FUNCTIONAL-ACTIVITY OF SPORAMIN FROM SWEET-POTATO (IPOMOEA-BATATAS LAM) - A TUBER STORAGE PROTEIN WITH TRYPSIN INHIBITORY ACTIVITY

Sporamin accounts for about 60% to 80% of total soluble protein in swe et potato tubers, and the predicted protein sequence of sporamin share s significant amino acid sequence identity with some Kunitz-type tryps in inhibitors. We constructed three recombinant plasmids with cDNAs th at encode preprosporamin, prosporamin, and sporamin, and these three w ere expressed in Escherichia coli cells as fusion proteins. All three forms of sporamin expressed in E. coil were shown to have strong inhib itory activity to trypsin in vitro, suggesting that post-translational modifications are not essential for trypsin inhibitory activity. Nort hern blot analysis showed that sporamin transcripts could be systemica lly induced in leaf tissue of sweet potato by wounding. Therefore, spo ramin may have a defense role as a protease inhibitor, in addition to its role as a storage protein.