Molecular Cell
Previews Re-Place Your BETs: The Dynamics of Super Enhancers Paulo P. Amaral1 and Andrew J. Bannister1,* 1Gurdon Institute and Department of Pathology, University of Cambridge, Tennis Court Road, Cambridge CB2 1QN, UK *Correspondence: [email protected] http://dx.doi.org/10.1016/j.molcel.2014.10.008
Super enhancers (SEs) have been linked to cell fate. Brown et al. (2014) now show the dynamic nature of super enhancers when endothelial cells are activated by TNF-a. The formation of de novo SEs and concomitant decommissioning of preexisting enhancers drive rapid inflammatory transcriptional responses and involve NF-kB-dependent redeployment of BRD4. Complex organisms comprise a multitude of specialized cells that work together to specify tissue functions and maintain homeostasis. Recently, so-called super enhancers (SEs)—extended regions containing clustered ‘‘typical’’ enhancer (TE) elements that amass a large proportion of key transcription factors (TFs) and enhancer chromatin marks—have been shown to be distinctive regulatory features of genes that specify cell type and function. SEs are also involved in regulation of genes strongly linked to cancer and other diseases, and thus they represent new potential therapeutic targets (Dawson and Kouzarides, 2012). Consequently, the functional mechanisms of SEs are currently under intense scrutiny. In this issue of Molecular Cell, Plutzky and colleagues extend our understanding of SE function by showing that they can act as fast switches enabling rapid cellstate transitions (Brown et al., 2014). They explored the tightly choreographed process of inflammatory activation of endothelial cells (ECs) by tumor necrosis factor alpha (TNF-a), which requires transcription factor NF-kB. Upon stimulation, NF-kB translocates to the nucleus, where it acts as a master regulator of inflammatory genes. Activated NF-kB interacts with and recruits coactivators, including the BET bromodomain factor BRD4, a reader of active gene-associated histone acetylation marks. In nonstimulated ECs, Brown et al. (2014) observed highly concentrated BRD4 at SEs that are heavily enriched for histone acetylation, particularly H3K27ac, and that regulate genes that define EC basal functions. This is completely consistent with other reports that indicate that
BRD4-enriched SEs help to establish and maintain particular cell states and lineages (Whyte et al., 2013; Shi and Vakoc, 2014). However, upon proinflammatory stimulation, there is significant recruitment of NF-kB to SEs proximal to inflammatory genes, leading to massive recruitment of BRD4, increased levels of H3K27ac, and immediate transcriptional activation of the genes. Remarkably, this involves rapid and large-scale redistribution of BRD4 from decommissioned basal SEs to the de novo licensed SEs in a strictly NF-kBdependent manner (Figure 1). The ensuing proinflammatory gene transcription occurs following recruitment of RNA Polymerase II (Pol II) and, importantly, increased transcriptional elongation rates of target genes. Similar to previous studies (Shi and Vakoc, 2014; Love´n et al., 2013), this is impaired upon treatment of the cells with the BET inhibitor compound JQ1, which disrupts BRD4 binding to acetylated histones preferentially at SEs. This clearly demonstrates that BRD4’s activating role is much more pronounced at SEs compared to TEs. Indeed, a chemical genomics approach identified chromatin regions directly affected by JQ1 activity as those that preferentially lose BRD4 binding in activated endothelium, leading to impaired gene activation of inflammatory genes. Intriguingly, only a modest increase in H3K27ac was detected in the newly activated SEs while histone acetylation persisted in the abandoned SEs. This indicates that H3K27ac is not sufficient for recruitment of BRD4 and enhancer activation. In fact, while the results obtained by Brown et al. (2014) significantly contribute to our understanding of the
dynamics of SE activation and its interplay with chromatin regulation, their data also raise many new questions (Figure 1). In particular, a mechanistic understanding of the sequence of events that lead to SE decommissioning, formation, and function is still lacking. For example, the observation that NF-kB drives BRD4 redistribution to SE regions that are enriched in the TF’s binding motifs is in line with previous studies, which showed that tissue-specific master TFs are involved in the regulation of SEs (e.g., Hnisz et al., 2013). However, upon inflammatory stimulus, NF-kB also occupies most active TEs but specifically targets the bulk of BRD4 to de novo SEs at the expense of basal ones. Is this simply driven by the sheer density of NF-kB binding sites in the inducible SEs, or are there additional ‘‘marks’’ at SEs for preferential recruitment of BRD4? Conversely, what