Biochemical characterisation of the baculovirus per os infectivity complex

Baculoviruses infect insect midgut epithelial cells via the occlusion-derived form of virus (ODVs). For Autographa californica multiple nucleopolyhedrovirus (AcMNPV) at least seven conserved ODV envelope proteins are essential for oral infectivity and are called called per os infectivity factors (PIFs). Previous work showed that most of these PIF’s are present in a multi-molecular complex. PIF1, PIF2 and PIF3 are essential for complex formation and PIF4 is important for stabilization of this complex. These four PIF’s are considered to be the core components of the PIF-complex. Another PIF-protein, P74, has been identified as interaction partner of PIF1 and is more loosely associated to the complex. Analysis of larvae-derived ODV’s (L-ODV) showed clear biochemical differences when compared with the cell culture-derived ODV’s (C-ODV). On western blots with L-ODV’s, the band representing the PIF-complex is less well defined and migrate faster than with C-ODV’s. Moreover, in C-ODV’s, PIF1 monomers were detected together with the complex while these were absent in L-ODV’s. These observations suggest that the composition, stoichiometry and/or conformation might be different depending on the origin of the virus. Furthermore, P74 has been shown to be cleaved only in L-ODV’s. This cleavage event is mediated by a still-to-be-identified endogenous protease, which is co-occluded with the L-ODV’s. Together, these observations suggest that the PIF-complex and interaction partners has different characteristics, depending on whether the virus is propagated in insect larvae or cell culture