The mouse mammary tumor virus long terminal repeat encodes A 47 kDa Glycoprotein with a short half-life in mammalian cells

The mouse mammary tumor virus proviral DNA contains an open reading frame in the 3' long terminal repeat which can code for a 36 kDa polypeptide with a putative transmembrane sequence and five Nlinked glycosylation sites. This gene is known to code for a superantigen which deletes a specific subset of CD4⁺ T lymphocytes in vivo. The superantigen encoded by the exogenous mouse mammary tumor virus of the GR strain acts specifically on Vβ 14 bearing T cells. We produced recombinant vaccinia viruses to express either the complete or a truncated ORF protein after infection of primate cells in culture. The complete ORF gene in mammalian cells leads to the production of a 47 kDa protein which is specifically detected with an anti-ORF-peptide antiserum. The 47 kDa protein can be labeled with d-[2-³H]mannose and its synthesis is inhibited by tunicamycin, an N-linked glycosylation inhibitor, indicating that it is a glycoprotein. The truncated ORF protein beginning at the second ATG of the open reading frame is also modified, but the C-terminal half of ORF, starting at the fifth ATG, has the expected size of the non modified polypeptide. Pulse-chase experiments indicate that the ORF protein has a short half-life of about 1.5-2hr.