promotes the rapid depletion of BRD4 from decommissioned basal SEs? Several potential factors could be at play. For instance, inflammatory SEs already harbor H3K27ac before TNF-a stimulation, indicating an earlier priming for activation in basal cells. Therefore, additional cell lineage-specific factors may landmark SEs to enable rapid transitions, in this case favoring recruitment of NF-kB and BRD4. These may include pioneer factors and chromatin regulatory proteins that increase DNA accessibility of SEs. In addition, pre-established higher-order topological structures may favor occupation of the marked SEs and also likely underlie activation of the associated proximal genes (e.g., by formation of looping structures and confinement in nuclear domains). In fact, nuclear
Molecular Cell 56, October 23, 2014 ª2014 Elsevier Inc. 187
Molecular Cell
Previews
Figure 1. Interplay between Different Mechanisms Underlying Expression Specification In addition to super enhancers, chromatin domains bearing broad specific histone modifications (e.g., H3K4me3 around gene TSSs and H3Y41ph throughout gene bodies; Benayoun et al., 2014; Dawson et al., 2012), various noncoding RNAs (ncRNAs) and chromatin topology have all been linked to controlling specific gene sets containing cell specification genes. TSS, transcription start site; TTS, transcription termination site; miRNA, microRNA; lncRNA, long noncoding RNA.
architecture and genome topological organization may help to explain the fast and ordered global shift of BRD4 from decommissioned to de novo active SEs (Figure 1). Such constrained regulatory topologies could be a more general feature of lineage commitment (and, similarly, reprogramming resistance), but this remains to be established. Another mechanistic aspect not explored in the current work is the potential role of Pol II recruited to enhancers. An additional intrinsic feature of SEs is the regulated transcription of enhancer RNAs (eRNAs), which have recently been strongly linked with SE
activity (e.g., IIott et al., 2014). Brown et al. (2014) detected a significant increase in Pol II occupancy in activated SEs, along with its depletion from basal ones, suggesting Pol II transcription and/ or the eRNAs are also relevant in this particular system (Figure 1). Indeed, the fast dynamic system used in the current work offers an opportunity to elucidate this and other factors involved in the kinetics and cascade of events that lead to SE activation. Thus, the insights provided by this work and the experimental opportunities it presents should help elucidate the general features of dynamic differentiation systems and cell-
188 Molecular Cell 56, October 23, 2014 ª2014 Elsevier Inc.
fate specification, with particular relevance to developmental processes and pathological conditions. Supporting this proposition, Brown et al. (2014) demonstrated phenotypic roles for BRD4-dependent activation of the TNF-a endothelial inflammatory response in relevant disease models, both in vitro and in vivo. Furthermore, they assessed the therapeutic potential of BRD4 inhibition in systemic vascular inflammation using a mouse model of cholesterol-induced atherosclerosis, which is preceded by atherogenesis mediated by EC activation. Remarkably, treatment of hypercholesterolemic mice
Molecular Cell
Previews with JQ1 attenuated atherogenesis in both early and late atherosclerosis development. In summary, the present work underscores the importance of SEs as preferential targets of BET-mediated gene regulation in fast transitions and pathological conditions, which could be targeted by BET inhibition therapies. This may underlie the intriguing potency and selectivity of BET inhibitors as reported in different cancers and other diseases (Dawson and Kouzarides, 2012; Shi and Vakoc, 2014; Love´n et al., 2013). It also reinforces the central role of SEs among the repertoire of molecular mechanisms underlying cell identity determination, as well as fast cell state transitions, which are crucial in development, homeostasis, stress response, and disease. It will be
important to determine their idiosyncrasies and interplay with other mechanisms regulating key genes, such as widespread chromatin modifications (Benayoun et al., 2014, Dawson et al., 2012) and stimuli-responsive and cell-specific small and long regulatory RNAs (reviewed in Amaral et al., 2013) (Figure 1). REFERENCES Amaral, P.P., Dinger, M.E., and Mattick, J.S. (2013). Brief Funct Genomics 12, 254–278. Benayoun, B.A., Pollina, E.A., Ucar, D., Mahmoudi, S., Karra, K., Wong, E.D., Devarajan, K., Daugherty, A.C., Kundaje, A.B., Mancini, E., et al. (2014). Cell 158, 673–688. Brown, J.D., Lin, C.Y., Duan, Q., Griffin, G., Federation, A., Paranal, R.M., Bair, S., Newton, G., Lichtman, A., Kung, A., et al. (2014). Mol. Cell 56, this issue, 219–231.
