Smad ubiquitylation regulatory factor 1/2 (Smurf1/2) promotes p53 degradation by stabilizing the E3 ligase MDM2.
ABSTRACT The tumor suppressor p53 protein is tightly regulated by a ubiquitin-proteasomal degradation mechanism. Several E3 ubiquitin ligases, including MDM2 (mouse double minute 2), have been reported to play an essential role in the regulation of p53 stability. However, it remains unclear how the activity of these E3 ligases is regulated. Here, we show that the HECT-type E3 ligase Smurf1/2 (Smad ubiquitylation regulatory factor 1/2) promotes p53 degradation by enhancing the activity of the E3 ligase MDM2. We provide evidence that the role of Smurf1/2 on the p53 stability is not dependent on the E3 activity of Smurf1/2 but rather is dependent on the activity of MDM2. We find that Smurf1/2 stabilizes MDM2 by enhancing the heterodimerization of MDM2 with MDMX, during which Smurf1/2 interacts with MDM2 and MDMX. We finally provide evidence that Smurf1/2 regulates apoptosis through p53. To our knowledge, this is the first report to demonstrate that Smurf1/2 functions as a factor to stabilize MDM2 protein rather than as a direct E3 ligase in regulation of p53 degradation.
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ABSTRACT: E3 ubiquitin ligases are final effectors of the enzyme cascade controlling ubiquitylation. A central issue in understanding their regulation is to decipher mechanisms of their assembly and activity. In contrast with RING-type E3s, fewer mechanisms are known for regulation of HECT-type E3s. Smad ubiquitylation regulatory factor 1 (Smurf1), a C2-WW-HECT-domain E3, is crucial for bone homeostasis, in which it suppresses osteoblast activity. However, whether and how its activity is regulated remains unclear. Here we show that Smurf1, but not Smurf2, interacts with casein kinase-2 interacting protein-1 (CKIP-1), resulting in an increase in its E3 ligase activity. Surprisingly, CKIP-1 targets specifically the linker region between the WW domains of Smurf1, thereby augmenting its affinity for and promoting ubiquitylation of the substrate. Moreover, CKIP-1-deficient mice undergo an age-dependent increase in bone mass as a result of accelerated osteogenesis and decreased Smurf1 activity. These findings provide evidence that the WW domains linker is important in complex assembly and in regulating activity of HECT-type E3s and that CKIP-1 functions as the first auxiliary factor to enhance the activation of Smurf1.Nature Cell Biology 08/2008; 10(8):994-1002. · 20.76 Impact Factor
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ABSTRACT: Only a few p53 regulators have been shown to participate in the selective control of p53-mediated cell cycle arrest or apoptosis. How p53-mediated apoptosis is negatively regulated remains largely unclear. Here we report that Apak (ATM and p53-associated KZNF protein), a Krüppel-associated box (KRAB)-type zinc-finger protein, binds directly to p53 in unstressed cells, specifically downregulates pro-apoptotic genes, and suppresses p53-mediated apoptosis by recruiting KRAB-box-associated protein (KAP)-1 and histone deacetylase 1 (HDAC1) to attenuate the acetylation of p53. Apak inhibits p53 activity by interacting with ATM, a previously identified p53 activator. In response to stress, Apak is phosphorylated by ATM and dissociates from p53, resulting in activation of p53 and induction of apoptosis. These findings revealed Apak to be a negative regulator of p53-mediated apoptosis and showed the dual role of ATM in p53 regulation.Nature Cell Biology 05/2009; 11(5):580-91. · 20.76 Impact Factor
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ABSTRACT: The INK4a-ARF locus encodes two unrelated proteins that both function in tumor suppression. p16INK4 binds to and inhibits the activity of CDK4 and CDK6, and ARF arrests the cell cycle in a p53-dependent manner. We show here that ARF binds to MDM2 and promotes the rapid degradation of MDM2. This interaction is mediated by the exon 1beta-encoded N-terminal domain of ARF and a C-terminal region of MDM2. ARF-promoted MDM2 degradation is associated with MDM2 modification and concurrent p53 stabilization and accumulation. The functional consequence of ARF-regulated p53 levels via MDM2 proteolysis is evidenced by the ability of ectopically expressed ARF to restore a p53-imposed G1 cell cycle arrest that is otherwise abrogated by MDM2. Thus, deletion of the ARF-INK4a locus simultaneously impairs both the INK4a-cyclin D/CDK4-RB and the ARF-MDM2-p53 pathways.Cell 03/1998; 92(6):725-34. · 31.96 Impact Factor