Coactivator binding promotes the specific interaction between ligand and the pregnane X receptor

Area of Science:

• Biochemistry
• Structural Biology
• Pharmacology

Background:

• The pregnane X receptor (PXR) is a key regulator of drug metabolism and excretion.
• PXR detects diverse endogenous and xenobiotic compounds.
• Understanding PXR's structural basis for activation is critical for drug development.

Purpose of the Study:

• To determine the crystal structure of the human PXR ligand-binding domain (LBD) complexed with SR12813 and a SRC-1 peptide.
• To elucidate the binding orientation of SR12813 and the interaction with coactivators.
• To investigate the role of coactivator binding in PXR conformational stability and activation.

Main Methods:

• X-ray crystallography (2.0Å resolution) to determine the PXR-SR12813-SRC-1 complex structure.
• Thermal denaturation studies to assess ligand and coactivator effects on PXR LBD stability.
• Analysis of PXR homodimerization and novel structural features.

Main Results:

• The crystal structure revealed a PXR homodimer with a novel alpha2 helix.
• The SRC-1 peptide bound adjacent to the AF-2 helix, forming two distinct helices.
• SR12813 bound in a single, unique orientation, contacting the AF-2 helix, unlike previous structures.
• Both SR12813 and SRC-1 individually stabilized PXR LBD, with an additive effect when combined.

Conclusions:

• Coactivator binding to PXR limits receptor flexibility, trapping an active conformation.
• The unique binding of SR12813 and coactivator highlights the requirement for specificity in PXR activation.
• These findings provide structural insights into PXR regulation and its role in drug metabolism.