The ligand-binding domains of the retinoid X receptor alpha (RXRalpha) and of the retinoic acid receptor beta (RARbeta) were overexpressed separately and copurified in the heterodimeric form. Using a crystallization solution containing sodium formate and PEG 3350 as precipitant, the heterodimer was cocrystallized with the promiscuous ligand 9-cis-retinoic acid (9C-RA) and a 13-residue fragment of the nuclear receptor interaction domain (NID) of the transcriptional coactivator TRAP220. The crystals grew in the trigonal space group P3(1)21, with unit-cell parameters a = b = 115.7, c = 247.2 angstroms and two heterodimers per asymmetric unit. X-ray diffraction data were collected to 2.9 angstroms resolution. The structure was solved by molecular replacement and is currently being refined.