Dawson, M.A., and Kouzarides, T. (2012). Cell 150, 12–27. Dawson, M.A., Foster, S.D., Bannister, A.J., Robson, S.C., Hannah, R., Wang, X., Xhemalce, B., Wood, A.D., Green, A.R., Go¨ttgens, B., and Kouzarides, T. (2012). Cell Rep 2, 470–477. Hnisz, D., Abraham, B.J., Lee, T.I., Lau, A., SaintAndre´, V., Sigova, A.A., Hoke, H.A., and Young, R.A. (2013). Cell 155, 934–947. IIott, N.E., Heward, J.A., Roux, B., Tsitsiou, E., Fenwick, P.S., Lenzi, L., Goodhead, I., Hertz-Fowler, C., Heger, A., Hall, N., et al. (2014). Nat. Commun. 5, 3979. Love´n, J., Hoke, H.A., Lin, C.Y., Lau, A., Orlando, D.A., Vakoc, C.R., Bradner, J.E., Lee, T.I., and Young, R.A. (2013). Cell 153, 320–334. Shi, J., and Vakoc, C.R. (2014). Mol. Cell 54, 728–736. Whyte, W.A., Orlando, D.A., Hnisz, D., Abraham, B.J., Lin, C.Y., Kagey, M.H., Rahl, P.B., Lee, T.I., and Young, R.A. (2013). Cell 153, 307–319.
Ubiquitin Chain Elongation: An Intriguing Strategy Kazuhiro Iwai1 and Keiji Tanaka2,* 1Department
of Molecular and Cellular Physiology, Graduate School of Medicine, Kyoto University, Kyoto 606-8501, Japan of Protein Metabolism, Tokyo Metropolitan Institute of Medical Science, Tokyo 156-8506, Japan *Correspondence: [email protected] http://dx.doi.org/10.1016/j.molcel.2014.10.009 2Laboratory
Using multidisciplinary analyses, in this issue Kelly et al. (2014) and Brown et al. (2014) reveal an unexpected role for the RING finger and substrate recognition adaptor proteins of the anaphase promoting complex/cyclosome (APC/C) in ubiquitin chain elongation. Recently, our knowledge about the ubiquitin conjugation system in controlling protein functions has expanded enormously. In general, the ubiquitin conjugation system attaches polyubiquitin chains to substrate proteins in order to regulate their functions. Because the type of ubiquitin chain determines the mode of protein regulation, the mechanisms underlying selective generation of specific ubiquitin chains are of great interest (Komander and Rape, 2012). Polyubiquitin chains are generated by the repeated actions of three classes of enzymes: ubiquitinactivating enzymes (E1), ubiquitin-conjugating enzymes (E2), and ubiquitin ligases (E3). However, the precise mechanism of polyubiquitin generation by the RING
E3s, the largest family of E3s, has not been convincingly determined. RING ligases recognize both E2s and substrates simultaneously via distinct recognition motifs and then conjugate polyubiquitin chains onto the substrates. A topological constraint seems to arise during chain elongation: the longer the chain, the further the spatial distance between the E3 and the reaction site (Deshaies and Joazeiro, 2009). In two papers in this issue, Kelly et al. (2014) and Brown et al. (2014) used multidisciplinary analyses, including structural, biochemical, and cell biological analyses, to dissect the mechanism of polyubiquitin chain generation by APC/C. This complex is a multisubunit RING finger E3 that mediates
ubiquitin-mediated degradation of regulators of chromosomal segregation, cytokinesis, and mitotic exit, as well as the initiation of DNA replication and other processes, in order to promote cell-cycle progression (Chang et al., 2014). APC/C recognizes substrates via the coordinated functions of APC10 and the adaptor protein CDC20 or CDH1 and conjugates K11- or K48-linked ubiquitin chains to substrates destined for proteasomal degradation. In conjunction with the E2 protein Ube2S, APC/C generates K11 chains by specifically conjugating the C terminus of donor ubiquitin onto the K11 residue of acceptor ubiquitin. The two groups found that CDC20 and CDH1, adaptor proteins involved in
Molecular Cell 56, October 23, 2014 ª2014 Elsevier Inc. 189
Previews Re-Place Your BETs: The Dynamics of Super Enhancers Paulo P. Amaral1 and Andrew J. Bannister1,* 1Gurdon Institute and Department of Pathology, University of Cambridge, Tennis Court Road, Cambridge CB2 1QN, UK *Correspondence: [email protected] http://dx.doi.org/10.1016/j.molcel.2014.10.008
