Two snapshots of electron transport across the membrane: insights into the structure and function of DsbD

In Escherichia coli, the periplasmic protein disulfide isomerase, DsbC, is maintained reduced by transfer of electrons from cytoplasmic thioredoxin-1 (Trx1) via the cytoplasmic membrane protein, DsbD. The transmembrane domain of DsbD (DsbDbeta), which comprises eight transmembrane segments (TMs), contains two redox-active cysteines (Cys-163 and Cys-285), each of which is water-exposed to both sides of the membrane. Cys-163 in TM1 and Cys-285 in TM4 can interact with cytoplasmic Trx1 and a periplasmic Trx-like domain of DsbD, respectively. When Cys-163 and Cys-285 are disulfide-bonded, the C-terminal halves of TM1 and TM4 are water-exposed, whereas the N-terminal halves of these TMs are not. To assess possible conformational changes of DsbDbeta when its two cysteines are reduced, we have determined the accessibility of portions of TM1 and TM4. We substituted cysteines for amino acids in these TM segments and determined alkylation accessibility. We find that the alkylation accessibility of single Cys replacements in TM1 and TM4 is the same in oxidized and reduced DsbDbeta, indicating a relatively static conformation of DsbDbeta between the two redox states. We also find that the accessibility of amino acids of TM2 and TM3 when Cys-163 and Cys-285 are oxidized or reduced shows no change. Together, these results support a relatively static structure of DsbDbeta in the switch between the oxidized and the reduced state but raise the possibility of conformational changes when interacting with Trx proteins. In addition, we also find water-exposed residues in the cytoplasmic proximal portion of TM3, allowing a more detailed characterization of the cavity in DsbDbeta.