Purification and characterization of a murine tumor cell surface glycoprotein of 75,000 daltons that is related to the major envelope glycoprotein of murine leukemia virus

Abstract

An antigen which competes with murine C-type viral glycoprotein in an interspecies radioimmunocompetition assay has been purified from the surface of EL-4 tumor cells. The EL-4 tumor cell is virus particle negative. The amount of p30 was extremely low and there was no detectable p12 or p15, confirming that the tumor cell was not expressing a murine oncornavirus. In homologous competition radioimmunoassays, the tumor cell was negative for Rauscher and AKR gp71, but in an interspecies assay employing 125I-Rauscher gp71 and anti-feline leukemia virus serum, there was a reactive antigen. The gp71-like antigen was on the surface of the tumor cell since, by immunofluorescence and immunoelectronmicroscopy, the EL-4 cell could be labeled with antiserum against Rauscher virus gp71. The gp71 cross-reactive antigen was purified by lithium diiodosalycilate extraction, DEAF; chromatography, and lentil lectin affinity chromatography. It is a glycoprotein of about 75,000 daltons which could be precipitated by various broadly reactive antisera to murine gp71s and antisera to murine virus. Based on radioimmunocompetition assays, the purified gp75 was not related to AKR or Rauscher gp71, but did compete in an assay using BALB/2 xenotropic gp71 and anti-C57/L virus serum. It also competed in an assay using 125I-Moloney virus gp71 and anti-Moloney virus serum, but not in a more type-specific assay assay 125I-Moloney virus gp71 and anti-Moloney gp71 serum. Tryptic peptide maps of iodinated proteins were prepared and the cell surface antigen was compared with AKR gp71, Moloney gp71, Rauscher gp71, and xenotropic BALB/2 and NZB gp71s. The EL-4 gp75 was not identical with or very similar structurally to any of the viral gp71s. The differences in the structures of the various viral gp71s shown here and also in other laboratories are consistent with the idea that the env genes of murine viruses which code for gp71s are sites of frequent recombinational events. Recombination between different endogenous viral sequences or between viral and host allelic genomes could have resulted in many immunologically related proteins on viruses, cell surfaces and, in sera of mice, which are immunologically related, about 70,000-dalton molecular weight, but have divergent structure.