SUMO-dependent regulation of thymine DNA glycosylase alters subnuclear localization and CBP/p300 recruitment

摘要

In vertebrates, cytosine methylation at CpG dinucleotides is an important mechanism regulating gene expression and chromatin structure. This epigenetic mechanism also contributes to genomic instability due to spontaneous conversion of methylcytosines to thymines, thereby generating potentially mutagenic guanine-thymine mispairs. Thymine DNA glycosylase (TDG) excises thymine and uracil mis-paired with guanine in a CpG context and also functions as a transcriptional cofactor. TDG localizes to euchromatin, and associates with transcription factors, including histone acetyltranferases CBP (CREB-binding protein) and p300. In vitro studies have shown that CBP/p300 and TDG form multi-enzyme complexes that are potentially involved in both DNA repair and transcriptional regulation. Furthermore, acetylation of TDG by CBP/p300 may serve as a regulatory switch between these functions. Interestingly, TDG translocates to discrete nuclear structures, known as promye-locytic leukemia protein "oncogenic domains (PODs),which are closely associated with euchromatin. A number of DNA repair and transcription factors, such as CBP, are known to localize to these subnuclear structures. We have shown that the SUMO-1 (small ubiquitin-like modifier-1) protein binding activity of TDG is essential for activation of CBP-de-pendent transcription and localization to PODs. SUMO-1 binding activity resides in 2 distinct amino- and carboxy-ter-minal motifs that are negatively regulated by DNA-binding and covalent SUMO-1 conjugation (sumoylation) to Lys341. TDG sumoylation also blocks interaction with CBP, preventing TDG acetylation in vitro. These findings highlight a central role of SUMO-dependent mechanisms in regulating cofactor recruitment and subnuclear localization of TDG.