Rifampicin-Independent Interactions between the Pregnane X Receptor Ligand Binding Domain and Peptide Fragments of Coactivator and Corepressor Proteins

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• 作者：Punya Navaratnarajah ; Bridgett L. Steele ; Matthew R. Redinbo ; Nancy L. Thompson
• 刊名：Biochemistry
• 出版年：2012
• 出版时间：January 10, 2012
• 年：2012
• 卷：51
• 期：1
• 页码：19-31
• 全文大小：372K
• 年卷期：v.51,no.1(January 10, 2012)
• ISSN：1520-4995

文摘

The pregnane X receptor (PXR), a member of the nuclear receptor superfamily, regulates the expression of drug-metabolizing enzymes in a ligand-dependent manner. The conventional view of nuclear receptor action is that ligand binding enhances the receptor鈥檚 affinity for coactivator proteins, while decreasing its affinity for corepressors. To date, however, no known rigorous biophysical studies have been conducted to investigate the interaction among PXR, its coregulators, and ligands. In this work, steady-state total internal reflection fluorescence microscopy (TIRFM) and total internal reflection with fluorescence recovery after photobleaching were used to measure the thermodynamics and kinetics of the interaction between the PXR ligand binding domain and a peptide fragment of the steroid receptor coactivator-1 (SRC-1) in the presence and absence of the established PXR agonist, rifampicin. Equilibrium dissociation and dissociation rate constants of 5 渭M and 2 s^鈥?, respectively, were obtained in the presence and absence of rifampicin, indicating that the ligand does not enhance the affinity of the PXR and SRC-1 fragments. Additionally, TIRFM was used to examine the interaction between PXR and a peptide fragment of the corepressor protein, the silencing mediator for retinoid and thyroid receptors (SMRT). An equilibrium dissociation constant of 70 渭M was obtained for SMRT in the presence and absence of rifampicin. These results strongly suggest that the mechanism of ligand-dependent activation in PXR differs significantly from that seen in many other nuclear receptors.