Function, oligomerization and N-linked glycosylation of the Helicoverpa armigera single nucleopolyhedrovirus envelope fusion protein

Abstract

In the familyBaculoviridae, two distinct envelope fusion proteins are identified in budded virions (BVs). GP64 is the major envelope fusion protein of group I nucleopolyhedrovirus (NPV) BVs. An unrelated type of envelope fusion protein, named F, is encoded by group II NPVs. The genome ofHelicoverpa armigera(Hear) NPV, a group II NPV of the single nucleocapsid or S type, also encodes an F-like protein: open reading frame 133 (Ha133). It was demonstrated by N-terminal sequencing of the major 59 kDa protein present in HearNPV BV that this protein is one of the two F subunits: F₁(transmembrane subunit of 59 kDa) and F₂(surface subunit of 20 kDa), both the result of cleavage by a proprotein convertase and disulfide-linked. The HearNPV F protein proved to be a functional analogue of GP64, as the infectivity of an AcMNPVgp64-deletion mutant was rescued by the introduction of the HearNPV F gene. It was also demonstrated by chemical cross-linking that HearNPV F is present in BVs as an oligomer whereby, unlike GP64, disulfide bonds are not involved. Deglycosylation assays indicated that both F₁and F₂possessN-linked glycans. However, when F was made in Hz2E5 cells, these glycans did not have anα-1-3 core fucose modification that usually occurs in insect cells. Asα-1-3 core fucose is a major inducer of an allergic response in humans, the present observation makes the HearNPV–Hz2E5 system an attractive alternative for the production of recombinant glycoproteins for therapeutic use in humans.