms of T per 3 X 10(6) cells. The former was highly soluble after Nonidet P-40 extraction of the infected cells, unlike its Escherichia coli-produced counterpart. Both SF9-produced proteins were of authentic size and could be readily immunoprecipitated by specific antibodies. Single-step immunoaffinity chromatography was used to purify the two proteins to near homogeneity, with yields averaging 70% in each case. Experiments to test the biological activity of the baculovirus SV40 proteins showed that SF9 t was capable of associating with two of the cellular proteins reported to bind to t in SV40-infected mammalian cells. Moreover, SF9 T had ATPase activity comparable to that of T produced in monkey cells, exhibited helicase activity and SV40 origin-specific DNA binding, and was active in the SV40 DNA replication assay in vitro. Thus, the SV40 T antigens produced in insect cells can be used in future studies of their biochemical roles in vitro and in vivo.