Autographa californica multiple nucleopolyhedrovirus AC83 is a per os infectivity factor (PIF) protein required for occlusion-derived virus (ODV) and budded virus nucleocapsid assembly as well as assembly of the PIF complex in ODV envelopes
Javed, M.A., Biswas, S., Willis, L.G., Harris, S., Pritchard, C., van Oers, M.M., Cameron Donly, B., Erlandson, M.A., Hegedus, D.D., Theilmann, D.A. (2017). Autographa californica multiple nucleopolyhedrovirus AC83 is a per os infectivity factor (PIF) protein required for occlusion-derived virus (ODV) and budded virus nucleocapsid assembly as well as assembly of the PIF complex in ODV envelopes. Journal of Virology, [online] 91(5), http://dx.doi.org/10.1128/JVI.02115-16
Plain language summary
Baculoviruses are natural insect viruses which primarily infect Lepidoptera (moths and butterflies). They initiate infection in their host caterpillar by binding to the cells lining the midgut, a process which is mediated by specific proteins called per os infectivity factors, or PIFs for short. The model baculovirus, AcMNPV, encodes seven PIF proteins and deletion of the gene for any of these PIFs results in the virus being unable to bind or enter midgut cells. This study analyzed the role of another viral protein, AC83, in oral infectivity by the virus to determine if it is also a PIF protein. AC83 was found to have all the properties of a PIF protein and therefore should be considered PIF8. Based on this conclusion, PIF proteins represent 29% of the known baculovirus core genes, which are the set of genes conserved in all baculovirus genomes sequenced to date, and they now appear to be part of an essential infection mechanism that is utilized by all baculoviruses and potentially other families of large nuclear-replicating invertebrate viruses as well. This fundamental information on the role of pif genes and their protein products in baculovirus infection is necessary to enable the optimization of the properties of these viruses so that they can be effectively implemented as biopesticide tools in agriculture.
Baculovirus occlusion-derived virus (ODV) initiates infection of lepidopteran larval hosts by binding to the midgut epithelia, which is mediated by per os infectivity factors (PIFs). Autographa californica multiple nucleopolyhedrovirus (AcMNPV) encodes seven PIF proteins, of which PIF1 to PIF4 form a core complex in ODV envelopes to which PIF0 and PIF6 loosely associate. Deletion of any pif gene results in ODV being unable to bind or enter midgut cells. AC83 also associates with the PIF complex, and this study further analyzed its role in oral infectivity to determine if it is a PIF protein. It had been proposed that AC83 possesses a chitin binding domain that enables transit through the peritrophic matrix; however, no chitin binding activity has ever been demonstrated. AC83 has been reported to be found only in the ODV envelopes, but in contrast, the Orgyia pseudotsugata MNPV AC83 homolog is associated with both ODV nucleocapsids and envelopes. In addition, unlike known pif genes, deletion of ac83 eliminates nucleocapsid formation. We propose a new model for AC83 function and show AC83 is associated with both ODV nucleocapsids and envelopes. We also further define the domain required for nucleocapsid assembly. The cysteine-rich region of AC83 is also shown not to be a chitin binding domain but a zinc finger domain required for the recruitment or assembly of the PIF complex to ODV envelopes. As such, AC83 has all the properties of a PIF protein and should be considered PIF8. In addition, pif7 (ac110) is reported as the 38th baculovirus core gene.