The Xeroderma Pigmentosum Group E Gene Product DDB2 Activates Nucleotide Excision Repair by Regulating the Level of p21Waf1/Cip1

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.