Thioredoxin-like protein TlpA from Bradyrhizobium japonicum is a reductant for the copper metallochaperone ScoI

TlpA and ScoI of Bradyrhizobium japonicum are membrane-anchored thioredoxin-like proteins oriented towards the periplasm. TlpA is a protein-disulfide reductase. ScoI is a copper chaperone for cytochrome oxidase biogenesis. TlpA with its negative redox potential (E-o' -256 mV) was shown here to reduce oxidized ScoI, for which we determined a less negative E-o' (-160 mV). The fast forward reaction (rate constant 9.4 x 10(4) M-1 s(-1)) was typical for physiologically relevant disulfide exchange reactions. A transient TlpA-ScoI heterodisulfide formed between Cys107 of TlpA's active site (C107XXC110) and Cys78 of ScoI's copper-binding site (C74XXXC78). We conclude that TlpA recycles ScoI to the dithiol form prior to metallation.