Amino-acid sequence of a trypsin/chymotrypsin inhibitor from giant taro ( Alocasiamacrorrhiza)

Abstract

Giant taro ( 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 taro 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 taro trypsin/chymotrypsin inhibitor is Leu-56.