The Nuclear Xenobiotic Receptor CARStructural Determinants of Constitutive Activation and Heterodimerization

Abstract

Constitutive androstane receptor (CAR) induces xenobiotic, bilirubin, and thyroid hormone metabolism as a heterodimer with the retinoid X receptor (RXR). Unlike ligand-dependent nuclear receptors, CAR is constitutively active. Here, we report the heterodimeric structure of the CAR and RXR ligand binding domains (LBDs), which reveals an unusually large dimerization interface and a small CAR ligand binding pocket. Constitutive CAR activity appears to be mediated by the compact nature of the CAR LBD that displays several unique features including a shortened AF2 helix and helix H10, which are linked by a two-turn helix that normally adopts an extended loop in other receptors, and an extended helix H2 that stabilizes the canonical LBD fold by packing tightly against helix H3. These structural observations provide a molecular framework for understanding the atypical transcriptional activation properties of CAR.

Molecular recognition of parathyroid hormone by its G protein-coupled receptor

“…The binding between PTH and the PTH1R ECD was determined by an AlphaScreen assay using a hexahistidine detection kit from PerkinElmer (52). The binding mixtures, containing 40 nM N-terminally biotinylated PTH-(7-34)-NH2, 40 nM of MBP-PTH1R-ECD-His6, 15 g/ml of streptavidin-coated ''donor'' beads, and Ni-chelate-coated ''acceptor'' beads, were incubated in a buffer of 50 mM Mops (pH 7.4), 100 mM NaCl, and 0.1 mg/ml BSA for 1 h. For competition assays, 100 M unlabeled peptides were added and incubated for 1 additional h before signal recording.…”

Phenobarbital Meets Phosphorylation of Nuclear Receptors

“…Chemicals that activate or repress CAR in cell-based assays are generally considered ligands. They were found to bind the LBD of CAR in X-ray crystal structures (Suino et al, 2004;Xu et al, 2004). Although EGF repressed ligand activation of CAR (e.g., by ligands such as TCPOBOP and CITCO), unlike PB, these ligands did not repress EGF signaling at the EGFR (Koike et al, 2007;Shizu et al, 2017).…”

Deciphering the roles of the constitutive androstane receptor in energy metabolism

“…Crystal structure analysis revealed that the LBD amino acids on CAR form a pocket that is responsible for the high binding affinity for the Leu-X-X-Leu-Leu (LXXLL) motif, a conserved motif found in many nuclear receptor coactivators [50] . A number of coactivators have been found to physically interact with CAR [51] .…”