Oryzacystatins as the first well-defined cystatins of plant origin and their target proteinases in rice seeds.

Two cystatins occur in mature seeds of the rice, Oryza sativa L. japonica, which are named oryzacystatin I (OC-I) and oryzacystatin II (OC-II). These are highly homologous to each other and are significantly homologous to cystatin superfamily members of animal origin, especially to family-2 cystatins. However, both lack disulfide bonds as in the case of family-1 cystatins (stefins). Each of OC-I and OC-II thus seems to be chimerical of family-1 and family-2 cystatins, and we propose that a new category such as "phytocystatin" be opened for these cystatins of plant origin. For specificity it was observed that OC-I inhibits papain 100 times more efficiently than cathepsin H, whereas, OC-II inhibits cathepsin H 100 times more efficiently than papain. A cysteine proteinase, named oryzain alpha, exists in germinating rice seeds. cDNA cloning studies have disclosed that two other related species, named oryzain beta and gamma, are also present. In respect to the amino acid sequence, oryzain alpha and beta are homologous to papain and oryzain gamma is homologous to cathepsin H. These observations suggest the possibility that either or both of oryzain alpha and beta are target enzymes of OC-I and oryzain gamma is a target enzyme of OC-II.