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The xeroderma pigmentosum group E gene product DDB2, a protein involved in nucleotide excision repair (NER), associates with the E3 ubiquitin ligase complex Cul4A-DDB1. But the precise role of these interactions in the NER activity of DDB2 is unclear. Several models, including DDB2-mediated ubiquitination of histones in UV-irradiated cells, have been proposed. But those models lack clear genetic evidence. Here we show that DDB2 participates in NER by regulating the cellular levels of p21Waf1/Cip1. We show that DDB2 enhances nuclear accumulation of DDB1, which binds to a modified form of p53 containing phosphorylation at Ser18 (p53S18P) and targets it for degradation in low-dose-UV-irradiated cells. DDB2-/- mouse embryonic fibroblasts (MEFs), unlike wild-type MEFs, are deficient in the proteolysis of p53S18P. Accumulation of p53S18P in DDB2-/- MEFs causes higher expression p21Waf1/Cip1. We show that the increased expression of p21Waf1/Cip1 is the cause NER deficiency in DDB2-/- cells because deletion or knockdown of p21Waf1/Cip1 reverses their NER-deficient phenotype. p21Waf1/Cip1 was shown to bind PCNA, which is required for both DNA replication and NER. Moreover, an increased level of p21Waf1/Cip1 was shown to inhibit NER both in vitro and in vivo. Our results provide genetic evidence linking the regulation of p21Waf1/Cip1 to the NER activity of DDB2. [ABSTRACT FROM AUTHOR]

DDB1, a subunit of the damaged-DNA binding protein DDB, has been shown to function also as an adaptor for Cul4A, a member of the cullin family of E3 ubiquitin ligase. The Cul4A-DDB1 complex remains associated with the COP9 signalosome, and that interaction is conserved from fission yeast to human. Studies with fission yeast suggested a role of the Pcu4-Ddb1-signalosome complex in the proteolysis of the replication inhibitor Spd1. Here we provide evidence that the function of replication inhibitor proteolysis is conserved in the mammalian DDB1-Cul4A-signalosome complex. We show that small interfering RNA-mediated knockdown of DDB1, CSN1 (a subunit of the signalosome), and Cul4A in mammalian cells causes an accumulation of p27Kip1. Moreover, expression of DDB1 reduces the level of p27Kip1 by increasing its decay rate. The DDB1-induced proteolysis of p27Kip1 requires signalosome and Cul4A, because DDB1 failed to increase the decay rate of p27Kip1 in cells deficient in CSN1 or Cul4A. Surprisingly, the DDB1-induced proteolysis of p27Kip1 also involves Skp2, an F-box protein that allows targeting of p27Kip1 for ubiquitination by the Skp1-Cul1-F-box complex. Moreover, we provide evidence for a physical association between Cul4A, DDB1, and Skp2. We speculate that the F-box protein Skp2, in addition to utilizing Cul1-Skp1, utilizes Cul4A-DDB1 to induce proteolysis of p27Kip1 [ABSTRACT FROM AUTHOR]

The xeroderma pigmentosum group E gene product DDB2, a protein involved in nucleotide excision repair (NER), associates with the E3 ubiquitin ligase complex Cul4A-DDB1. But the precise role of these interactions in the NER activity of DDB2 is unclear. Several models, including DDB2-mediated ubiquitination of histones in UV-irradiated cells, have been proposed. But those models lack clear genetic evidence. Here we show that DDB2 participates in NER by regulating the cellular levels of p21Waf1/Cip1. We show that DDB2 enhances nuclear accumulation of DDB1, which binds to a modified form of p53 containing phosphorylation at Ser18 (p53S18P) and targets it for degradation in low-dose-UV-irradiated cells. DDB2-/- mouse embryonic fibroblasts (MEFs), unlike wild-type MEFs, are deficient in the proteolysis of p53S18P. Accumulation of p53S18P in DDB2-/- MEFs causes higher expression p21Waf1/Cip1. We show that the increased expression of p21Waf1/Cip1 is the cause NER deficiency in DDB2-/- cells because deletion or knockdown of p21Waf1/Cip1 reverses their NER-deficient phenotype. p21Waf1/Cip1 was shown to bind PCNA, which is required for both DNA replication and NER. Moreover, an increased level of p21Waf1/Cip1 was shown to inhibit NER both in vitro and in vivo. Our results provide genetic evidence linking the regulation of p21Waf1/Cip1 to the NER activity of DDB2. [ABSTRACT FROM AUTHOR]