The atypical thiol-disulfide exchange protein α-DsbA2 from Wolbachia pipientis is a homotrimeric disulfide isomerase

Abstract DiSulfide Bond (DSB) oxidative folding enzymes are master regulators of virulence localized to the periplasm of many Gram-negative bacteria. The archetypal DSB machinery from Escherichia coli K12 has a dithiol oxidizing redox relay pair (DsbA/B), a disulfide isomerizing redox relay pair (DsbC/D) and specialist reducing enzymes DsbE and DsbG that also interact with DsbD. By contrast the Gram-negative bacterium Wolbachia pipientis encodes just three DSB enzymes. Two of these α-DsbA1 and α-DsbB form a redox relay pair analogous to E. coli DsbA/B. The third enzyme α-DsbA2 incorporates a DsbA-like sequence but does not interact with α-DsbB. In comparison with other DsbA enzymes, α-DsbA2 has ∼50 extra N-terminal residues. The crystal structure of α-DsbA2ΔN, the N-terminally truncated form in which these residues are removed confirms the DsbA-like nature of this domain. However, α-DsbA2 does not have DsbA-like activity: it is structurally and functionally different as a consequence of its N-terminal residues. First, α-DsbA2 is a powerful disulfide isomerase and a poor dithiol oxidase – ie its role is to shuffle rather than introduce disulfide bonds. Moreover, small-angle X-ray scattering of α-DsbA2 reveals a homotrimeric arrangement. Our results allow us to draw conclusions about the factors required for functionally equivalent enzymatic activity across structurally diverse protein architectures.