[A new protein system protects single cysteines against oxidative stress]

The Escherichia coli periplasm contains several proteins from the thioredoxin family. DsbA and Dsbc interact with unfolded proteins to catalyze disulfide bond formation or isomerisation, respectively. The function of a third protein, DsbG, had remained elusive. By trapping DsbG attached to three of its substrates, we made the intriguing discovery that DsbG interacts with folded proteins possessing only one cysteine residue in their sequence. This residue is vulnerable to oxidation and forms a sulfenic acid in vitro. We sought to determine whether this cysteine is also sulfenylated in vivo, which led us to observe extensive sulfenic acid formation in the periplasm, especially in dsbcdsbG strains. Thus, by chasing the substrates of DsbG, we uncovered a new reducing system that is involved in sulfenic acid reduction on a global level (Depuydt et al., Science 326 (2009), 1109-1111). DsbG appears to be a key player in that system. Our work reveals one potentially widespread mechanism whereby the very reactive sulfenic acid modification can be controlled in the cellular environment.