Dimerization and proper degradation of BmNPV IE2 are required for efficient virus growth in larvae of Bombyx mori(Lepidoptera: Bombycidae)

The baculovirus ie2gene is one of the immediate early genes, and its product is known to transactivate viral promoters. However, the roles of Bombyx morinucleopolyhedrovirus (BmNPV) ie2in insect larvae are poorly understood. Here we investigated the functions of BmNPV IE2 in cultured cells and in insect larvae using two mutant viruses, BmIE2D and BmIE2CS. BmIE2D lacks the IE2 C-terminal coiled-coil domain that is required for IE2 dimerization. The other mutant BmIE2CS expresses an E3 ligase activity-deficient IE2 derivative, which is degraded more slowly compared with wild-type IE2. We found that ie2mutations had little effect on BmNPV infection in cultured cells, whereas budded virus and occlusion body production was significantly reduced in the hemolymph of B. morilarvae infected with ie2mutants. These results indicate that both dimerization and proper degradation of BmNPV IE2 are crucial steps for efficient virus growth in B. morilarvae, but not in cultured cells. Oral infection assays also revealed that the infectivity of the occluded form of ie2mutants was normal in B. morilarvae, which is inconsistent with the results reported from ie2mutants of Autographa californicaNPV. This suggests that loss of IE2 function causes virus-specific effects in host insects.