CHARACTERIZATION OF A GENOMIC SEQUENCE CODING FOR POTATO MULTICYSTATIN, AN 8-DOMAIN CYSTEINE PROTEINASE-INHIBITOR

A gene coding for potato multicystatin (PMC), the crystalline inhibito r of cysteine proteases which is found in tubers, was isolated and cha racterized. The deduced polypeptide product of this genomic sequence i s 757 amino acids long and has a molecular mass of 86,778 Da. It consi sts exclusively of eight closely related domains, with 53-89% identity of residues. Each repeated unit is homologous to the cystatin superfa mily of cysteine protease inhibitors. To date, no other member of this family has been found to contain so many inhibitor domains in one pol ypeptide. Eight introns are proposed in the 3.5 kb of genomic DNA codi ng for PMC, one in each cystatin unit. There is a family of 4 to 6 suc h large genes in potato, while in pea and maize the homologues are muc h smaller, and probably code for single-domain cystatins. PMC transcri pts are abundant in tubers, but scarce in undamaged leaves or stems of field-grown potatoes. The tuber messages are derived from at least fo ur genes (including the cloned example). The pattern of gene expressio n, as well as the properties of the protein, suggest that PMC has a ro le in the plant's defense system.