Specific Binding of Autographa californica M Nucleopolyhedrovirus Occlusion-Derived Virus to Midgut Cells of Heliothis virescens Larvae Is Mediated by Products of pif Genes Ac119 and Ac022 but Not by Ac115

Per os infectivity factors PIF1 (Ac119) and PIF2 (Ac022), like P74, are essential for oral infection of lepidopteran larval hosts ofAutographa californica M nucleopolyhedrovirus (AcM NPV). Here we show that Ac115 also is a PIF (PIF3) and that, unlike PIF1 and PIF2, it does not mediate specific binding of AcM NPV occlusion-derived virus (ODV) to midgut target cells. We used an improved in vivo fluorescence dequenching assay to compare binding, fusion, and competition among control AcM NPV ODV and the ODVs of AcM NPV PIF1, PIF2, and PIF3 deletion mutants. Our results showed that binding and fusion of PIF1 and PIF2 mutants, but not the PIF3 mutant, were both qualitatively and quantitatively different from those of control ODV. Unlike control and PIF3-deficient ODV, an excess of PIF1- or PIF2-deficient ODV failed to compete effectively with control ODV's binding to specific receptors on midgut epithelial cells. Moreover, the levels of PIF1- and PIF2-deficient ODV binding were depressed threefold compared to control levels. Binding, fusion, and competition by PIF3-deficient ODV, however, were all indistinguishable from those of control ODV. These results implicated PIF1 and PIF2 as ODV envelope attachment proteins that mediate specific binding to primary target cells within the midgut. In contrast, PIF3 mediates another unidentified, but critical, early event during primary infection.