F proteins of group II nucleopolyhedroviruses (NPVs) are envelope fusion proteins essential for virus entry and egress. An F-null Helicoverpa armigera single nucleocapsid NPV (HearNPV) bacmid, HaBacΔF, was constructed. This bacmid could not produce infectious budded virus (BV) when transfected into HzAM1 cells, showing that F protein is essential for cell-to-cell transmission of BVs. When HaBacΔF was pseudotyped with the homologous F protein (HaBacΔF-HaF, positive control) or with the heterologous F protein from Spodoptera exigua multinucleocapsid NPV (SeMNPV) (HaBacΔF-SeF), infectious BVs were produced with similar kinetics. In the late phase of infection, the BV titre of HaBacΔF-SeF virus was about ten times lower than that of HaBacΔF-HaF virus. Both pseudotyped viruses were able to fuse HzAM1 cells in a similar fashion. The F proteins of both HearNPV and SeMNPV were completely cleaved into F1 and F2 in the BVs of vHaBacΔF-HaF and vHaBacΔF-SeF, respectively, but the cleavage of SeF in vHaBacΔF-SeF-infected HzAM1 cells was incomplete, explaining the lower BV titre of vHaBacΔF-SeF. Polyclonal antisera against HaF1 and SeF1 specifically neutralized the infection of vHaBacΔF-HaF and vHaBacΔF-SeF, respectively. HaF1 antiserum showed some cross-neutralization with vHaBacΔF-SeF. These results demonstrate that group II NPV F proteins can be functionally replaced with a homologue of other group II NPVs, suggesting that the interaction of F with other viral or host proteins is not absolutely species-specific.
Vip3A is responsible for the potency of Bacillus thuringiensis 9816C culture supernatant against Helicoverpa armigera and Spodoptera exigua
