Identification and characterization of a rice cysteine endopeptidase that digests glutelin

Little or no endopeptidase activity was detected in extracts from storage organs of dark-grown rice seeds until day 6 of post-imbibition, and the activity expressed per seed increased notably after day 9, reached a maximum on day 18, then decreased. Two major endopeptidases, REP-1 and REP-2, were present in the 40-75% saturated ammonium sulfate fraction from day-9 germinated seeds, and could be separated by hydrophobic column chromatography. REP-1 was further purified to a single polypeptide of 36 kDa. REP-1 digested in vitro both the acidic and basic subunits of rice glutelin, the major seed storage protein of rice. Determination of the N-terminal amino acid sequence and experiments with protease inhibitors indicated that REP-1 is a cysteine endopeptidase. The nucleotide sequence of a full-length REP-1 cDNA was determined by a combination of screening of cDNA libraries from rice seeds and the 5' rapid amplification of cDNA ends technique.