Purification and crystallization of the human RXRα ligand-binding domain-9-cisRA complex

The purification and crystallization of the stoichiometric complex of human RXR alpha ligand-binding domain (hRXR alpha LBD) bound to its natural ligand 9-cis retinoic acid (9-cisRA) are described. A three-step purification yields a pure and homogenous complex. Based on the crystallization conditions of several other nuclear receptors, an exhaustive crystallization screening using carboxylic acids as precipitating agents was performed in association with the use of polyhydric alcohols acting as cosmotropic solutes. Crystals of the hRXR alpha LBD-9-cisRA complex grew in a tripartite mixture containing sodium formate, glycerol and propane-1,2-diol. Micro- and macroseeding were necessary to improve both the size and the quality of crystals in order to make them suitable for structure determination.