The Protease Inhibitors of Seeds

Abstract

Seeds commonly contain relatively high levels of protein proteinase inhibitors. The majority of the well characterized inhibitors are active against serine endopeptidases, while a small number are inhibitors of cysteine endopeptidases. Sufficient amino acid sequence data have accumulated to allow their grouping into nine families. Members of eight families inhibit serine endopeptidases: the Kunitz Soybean Trypsin Inhibitor, the Bowman-Birk Trypsin Inhibitor, the Squash Trypsin Inhibitor, the Potato Protease Inhibitor I, the CM Protein/Napin Protease Inhibitor, the Protein Z/Serpin, and the Maize Bifunctional Inhibitor/Thaumatin families. One inhibitor family, the Phytocystatins, inhibits cysteine endopeptidases. Less well characterized inhibitors of metalloproteinases have also been described, but their sequence data are not yet available. The protease inhibitors appear to be involved primarily in defense of the seed from exogenous proteases such as those secreted by pathogenic microbes or by insect pests. In some cases the inhibitors may also act as storage depots for sulfur-containing amino acids. During germination and early seedling growth, these inhibitors are degraded much like the bulk storage proteins. In a relatively small number of instances the seed protease inhibitors may serve to regulate endogenous proteases. Transgenic plants expressing the appropriate seed protease inhibitors in aerial tissue have been found to have increased resistance to insect depredation. This promises to be an effective way to control insect pests without the use of environmentally damaging synthetic insecticides.