Materials and Methods
Differential Scanning Fluorimetry (DSF)
Thermal melting experiments were carried out using an Mx3005p Real Time PCR machine (Stratagene). Proteins were buffered in 10 mM HEPES pH 7.5, 500 mM NaCl and assayed in a 96-well plate at a final concentration of 2 µM in 20 µl volume. PFI-1 was added at a final concentration of 10 µM. SYPRO Orange (Molecular Probes) was added as a fluorescence probe at a dilution of 1:1000. Excitation and emission filters for the SYPRO-Orange dye were set to 465 nm and 590 nm, respectively. The temperature was raised with a step of 3 °C per minute from 25 °C to 96 °C and fluorescence readings were taken at each interval. The temperature dependence of the fluorescence during the protein denaturation process was approximated by the equation where ?u G (T) is the difference in unfolding free energy between the folded and unfolded state, R is the gas constant and y F and y U are the fluorescence intensity of the probe in the presence of completely folded and unfolded protein respectively. The baselines of the denatured and native states were approximated by a linear fit. The observed temperature shifts, ? T m obs, were recorded as the difference between the transition midpoints of sample and reference wells containing protein without ligand in the same plate and determined by non-linear least squares fit.
Isothermal Titration Calorimetry
Experiments were carried out on a VP-ITC titration microcalorimeter from MicroCal TM, LLC (Northampton, MA) at 15 °C while stirring at 295 rpm, in ITC buffer (50 mM HEPES pH 7.4 at 25 °C, 150 mM NaCl). The injection syringe (250 µl) was loaded with a solution of the protein sample (300 µM protein, in ITC buffer). All titrations were conducted using an initial injection of 2 µl followed by 34 identical injections of 8 µl with a duration of 16 sec (per injection) and a spacing of 250 sec between injections. Heats of dilution were determined by independent titrations (protein into buffer) and were subtracted from the experimental data. Data analysis was carried out using the MicroCal TM Origin software supplied with the instrument to yield enthalpies of binding (?H) and binding constants (K B). Thermodynamic parameters were calculated (? G = ? H - T? S = -R TlnK B, where ? G, ? H and ? S are the changes in free energy, enthalpy and entropy of binding respectively). In all cases a single binding site model was employed.
PFi-1 (10 µm) was screened against a panel of 15 ligand receptors, ion channels and transports using an established and widely utilized commercial assay platform (ExpresSProfile; CEREP, Paris, FRANCE); <50% inhibition was observed.
Alpha screen Assay
All reagents were diluted in 50 mM HEPES, 100 mM NaCl, 0.1 % BSA, pH 7.4 supplemented with 0.05 % CHAPS and allowed to equilibrate to room temperature prior to addition to plates. A 24-point 1:2 serial dilution of the ligands was prepared over the range of 1500 µM and 4 µl transferred to low-volume 384-well plates (ProxiPlateTM-384 Plus, PerkinElmer, USA), followed by 4 µl of HIS-tagged protein (BRD4(1), 50 nM, BRD4(2), 50 nM). Plates were sealed and incubated at room temperature for 30 minutes, before the addition of 4 µl of biotinylated peptide at equimolar concentration to the protein [peptide for BETs H4K5acK8acK12acK16ac, H-SGRGK(Ac)GGK(Ac)GLGK(Ac)GGAK(Ac)RHRK(Biotin)-OH. Plates were sealed and incubated for a further 30 minutes, before the addition of 4 µl of streptavidin-coated donor beads (25 µg/ml) and 4 µl nickel chelate acceptor beads (25 µg/ml) under low light conditions. Plates were foil-sealed to protect from light, incubated at room temperature for 60 minutes and read on a PHERAstar FS plate reader (BMG Labtech, Germany) using an AlphaScreen 680 excitation/570 emission filter set. IC50 values were calculated in Prism 5 after normalization against corresponding DMSO controls and are given as the final concentration of compound in the 20 µl reaction volume.
Kinetic measurements were done using OctetRed384 instrument (ForteBio Inc, CA, USA). Biotinylated protein was immobilized on Super Streptavidin Biosensors at 2µg/ml concentration. Association and dissociation measurements were done in 50 mM HEPES, 100 mM NaCl, pH 7.4 buffer supplemented with 0.01 % Tween. Experiments were performed at 25°C with association and dissociation times of 240 sec. Compounds were prepared as one in two dilutions starting from 128µM. Binding to the reference sensors (no protein attached) was subtracted before calculations. Binding constants were calculated using ForteBio Analysis software.