The complete genome sequences of a number of diverse members of the Baculoviridae including both nucleopolyhedroviruses (NPVs) and granuloviruses (GVs) revealed that they lack a homolog of GP64, the envelope fusion protein of the budded form of Autographa californica multinucleocapsid NPV (AcMNPV) and its close relatives. Computer-assisted analyses of the genome of one of these viruses, Lymantria dispar MNPV The Baculoviridae are a large family of occluded, rod-shaped viruses with circular, supercoiled, double-stranded DNA genomes of 100 to 180 kb depending on the virus strain. Two baculovirus genera have been described; the nucleopolyhedroviruses (NPVs) (34) have multiple virions present in large polyhedron-shaped occlusion bodies, whereas the granuloviruses (GVs) (42) normally display a single nucleocapsid embedded in a small granular occlusion body. NPVs are characterized by the production of two virion forms, or phenotypes (40). Occlusion-derived virions are found in occlusion bodies and initiate infection in midgut cells upon ingestion by susceptible hosts. In contrast, the budded virus (BV) form is not occluded, is produced early in infection, and spreads the infection within the infected insect prior to occlusion body production. Although the nucleocapsids for the two virion forms appear to have similar polypeptide contents, the compositions and structures of their envelopes appear to be distinct (7). BV infection by both Autographa californica multinucleocapsid NPV (AcMNPV) and Orgyia pseudotsugata MNPV (OpMNPV) has been investigated, and a glycoprotein called GP64 has been found to be associated with the BV form of both these viruses (3, 39, 41). GP64 is expressed both early and late in infection and is transported to and incorporated into the cell membrane. As nucleocapsids bud through the cell membrane and exit the cell, they become enveloped in the GP64-modified cell membrane. GP64 appears to be required for the spread of the infection to other cells and for the virus to exit from an infected cell (23,26,28). BV enters cells via an endocytic pathway, and, upon acidification of the endocytic vesicle, the structure of GP64 is altered and fusion of the viral and endosomal membranes occurs (17,20,22). This results in the release of the nucleocapsid into the cytoplasm. Surprisingly, GP64 is closely related to the envelope fusion protein of the Thogoto virus genus of the Orthomyxoviridae, which are minus strand RNA viruses (24,25).Recently the sequences of the 161-kb Lymantria dispar MNPV (LdMNPV) (18), 136-kb Spodoptera exigua MNPV (15), and 178-kb Xestia c-Nigrum GV (XcGV) (11) genomes have been reported. A striking feature of these genomes is the lack of an identifiable homolog to gp64. The fact that the life cycle of LdMNPV is similar to those of AcMNPV and OpMNPV suggested that it must express a different envelope fusion protein. Computer-assisted analyses of the LdMNPV genome identified a single 676-amino-acid product of LdMNPV open reading frame (ORF) 130 (ld130), which was predicted to have both a ...