Tryptic peptide analysis and NH2-terminal amino acid sequences of polyhedrins of two baculoviruses from Orgyia pseudotsugata

Atomic structure of a nudivirus occlusion body protein determined from a 70-year-old crystal sample

Infectious protein crystals are an essential part of the viral lifecycle for double-stranded DNA Baculoviridae and double-stranded RNA cypoviruses. These viral protein crystals, termed occlusion bodies or polyhedra, are dense protein assemblies that form a crystalline array, encasing newly formed virions. Here, using X-ray crystallography we determine the structure of a polyhedrin from Nudiviridae. This double-stranded DNA virus family is a sister-group to the baculoviruses, whose members were thought to lack occlusion bodies. The 70-year-old sample contains a well-ordered lattice formed by a predominantly α-helical building block that assembles into a dense, highly interconnected protein crystal. The lattice is maintained by extensive hydrophobic and electrostatic interactions, disulfide bonds, and domain switching. The resulting lattice is resistant to most environmental stresses. Comparison of this structure to baculovirus or cypovirus polyhedra shows a distinct protein structure, crystal space group, and unit cell dimensions, however, all polyhedra utilise common principles of occlusion body assembly.

Bacterial, viral, and fungal insecticides

Microorganisms that are pathogenic to insects provide a wealth of biological material that can be exploited by humans to control insect pests. Innovative applications of a few such entomopathogens are found throughout the world, but widespread commercial production of microbial insecticides awaits further studies of the biology, ecology, and pathogenicity of the agents. Genetic engineering techniques may be used to increase the virulence of these microorganisms, as well as to make them more tolerant of physical and chemical conditions and perhaps to broaden their host ranges. The use of microbial insecticides could decrease our dependence on chemical pesticides.

Characterisation and partial sequence analysis of two novel cypoviruses isolated from the winter moth Operophtera brumata (Lepidoptera: Geometridae)

The complete nucleotide sequences of genomic segments S5 to S10 from Operophtera brumata cypovirus 18 (OpbuCPV18), and the complete nucleotide sequences of genomic segments S2, S5, S9 and S10 from Operophtera brumata cypovirus 19 (OpbuCPV19) have been determined. Each genome segment contained a single open reading frame (ORF). Conserved motifs 5' (AGUAAA....GUUAGCU) 3' were found at the ends of each segment of OpbuCPV18, whilst conserved motifs 5' (AACAAA....UUUGC) 3' were found at each segment terminus of OpbuCPV19. The putative proteins were compared with those of other members of the Reoviridae family. Phylogenetic analysis using the polyhedrin gene (S10) indicated that OpbuCPV18 was most closely related to Dendrolimus punctatus cypovirus 1, whilst OpbuCPV19 was most closely related to Trichoplusia ni cypovirus 15. In addition, analysis of S2, which encoded a putative RNA-dependant RNA polymerase gene, confirmed OpbuCPV19 belonged to the genus Cypovirus. Following the expression of the ORF from OpbuCPV19 S10, using a baculovirus expression vector, occlusion bodies were observed in insect cell culture. This demonstrated that segment 10 coded for the polyhedrin gene, capable of forming a polyhedral crystalline matrix.

Milestones leading to the genetic engineering of baculoviruses as expression vector systems and viral pesticides

The baculovirus expression vector system (BEVS) is widely established as a highly useful and effective eukaryotic expression system. Thousands of soluble and membrane proteins that, in general, are correctly folded, modified, sorted and assembled to produce highly authentic recombinant proteins have been cloned and expressed. This historical chronology and perspective will focus on the original, peer-reviewed discoveries that were pioneering and seminal to the development of the BEVS and that provided the basis for subsequent and more recent developments and applications.

Molecular analysis of an occlusion body protein from Culex nigripalpus nucleopolyhedrovirus (CuniNPV)

CUN085 is an occlusion body (OB) protein from the nucleopolyhedrovirus of Culex nigripalpus (CuniNPV). SDS-PAGE analysis indicated that the CuniNPV OB protein is about 3 times the size (approximately 90 kDa) of characterized nucleopolyhedrovirus (NPVs) and granulovirus OB proteins. Rapid amplification of cDNA ends (RACE), RNase protection assay, real-time PCR, and protein sequencing were used to characterize CUN085 from CuniNPV. RACE data revealed that the transcriptional start and termination sites for the CUN085 gene yielded a polypeptide comprised of 822 amino acids indicating that translation initiates within a larger 882 amino acid open reading frame that was originally predicted from the CuniNPV genome sequence. Transcription of CUN085 started at a consensus baculovirus late transcription start site TAAG at nucleotide position 75433 of the CuniNPV genome sequence. RNase protection assays and quantitative real-time PCR show that the CUN085 transcript is first detected in mosquito larvae at approximately 6 h after infection with CuniNPV and its prevalence increased progressively over the subsequent 18 h.

