Nuclear receptors (NR) classically regulate gene expression by stimulating transcription upon binding to their cognate ligands. It is now well established that NR-mediated transcriptional activation requires the recruitment of coregulator complexes, which facilitate recruitment of the basal transcription machinery through direct interactions with the basal transcription machinery and/or through chromatin remodeling. However, a number of recently described NR coactivators have been implicated in cross-talk with other nuclear processes including RNA splicing and DNA repair. T:G mismatch-specific thymine DNA glycosylase (TDG) is required for base excision repair of deaminated methylcytosine. Here we show that TDG is a coactivator for estrogen receptor ␣ (ER␣). We demonstrate that TDG interacts with ER␣ in vitro and in vivo and suggest a separate role for TDG to its established role in DNA repair. We show that this involves helix 12 of ER␣. The region of interaction in TDG is mapped to a putative ␣-helical motif containing a motif distinct from but similar to the LXXLL motif that mediates interaction with NR. Together with recent reports linking TFIIH in regulating NR function, our findings provide new data to further support an important link between DNA repair proteins and nuclear receptor function.Nuclear receptors (NR) 1 regulate gene expression upon binding to small lipophilic molecules. The NR superfamily encompasses high affinity receptors for the steroid hormones, vitamin D3, thyroid hormone, and retinoic acid as well as so-called "orphan receptors" that bind with low affinity to dietary lipids such as fatty acids, oxysterols, bile acids, and xenobiotics and a large number of receptors with no known ligand (1). Two related estrogen receptors (ER␣) are responsible for the major estrogenic responses in mammals, being required for male and female reproductive function, but also for bone maintenance in the cardiovascular system and in regulating certain brain functions (2, 3).NR share a common modular structure with a core DNA binding domain and a C-terminal ligand binding domain (LBD). The LBD functions as a ligand-dependent transcription activation domain (AF2) as well as providing a surface for interaction with other NR. Transcription activation is mediated by synergism between AF2 and a second transactivation domain (AF1) located N-terminal to the DNA binding domain (4,5). Whereas AF2 activity requires ligand binding, AF1 activity is subject to regulation by phosphorylation (6 -8).Transcription regulation by NR occurs through direct interaction with the transcription machinery and/or by recruitment of coregulator proteins that facilitate interaction with the transcription machinery. Moreover, chromatin-remodeling complexes are now known to play an important role in gene transcription (9 -12). Several coactivator proteins associate with ligand-bound NR and include the p160 proteins, SRC1, TIF2/ GRIP1, and pCIP/ACTR/AIB1/RAC3/TRAM-1 (13). The thyroid hormone receptor-associated protein complex (14), which is v...

Gene activation involves protein complexes with diverse enzymatic activities, some of which are involved in chromatin modification. We have shown previously that the base excision repair enzyme thymine DNA glycosylase (TDG) acts as a potent coactivator for estrogen receptor-α. To further understand how TDG acts in this context, we studied its interaction with known coactivators of nuclear receptors. We find that TDG interacts in vitro and in vivo with the p160 coactivator SRC1, with the interaction being mediated by a previously undescribed motif encoding four equally spaced tyrosine residues in TDG, each tyrosine being separated by three amino acids. This is found to interact with two motifs in SRC1 also containing tyrosine residues separated by three amino acids. Site-directed mutagenesis shows that the tyrosines encoded in these motifs are critical for the interaction. The related p160 protein TIF2 does not interact with TDG and has the altered sequence, F-X-X-X-Y, at the equivalent positions relative to SRC1. Substitution of the phenylalanines to tyrosines is sufficient to bring about interaction of TIF2 with TDG. These findings highlight a new protein–protein interaction motif based on Y-X-X-X-Y and provide new insight into the interaction of diverse proteins in coactivator complexes.

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