As indicated by Rapić-Otrin et al. in 2003, the DDB2 gene is located on human chromosome 11p11.2, spans a region of approximately 24 – 26 kb and includes 10 exons. The DDB2 protein contains five putative WD40 repeats positioned downstream from the second exon. The WD40 motif identified in DDB2 is characteristic of proteins involved in the recognition of chromatin proteins. The C-terminal region of DDB2 is essential for binding to DDB1. Together, the two proteins form a UV-damaged DNA binding protein complex.
If humans have a mutation in each copy of their DDB2 gene, this causes a mild form of the human diseasexeroderma pigmentosum, called XPE. Patients in the XPE group have mild dermatological manifestations and are neurologically unaffected. Mutation in the DDB2 gene causes a deficiency in nucleotide excision repair of DNA. This deficiency is also mild, showing 40 to 60% of normal repair capability and a modest sensitivity to UV light in comparison to the sensitivities of cells defective in the other XP genes XPA, XPB, XPC, XPD, XPF and XPG.
Function
Binding to damaged DNA
As shown by Wittschieben et al., when DDB2 is in a complex with DDB1, forming the heterodimer DDB, this complex binds strongly to DNA containing one type of UV light-induced photoproduct , to DNA with an abasic site, to DNA containing mismatches without a covalent lesion, and to “compound” lesions containing both mismatches and lesions. The heterodimer DDB binds with intermediate strength to DNA containing another UV light-induced photoproduct, and binds weakly to DNA that has no DNA damage. The DDB2 component of the heterodimer contains the specificity for binding to damaged DNA, since a heterodimer DDB complex containing amino acid substitutions in the DDB2 subunit, as found in XP-E patients, is very deficient in binding to damaged DNA. DDB1 and DDB2, each acting alone, do not bind DNA.
Chromatin remodeling
The packaging of eukaryotic DNA into chromatin presents a barrier to all DNA-based processes that require recruitment of enzymes to their sites of action. To allow the critical cellular process of DNA repair, the chromatin must be relaxed. DDB2, in its heterodimeric complex with DDB1, and further complexed with the ubiquitin ligase protein CUL4A and with PARP1 rapidly associates with UV-induced damage within chromatin, with half-maximum association completed in 40 seconds. The PARP1 protein, attached to both DDB1 and DDB2, then PARylates on DDB2 that attracts the DNA remodeling protein ALC1. Action of ALC1 relaxes the chromatin at the site of UV damage to DNA. This relaxation allows other proteins in the nucleotide excision repair pathway to enter the chromatin and repair the DNA damaged by the UV-induced presence of cyclobutane pyrimidine dimers.
Other functions
In 2015, Zhu et al. showed that DDB2 down-regulates the acetylation of lysine 56 in histone H3 after UV-induced DNA damage through DDB2 interaction with histone deacetylases 1 and 2. Decreased acetylation of histones decreases transcription of associated genes in the DNA wrapped around the histones. In 2016, Zou et al. showed that DDB2 is involved in cell cycle arrest and homologous recombinational DNA repair after cells are subjected to ionizing radiation. In 2016, Christmann et al. showed that exposure of cells to the carcinogenic benzopyrene metabolite BPDE caused prompt and sustained upregulation of DDB2. This contributed to enhanced removal of BPDE adducts from DNA. In 2017, Fantini et al. showed that DDB2, in association with XRCC5 and XRCC6, has transcriptional activities. The DDB2/Ku effects on transcription are separate from the actions of the Ku heterodimer in non-homologous end joining DNA repair.