Sequence analysis and expression of the polyhedrin gene of Choristoneura fumiferana cytoplasmic polyhedrosis virus (CfCPV)

The segmented double-stranded RNA genome of Choristoneura fumiferana cytoplasmic polyhedrosis virus (CfCPV) was extracted, polyadenylated, reverse-transcribed into cDNA and cloned. The cDNA clones that hybridized to the smallest genomic segment (segment 10) were identified, and its nucleotide sequence was determined. Genome segment 10 of CfCPV was found to be 1171 nucleotides in length with a single open reading frame in one strand capable of coding a predicted protein of 258 residues (Mr of 29,795), consistent with an apparent Mr of 30.5 kDa determined by SDS-PAGE of purified polyhedrin. Comparison of the nucleotide and amino acid sequences of the polyhedrin gene of CfCPV with those of other CPVs and with several nuclear polyhedrosis viruses revealed no particular homology. Analysis of the hydrophilic profiles and predicted secondary structures of Bombyx mori (BmCPV), Euxoa scandens (EsCPV) and CfCPV indicated the presence of seven similar regions located at the amino terminus of the polyhedrin polypeptide of the three viruses. The expression of the cloned CfCPV polyhedrin gene in Escherichia coli demonstrated that this polyhedrin has the property of self-assembly, since the production of crystal-like occlusion with a well-defined crystalline lattice structure was observed.

Molecular cloning and characterization of cytoplasmic polyhedrosis virus polyhedrin and a viable deletion mutant gene

The double-stranded RNA genome of Bombyx mori cytoplasmic polyhedrosis virus (CPV) was converted to double-stranded DNA and cloned into plasmid pBR322. The complete nucleotide sequence of cloned genome segment 10, which encodes virus polyhedrin polypeptide, was determined. The CPV polyhedrin gene consists of 942 based pairs and possesses a long open reading frame that codes for a polypeptide of 248 amino acids (molecular weight, 28,500), consistent with an apparent molecular weight of 28,000 previously determined for purified polyhedrin. No sequence homology was found between CPV polyhedrin and polyhedrins from several nuclear polyhedrosis viruses. In addition to the polyhedrin gene, we completed the sequence analysis of a small deletion mutant gene derived from the polyhedrin gene. This mutant gene consists of two subset domains of the polyhedrin gene, i.e., the 5'-terminal 121 base pairs and the 3'-terminal 200 base pairs. An in vitro transcription demonstrated that the small mutant gene is transcribed by virion-associated RNA polymerases. These data confirm the importance of CPV terminal sequences in virus genome replication.

Delta endotoxin of Bacillus thuringiensis subsp. israelensis

From Bacillus thuringiensis subsp. israelensis, a proteinase-resistant protein was purified which exhibited toxicity to larval mosquitoes and cultured mosquito cells, lysed erythrocytes, and was lethal to mice. To extract the protein, a sporulating culture of B. thuringiensis subsp. israelensis was treated with alkali, neutralized, and incubated with trypsin and proteinase K. It was then purified by gel filtration and DEAE column chromatography. Up to 240 micrograms of toxic protein was purified from 1 g (wet weight) of culture pellet. Two closely related forms of toxic protein were obtained: the 25a and 25b proteins. The two forms comigrated near 25,000 daltons in a sodium dodecyl sulfate-polyacrylamide gel, were serologically related, and showed similar partial protease digestion profiles, but were distinguishable by DEAE chromatography and nondenaturing polyacrylamide gel electrophoresis. Protein sequencing data indicated the 25b protein lacked the two amino acids at the amino terminus of the 25a protein. A Western blot enzyme-linked immunosorbent assay of alkali-solubilized proteins that were not treated with proteases suggested the toxic 25a and 25b proteins were proteolytically derived from a larger molecule of about 28,000 daltons. Alkali-solubilized proteins from an acrystalliferous strain of B. thuringiensis subsp. israelensis and from B. thuringiensis subsp. kurstaki failed to cross-react with antibodies to the 25a protein.

Production of polyhedrin monoclonal antibodies for distinguishing two Orgyia pseudotsugata baculoviruses

Monoclonal antibodies were produced to polyhedrins from Orgyia pseudotsugata multicapsid nuclear polyhedrosis virus (OpMNPV) and single-capsid nuclear polyhedrosis virus (OpSNPV). Although the polyhedrins are closely related, antibodies were selected which allowed differentiation between the two viruses. In an indirect enzyme-linked immunosorbent assay, purified OpMNPV and OpSNPV polyhedrins could be detected by specific monoclonal antibodies at concentrations as low as 2 and 5 ng/ml, respectively. The antibodies were also capable of identifying their homologous polyhedrin in extracts of infected insects. These antibodies would be useful for monitoring production of the viral insecticide, TM Biocontrol-1, which by license must contain only OpMNPV, and to confirm that insect mortality after aerial spraying with this insecticide is attributable to OpMNPV infection.

