Baculoviruses are arthropod-specific large DNA viruses that orally infect the larvae of lepidopteran, hymenopteran and dipteran insect species. These larvae become infected when they eat a food source that is contaminated with viral occlusion bodies (OBs). These OBs contain occlusion-derived viruses (ODVs), which are released upon ingestion of the OBs and infect the endothelial midgut cells. At least nine different ODV envelope proteins are essential for this oral infectivity and these are denoted per os infectivity factors (PIFs). Seven of these PIFs form a complex, consisting of PIF1, 2, 3 and 4 that form a stable core complex and PIF0 (P74), PIF6 and PIF8 (P95) that associate with this complex with lower affinity than the core components. The existence of a PIF complex and the fact that the pif genes are conserved in baculovirus genomes suggests that PIF-proteins cooperatively mediate oral infectivity rather than as individual functional entities. This review therefore discusses the knowledge obtained for individual PIFs in light of their relationship with other members of the PIF complex.
Keywords: ODV entry complex; ODV-E56; ODV-E66; PIF; R18 de-quenching assay; baculovirus; membrane fusion; per os infectivity factors; protein trafficking.