Super enhancers (SEs) have been linked to cell fate. Brown et al. (2014) now show the dynamic nature of super enhancers when endothelial cells are activated by TNF-a. The formation of de novo SEs and concomitant decommissioning of preexisting enhancers drive rapid inflammatory transcriptional responses and involve NF-kB-dependent redeployment of BRD4. Complex organisms comprise a multitude of specialized cells that work together to specify tissue functions and maintain homeostasis. Recently, so-called super enhancers (SEs)—extended regions containing clustered ‘‘typical’’ enhancer (TE) elements that amass a large proportion of key transcription factors (TFs) and enhancer chromatin marks—have been shown to be distinctive regulatory features of genes that specify cell type and function. SEs are also involved in regulation of genes strongly linked to cancer and other diseases, and thus they represent new potential therapeutic targets (Dawson and Kouzarides, 2012). Consequently, the functional mechanisms of SEs are currently under intense scrutiny. In this issue of Molecular Cell, Plutzky and colleagues extend our understanding of SE function by showing that they can act as fast switches enabling rapid cellstate transitions (Brown et al., 2014). They explored the tightly choreographed process of inflammatory activation of endothelial cells (ECs) by tumor necrosis factor alpha (TNF-a), which requires transcription factor NF-kB. Upon stimulation, NF-kB translocates to the nucleus, where it acts as a master regulator of inflammatory genes. Activated NF-kB interacts with and recruits coactivators, including the BET bromodomain factor BRD4, a reader of active gene-associated histone acetylation marks. In nonstimulated ECs, Brown et al. (2014) observed highly concentrated BRD4 at SEs that are heavily enriched for histone acetylation, particularly H3K27ac, and that regulate genes that define EC basal functions. This is completely consistent with other reports that indicate that
BRD4-enriched SEs help to establish and maintain particular cell states and lineages (Whyte et al., 2013; Shi and Vakoc, 2014). However, upon proinflammatory stimulation, there is significant recruitment of NF-kB to SEs proximal to inflammatory genes, leading to massive recruitment of BRD4, increased levels of H3K27ac, and immediate transcriptional activation of the genes. Remarkably, this involves rapid and large-scale redistribution of BRD4 from decommissioned basal SEs to the de novo licensed SEs in a strictly NF-kBdependent manner (Figure 1). The ensuing proinflammatory gene transcription occurs following recruitment of RNA Polymerase II (Pol II) and, importantly, increased transcriptional elongation rates of target genes. Similar to previous studies (Shi and Vakoc, 2014; Love´n et al., 2013), this is impaired upon treatment of the cells with the BET inhibitor compound JQ1, which disrupts BRD4 binding to acetylated histones preferentially at SEs. This clearly demonstrates that BRD4’s activating role is much more pronounced at SEs compared to TEs. Indeed, a chemical genomics approach identified chromatin regions directly affected by JQ1 activity as those that preferentially lose BRD4 binding in activated endothelium, leading to impaired gene activation of inflammatory genes. Intriguingly, only a modest increase in H3K27ac was detected in the newly activated SEs while histone acetylation persisted in the abandoned SEs. This indicates that H3K27ac is not sufficient for recruitment of BRD4 and enhancer activation. In fact, while the results obtained by Brown et al. (2014) significantly contribute to our understanding of the
dynamics of SE activation and its interplay with chromatin regulation, their data also raise many new questions (Figure 1). In particular, a mechanistic understanding of the sequence of events that lead to SE decommissioning, formation, and function is still lacking. For example, the observation that NF-kB drives BRD4 redistribution to SE regions that are enriched in the TF’s binding motifs is in line with previous studies, which showed that tissue-specific master TFs are involved in the regulation of SEs (e.g., Hnisz et al., 2013). However, upon inflammatory stimulus, NF-kB also occupies most active TEs but specifically targets the bulk of BRD4 to de novo SEs at the expense of basal ones. Is this simply driven by the sheer density of NF-kB binding sites in the inducible SEs, or are there additional ‘‘marks’’ at SEs for preferential recruitment of BRD4? Conversely, what promotes the rapid depletion of BRD4 from decommissioned basal SEs? Several potential factors could be at play. For instance, inflammatory SEs already harbor H3K27ac before TNF-a stimulation, indicating an earlier priming for activation in basal cells. Therefore, additional cell lineage-specific factors may landmark SEs to enable rapid transitions, in this case favoring recruitment of NF-kB and BRD4. These may include pioneer factors and chromatin regulatory proteins that increase DNA accessibility of SEs. In addition, pre-established higher-order topological structures may favor occupation of the marked SEs and also likely underlie activation of the associated proximal genes (e.g., by formation of looping structures and confinement in nuclear domains). In fact, nuclear
