A Structural Model of the Constitutive Androstane Receptor Defines Novel Interactions That Mediate Ligand-Independent Activity

Abstract

Unlike classical nuclear receptors that require ligand for transcriptional activity, the constitutive androstane receptor (CAR) is active in the absence of ligand. To determine the molecular contacts that underlie this constitutive activity, we created a three-dimensional model of CAR and verified critical structural features by mutational analysis. We found that the same motifs that facilitate ligand-dependent activity in classical receptors also mediated constitutive activity in CAR. This raises a critical question: how are these motifs maintained in an active conformation in unliganded CAR? The model identified several novel interactions that account for this activity. First, CAR possesses a short loop between helix 11 and the transactivation domain (helix 12), as well as a short carboxy-terminal helix. Together, these features favor ligand-independent docking of the transactivation domain in a position that is characteristic of ligand-activated receptors. Second, this active conformation is further stabilized by a charge-charge interaction that anchors the carboxy-terminal activation domain to helix 4. Mutational analysis of these interactions provides direct experimental support for this model. We also show that ligand-mediated repression of constitutive activity reflects both a displacement of coactivator and a recruitment of corepressor. Our data demonstrate that CAR utilizes the same conserved structural motifs and coregulator proteins as originally defined for classical nuclear receptors. Despite these remarkable similarities, our model demonstrates how a few critical changes in CAR can dramatically reverse the transcriptional activity of this protein.Nuclear hormone receptors are transcription factors essential for virtually all aspects of physiology, including normal differentiation, development, and homeostasis. The transcriptional activity of these receptors is modulated by small lipophilic ligands, including the classical steroid hormones, thyroid hormone, retinoids, and other lipid metabolites (33). Upon binding ligand, classical nuclear receptors undergo a conformation change that results in the recruitment or displacement of a variety of coregulator proteins (15, 55). These coregulators include coactivators (PBP/DRIP205/TRAP220/ TRIP2, SRC-1/NCoA-1, GRIP1/TIF2/NCoA-2, and ACTR/ pCIP/AIB1/NCoA-3) (2,4,19,30,32,38,47,56,59) and corepressors (nuclear receptor corepressor [NCoR] and silencing mediator of retinoic acid and thyroid hormone receptors [SMRT]) (5, 21, 42). Both classes of coregulators utilize ␣-helical motifs (receptor interaction domains [RIDs]) to make direct contacts with a hydrophobic cleft (8, 11, 37, 43) on the surface of nuclear receptors (18,22,47). These coregulators form complexes with other proteins that function either by remodeling chromatin or by providing a bridge between the nuclear receptor and the basal transcription machinery. These interactions allow nuclear receptor ligands to activate or repress transcription of specific target genes (17).Nuclear receptors have a c...

Agonist-dependent and Agonist-independent Transactivations of the Human Constitutive Androstane Receptor Are Modulated by Specific Amino Acid Pairs

“…A recent report on mouse CAR (34) indicated that an interaction of the negatively charged C terminus of helix 12 with the positively charged Lys-205 (Lys-195 in human CAR) in helix 4 may contribute to the stabilization of helix 12 (see Fig. 2B).…”

Deciphering the roles of the constitutive androstane receptor in energy metabolism

“…The LXXLL-binding pocket can also be targeted by some corepressors. Like many other nuclear receptors, CAR is also bound and repressed by the following common corepressors: nuclear receptor corepressor 1 (NCoR1), NCoR2/SMRT, and DAX-1 (NR0B1) [53,54,57,62] . Collectively, the LXXLL-mediated direct interaction of CAR and its cofactors is very important for the activity of CAR.…”

“…A number of coactivators have been found to physically interact with CAR [51] . The three members of p160 coactivators, SRC1, SRC2, and SRC3, as well as PGC1α have been reported to enhance the activity of CAR based on reporter gene assays [52][53][54][55][56][57] . The function of individual coactivators seems to be redundant, at least for the induction of mouse Cyp2b10 [58] .…”

Interaction of PER2 with the Constitutive Androstane Receptor Possibly Links Circadian Rhythms to Metabolism

“…39 It would be worth exploring if it could interact with the charge clamp of NRs. 9,31,[40][41][42] Another interesting observation is the presence of a band at approximately 40 kDa at the western blot with anti-V5 Ab. Similar sized bands have been observed in our previous work, where a >40 kDa band was detected with western blot from mouse liver samples, with the use of a different Ab (Abcam ab467).…”