| Assay Method Information | |
| | FRET Activity Assay |
| Description: | Determination of a ligand mediated cofactor peptide interaction to quantify ligand binding to the nuclear receptor FXR was performed as follows.Preparation of human FXR alpha ligand binding domain: The human FXRalpha LBD was expressed in E. coli strain BL21(DE3) as an N-terminally GST tagged fusion protein. The DNA encoding the FXR ligand binding domain was cloned into vector pDEST15 (Invitrogen). Expression was under control of an IPTG inducible T7 promoter. The amino acid boundaries of the ligand binding domain were amino acids 187-472 of Database entry NM_005123 (RefSeq). Expression and purification of the FXR-LBD: An overnight preculture of a transformed E. coli strain was diluted 1:20 in LB-Ampicillin medium and grown at 30° C. to an optical density of OD600=0.4-0.6. Gene expression was then induced by addition of 0.5 mM IPTG. Cells were incubated an additional 6 h at 30° C., 180 rpm. Cells were collected by centrifugation (7000×g, 7 min, rt). Per liter of original cell culture, cells were resuspended in 10 mL lysis buffer (50 mM Glucose, 50 mM Tris pH 7.9, 1 mM EDTA and 4 mg/mL lysozyme) and left on ice for 30 min. Cells were then subjected to sonication and cell debris removed via centrifugation (22000×g, 30 min, 4° C.). Per 10 mL of supernatant 0.5 mL prewashed Glutathione 4B sepharose slurry (Qiagen) was added and the suspension kept slowly rotating for 1 h at 4° C. Glutathione 4B sepharose beads were pelleted by centrifugation (2000×g, 15 sec, 4° C.) and washed twice in wash buffer (25 mM Tris, 50 mM KCl, 4 mM MgCl2 and 1M NaCl). The pellet was resuspended in 3 mL elution buffer per liter of original culture (elution buffer: 20 mM Tris, 60 mM KCl, 5 mM MgCl2 and 80 mM glutathione added immediately prior to use as powder). The suspension was left rotating for 15 min at 4° C., the beads pelleted and eluted again with half the volume of elution buffer than the first time. The eluates were pooled and dialysed overnight in 20 mM Hepes buffer (pH 7.5) containing 60 mM KCl, 5 mM MgCl2 as well as 1 mM dithiothreitol and 10% (v/v) glycerol. The protein was analysed by SDS-Page.The method measures the ability of putative ligands to modulate the interaction between the purified bacterial expressed FXR ligand binding domain (LBD) and a synthetic biotinylated peptide based on residues 676-700 of SRC-1 (LCD2, 676-700). The sequence of the peptide used was B-CPSSHSSLTERHKILHRLLQEGSPS-COOH (SEQ ID NO: 1) where the N-terminus was biotinylated (B). The ligand binding domain (LBD) of FXR was expressed as fusion protein with GST in BL-21 cells using the vector pDEST15. Cells were lysed by sonication, and the fusion proteins purified over glutathione sepharose (Pharmacia) according to the manufacturers instructions. For screening of compounds for their influence on the FXR-peptide interaction, the Perkin Elmer LANCE technology was applied. This method relies on the binding dependent energy transfer from a donor to an acceptor fluorophor attached to the binding partner of interest. For ease of handling and reduction of background from compound fluorescence LANCE technology makes use of generic fluorophore labels and time resolved detection Assays were done in a final volume of 25 μL in a 384 well plate, in a Tris-based buffer (20 mM Tris-HCl pH 7.5; 60 mM KCl, 5 mM MgCl2; 35 ng/μL BSA), containing 20-60 ng/well recombinantly expressed FXR-LBD fused to GST, 200-600 nM N-terminally biotinylated peptide, representing SRC1 aminoacids 676-700, 200 ng/well Streptavidin-xlAPC conjugate (Prozyme) and 6-10 ng/well Eu W1024-antiGST (Perkin Elmer). DMSO content of the samples was kept at 1%. After generation of the assay mix and diluting the potentially FXR modulating ligands, the assay was equilibrated for 1 h in the dark at rt in FIA-plates black 384 well (Greiner). The LANCE signal was detected by a Perkin Elmer VICTOR2VTM Multilabel Counter. |
| Affinity data for this assay | |
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