Mouse mammary tumor virus and murine leukemia virus cell surface antigens on virus producer and nonproducer mammary epithelial tumor cells.

Mouse mammary tumor virus (MMTV)- and murine leukemia virus (MuLV)- specific cell surface antigens (CSA) on virus producer and nonproducer mammary epithelial tumor cells were studied using the techniques of lactoperoxidase catalyzed iodination of cell surface proteins followed by radioimmune precipitation with monospecific antisera to the major MMTV proteins gp52, gp36, p27, and p10 and to the major MuLV proteins gp70 and p30. The incorporation of iodinated CSA into extracellular virus was determined by analyzing labeled proteins in purified virus. On cells producing only MMTV both gp52 and gp70 were present on the cell surface. Furthermore, gp52 was the only labeled protein in extracellular MMTV produced by these cells. On cells producing both MMTV and MuLV, both gp52 and gp70 were present on the cell surface, and were the only labeled proteins present in their respective extracellular viruses indicating that gp70 and gp52 are present on mutually exclusive cellular viral budding sites. In addition, MuLV anti-p30 serum precipitated two iodinated proteins with molecular weights of 85,000 and 95,000 daltons, analogous to the Gross cell surface antigen (GCSA). Labeled gp52 and gp70 represent true CSA as demonstrated by the fact that they were also present on the surface of cells producing no virus, but producing large amounts of MMTV glycoproteins and nonglyco-proteins. These results further demonstrate that the precursor to the MMTV glycoproteins (gPr75-MMTV env) is cleaved prior to the appearance of gp52 on the cell surface.