Structure of the soluble domain of a membrane-anchored thioredoxin-like protein from Bradyrhizobium japonicum reveals unusual properties

TlpA is an unusual thioredoxin-like protein present in the nitrogen-fixing soil bacterium Bradyrhizobium japonicum. A hydrophobic N-terminal transmembrane domain anchors it to the cytoplasmic membrane, whereby the hydrophilic thioredoxin domain becomes exposed to the periplasmic space. There, TlpA catalyses an essential reaction, probably a reduction, in the biogenesis of cytochrome aa3. The soluble thioredoxin domain (TlpAsol), devoid of the membrane anchor, was purified and crystallized. Oxidized TlpAsol crystallized as a non-covalent dimer in the space group P212121. The X-ray structure analysis was carried out by isomorphous replacement using a xenon derivative. This resulted in a high-resolution (1.6 Å) three-dimensional structure that displayed all of the features of a classical thioredoxin fold. A number of peculiar structural details were uncovered: (i) Only one of the two active-site-cysteine sulphurs (Cys72, the one closer to the N terminus) is exposed on the surface, making it the likely nucleophile for the reduction of target proteins. (ii) TlpAsol possesses a unique structural disulphide bond, formed between Cys10 and Cys155, which connects an unprecedented N-terminal α helix with a β sheet near the C terminus. (iii) An insertion of about 25 amino acid residues, not found in the thioredoxin prototype of Escherichia coli, contributes only marginally to the thioredoxin fold, but forms an extra, surface-exposed α helix. This region plus another surface-exposed stretch (-Ile-Gly-Arg-Ala-), which is absent even in the closest TlpA relatives, might be considered as specificity determinants for the recognition of target proteins in the periplasm. The TlpAsol structure paves the way towards unraveling important structure-function relationships by rational mutagenesis.