Losmapimod (GSK-AHAB) (GW856553X; SB856553; GSK-AHAB) is a specific, potent, and orally active p38 MAPK inhibitor (pKi: 8.1/7.6 for p38α/β).

p38α:8.1(pKi), p38β:7.6(pKi)

DNA binding assays: All reactions are performed in black non-binding polystyrene 384-well plates with reaction volumes of 30–100 μL. Purified DNA strand exchange proteins and chemical compounds are pre-incubated at room temperature for 5 minutes; they are then further incubated at 37°C for 30 min with 100 nM of ssDNA substrate, consisting of a 45-mer poly-dT tagged with Alexa 488 at the 5' terminus (synthesized and purified by Integrated DNA Technologies). Reactions are performed in 20 mM HEPES pH 7.5, 10 mM MgCl2, 0.25 μM BSA, 2% glycerol, 30 mM NaCl, 4% DMSO and 2 mM ATP. Some conditions included DTT or TCEP (tris(2-carboxyethyl)phosphine) as indicated. DNA binding is measured as a function of fluorescence polarization (FP) with a Safire2 plate reader, using the following settings: excitation 470±5 nM, emission 530±5 nM, 10 reads/well, Z height and G factor auto-calibrated from control wells. Displayed error bars represent standard deviation. For experiments involving a titration of protein concentrations, data are fit to an equation that accounts for the cooperative nature by which recombinase proteins bind DNA. For experiments involving a titration of RI-1, protein concentrations are selected to give an ～80% saturation of the FP signal in the absence of RI-1.