A novel two-chain proteinase inhibitor generated by circularization of a multidomain precursor protein

Female reproductive tissues of the ornamental tobacco amass high levels of serine proteinase inhibitors (PIs) for protection against pests and pathoge ns. These PIs are produced from a precursor protein composed of six repeats each with a protease reactive site. Here we show that proteolytic processi ng of the precursor generates five single-chain PIs and a remarkable two-ch ain inhibitor formed by disulfide-bond Linkage of Nand C-terminal peptide f ragments. Surprisingly, PI precursors adopt this circular structure regardl ess of the number of inhibitor domains, suggesting this bracelet-like confo rmation is characteristic of the widespread potato inhibitor II (Pot II) pr otein family.