Functional characterization of Autographa californica multiple nucleopolyhedrovirus ORF43 and phenotypic changes of ORF43-knockout mutant

ORF43 (ac43) of Autographa californica multiple nucleopolyhedrovirus (AcMNPV) is a highly conserved baculovirus gene of unknown function. To investigate the role of ac43 in the baculovirus lifecycle, we constructed an ac43-deleted mutant AcMNPV, Ac43KO. After transfection into Spodoptera frugiperda cells, Ac43KO produced polyhedra much larger in size than those of wild-type AcMNPV. Interestingly, some of the nucleocapsids were singly enveloped in the polyhedrin matrix while the nucleocapsids of AcMNPV are known to be multiply enveloped. Furthermore, Ac43KO led to a defect in the transcription and expression of polyhedrin, which resulted in reduced occlusion body production. However, Ac43KO did not affect production of budded virus as there was no remarkable difference in budded virus titer. These results suggest that ac43 plays an important role in the expression of polyhedrin, the morphogenesis of occlusion body, and the assembly of virions occluded in occlusion bodies.