RARbeta ligand-binding domain bound to an SRC-1 co-activator peptide: purification, crystallization and preliminary X-ray diffraction analysis.

Retinoids have demonstrated therapeutic efficacy in the treatment of acute promyelocytic leukaemia and in the chemoprevention of a large number of cancers. As the cellular signalling pathway of retinoids can be transduced by the three retinoic acid receptor (RAR) isotypes alpha, beta and gamma, the side effects of these treatments induced efforts to generate isotype-selective ligands. Despite knowledge of the crystal structures of RARalpha and RARgamma ligand-binding domains (LBDs), the rational design of such ligands has been hampered by the absence of RARbeta LBD structural data. Here, a strategy used to express a large-scale soluble fraction of the human RARbeta LBD suitable for biophysical analysis is reported, as well as a procedure for crystallizing it bound to a synthetic retinoid (TTNPB) with or without a co-activator peptide (SRC-1). Preliminary X-ray analysis revealed that both complexes crystallized in the orthorhombic space group P2(1)2(1)2(1). The unit-cell parameters are a = 47.81, b = 58.52, c = 92.83 A for the TTNPB-hRARbeta LBD crystal and a = 58.14, b = 84.07, c = 102.37 A when the SRC-1 peptide is also bound.