A block in virus-like particle maturation following assembly of murine leukaemia virus in insect cells

Expression of the murine leukaemia virus (MLV) major Gag antigen p65Gag using the baculovirus expression system leads to efficient assembly and release of virus-like particles (VLP) representative of immature MLV. Expression of p180Gag-Pol, facilitated normally in mammalian cells by readthrough of the p65Gag termination codon, also occurs efficiently in insect cells to provide a source of the MLV protease and a pattern of p65Gag processing similar to that observed in mammalian cells. VLP release from p180Gag-Pol-expressing cells however remains essentially immature with disproportionate levels of the uncleaved p65Gag precursor when compared to the intracellular Gag profile. Changing the p65Gag termination codon altered the level of p65Gag and p180Gag-Pol within expressing cells but did not alter the pattern of released VLP, which remained immature. Coexpression of p65Gag with a fixed readthrough p180Gag-Pol also led to only immature VLP release despite high intracellular protease levels. Our data suggest a mechanism that preferentially selects uncleaved p65Gag for the assembly of MLV in this heterologous expression system and implies that, in addition to their relative levels, active sorting of the correct p65Gag and p180Gag-Pol ratios may occur in producer cells.