The quantification of protein solution for binding assay was determined by the UV absorption method. The purified IPO protein was dialyzed against 20 mM Tris-HCl, 150 mM NaCl (pH 7.0) at 4uC overnight. The concentration of IPO and DN10IPO was determined by UV absorption spectroscopy at 280 nm with the specific extinction coefficient e of 22,920 M 21 cm 21 , which was determined from the prediction of IPO primary sequence. From Beer-Lambert law, A = e6b6C where A is the absorbance of the sample at 280 nm, b is the pathlength in 1 cm, and C is the protein concentration (M).