Assay Method Information

Assay Name:  Ligand KI Assay
Description:  The labeled ligand specifically binds BRD4-1 and BRD4-2 and can be displaced by a small molecule inhibitor that shares a similar or overlapping binding site. BRD4-I and BRD4-2 were expressed and purified from Escherichia coli as N-terminal His6, tagged proteins. A Eu-cryptate labeled anti-His antibody (Perkin Elmer) was used to specifically bind BRD4. Binding of BRD4 to the labeled probe/ligand resulted in an increase in FRET signal whereas displacement of this labeled ligand from BRD4 with a small molecule inhibitor resulted in a decrease in FRET signal. Assays were performed in 50 mM Hepes (pH 7.5), 150 mM NaCl, 0.1 mg/ml BSA, 0.01% (v/v) Brij, 0.5% (v/v) DMSO and 10 nM labeled ligand at the following concentrations for each BRD4 isoform: 2 nM BRD4-1 and 0.5 nM BRD4-2. After an assay reaction time of 60 minutes at 25° C., binding was measured with 2 nM Eu-cryptate labeled anti-His antibody. TR-FRET signal was detected on an Envision plate reader (Ex: 320 nm; Em: 615/665 nm; 100 us delay and 200 us read window). Data were normalized based on a positive (2 uM I-BET) and negative (DMSO) controls and IC50 values were calculated from the fit of the dose-response curves to a four-parameter equation. All IC50 values represent geometric mean values of a minimum of four determinations. The IC50 values were converted to Ki values (dissociation constant for BRD4-inhibitor complex) using the Cheng and Prusoff equation for a competitive inhibitor mode of action.
Affinity data for this assay
 

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