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PDBsum entry 1xnx
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Ligand receptor/transcription regulation
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PDB id
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1xnx
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References listed in PDB file
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Key reference
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Title
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Structure of the murine constitutive androstane receptor complexed to androstenol: a molecular basis for inverse agonism.
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Authors
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L.Shan,
J.Vincent,
J.S.Brunzelle,
I.Dussault,
M.Lin,
I.Ianculescu,
M.A.Sherman,
B.M.Forman,
E.J.Fernandez.
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Ref.
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Mol Cell, 2004,
16,
907-917.
[DOI no: ]
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PubMed id
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Abstract
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The nuclear receptor CAR is a xenobiotic responsive transcription factor that
plays a central role in the clearance of drugs and bilirubin while promoting
cocaine and acetaminophen toxicity. In addition, CAR has established a
"reverse" paradigm of nuclear receptor action where the receptor is
active in the absence of ligand and inactive when bound to inverse agonists. We
now report the crystal structure of murine CAR bound to the inverse agonist
androstenol. Androstenol binds within the ligand binding pocket, but unlike many
nuclear receptor ligands, it makes no contacts with helix H12/AF2. The
transition from constitutive to basal activity (androstenol bound) appears to be
associated with a ligand-induced kink between helices H10 and H11. This disrupts
the previously predicted salt bridge that locks H12 in the transcriptionally
active conformation. This mechanism of inverse agonism is distinct from
traditional nuclear receptor antagonists thereby offering a new approach to
receptor modulation.
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Figure 1.
Figure 1. Structure of mCAR LBD/Androstenol Complex(A and
B) Ribbon representation of each CAR molecule within the
asymmetric unit. The arrow indicates the region of missing
density between helices H11 and H12 in molecule B. The bound
ligand is also displayed in each molecule as ball-and-stick
representations.(C) F[o]-F[c] omit map contoured at 5σ for
androstenol within the binding pocket. The model is superimposed
on the electron density, and the androstenol is colored blue
(carbon) and red (hydroxyl).(D) Electrostatic surface
representation of the ligand binding pocket. Androstenol is
shown in black (carbon) and red (hydroxyl).
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Figure 4.
Figure 4. Model for Androstenol-Induced Repression of
CAR(A) A stereo model to show that when bound to androstenol,
the CAR helix H11 (purple) adopts a conformation that is similar
to apo inactive RXR (gray), which is in contrast to the fused
H10-H11 helix in the agonist bound active nuclear receptor
conformations such as in PXR (blue).(B) A Glu339-Gln245 backbone
amide hydrogen bond acts as a “pin” about which helix H11
can twist toward the ligand binding pocket in CAR (purple). This
interaction is conserved in most nuclear receptors even when in
the active state conformation as in the VDR structure (cyan,
Gln400). Residues in parentheses are from VDR.(C) The
Glu339-Gln245 backbone amide hydrogen bond is important for
maintaining the integrity of the ligand binding pocket.
Relative to wild-type CAR, dose response experiments demonstrate
that the E339A mutant displays a 10-fold higher EC[50] for
androstenol (RE2×2-TK-Luc reporter construct).(D) A
conceptual model of CAR active-to-inactive state transition. Apo
CAR is in green showing the predicted K205-C terminus
interaction that stabilizes AF2 in the active conformation.
Binding to androstenol leads to a movement (double-headed arrow)
of helix H11 toward the binding pocket, generation of the
H10-H11 kink, and dissociation of H12 from the body of the
protein as in the CAR/androstenol structure presented here.
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The above figures are
reprinted
by permission from Cell Press:
Mol Cell
(2004,
16,
907-917)
copyright 2004.
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