Biosynthesis and metabolism of viral proteins expressed on the surface of murine leukemia virus-infected cells.

Ledbetter, J A and Nowinski, R C and Eisenman, R N (1978) Biosynthesis and metabolism of viral proteins expressed on the surface of murine leukemia virus-infected cells. Virology, 91 (1). pp. 116-129. ISSN 0042-6822

Abstract

Viral proteins expressed on the surface of the Gross virus-induced leukemia E♂G2 were detected by immune precipitation assays from detergent lysates of cells that were labeled by the [125I]lactoperoxidase method. These proteins included gp70 and two glycosylated polyproteins that contained antigens coded by the gag gene. The 95,000-dalton polyprotein (gpP95gag) reacted with antisera against p30, p12, and p10, while the 85,000-dalton polyprotein (gpP85gag) reacted with antisera against p30 and p12, but not against p10. Neither polyprotein reacted with antiserum against reverse transcriptase. GpP95gag and gpP85gag contained glucosamine, but not fucose, while gp70 contained both sugars. Tryptic peptide maps of the iodinated polyproteins indicated that gpP95gag and gpP85gag were highly related. Viral polypeptides, having identical electrophoretic mobilities as the cell surface gpP95gag and gpP85gag, were also detected by immune precipitation of lysates from cells that were metabolically labeled with radioactive methionine or arginine. In kinetic studies, gpP95gag was labeled rapidly following a 10-min pulse; in contrast, gpP85gag was not detected until late in the chase period, suggesting that this glycosylated polyprotein was a processed product of gpP95gag. Mapping of arginine-labeled tryptic peptides of different gag polyproteins supported this conjecture. In fact, peptide maps with isolated viral proteins further demonstrated that gpP95gag and gpP85gag contained peptides of p30,p15/p12, and p10. The presence of p10 peptides in both polyproteins was an unanticipated finding since the gpP85gag did not react in immune precipitation assays with anti-p10 serum. By peptide mapping, both of the glycosylated polyproteins also appeared highly related to Pr75gag, the intracellular precursor to virion gag proteins. Our data suggest that gpP95gag is a glycosylated cell surface form of the primary gag precursor Pr75gag, and that processing of the gpP95gag results in the production of gpP85gag.

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