title: Coactivator binding promotes the specific interaction between ligand and the pregnane X receptor.
abstract: The pregnane X receptor (PXR) detects the presence of a wide variety of endogenous and xenobiotic compounds, and is a master regulator of the expression of genes central to drug metabolism and excretion. We present the 2.0A crystal structure of the human PXR ligand-binding domain (LBD) in complex with the cholesterol-lowering compound SR12813 and a 25 amino acid residue fragment of the human steroid receptor coactivator-1 (SRC-1) containing one LXXLL motif. PXR crystallizes as a homodimer in the asymmetric unit in this structure and possesses a novel alpha2 helix adjacent to its ligand-binding cavity. The SRC-1 peptide forms two distinct helices and binds adjacent to the ligand-dependent transactivation AF-2 helix on the surface of PXR. In contrast with previous PXR structures, in which SR12813 bound in multiple orientations, the small SR12813 agonist in this structure binds in a single, unique orientation within the receptor's ligand-binding pocket and contacts the AF-2 helix. Thermal denaturation studies reveal that the SR12813 ligand and SRC-1 coactivator peptide each stabilize the LBD of PXR, and that together they exert an additive effect on the stability of the receptor. These results indicate that the binding of coactivator to the surface of PXR limits the ability of this promiscuous receptor to "breathe" and helps to trap a single, active conformation of SR12813. They further reveal that specificity is required for PXR activation.
authors.affiliation: Departments of Chemistry and Biochemistry & Biophysics, and the Lineberger Comprehensive Cancer Center, The University of North Carolina at Chapel Hill, Chapel Hill, NC 27599, USA.
authors.forename: Ryan E
authors.initials: RE
authors.lastname: Watkins
authors.forename: Paula R
authors.initials: PR
authors.lastname: Davis-Searles
authors.forename: Mill H
authors.initials: MH
authors.lastname: Lambert
authors.forename: Matthew R
authors.initials: MR
authors.lastname: Redinbo
mesh_terms: D000595:Amino Acid Sequence; D001665:Binding Sites; D002942:Circular Dichroism; D018360:Crystallography, X-Ray; D019281:Dimerization; D004164:Diphosphonates / Q000737:chemistry / Q000378:metabolism / Q000494:pharmacology; D051548:Histone Acetyltransferases; D006801:Humans; D008024:Ligands; D008958:Models, Molecular; D008969:Molecular Sequence Data; D056920:Nuclear Receptor Coactivator 1; D010446:Peptide Fragments / Q000737:chemistry / Q000378:metabolism / Q000494:pharmacology; D000077297:Pregnane X Receptor; D011485:Protein Binding; D011489:Protein Denaturation / Q000187:drug effects; D017434:Protein Structure, Tertiary; D018160:Receptors, Cytoplasmic and Nuclear / Q000737:chemistry* / Q000378:metabolism*; D011987:Receptors, Steroid / Q000737:chemistry* / Q000378:metabolism*; D013816:Thermodynamics; D014157:Transcription Factors / Q000737:chemistry* / Q000378:metabolism* / Q000494:pharmacology
chemical_list: D004164:Diphosphonates; D008024:Ligands; D010446:Peptide Fragments; D000077297:Pregnane X Receptor; D018160:Receptors, Cytoplasmic and Nuclear; D011987:Receptors, Steroid; C100355:SR 12813; D014157:Transcription Factors; D051548:Histone Acetyltransferases; C538859:NCOA1 protein, human; D056920:Nuclear Receptor Coactivator 1