Mutation of two residues converts the ligand-binding domain of RXRα into a uniform monomer without impairing the binding of retinoic acid and cofactors

Retinoid X receptor (RXR alpha) is a nuclear receptor (NR) for retinoic acid (RA) and regulates various NR signaling pathways. Ligand-binding domain (LBD) of RXR alpha can bind with its ligand 9-cis-RA and cofactors, and mediate the forming of homodimer and homotetramer of RXR alpha and its heterodimer with other NRs, conferring RXR alpha the ability to play complicated roles in development and diseases. Due to the coexistence of monomer, dimer and tetramer, there are difficulties to study the structure and interaction of RXR alpha-LBD with its ligands and cofactors in solution and to distinguish the roles of different forms of RXR alpha in cell. Here, through analyzing available structures of RXR alpha-LBD, we selected two residues, D379 and L420, in the homodimer interface to design three mutants of RXR alpha-LBD. Recombinant proteins of the three mutants showed decreased proportions of dimer and tetramer but unchanged overall structure and binding affinities to 9-cis-RA, corepressor SMRT, and coactivator SRC2. Especially, the double-site mutant RXR alpha-LBDD379A-L420G existed as a uniform monomer. Furthermore, L420 was found to play a similar role in forming RXR alpha-LBD homodimer and its heterodimer with various NRs, while the role of D379 varies a lot, as it shows almost no interaction with RAR alpha/b, LXR alpha/b, and THR alpha/b. This study provides a new insight into the mechanism for forming RXR alpha-LBD homodimer and its heterodimer with other NRs, and will facilitate the studies on the structure and interaction of RXR alpha-LBD with ligands, cofactors and drugs in solution, and the broad physiological functions of RXR alpha cooperating with various NRs in cell. (c) 2022 Elsevier Inc. All rights reserved.

Automated summary

It has been found that the nuclear receptor RXR alpha can bind with various ligands and cofactors, forming homodimer and homotetramer of RXR alpha and its heterodimer with other nuclear receptors, and playing complex roles in development and diseases. Two residues, D379 and L420, in the homodimer interface of RXR alpha have been identified to be crucial for its formation. The study also reveals the role of L420 in both homodimer and heterodimer formation, while D379 shows diverse interactions with different nuclear receptors. This study provides new insights into the mechanisms of RXR alpha homodimer and heterodimer formation and facilitates further research on the structure, interaction, and physiological functions of RXR alpha.