AMINO-ACID-SEQUENCE OF A TRYPSIN/CHYMOTRYPSIN INHIBITOR FROM GIANT TARO (ALOCASIA-MACRORRHIZA)

Giant tare (Alocasia macrorrhiza) contains a protein which inhibits both trypsin and chymotrypsin. This trypsin/chymotrypsin inhibitor exists as a dimer of two identical monomers each with slight polymorphism and is an attractive candidate for conferring insect resistance in transgenic plants. The 184 amino-acid sequence (molecular mass of 19774 Da for the Met-24, Glu-50 form) has been determined and is compared with those of other Kunitz-type trypsin, chymotrypsin and subtilisin inhibitors. There appears to be greater 'homology' between the giant tare inhibitor and those inhibitors from other monocotyledons than inhibitors from dicotyledons. The P-1 loop region is different from that of other Kunitz-type inhibitors and contains a sequence Leu-Ala-Phe-Phe-Pro at residues 56-60. This section of sequence differs only by a Leu/Ile replacement to a tight binding inhibitor of neutrophil elastase, recently produced by genetic engineering. The most likely candidate for the P-1 residue in the giant tare trypsin/chymotrypsin inhibitor is Leu-56.