Abstract：
Retinoid X receptor (RXRα) is a nuclear receptor (NR) for retinoic acid (RA) and regulates various NR signaling pathways. Ligand-binding domain (LBD) of RXRα can bind with its ligand 9-cis-RA and cofactors, and mediate the forming of homodimer and homotetramer of RXRα and its heterodimer with other NRs, conferring RXRα 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 RXRα-LBD with its ligands and cofactors in solution and to distinguish the roles of different forms of RXRα in cell. Here, through analyzing available structures of RXRα-LBD, we selected two residues, D379 and L420, in the homodimer interface to design three mutants of RXRα-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 RXRα-LBDD379A-L420G existed as a uniform monomer. Furthermore, L420 was found to play a similar role in forming RXRα-LBD homodimer and its heterodimer with various NRs, while the role of D379 varies a lot, as it shows almost no interaction with RARα/β, LXRα/β, and THRα/β. This study provides a new insight into the mechanism for forming RXRα-LBD homodimer and its heterodimer with other NRs, and will facilitate the studies on the structure and interaction of RXRα-LBD with ligands, cofactors and drugs in solution, and the broad physiological functions of RXRα cooperating with various NRs in cell.
摘要：
类视黄醇X受体(RXRα)是视黄酸(RA)的核受体(NR),调节多种NR信号通路。RXRα的配体结合域（LBD）可以与其配体9-cis-RA和辅因子结合，并介导RXRα的同源二聚体和同源四聚体及其与其他NRs的异源二聚体的形成，赋予RXRα在发育和疾病中发挥复杂作用的能力。由于单体的共存，二聚体和四聚体，研究RXRα-LBD与其配体和辅因子在溶液中的结构和相互作用以及区分不同形式的RXRα在细胞中的作用存在困难。这里,通过分析RXRα-LBD的可用结构，我们选择了两个残基，D379和L420，在同源二聚体界面上设计了三个RXRα-LBD突变体。三种突变体的重组蛋白显示二聚体和四聚体的比例降低，但总体结构和对9-cis-RA的结合亲和力不变，辅抑制子SMRT,和助活化剂SRC2。尤其是,双位点突变体RXRα-LBDD379A-L420G作为均匀单体存在。此外，发现L420在形成RXRα-LBD同源二聚体及其与各种NRs的异源二聚体中起类似作用，虽然D379的作用变化很大，因为它几乎没有与RARα/β相互作用，LXRα/β，和THRα/β。这项研究为RXRα-LBD同源二聚体及其与其他NRs的异源二聚体的形成机制提供了新的见解。并将有助于研究RXRα-LBD与配体的结构和相互作用，溶液中的辅因子和药物，以及RXRα与细胞中各种NRs协同作用的广泛生理功能。
