Cysteine proteinase (CPR) cDNA clone (SPCPRPP) of sweet potato (Ipomoea batatas [L.] Lam 'Tainong 57') storage roots were isolated by differential display. The open reading frame in this cDNA encodes a pre-proprotein of 371 amino acids with conserved catalytic amino acids of papain. Examination of the expression patterns in sweet potato by Northern blot analyses revealed that the transcripts of SPCPRPP were specifically induced in the storage roots. Recombinant SPCPRPP protein overproduced in Escherichia coli (M15) was purified by Ni2+-chelated affinity chromatography. Active recombinant SPCPRPP protein was able to digest the 22 kDa sweet potato trypsin inhibitor (TI) protein when the latter was reduced by DTT (dithiothreitol) or NTS (NADPH/thioredoxin system). A smaller peptide (14 kDa) was obtained as a digestion product. These results suggest that CPR is responsible for initiation of degradation and re-mobilization of stored 22 kDa TI during sprouting of SP storage roots after the reduction of 22 kDa TI by the NTS.
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