Xeroderma pigmentosum complementation group E protein (XPE/DDB2): purification of various complexes of XPE and analyses of their damaged DNA binding and putative DNA repair properties

Xeroderma pigmentosum is characterized by increased sensitivity of the affected individuals to sunlight and light-induced skin cancers and, in some cases, to neurological abnormalities. The disease is caused by a mutation in genes XPA through XPG and the XP variant (XPV) gene. The proteins encoded by the XPA, -B, -C, -D, -F, and -G genes are required for nucleotide excision repair, and the XPV gene encodes DNA polymerase eta, which carries out translesion DNA synthesis. In contrast, the mechanism by which the XPE gene product prevents sunlight-induced cancers is not known. The gene (XPE/DDB2) encodes the small subunit of a heterodimeric DNA binding protein with high affinity to UV-damaged DNA (UV-damaged DNA binding protein UV-DDB]). The DDB2 protein exists in at least four forms in the cell: monomeric DDB2, DDB1-DDB2 heterodimer (UV-DDB), and as a protein associated with both the Cullin 4A (CUL4A) complex and the COP9 signalosome. To better define the role of DDB2 in the cellular response to DNA damage, we purified all four forms of DDB2 and analyzed their DNA binding properties and their effects on mammalian nucleotide excision repair. We find that DDB2 has an intrinsic damaged DNA binding activity and that under our assay conditions neither DDB2 nor complexes that contain DDB2 (UV-DDB, CUL4A, and COP9) participate in nucleotide excision repair carried out by the six-factor human excision nuclease.