Structural studies on the disulfide bond isomerisation pathway

Abstract

Disulfide bonds are important for the structure and function and many proteins, including hormones, receptors, antibodies and bacterial toxins. Two complementary pathways facilitate the formation of correct disulfide bonds during oxidative protein folding in the periplasm of Escherichia coli. New disulfide bonds are non-specifically formed by the disulfide bond formation pathway. The disulfide bond isomerisation pathway, consisting of DsbC and DsbD, rearranges incorrect disulfide bonds that trap proteins in non-functional conformations. The protein disulfide bond isomerase DsbC interacts with these misfolded proteins and facilitates the refolding by isomerising incorrect disulfide bonds. To improve the current understanding of disulfide bond isomerisation the crystal structures of oxidised DsbC and DsbC Cl0lS, a mimic of the active reduced form, have been determined. The 1.9Å structure of DsbC revealed a V-shaped homodimeric molecule with each monomer consisting of two domains connected by a hinged linker helix. The N-terminal domains meet at the base of the V, forming the dimer interface. The catalytic C-terminal domains are located at the outer arms of the V and contain a thioredoxin fold with an active site CGYC motif. A 40 x 4O x 25 Å3 central uncharged cleft between the two active sites is proposed to bind substrate proteins, suggesting a model for DsbC catalysed disulfide bond isomerisation. DsbC is specifically activated by the N-terminal α domain of the transmembrane electron transporter DsbD. An intermediate of the electron transport reaction was trapped, yielding a covalent DsbC-DsbDα complex. The 2.3 Å crystal structure of the complex shows DsbDα binding into the central cleft of the V-shaped DsbC dimer, which assumes a closed conformation on complex formation. DsbDα is a new type of thiol oxidoreductase with an Ig fold. Comparison of the complex with oxidised DsbDα reveals major conformational changes in a cap structure that regulates the accessibility of the DsbDα active site. These results provide a first insight into a novel electron transport chain across the inner membrane and explain how DsbC is selectively activated by DsbD using electrons derived from the cytoplasm.