THIOCALSIN - A THIOREDOXIN-LINKED, SUBSTRATE-SPECIFIC PROTEASE DEPENDENT ON CALCIUM

We describe a protease, named ''thiocalsin,'' that is activated by cal cium but only after reductive activation by thioredoxin, a small prote in with a redox-active disulfide group that functions widely in regula tion, Thiocalsin appeared to be a 14-kDa serine protease that function s independently of calmodulin, The enzyme, purified from germinating w heat grain, specifically cleaved the major indigenous storage proteins , gliadins and glutenins, after they too had been reduced, preferentia lly by thioredoxin, The disulfide groups of the enzyme, as well as its protein substrates, were reduced by thioredoxin via NADPH and the ass ociated enzyme, NADP-thioredoxin reductase, The results broaden the ro les of thioredoxin and calcium and suggest a joint function in activat ing thiocalsin, thereby providing amino acids for germination and seed ling development.