A Novel Baculovirus Envelope Fusion Protein with a Proprotein Convertase Cleavage Site

Abstract

The entry mechanism of Spodoptera exigua multicapsid nucleopolyhedrovirus (SeMNPV), a group II NPV, in cultured cells was examined. SeMNPV budded virus (BV) enters by endocytosis as do the BVs of the group I NPVs, Autographa californica (Ac) MNPV and Orgyia pseudotsugata (Op) MNPV. In group I NPVs, upon infection acidification of the endosome triggers fusion of the viral and endosomal membrane, which is mediated by the BV envelope glycoprotein GP64. However, the SeMNPV genome lacks a homolog of GP64 envelope fusion protein (EFP). A functional homolog of the OpMNPV GP64 EFP was identified in SeMNPV ORF8 (Se8; 76 kDa) and appeared to be the major BV envelope protein. Surprisingly, a 60-kDa cleavage product of this protein is present in the BV envelope. A furin-like proprotein convertase cleavage site (R-X-K/R-R) was identified immediately upstream of the N-terminus of the mature Se8 protein and this site was also conserved in the Lymantria dispar (Ld) MNPV homolog (Ld130) of Se8. Syncytium formation assays showed that Se8 and Ld130 alone were sufficient to mediate membrane fusion upon acidification of the medium. Furthermore, C-terminal GFP-fusion proteins of Se8 and Ld130 were primarily localized in the plasma membrane of insect cells. This is consistent with their fusogenic activity and supports the conclusion that the Se8 gene product is a functional homolog of the GP64 EFP.

Functional studies of per os infectivity factors of Helicoverpa armigera single nucleocapsid nucleopolyhedrovirus

“…BVs enter the insect cells through adsorptive endocytosis dependent on endosomal low-pH activation (Volkman & Goldsmith, 1985). The main envelope protein, GP64 of group I (Hefferon et al, 1999) or F protein of group II (IJkel et al, 2000;Pearson et al, 2002), is responsible for BVs entry (Volkman & Goldsmith, 1985). In contrast to BV, ODV is more specialized.…”

Free-living pathogens: Life-history constraints and strain competition

“…A number of these viruses have been deployed as biological controls in forestry or agriculture (Godfray et al, 1997). The Baculoviridae comprise two genera, the granuloviruses (GVs) and the nucleopolyhedroviruses (NPVs), distinguished by the structure of their free-living stage (IJkel et al, 2000). Free viral particles must stabilize both their genetic material and receptor-binding sites.…”

“…GV occlusion bodies are small and usually contain only a single virion. NPV occlusion bodies are larger structures containing many virions (IJkel et al, 2000). Occlusion bodies can protect virions for lengthy periods, until consumption by an insect larva leads to virion release and infection (Anderson and May, 1981;Godfray et al, 1997).…”

Pseudotyping Autographa californica Multicapsid Nucleopolyhedrovirus (Ac M NPV): F Proteins from Group II NPVs Are Functionally Analogous to Ac M NPV GP64

“…Although GP64 proteins have been identified in other group I NPVs, recent data from the complete genomic sequences of a number of baculoviruses suggest that gp64 genes are not present in group II NPVs or GVs. An envelope protein gene that is unrelated to gp64 was recently identified in Lymantria dispar MNPV (LdMNPV) (27) and Spodoptera exigua MNPV (SeMNPV) (15), and homologs of this gene have been identified in other group II NPVs and GVs. This protein has been named the F (fusion) protein (33).…”

“…This protein has been named the F (fusion) protein (33). F proteins from LdMNPV (ORF 130, or Ld130) and SeMNPV (ORF 8,or Se8) are present in the virion membrane and mediate pH-triggered membrane fusion (15,27). Like envelope proteins from some other virus groups, the baculovirus F proteins are cleaved internally by cellular proprotein convertases such as furin, and cleavage is necessary for fusion activity (33).…”