Interaction of transcriptional intermediary factor 2 nuclear receptor box peptides with the coactivator binding site of estrogen receptor α

The activation function 2/ligand-dependent interaction between nuclear receptors and their coregulators is mediated by a short consensus motif, the so-called nuclear receptor (NR) box. Nuclear receptors exhibit distinct preferences for such motifs depending both on the bound ligand and on the NR box sequence. To better understand the structural basis of motif recognition, we characterized the interaction between estrogen receptor a and the NR box regions of the p160 coactivator TIF2. We have determined the crystal structures of complexes between the ligand-binding domain of estrogen receptor a and 12-mer peptides from the Box 2 and Box 3 regions of TIF2. Surprisingly, the Box 3 module displays an unexpected binding mode that is distinct from the canonical LXXLL interaction observed in other ligand-binding domain/NR box crystal structures. The peptide is shifted along the coactivator binding site in such a way that the interaction motif becomes LXXLL rather than the classical LXXLL. However, analysis of the binding properties of wild type NR box peptides, as well as mutant peptides designed to probe the Box 3 orientation, suggests that the Box 3 peptide primarily adopts the classical LXXLL orientation in solution. These results highlight the potential difficulties in interpretation of protein-protein interactions based on co-crystal structures using short peptide motifs.