The Protein-disulfide Isomerase DsbC Cooperates with SurA and DsbA in the Assembly of the Essential β-Barrel Protein LptD

The assembly of the beta-barrel proteins present in the outermembrane (OM) of Gram-negative bacteria is poorly characterized. After translocation across the inner membrane, unfolded beta-barrel proteins are escorted across the periplasm by chaperones that reside within this compartment. Two partially redundant chaperones, SurA and Skp, are considered to transport the bulk mass of beta-barrel proteins. We found that the periplasmic disulfide isomerase DsbC cooperates with SurA and the thiol oxidase DsbA in the folding of the essential beta-barrel protein LptD. LptD inserts lipopolysaccharides in the OM. It is also the only beta-barrel protein with more than two cysteine residues. We found that surAdsbC mutants, but not skpdsbC mutants, exhibit a synthetic phenotype. They have a decreased OM integrity, which is due to the lack of the isomerase activity of DsbC. We also isolated DsbC in a mixed disulfide complex with LptD. As such, LptD is identified as the first substrate of DsbC that is localized in the OM. Thus, electrons flowing from the cytoplasmic thioredoxin system maintain the integrity of the OM by assisting the folding of one of the most important beta-barrel proteins.