| Alphabaculoviruses of the family Baculoviridae infect the larvae of the Lepidoptera (moths and butterflies) when the occlusion-derived virus (ODV) released from the occlusion body (OB) binds and fuses to the midgut epithelium. Most alphabaculoviruses readily infect only a few species of caterpillars. The ODV contain more than 30 proteins, twelve of which are conserved across the alphabaculoviruses, including the envelope proteins ODV-E56 and ODV-E66. The mechanism of viral fusion and entry, as well as, the ODV envelope proteins implicated in this process, are unknown. A family of ODV envelope per os infectivity factors (PIFs) has been identified, which includes seven proteins that have significant effects on oral infectivity. Here, we assess the potential roles of ODV-E56 and ODV-E66 in oral infectivity. Bioassays showed that ODV-E56-negative virus was significantly less infectious per os, in both ODV and OB. However, the ODV-E56 negative purified ODV exhibited a two-log reduction in oral infectivity compared to ODV-E56 positive virus, whereas the ODV-E56 negative OBs exhibited a five-log reduction in infectivity. This suggests ODV-E56 may function in early interactions within the gut. The ODV-E56 negative viruses exhibited wild-type levels of binding and fusion, but viral DNA was not transcribed. ODV-E56 bound to 97 kDA protein from H. virescens midguts. Thus, ODV-E56 is not essential for cell fusion, but may function in cell signaling and post-fusion events. These results indicate that ODV-E56 is PIF-5.;Fifteen H. virescens gut-binding peptides were isolated using a phage display library, and two peptides showed similarity to ODV-E66. One phage peptide, HV1, exhibited strong binding to cryosections of fourth instar H. virescens midguts, and in competition assays with baculovirus resulted in decreased mortality and increased survival time. The homolog AcE66A to the second phage peptide (HV2), exhibited strong binding, but had no effect on mortality in competition assays. ODV-E56 and ODV-E66 are conserved proteins that localize to a very complex virion envelope, and further studies are needed to detail their functions. Increased knowledge of the determinants of virus infection may facilitate further development of these viruses for use as environmentally benign insecticides, as well as, greater understanding of viral mechanisms. |
