Mutagenesis and Nuclear Magnetic Resonance Analyses of the Fusion Peptide of Helicoverpa armigera Single Nucleocapsid Nucleopolyhedrovirus F Protein

ABSTRACT The entry of enveloped viruses into cells is normally mediated by fusion between viral and cellular membranes, in which the fusion peptide plays a crucial role. The fusion peptides of group II nucleopolyhedrovirus (NPV) F proteins are quite conserved, with a hydrophobic region located at the N terminal of the F1 fragment. For this report, we used mutagenesis and nuclear magnetic resonance (NMR) to study the structure and function of the fusion peptide of the Helicoverpa armigera single-nucleocapsid NPV (HearNPV) F protein (HaF). Five mutations in the fusion peptide of HaF, N1G, N1L, I2N, G3L, and D11L, were generated separately, and the mutated f genes were transformed into the f-null HearNPV bacmid. The mutations N1L, I2N, and D11L were found to completely abolish the ability of the recombinant bacmids to produce infectious budded virus, while the mutations N1G and G3L did not. The low-pH-induced envelope fusion assay demonstrated that the N1G substitution increased the fusogenicity of HaF, while the G3L substitution reduced its fusogenicity. NMR spectroscopy was used to determine the structure of a synthetic fusion peptide of HaF in the presence of sodium dodecyl sulfate micelles at pH 5.0. The fusion peptide appeared to be an amphiphilic structure composed of a flexible coil in the N terminus from N1 to N5, a 310-helix from F6 to G8, a turn at S9, and a regular α-helix from V10 to D19. The data provide the first NMR structure of a baculovirus fusion peptide and allow us to further understand the relationship of structure and function of the fusion peptide.