A threshold level of gag expression is required for particle formation in rat cells transformed by avian retroviruses

To test the hypothesis that the block of polyprotein precursor processing and particle formation in RSV-transformed mammalian cells is due to a low level of pr76gag expression, rat tumor cell lines with different amounts of precursor molecules were used. The wild-type forms of pr76gag have been expressed at a high level by use of SV40-based vector and thirty-two stable transfected cell clones were isolated. The gag protein expression was detected in the cell lysate by immunoblotting. Untransfected cells released no proteins that could be detected by immunoprecipitation with anti-RSV serum. Membrane-enclosed gag precursor-polyprotein molecules and infectious virus particles from different stably transfected clones have been found in the medium. Both immature and mature virions of type C morphology were directly detected by transmission electron microscopy. Surprisingly, virus-like particles of morphology similar to mature type C retroviruses were found enclosed within intracellular membranes in a stably transfected nonproducing clone.