Identification of the Lymantria dispar Nucleopolyhedrovirus Envelope Fusion Protein Provides Evidence for a Phylogenetic Division of the Baculoviridae

The complete genome sequences of a number of diverse members of theBaculoviridae including both nucleopolyhedroviruses (NPVs) and granuloviruses (GVs) revealed that they lack a homolog of GP64, the envelope fusion protein of the budded form ofAutographa californica multinucleocapsid NPV (AcM NPV) and its close relatives. Computer-assisted analyses of the genome of one of these viruses,Lymantria dispar M NPV (LdM NPV), revealed a single open reading frame (ld130 ) whose product had the predicted properties of a membrane protein. Characterization of the localization of the products of the full-lengthld130 gene and of anld130 -enhanced green fluorescent protein gene (egfp ) fusion using both immunofluorescence and fluorescence microscopy revealed that LD130 accumulates at the plasma membranes of cells infected with LdM NPV or transfected withld130-egfp . In addition, cells transfected with eitherld130 orld130-egfp or infected with wild-type virus undergo membrane fusion at pH 5. Western blot analyses indicate that LD130 is present in infected cells as an 83-kDa protein and is also present in budded virions as a protein doublet containing bands of 81 and 83 kDa. Tunicamycin treatment of infected cells resulted in an immunoreactive band of about 72 kDa, indicating that LD130 is N-glycosylated. Whereas the distribution ofgp64 appears to be confined to a relatively closely related group of NPVs, homologs ofld130 are present in a diverse number of both NPVs and GVs. This suggests that LD130 may be the primordial baculovirus envelope fusion protein.