The ricexip gene, which encodes the rice xylanase-inhibiting protein (riceXIP), was recombinantly expressed in Pichia pastoris GS115 under the control of AOX1 promoter. Recombinant riceXlP (rePriceXlP) was secreted into the supernatant and purified to homogeneity with the use of Ni-affinity resin. The molecular mass of rePriceXlP was approximately 44.0 kDa. RePriceXIP significantly inhibited the activity of GH11 xylanases in a concentration-dependent manner. The optimal inhibitory activity of rePriceXIP on GH11 xylanases (TfxA_CD214, reBaxA454, and TNA_CD526) occurred at 40 degrees C for 30 min. The IC50 values of rePriceXIP inhibiting TNA_CD214, reBaxA454, and TfxA_CD526 (0.5 U) were 45, 40, and 40 nM, respectively. The K-i of rePriceXlP on TNA_CD214 xylanase was 12.2 nM. Increasing the concentration of rePriceXIP did not change V-max but increased K-m, thereby suggesting that the inhibition was competitive. Analysis of the fluorescence intensities revealed that the K-q values for TNA_CD214, reBaxA454, and TNA_CD526 exceeded 2.0 x 10(10) L mo1(-1). s(-1), implying that static quenching occurred. Circular dichroism spectroscopy demonstrated that rePriceXIP bound to xylanase, thereby changing the secondary structure and reducing the catalytic activity of xylanase. Lower levels of hydrolytes are released from beechwood xylan by xylanases in the presence of rePriceXIP.