Ultrastructural and biochemical evidence that the L929 cell retrovirus lacks the env gene translation product

LCV, a murine retrovirus released by L929 mouse cell fibroblasts, is non-infectious when inoculated into SC-1, mink, D-17 or Vero cells. Ultrastructural examination by thin sectioning, freeze-etching or negative staining revealed the absence, on the viral envelope, of the radially disposed spikes. Polyacrylamide gel electrophoresis of radiolabelled viral components showed the absence of the glycosylated protein gp70 as well as of the p15E cleavage product of the polyprotein precursor gPr90env. The premature loss of the gp70 molecule from LCV to the culture medium was ruled out since no peak of D-[14C]glucosamine-labelled glycoprotein was detected by affinity chromatography or immunoprecipitation of concentrated medium. The ultrastructural and biochemical results all supported the hypothesis that the absence of infectivity was due to the lack of gp70 glycoprotein in the envelope of LCV. A possible block at a translational or post-translational level was also investigated by immunofluorescence studies with antisera directed against ecotropic or xenotropic gp70; Moloney murine leukaemia virus-infected or NZB cells were used as positive controls for eco- or xenotropic viruses respectively. The absence of fluorescent stain in L929 cells further supported these results and suggested that LCV and the L929 parental cell line lack the uncleaved precursor and the final product of the env gene translation process.