PROTEIN-FOLDING ACTIVITIES OF ESCHERICHIA-COLI PROTEIN DISULFIDE-ISOMERASE

DsbA is an Escherichia coli periplasmic protein that mediates disulfid e bond formation in newly secreted proteins in vivo. Addition of thiol reagents to purified dsbA reduces its disulfide bond and yields disul fide isomerase activity after removal of the thiol reagent. DsbA can c atalyze the conversion of a stable misfolded protein, misfolded IGF-I (mis-IGF-I), to its correctly folded conformation under physiological conditions. This conversion is the result of breaking and re-forming t wo disulfide bonds. The uncatalyzed rate of this reaction is undetecta ble. Kinetic analysis of the reaction yielded a K(m) of 43 muM and a k (cat) of 0.2 min-1. The oxidized form of dsbA stimulates the oxidative folding of completely reduced IGF-I at pII 7.0. Thus, dsbA has two po ssible functions depending on its redox state. The reduced form of the protein is a disulfide isomerase while the oxidized protein can assis t formation of disulfide bonds in reduced substrates under physiologic al conditions.