Isolation and Analysis of cDNA Encoding a Precursor of Canavalia ensiformis Asparaginyl Endopeptidase (Legumain)

Recently, asparaginyl endopeptidase was purified to homogeneity from jack bean (Canavalia ensiformis) seeds, and its NH2-terminal amino acid sequence was determined for 25 residues [Abe, Y. et al. (1993) J. Biol. Chem. 268, 3525-3529]. On the basis of this sequence information, we searched for seed cDNAs encoding this enzyme. Seven clones were obtained and sequenced. By combining four of them, we obtained a cDNA for a precursor of the enzyme containing the reported NH2-terminal sequence. The other three clones seemed to be for precursors of its isozymes. When the deduced amino acid sequences of these enzyme precursors were compared with those in the GeneBank, EMBL, and NBRF databases, only one protein was found with a homologous sequence. It was a precursor of hemoglobinase from a blood fluke (Schistosoma mansoni). Significant homology was observed only in the range of sequence for the mature form. Although hemoglobinase and asparaginyl endopeptidase behave as cysteine proteinases, their protein natures are distinct from either papain-type proteinases or clostripain. Various plant seeds have been reported to contain asparaginyl endopeptidases. This may be the first report, however, that deals with the primary structure of such a proteinase.