Summary auto-generated
This study investigated how the polyhedrin protein sequence determines the shape of occlusion bodies (OBs) in baculoviruses, using Thysanoplusia orichalcea nucleopolyhedrovirus (ThorNPV) as a model. ThorNPV naturally produces tetrahedral OBs, unlike most NPVs which form polyhedral structures. The researchers cloned and sequenced ThorNPV's polyhedrin gene, identifying a 738-nucleotide open reading frame encoding 246 amino acids. By replacing the polyhedrin gene of Autographa californica NPV (AcMNPV)—which normally forms polyhedral OBs—with ThorNPV's polyhedrin gene, they successfully generated recombinant viruses that produced tetrahedral OBs with properly enclosed virions. Site-directed mutagenesis revealed that a single amino acid, leucine at position 43, is responsible for the tetrahedral shape. When leucine was changed to isoleucine (matching AcMNPV), the OBs reverted to polyhedral form. The study demonstrates that polyhedrin protein sequence, not cellular factors, determines OB shape. However, recombinant viruses showed reduced infectivity and slightly greater alkaline susceptibility compared to wild-type strains, suggesting that other factors influence virion occlusion efficiency.
Key findings
- Leucine at position 43 of ThorNPV polyhedrin protein is solely responsible for determining tetrahedral occlusion body shape
- Replacing AcMNPV polyhedrin with ThorNPV polyhedrin in recombinant virus produces tetrahedral OBs, proving polyhedrin sequence governs OB morphology rather than cellular factors
- Recombinant virus expressing ThorNPV polyhedrin shows eightfold lower infectivity to beet armyworm compared to wild-type AcMNPV, despite proper virion occlusion
- The tetrahedral OB shape serves as a useful morphological marker for identifying mixed viral infections in insects
This summary was generated automatically from the article PDF and is not part of the original publication. Refer to the PDF for the authoritative text.
Abstract
A nucleopolyhedrovirus (NPV) isolated from the looper Thysanoplusia orichalcea L. (Lepidoptera: Noctuidae) (ThorNPV) is occluded in a tetrahedral protein matrix. The ORF of the ThorNPV polyhedrin gene contains 738 nt which code for 246 amino acids of the putative polyhedrin protein with an estimated molecular mass of 28,778 Da. The promoter of this gene is similar in length to the promoter of Spodoptera frugiperda NPV (SfMNPV), with a 5 nt deletion before the start codon compared to those of other NPVs. When the polyhedrin gene of Autographa californica NPV (AcMNPV), whose occlusion bodies (OBs) are polyhedral, was replaced by the polyhedrin gene of ThorNPV, which produces tetrahedral OBs, tetrahedral polyhedra with properly occluded virions were produced. This work establishes the importance of the polyhedrin protein sequence in determining OB shape. Leucine at position 43 of ThorNPV polyhedrin was identified as responsible for the tetrahedral shape of ThorNPV OBs by PCR-based site-directed mutagenesis. Susceptibility to alkaline buffer of OBs formed by recombinant AcMNPV (RECAcV) carrying the polyhedrin gene of ThorNPV was slightly greater than that of native ThorNPV OBs. The LD50 of RECAcV for third-instar beet armyworm (Spodoptera exigua) was significantly lower than that of AcMNPV (253 and 31 OBs per larva, respectively).