Molecular Cell 56, October 23, 2014 ª2014 Elsevier Inc. 187
Molecular Cell
Previews
Figure 1. Interplay between Different Mechanisms Underlying Expression Specification In addition to super enhancers, chromatin domains bearing broad specific histone modifications (e.g., H3K4me3 around gene TSSs and H3Y41ph throughout gene bodies; Benayoun et al., 2014; Dawson et al., 2012), various noncoding RNAs (ncRNAs) and chromatin topology have all been linked to controlling specific gene sets containing cell specification genes. TSS, transcription start site; TTS, transcription termination site; miRNA, microRNA; lncRNA, long noncoding RNA.
architecture and genome topological organization may help to explain the fast and ordered global shift of BRD4 from decommissioned to de novo active SEs (Figure 1). Such constrained regulatory topologies could be a more general feature of lineage commitment (and, similarly, reprogramming resistance), but this remains to be established. Another mechanistic aspect not explored in the current work is the potential role of Pol II recruited to enhancers. An additional intrinsic feature of SEs is the regulated transcription of enhancer RNAs (eRNAs), which have recently been strongly linked with SE
activity (e.g., IIott et al., 2014). Brown et al. (2014) detected a significant increase in Pol II occupancy in activated SEs, along with its depletion from basal ones, suggesting Pol II transcription and/ or the eRNAs are also relevant in this particular system (Figure 1). Indeed, the fast dynamic system used in the current work offers an opportunity to elucidate this and other factors involved in the kinetics and cascade of events that lead to SE activation. Thus, the insights provided by this work and the experimental opportunities it presents should help elucidate the general features of dynamic differentiation systems and cell-
188 Molecular Cell 56, October 23, 2014 ª2014 Elsevier Inc.
fate specification, with particular relevance to developmental processes and pathological conditions. Supporting this proposition, Brown et al. (2014) demonstrated phenotypic roles for BRD4-dependent activation of the TNF-a endothelial inflammatory response in relevant disease models, both in vitro and in vivo. Furthermore, they assessed the therapeutic potential of BRD4 inhibition in systemic vascular inflammation using a mouse model of cholesterol-induced atherosclerosis, which is preceded by atherogenesis mediated by EC activation. Remarkably, treatment of hypercholesterolemic mice
Molecular Cell
Previews with JQ1 attenuated atherogenesis in both early and late atherosclerosis development. In summary, the present work underscores the importance of SEs as preferential targets of BET-mediated gene regulation in fast transitions and pathological conditions, which could be targeted by BET inhibition therapies. This may underlie the intriguing potency and selectivity of BET inhibitors as reported in different cancers and other diseases (Dawson and Kouzarides, 2012; Shi and Vakoc, 2014; Love´n et al., 2013). It also reinforces the central role of SEs among the repertoire of molecular mechanisms underlying cell identity determination, as well as fast cell state transitions, which are crucial in development, homeostasis, stress response, and disease. It will be
important to determine their idiosyncrasies and interplay with other mechanisms regulating key genes, such as widespread chromatin modifications (Benayoun et al., 2014, Dawson et al., 2012) and stimuli-responsive and cell-specific small and long regulatory RNAs (reviewed in Amaral et al., 2013) (Figure 1). REFERENCES Amaral, P.P., Dinger, M.E., and Mattick, J.S. (2013). Brief Funct Genomics 12, 254–278. Benayoun, B.A., Pollina, E.A., Ucar, D., Mahmoudi, S., Karra, K., Wong, E.D., Devarajan, K., Daugherty, A.C., Kundaje, A.B., Mancini, E., et al. (2014). Cell 158, 673–688. Brown, J.D., Lin, C.Y., Duan, Q., Griffin, G., Federation, A., Paranal, R.M., Bair, S., Newton, G., Lichtman, A., Kung, A., et al. (2014). Mol. Cell 56, this issue, 219–231.