Strong and regulated expression of Escherichia coli beta-galactosidase in insect cells with a baculovirus vector

The N-terminal region of the gene encoding polyhedrin, the major occlusion protein of the insect baculovirus Autographa californica nuclear polyhedrosis virus (AcNPV), has been fused to DNA encoding Escherichia coli beta-galactosidase. The fused gene was inserted into the AcNPV DNA genome by cotransfection of insect cells with recombinant plasmid DNA and wild-type AcNPV genomic DNA. Recombinant viruses were selected as blue plaques in the presence of a beta-galactosidase indicator, 5-bromo-4-chloro-3-indolyl-beta-D-galactopyranoside. Studies of one such virus, L1GP-gal3, indicated that the synthesis of beta-galactosidase is temporally controlled beginning late (20 h) in infection after the release of infectious virus particles from the cell. By 48 h postinfection, a remarkably high level of expression is achieved. On the basis of these results, AcNPV should be a useful vector for the stable propagation and expression of passenger genes in a lepidopteran cell background. A generalized transplacement vector that facilitates the construction and selection of recombinant viruses carrying passenger genes under their own promoter control has also been developed.

Strong and regulated expression of Escherichia coli beta-galactosidase in insect cells with a baculovirus vector

The N-terminal region of the gene encoding polyhedrin, the major occlusion protein of the insect baculovirus Autographa californica nuclear polyhedrosis virus (AcNPV), has been fused to DNA encoding Escherichia coli beta-galactosidase. The fused gene was inserted into the AcNPV DNA genome by cotransfection of insect cells with recombinant plasmid DNA and wild-type AcNPV genomic DNA. Recombinant viruses were selected as blue plaques in the presence of a beta-galactosidase indicator, 5-bromo-4-chloro-3-indolyl-beta-D-galactopyranoside. Studies of one such virus, L1GP-gal3, indicated that the synthesis of beta-galactosidase is temporally controlled beginning late (20 h) in infection after the release of infectious virus particles from the cell. By 48 h postinfection, a remarkably high level of expression is achieved. On the basis of these results, AcNPV should be a useful vector for the stable propagation and expression of passenger genes in a lepidopteran cell background. A generalized transplacement vector that facilitates the construction and selection of recombinant viruses carrying passenger genes under their own promoter control has also been developed.

N-Terminal polyhedrin sequences and occluded Baculovirus evolution

A phylogenetic tree for occluded baculoviruses was constructed based on the N-terminal amino acid sequence of occlusion body proteins from six baculoviruses including three lepidopteran nuclear polyhedrosis viruses (NPVs), [two unicapsid (Bombyx mori and Orgyia pseudotsugata) and one multicapsid (Orgyia pseudotsugata)]; one granulosis virus (Pieris brassicae); and NPVs from a hymenopteran (Neodiprion sertifer) and a dipteran (Tipula paludosa). Amino acid sequence data for the B. mori NPV were from a report by Serebryani et al. (1977) and that for the O. pseudotsugata NPVs were reported previously by us (Rohrmann et al. 1979). The other N-terminal amino acid sequences are presented in this paper. The phylogenetic relationships determined based on the molecular evolution of polyhedrin were also investigated by antigenic comparisons of the proteins using a solid phase radioimmune assay. The results indicate that the lepidopteran NPVs are the most closely related of the above group of viruses and are related to these viruses in the following order: N. sertifer NPV, P. brassicae granulosis virus, and T. paludosa NPV. These data, in conjunction with Baculovirus distribution and evidence concerning insect phylogeny, suggest that the Baculovirus have an ancient association with insects and may havae evolved along with them.

Application of a Novel Radioimmunoassay to Identify Baculovirus Structural Proteins That Share Interspecies Antigenic Determinants

Immunological comparisons were made of baculovirus structural proteins by using a modification of the radioimmunological techniques described by Renart et al. (Proc. Natl. Acad. Sci. U.S.A. 76: 3116-3120, 1979) and Towbin et al. (Proc. Natl. Acad. Sci. U.S.A. 76: 4350-4354, 1979). Viral proteins were electrophoresed in polyacrylamide gels, transferred to nitrocellulose, and incubated with viral antisera, and the antibodies were detected with 125 I-labeled Staphylococcus aureus protein A. Antisera were prepared to purified and intact virions from five baculoviruses: Autographa californica, Porthetria dispar, Trichoplusia ni , and Heliothis zea nuclear polyhedrosis viruses (NPVs) and T. ni granulosis virus (GV). These antisera were tested against the virion structural polypeptides of 17 different species of baculoviruses. Specific multiple-nucleocapsid NPV (MNPV), single-nucleocapsid NPV (SNPV), and GV virion polypeptides were shown to have similar antigenic determinants and thus be immunologically related. The molecular weights of the virion polypeptides with cross-reacting antigenic determinants were identified. Antisera prepared to purified A. californica and H. zea MNPV polyhedrin (the occlusion body protein from NPVs) recognized antigenic determinants on all the polyhedrins and granulins (occlusion body protein from GVs) that were tested. No immunological relationship was detected between A. californica MNPV polyhedrin and any of the A. californica MNPV virion structural polypeptides present on either the virus isolated from occlusion bodies or A. californica MNPV extracellular virus from infected-cell cultures.