CORN KERNEL CYSTEINE PROTEINASE-INHIBITOR AS A NOVEL CYSTATIN SUPERFAMILY MEMBER OF PLANT-ORIGIN - MOLECULAR-CLONING AND EXPRESSION STUDIES

A full-length cDNA clone for a cysteine proteinase inhibitor (cystatin) was isolated from a lambdagt10 cDNA library of immature com kernels by screening with a mixture of cDNA inserts for oryzacystatins I and II. The cDNA clone spans 960 base pairs, encoding a 135-amino-acid protein containing a signal peptide fragment. The protein, named corn cystatin I, is considered to be a member of the cystatin superfamily, since it contains the commonly conserved Gln-Val-Val-Ala-Gly region that exists in most known cystatins as a probable binding site and is significantly similar to other cystatins in its overall amino acid sequence. Com cystatin I expressed in Escherichia coli showed a strong papain-inhibitory activity. Northern blot analysis showed that the amount of mRNA for com cystatin I reaches a maximum 2 weeks after flowering and then decreases gradually.