A Novel, Non-apoptotic Role for Scythe/BAT3: a Functional Switch between Pro- and Anti-proliferative Roles of p21 during the Cell Cycle

Abstract

<p><p>Scythe/BAT3 is a member of the BAG protein family whose role in apoptosis, a form of programmed cell death, has been extensively studied. However, since the developmental defects observed in <i>Bat3</i>-null mouse embryos cannot be explained solely by defects in apoptosis, we investigated whether BAT3 is also involved in cell-cycle progression.</p></p><p><p>Using a stable-inducible <i>Bat3</i>-knockdown cellular system, we demonstrated that reduced BAT3 protein levels cause a delay in both the G1/S transition and G2/M progression. Concurrent with these changes in cell-cycle progression, we observed a reduction in the turnover and phosphorylation of the CDK inhibitor p21, which is best known as an inhibitor of DNA replication; however, phosphorylated p21 has also been shown to promote G2/M progression. Additionally, we observed that the p21 turnover rate was also reduced in <i>Bat3</i>-knockdown cells released from G2/M synchronization. Our findings indicate that in <i>Bat3</i>-knockdown cells, p21 continues to be synthesized during cell-cycle phases that do not normally require p21, resulting in p21 protein accumulation and subsequent cell-cycle delay. Finally, we showed that BAT3 co-localizes with p21 during the cell cycle and is required for the translocation of p21 from the cytoplasm to the nucleus during the G1/S transition and G2/M progression. </p></p><p><p>Our study reveals a novel, non-apoptotic role for BAT3 in cell-cycle regulation. By maintaining low p21 protein levels during G1/S transition, BAT3 counteracts the inhibitory effect of p21 on DNA replication and thus enables the cells to progress from G1 to S phase. Conversely, during G2/M progression, BAT3 facilitates p21 phosphorylation, an event required for G2/M progression. BAT3 modulates these pro- and anti-proliferative roles of p21 at least in part by regulating the translocation of p21 between the cytoplasm and nucleus of the cells to ensure proper functioning and regulation of p21 in the appropriate intracellular compartments during different cell-cycle phases. </p></p>