Isolation and characterization of two distinct cDNA clones encoding corn seed cysteine proteinases

We obtained two cDNA clones encoding corn seed cysteine proteinases (CCP1 and CCP2). Sequence analysis showed that CCP1 consists of 371 amino acid residues, in a prepro-protein form, with two unique short insertions in the mature protein region that are not found in papain or other common CPs. CCP2 consists of 360 amino acid residues with a vacuole sorting signal in the pro-sequence region. An amino acid sequence similarity of 42% was found between the mature protein region of CCP1 and that of CCP2. Although CCP1 is highly homologous to pea 15a CP (72%) and Arabidopsis thaliana RD 19 CP (79%), both of which are known to be induced only when the plant is exposed to a dehydrated environment, it showed very low homologies to other known cysteine proteinases (38-43%). CCP2 showed as much as 87% and 89% identity to rice oryzain gamma and barley aleurain, respectively. We also observed that the CCP1 mRNA is expressed in ripened corn seeds, although its expression reaches a maximum 5 days after the onset of germination. On the other hand, the CCP2 mRNA is expressed only during germination, with maximum expression at the 3-day stage. These results suggest the presence of at least two cysteine proteinases playing differential roles in the corn seeds.