Functional Study of the Retrotransposon-Derived Human PEG10 Protease

Paternally expressed gene 10 (PEG10) is a human retrotransposon-derived imprinted gene. The mRNA of PEG10 encodes two protein isoforms: the Gag-like protein (RF1PEG10) is coded by reading frame 1, while the Gag-Pol-like polyprotein (RF1/RF2PEG10) is coded by reading frames 1 and 2. The proteins are translated by a typical retroviral frameshift mechanism. The protease (PR) domain of RF2PEG10 contains an -Asp-Ser-Gly- sequence, which corresponds to the consensus -Asp-Ser/Thr-Gly- active-site motif of retroviral aspartic proteases. The function of the aspartic protease domain of RF2PEG10 remains unclear. To elucidate the function of PEG10 protease (PRPEG10), we designed a frameshift mutant (fsRF1/RF2PEG10) for comparison with the RF1/RF2PEG10 form. To study the effects of PRPEG10 on cellular proliferation and viability, mammalian HEK293T and HaCaT cells were transfected with plasmids coding for either RF1/RF2PEG10, the frameshift mutant (fsRF1/RF2PEG10), or a PR active-site (D370A) mutant fsRF1/RF2PEG10. Our results indicate that fsRF1/RF2PEG10 overexpression results in increased cellular proliferation. Remarkably, transfection with fsRF1/RF2PEG10 had a detrimental effect on cell viability. We hypothesize that PRPEG10 plays an important role in the function of this retroviral remnant, mediating the proliferation of cells and possibly implicating it in the inhibition of apoptosis.