PRWz and PRWWz transfectants expressed readily detectable Gag, but produced barely detectable virus-associated Gag, suggesting a severe defect in virus assembly or release ( Fig. 2A , lanes 3 and 4). In contrast, cells transfected with PRKz or PRKKz released readily detectable (though incompletely processed) Gag (lanes 5 and 6), similar to the Gag/PR scenario. VLP levels produced by PRWKz and PRKWz (lanes 7 and 8) were inbetween those produced by PRWz and PRKz. Since enhanced or premature Gag cleavage by PR can lead to significantly reduced virus release, and since the wt LZ fusion-containing constructs exhibited higher ratios of cellular p24 gag to Pr55 gag compared to those found in Gag/PR cell lysates ( Fig. 2A , lanes 3-4 vs. lane 2), we suggest that the LZ-associated virus production defect was largely due to enhanced Pr55 gag cleavage efficiency.