Human PXR forms a tryptophan zipper-mediated homodimer

Area of Science:

• Biochemistry
• Molecular Biology
• Structural Biology

Background:

• The human pregnane X receptor (PXR) is a nuclear receptor that regulates genes involved in xenobiotic and endobiotic metabolism.
• PXR plays a critical role in detoxification pathways by responding to a diverse range of potentially harmful chemicals.

Purpose of the Study:

• To elucidate the structural basis and functional significance of PXR ligand binding domain (LBD) homodimerization.
• To investigate the role of the novel tryptophan-zipper (Trp-Zip) interaction in PXR homodimer formation and function.

Main Methods:

• X-ray crystallography to determine the structure of the PXR LBD homodimer.
• Analytical ultracentrifugation to confirm homodimer formation in solution.
• Site-directed mutagenesis to disrupt the Trp-Zip interaction.
• Transient transfection assays and coactivator recruitment assays (in vitro and in vivo) to assess receptor activity.

Main Results:

• Structural analysis revealed a unique PXR LBD homodimer stabilized by an intermolecular beta-sheet and a novel tryptophan-zipper (Trp-Zip) interaction.
• Analytical ultracentrifugation confirmed PXR LBD homodimerization in solution.
• Disruption of the Trp-Zip interaction abolished homodimer formation but did not affect DNA binding, RXRalpha interaction, or ligand binding.
• Abolishing homodimerization significantly reduced PXR transcriptional activity and coactivator SRC-1 recruitment.

Conclusions:

• The PXR LBD forms a unique homodimer mediated by a Trp-Zip interaction.
• This homodimerization is essential for PXR's transcriptional activity and coactivator recruitment, playing a key role in the receptor's function.
• The findings provide novel insights into the structural and functional mechanisms of nuclear receptor-mediated xenobiotic metabolism.