Expression and inhibitory activity analysis of a 25-kD Bowman-Birk protease inhibitor in rice

Rice Bowman-Birk inhibitors (RBBI), with one (8 kD) or two homologous domains (16 kD), were found to be effective trypsin inhibitors in vitro. In this study, we demonstrate that the 25-kD protein corresponding to the three-domain RBBI indeed exists in rice in planta, and that the RBBIs are regulated by development and wounding. We also found by inhibitory activity assay that the 3:13 disulfide bond, but not the 4:5 disulfide bond, suppresses the trypsin-inhibitory activity, and the D3 domain of RBBI3-1 has no inhibitory activity against trypsin, chymotrypsin, paparin or subtilisin. Mutation analyses showed that conversion from Lys to Leu or Tyr in the N-terminal P1 site in D1 domain did not create chymotrypsin-inhibitory activity, suggesting that the structure of the reactive loop in D1 domain hinder the new inhibitory specificity at P1 site, and the chymotrypsin-inhibitory activity might need the participation of other structures, e.g. 3:13 disulfide bond.