The env-like open reading frame of the baculovirus-integrated retrotransposon TED encodes a retrovirus-like envelope protein

Abstract

TED is a 7.5-kbp member of the gypsy family of retrotransposons that was first identified by its integration within the baculovirus DNA genome. This lepidopteran (moth) transposon contains three retrovirus-like genes, including functional gag and pol that yield reverse transcriptase-containing virus-like particles. To identify and characterize the product(s) of the third env-like open reading frame, TED ORF3 was expressed in homologous lepidopteran cells by using a baculovirus vector, vENV. Immunoblots and immunoprecipitations with antiserum raised against a bacterial ORF3-fusion protein detected two ORF3-encoded proteins, p68(env) and gp75(env). On the basis of selective incorporation of [3H]mannose and inhibition of modification by tunicamycin which blocks N-linked glycosylation, gp75(env) is a glycoprotein derived from core precursor p68(env). As predicted by the presence of a transmembrane domain near the carboxyl terminus, both p68(env) and gp75(env) were associated with heavy membranes of vENV-infected cells. Thus, TED ORF3 encodes a membrane glycoprotein with properties characteristic of retroviral env proteins. These data are consistent with the hypothesis that TED is an invertebrate retrovirus. Moreover, TED integration within the baculovirus genome provides an example of retroelement-mediated acquisition of host genes that may contribute to virus evolution.