The ligand binding domain of the retinoid X receptor and Ultraspiracle, members of nuclear receptor superfamily.
The ligand binding domain of the retinoid X receptor (RXR) and Ultraspiracle (USP): This family includes two evolutionary related nuclear receptors: retinoid X receptor (RXR) and Ultraspiracle (USP). RXR is a nuclear receptor in mammalian and USP is its counterpart in invertebrates. The native ligand of retinoid X receptor is 9-cis retinoic acid (RA). RXR functions as a DNA binding partner by forming heterodimers with other nuclear receptors including CAR, FXR, LXR, PPAR, PXR, RAR, TR, and VDR. RXRs can play different roles in these heterodimers. It acts either as a structural component of the heterodimer complex, required for DNA binding but not acting as a receptor or as both a structural and a functional component of the heterodimer, allowing 9-cis RA to signal through the corresponding heterodimer. In addition, RXR can also form homodimers, functioning as a receptor for 9-cis RA, independently of other nuclear receptors. Ultraspiracle (USP) plays similar roles as DNA binding partner of other nuclear rec eptors in invertebrates. USP has no known high-affinity ligand and is thought to be a silent component in the heterodimeric complex with partner receptors. Like other members of the nuclear receptor (NR) superfamily of ligand-activated transcription factors, RXR and USP have a central well conserved DNA binding domain (DBD), a variable N-terminal domain, a flexible hinge and a C-terminal ligand binding domain (LBD).