Mature viral cathepsin is required for release of viral occlusion bodies from Autographa californica multiple nucleopolyhedrovirus-infected cells

Baculovirus-infected larvae release progeny viral occlusion bodies (OBs) to enable cyclical virus transmission to new hosts. The alphabaculovirus chitinase and cathepsin enzymes cause terminal liquefaction of host insect cadavers, aiding OB dispersal. The mechanism of cell lysis required to release the OBs is unclear but here we show Autographa californica multiple nucleopolyhedrovirus cathepsin protease activity is required for efficient release of the host tissue-degrading chitinase and cathepsin enzymes and critical for release of progeny OBs from virus-infected cells. Comparisons between viruses containing or lacking cathepsin indicate that cathepsin was necessary for OB release into cultured cell media or hemolymph of insects. In addition, pharmacological inhibition of cysteine protease activity in cells during infection blocked maturation of active cathepsin and OB release from infected cells. Together, these results suggest an important link between baculovirus-induced cell lysis, the concomitant maturation of cathepsin, and cellular release of chitinase, cathepsin and progeny OBs from cells.

Functional Characterization of the Group I Alphabaculovirus Specific Gene ac73

Baculoviridae is a family of large DNA viruses that specifically infect insects. It contains four genera, Alpha-, Beta-, Gamma-, and Deltabaculovirus. Alphabaculovirus is further divided into Group I and II, and Group I appears to be emerged most recently among all baculoviruses. Interestingly, there are 12 Group I specific genes that are only found in this lineage. Studying these genes is helpful to understand how baculoviruses evolved. Here, we reported the functional analyzing results of ac73, a function unknown Group I specific gene of Autographa californica multiple nucleopolyhedrovirus (AcMNPV) which is the type species of baculovirus. The AC73 protein encoded by ac73 was found to be expressed during the late stage of infection and incorporated into the nucleocapsids of budded virus (BV) and occlusion-derived virus (ODV). In infected cells, AC73 resided mainly in the ring zone region of the nucleus, and appeared to be assembled into occlusion bodies (OBs). The ac73 knockout and repaired viruses were constructed and studied by in vitro and in vivo infection. Although ac73 was not essential for BV and ODV or OB formation, the BV titer and viral infectivity in insect larvae of ac73 knockout AcMNPV decreased by about 5-8 and 3-4 fold compared to those of wild type virus, respectively, suggesting ac73 contributed to infectious BV production and viral infectivity in vivo. This research provides new insight into the function of this Group I specific gene.

Bombyx mori nucleopolyhedrovirus protein Bm11 is involved in occlusion body production and occlusion-derived virus embedding

Bombyx mori nucleopolyhedrovirus (BmNPV) orf11 (bm11) is a highly conserved gene with unknown function. It is homologous to AcMNPV orf19. In this study, a bm11 knockout virus was constructed and its role was investigated. Expression analysis indicated that bm11 is a late gene and confocal microscopy analysis demonstrated that Bm11 localizes predominantly in the nuclear ring zone at the late phase of infection. The bm11 deletion did not affect budded virus (BV) production or viral genome replication, but markedly reduced the production of occlusion bodies (OBs) and the embedding of occlusion-derived viruses (ODVs). Bio-assays showed that Bm11 was involved in BmNPV infectivity in vivo by direct injection. In conclusion, our results demonstrated that although Bm11 is not essential for BV production or mature ODV formation, it affects OB production and ODV occlusion.