Crystallization and initial crystallographic analysis of the disulfide bond isomerase DsbC in complex with the alpha domain of the electron transporter DsbD.

The protein disulfide bond isomerase DsbC catalyzes the rearrangement of incorrect disulfide bonds during oxidative protein folding in the periplasm of Escherichia coli. The active site cysteines of DsbC are maintained in the active reduced form by the transmembrane electron transporter DsbD. DsbD obtains electrons from the cytoplasm, transports them across the inner membrane, and passes them onto periplasmic substrates, such as DsbC. The electron transport process involves several thiol disulfide exchange reactions between different classes of thiol oxidoreductase. We were able to trap the final electron transport reaction using active site mutants yielding a stable DsbC-DsbDalpha complex. This disulfide cross-linked complex was purified to homogeneity and crystallized. Dehydration of the tetragonal crystals changed the unit cell dimensions from a approximately b = 73 A, c = 267.5 A to a = b = 68.9 A, c = 230.3 A, reducing the cell volume by 23% and the solvent content from 55 to 41%. Crystal dehydration and cryo-cooling improved the diffraction quality of the crystals from 7 to 2.3 A resolution.