Expression of bovine immunodeficiency-like virus envelope glycoproteins by a recombinant baculovirus in insect cells

The bovine immunodeficiency-like virus (BIV) env open reading frame (ORF) contains both sequences encoding env and sequences for exon 1 of the putative rev gene. Recombinant baculoviruses incorporating BIV env ORF sequences were constructed to characterize the expression, processing, and immunogenicity of products of the BIV env ORF in insect cells and to develop reagents to study native BIV Env glycoproteins. A recombinant baculovirus containing the entire env ORF synthesized a nonglycosylated, 20-kDa, BIV-specific protein, apparently unrelated to native BIV Env proteins. In contrast, a recombinant baculovirus containing a truncated env ORF in which the coding sequences for rev exon 1 were deleted synthesized three size classes of glycosylated proteins in insect cells related to the BIV Env precursor (gp145), surface (gp100), and transmembrane (gp45) glycoproteins observed in BIV-infected mammalian cells. Oligomers of recombinant BIV Env proteins also formed in these baculovirus-infected insect cells. Immunofluorescence staining of intact insect cells infected by the baculovirus expressing BIV Env with BIV-specific serum demonstrated that the recombinant Env glycoproteins were expressed on the cell surface. Antisera raised to recombinant Env glycoproteins immunoprecipitated native gp145, gp100, and gp45 in BIV-infected bovine cells similar to sera from animals naturally or experimentally infected with BIV.