Figure 1. Stabilized dE2F1 can induce apoptosis through transcriptional and non-transcriptional mechanisms. (1) During S-phase, a dE2F1 protein without a PIP-degron but with an intact DNA–binding domain will promote apoptosis through transcriptional activation of pro- apoptotic genes, such as hid. (2) A dE2F1 protein without a PIP-degron and lacking an intact DNA-binding domain can promote apoptosis through an unknown mechanism that requires a physical interaction with the RBF1 protein; this unknown mechanism is also dependent upon hid. doi:10.1371/journal.pgen.1002909.g001 question will be whether a similar mech- anism is operative in mammalian cells. Although the PIP-degron is not conserved in E2F proteins in other species, other interactions have been described that lead to the turnover of mammalian E2Fs after References 1. Biswas AK, Johnson DG (2012) Transcriptional and nontranscriptional functions of E2F1 in response to DNA damage. Cancer Res 72(1): 13–17. 2. Stevens C, La Thangue NB (2004) The emerging role of E2F-1 in the DNA damage response and checkpoint control. DNA Repair (Amst) 3(8–9): 1071–1079. 3. Asano M, Nevins JR, Wharton RP (1996) Ectopic E2F expression induces S phase and apoptosis in Drosophila imaginal discs. Genes Dev 10(11): 1422–32. 4. Shibutani S, Swanhart LM, Duronio RJ (2007) Rbf1-independent termination of E2f1-target the G1/S transition [9,10]. It will be extremely exciting if a conserved mecha- nism of E2F action exists that occurs independently of E2F’s ability to bind to DNA and that enables deregulated E2F to trigger cell death. gene expression during early Drosophila embryo- genesis. Development 134: 467–478. 5. Shibutani ST, de la Cruz AF, Tran V, Turbyfill WJ, 3rd, Reis T, et al. (2008) Intrinsic negative cell cycle regulation provided by PIP box- and Cul4Cdt2-mediated destruction of E2f1 during S phase. Dev Cell 15(6): 890–900. 6. Davidson JM, Duronio RJ (2012) S phase- coupled E2f1 destruction ensures homeostasis in proliferating tissues. PLoS Genet 8(8): e1002831. doi:10.1371/journal.pgen.1002831. 7. Tanaka-Matakatsu M, Xu J, Cheng L, Du W (2009) Regulation of apoptosis of rbf mutant cells during Drosophila development. Dev Biol 326(2): 347–356. Acknowledgments We thank our colleagues at the MGH Cancer Center for helpful discussions. 8. Moon NS, Di Stefano L, Morris EJ, Patel R, White K, et al. (2008) E2F and p53 induce apoptosis independently during Drosophila de- velopment but intersect in the context of DNA d a m a g e . P L o S G e n e t 4 ( 8 ) : e 1 0 0 0 1 5 3 . doi:10.1371/journal.pgen.1000153. 9. Krek W, Xu G, Livingston DM (1995) Cyclin A- kinase regulation of E2F1 DNA binding function underlies suppression of an S-phase checkpoint. Cell 83(7): 1149–1158. 10. Campanero MR, Flemington EK (1997) Regulation of E2F through ubiquitin-proteasome-dependent degra- dation: stabilization by the pRB tumor suppressor protein. Proc Natl Acad Sci U S A 94(6): 2221–2226. PLOS Genetics | www.plosgenetics.org 2 August 2012 | Volume 8 | Issue 8 | e1002909