Dawson, M.A., and Kouzarides, T. (2012). Cell 150, 12–27. Dawson, M.A., Foster, S.D., Bannister, A.J., Robson, S.C., Hannah, R., Wang, X., Xhemalce, B., Wood, A.D., Green, A.R., Go¨ttgens, B., and Kouzarides, T. (2012). Cell Rep 2, 470–477. Hnisz, D., Abraham, B.J., Lee, T.I., Lau, A., SaintAndre´, V., Sigova, A.A., Hoke, H.A., and Young, R.A. (2013). Cell 155, 934–947. IIott, N.E., Heward, J.A., Roux, B., Tsitsiou, E., Fenwick, P.S., Lenzi, L., Goodhead, I., Hertz-Fowler, C., Heger, A., Hall, N., et al. (2014). Nat. Commun. 5, 3979. Love´n, J., Hoke, H.A., Lin, C.Y., Lau, A., Orlando, D.A., Vakoc, C.R., Bradner, J.E., Lee, T.I., and Young, R.A. (2013). Cell 153, 320–334. Shi, J., and Vakoc, C.R. (2014). Mol. Cell 54, 728–736. Whyte, W.A., Orlando, D.A., Hnisz, D., Abraham, B.J., Lin, C.Y., Kagey, M.H., Rahl, P.B., Lee, T.I., and Young, R.A. (2013). Cell 153, 307–319.
Ubiquitin Chain Elongation: An Intriguing Strategy Kazuhiro Iwai1 and Keiji Tanaka2,* 1Department
of Molecular and Cellular Physiology, Graduate School of Medicine, Kyoto University, Kyoto 606-8501, Japan of Protein Metabolism, Tokyo Metropolitan Institute of Medical Science, Tokyo 156-8506, Japan *Correspondence: [email protected] http://dx.doi.org/10.1016/j.molcel.2014.10.009 2Laboratory
Using multidisciplinary analyses, in this issue Kelly et al. (2014) and Brown et al. (2014) reveal an unexpected role for the RING finger and substrate recognition adaptor proteins of the anaphase promoting complex/cyclosome (APC/C) in ubiquitin chain elongation. Recently, our knowledge about the ubiquitin conjugation system in controlling protein functions has expanded enormously. In general, the ubiquitin conjugation system attaches polyubiquitin chains to substrate proteins in order to regulate their functions. Because the type of ubiquitin chain determines the mode of protein regulation, the mechanisms underlying selective generation of specific ubiquitin chains are of great interest (Komander and Rape, 2012). Polyubiquitin chains are generated by the repeated actions of three classes of enzymes: ubiquitinactivating enzymes (E1), ubiquitin-conjugating enzymes (E2), and ubiquitin ligases (E3). However, the precise mechanism of polyubiquitin generation by the RING
E3s, the largest family of E3s, has not been convincingly determined. RING ligases recognize both E2s and substrates simultaneously via distinct recognition motifs and then conjugate polyubiquitin chains onto the substrates. A topological constraint seems to arise during chain elongation: the longer the chain, the further the spatial distance between the E3 and the reaction site (Deshaies and Joazeiro, 2009). In two papers in this issue, Kelly et al. (2014) and Brown et al. (2014) used multidisciplinary analyses, including structural, biochemical, and cell biological analyses, to dissect the mechanism of polyubiquitin chain generation by APC/C. This complex is a multisubunit RING finger E3 that mediates
ubiquitin-mediated degradation of regulators of chromosomal segregation, cytokinesis, and mitotic exit, as well as the initiation of DNA replication and other processes, in order to promote cell-cycle progression (Chang et al., 2014). APC/C recognizes substrates via the coordinated functions of APC10 and the adaptor protein CDC20 or CDH1 and conjugates K11- or K48-linked ubiquitin chains to substrates destined for proteasomal degradation. In conjunction with the E2 protein Ube2S, APC/C generates K11 chains by specifically conjugating the C terminus of donor ubiquitin onto the K11 residue of acceptor ubiquitin. The two groups found that CDC20 and CDH1, adaptor proteins involved in
Molecular Cell 56, October 23, 2014 ª2014 Elsevier Inc. 189
