Description
Retinoid X receptor (RXR) serves as a promiscuous heterodimerization partner for many nuclear receptors through the identity box, a 40-amino acid subregion within the ligand binding domain. RXR partners include thyroid hormone receptors (TRs), retinoic acid receptors (RARs), peroxisome proliferator-activated receptor (PPAR), several constitutive active orphan nuclear receptors, e.g. nuclear growth factor I-B (NGFI-B), oxysterol receptors (FXR,LXR), and constitutive androstane receptors (CAR). RXRs also form homodimers to mediate the effects of 9-cis-retinoic acid (9-cRA). Depending on these protein-protein interactions, RXR-containing complexes have distinct ligand-dependent and constitutive functions. RXR-β was originally identified as a protein able to bind to the regulatory region II of the murine major histocompatibility complex (MHC) class I promoter and referred to as H2RIIBP. The human RXR-β gene has been mapped to the short arm or centromeric region of chromosome 6 (6pter-q13) which also contains the MHC I locus. Two major RXR-b isoforms in the mouse, designated RXR-β1 and RXR-β2, differ in the N-terminal (A) domain with RXR-β1 containing an extra 72 amino acids. These isoform mRNAs are transcribed from two CpG island promoters and the different N-terminal exons are spliced to exons common to both isoforms from the remainder of the gene. Although the two mouse RXR isoforms have been characterised in some detail, less information is available about their human counterparts