A treshold 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 a nd particle formation in RSV-transformed mammalian cells is due to a low le vel of pr76(gag) expression, rat tumor cell lines with different amounts of precursor molecules were used. The wildtype forms of pr76(gag) have been e xpressed at a high level by use of SV40-based vector and thirty-two stable transfected cell clones were isolated. The gag protein expression was detec ted in the cell lysate by immunoblotting. Untransfected cells released no p roteins 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 m edium. Both immature and mature virions of type C morphology were directly detected by transmission electron microscopy, Surprisingly, virus-like part icles of morphology similar to mature type C retroviruses were found enclos ed within intracellular membranes in a stably transfected nonproducing clon e.