Gel Mobility Shift Assays Varying amounts of purified recombinant protein were incubated with 1 nM radiolabelled substrate at 20°C for 30 min in binding buffer (Supplemental Information). Following addition of 2.5% Ficoll-400, samples were run on 8% native polyacrylamide gels in TBE buffer. Gels were dried, exposed to storage phosphor screens, quantified by a Typhoon FLA 9500 (GE Healthcare) phosphorimager, and analyzed with the ImageQuant software.