Purification and characterization of bowman-birk protease inhibitor from rice coleoptiles

A 15 kDa rice Bowman-Birk inhibitor from fast elongating coleoptiles has been purified and identified using partial N-terminal sequence, LC-MS, and MALDI-TOF MS as a 133 amino acid polypeptide (BBIrc1). The kinetic study shows this protease inhibitor displays competitive inhibition toward trypsin with Ki of 4.0 × 10^-7 M and non-competitive inhibition toward α-chymotrypsin with Ki of 9.3 940-948 10^-6 M. The Western blotting results of the anti-sera raised against this 15 kDa protein showed that this anti-serum recognized two BBI proteins with molecular size around 15 kDa (BBIrc1) and 25 kDa (BBIrc2) and the quantity of the expression of 15 kDa was nearly constant under both aerobic and hypoxia conditions; however, the 25 kDa expression was greatly up-regulated when the fast elongating coleoptiles were transferred from hypoxia conditions to the aerobic conditions. The results indicate that the expression pattern of BBIs proteins correlated to the developmental stage in terms of morphological changes. The partial N-terminal sequence of the first 9 amino acids of 25 kDa was AEAPPRPPK, which is the same as the amino acid sequence of 37th to 45th of RBBI3-1 and LC-MS study shows that several mass fragments fit to RBBI3-1. The 25 kDa protein also shows specific binding to bovine trypsin. This expression pattern demonstrates for the first time that environmental factor, oxygen, can select and enhance specific BBI gene expression. The results of this study suggest BBI proteins might play multiple biological functions inside rice coleoptiles.