Over the last years it has

Over the last 5 years, it has become clear that proper regulation of apoptosis is dependent on appropriate ubiquitylation of various cellular components (Vucic, Dixit, & Wertz, 2011). Here, we have focused on IAP proteins that contain a RING domain, but other E3 ligases such as MDM2, which is the primary E3 that regulates the abundance of the tumor suppressor protein p53, also have critical roles in regulating apoptosis. The RING domains from MDM2 and IAPs form similar dimers (Kostic et al., 2006, Linke et al., 2008, Mace et al., 2008), and it is anticipated that their mechanism of action will be comparable. However, other structurally distinct E3s, such as linear ubiquitin chain assembling complex (Stieglitz et al., 2013) also play important roles in regulating apoptosis and a detailed understanding of their interactions with the E2~Ub conjugate will be of considerable interest. Recent studies have also uncovered additional roles for E2 9-amino Camptothecin and E2~Ub conjugates in modulating the activity of deubiquitinating enzymes (DUBs), such as OTUB1 (Juang et al., 2012, Wiener et al., 2013, Wiener et al., 2012). Interestingly, OTUB1 has also been shown to associate and modulate the stability of cIAP1 (Goncharov et al., 2013). Together, these studies highlight the central role of E2~Ub conjugates in regulating cell death and it will be of considerable interest to understand the factors that influence the balance between E3-E2~Ub and DUB-E2~Ub interactions. We anticipate that biochemically defined assays that require preparations of pure reagents, such as those described here, will be essential for this understanding.
Acknowledgments
Introduction
CRL4 E3 ligases in tumorigenesis
Small molecule-based PROTACs
PROTAC: Major biological impacts
Acknowledgements The authors sincerely apologize to all those colleagues whose important work was not cited in this paper owing to space limitations. They thank the members of Wei laboratory for critical reading and discussion of the manuscript. W.W. is a Leukemia & Lymphoma Society (LLS) research scholar. This work was supported in part by Scientific Research Training Program for Young Talents (Union Hospital, Tongji Medical College, Huazhong University of Science and Technology) to J.C., by National Natural Science Foundation of China (81572413) to K.T., by the V Foundation for Cancer Research to P.Z., and by US National Institutes of Health (NIH) grants to P. Z. (CA159925 and CA213992) and W.W. (GM094777 and CA177910).
Introduction Progression through the cell cycle has captivated cell biologists for more than a century. The discrete steps involving biosynthesis of cellular macromolecules, chromosome and organelle duplication, and subsequent mitosis rely on biochemical reactions occurring in proper sequence. In the mid-1990s, numerous discoveries converged on a new paradigm that these events are ordered in part by the timely Ub-mediated proteolysis of cell cycle proteins 1, 2. Indeed, it is now widely appreciated that cell cycle transitions are temporally controlled when crucial regulatory enzymes are activated through Ub-mediated proteolysis of their inhibitors. As examples, anaphase is initiated when the cohesin complex that binds sister chromosomes is cleaved by separase upon Ub-mediated degradation of the inhibitor securin, and the G1–S transition is regulated by activation of cyclin-dependent kinases (CDKs) upon degradation of inhibitors p21 and p27. Another role of Ub-mediated proteolysis is the termination of proteins, including cyclins, when their tasks in the cell cycle are completed. This is crucial for preventing errant recurrence of processes such as DNA replication or cytokinesis.