3D crystal structure of RARβ and RXRα LBDs in complex with LG754 and a coactivator peptides. (A) cartoon representation of RARβ LBD. Helices are numbered from H1 to H12, the latest being the activation helix that adopts the agonist conformation. The coactivator peptide (SCR1 NR2) bound to the surface formed by H3, H4, and H12 of RARβ is shown in yellow. The ligand LG754 is shown by stick representation; (B) cartoon representation of RXRα LBD. The coactivator peptide (TIF2 NR2) bound to the protein is shown in yellow; (C) superposition of the two complexes of RARβ LBD bound to LG754 and coactivator peptide observed in the asymmetric unit; (D) superposition of our complex of RXRα LBD bound to LG754 and coactivator peptide (same as in B) with the crystal structure of RXRα LBD bound to LG754 in the context of the heterodimer RARα-RXRα [69] (PDB code 3a9e). In the latest structure, the activation helix H12 adopts an antagonist position protruding outside the LBD, whereas, in the present structure (PDB code 6sti), helix H12 (shown in red) folds back on the LBD in the agonist position.