3. in which UV-dependent degradation of DDB2 is important for the release of DDB1 from continuous association to unrepaired DNA and makes DDB1 available for its other DNA damage response functions. Nucleotide excision repair (NER) removes a wide spectrum of bulky DNA lesions induced by UV irradiation and chemical mutagens (21,29). Inherited mutations in NER genes result in the UV-sensitive and cancer-prone syndrome xeroderma pigmentosum (11). NER is divided into two subpathways: transcription-coupled NER (TCR) and global genome NER (GG-NER). TCR repairs DNA lesions in the transcribed strand of active genes (16), whereas GG-NER removes damage throughout the genome (24). The two subpathways differ only in the first step, damage recognition. In TCR, blocking of RNA polymerase II transcription elongation at the DNA lesion serves as a damage recognition signal and stimulates accumulation of downstream repair factors (17,60). In GG-NER, the principal damage recognition factor is the XPC-HR23B-CEN2 complex (3,58,64), which is essential for GG-NER (1,6). Several studies demonstrated that the DDB1-DDB2 heterodimer (UV-DDB protein) is an auxiliary damage-recognizing factor of GG-NER stimulating the binding of XPC to UV damage sites. Unlike other NER factors (TFIIH, XPG, XPA-RPA, and ERCC1-XPF), which require functional XPC to bind DNA damage sites (64), DDB2 binds to DNA lesions in XPC-deficient cells (65). Recruitment of XPC to UV lesions in human cells was significantly decreased in the absence of functional DDB2 (43,68), whereas overexpression of DDB2 results in enhanced recruitment of XPC to cyclobutane pyrimidine dimers (CPD) (15). The UV-DDB protein has affinity for Mouse monoclonal to PTH1R the two major cytotoxic/mutagenic types of lesions introduced in DNA by UV irradiation [pyrimidine (6-4) pyrimidone photoproducts (6-4PP) and CPD] as well as for other bulky DNA lesions (18,70). DDB2 mutations were found in the mildest form of xeroderma pigmentosum syndrome, complementation group E (XP-E) (10,46). Cells of XP-E patients are deficient in GG-NER of CPD (33) and show delayed repair of 6-4PP (35,43). Knockdown of DDB1 in human fibroblasts by small interfering RNA (siRNA) also resulted in deficiency in GG-NER of CPD (39). Overexpression Cytarabine hydrochloride of DDB2 in murine cells, which are usually expressing a very small amount of DDB2 (2,62) and are deficient in GG-NER of CPD (9,55), increases the rates of repair of both CPD and 6-4PP (2). Together, these studies show the significance of UV-DDB in GG-NER. DDB1 and DDB2 are subunits of a larger protein complex, which also contains cullin 4A (CUL4) and Roc1 and possesses ubiquitin ligase (E3) activity (23). The DDB1-DDB2-CUL4A-Roc1 E3 ligase complex is inactive in nonirradiated cells but becomes active in response to UV irradiation (23) and polyubiquitylates XPC (59). This ubiquitylation positively regulates the Cytarabine hydrochloride NER function of XPC and links the ubiquitin ligase and NER functions of UV-DDB (59,67). Other targets for ubiquitylation by DDB2-contaning ubiquitin ligase are histones H2A, H3, and H4 (36,66). These findings suggest a possible role for this ubiquitin ligase in post-UV irradiation chromatin remodeling in order to make it accessible for the downstream factors. DDB2 itself is also a substrate for ubiquitylation, which results in its proteosomal degradation by the 26S proteasome (8,45,53). This degradation of Cytarabine hydrochloride DDB2 is independent of downstream NER factors (53). The biological significance of this paradoxical UV-dependent degradation of a factor functionally involved in the repair of UV-induced lesions is still not understood. It was suggested that this degradation.