The atypical thiol-disulfide exchange protein {alpha}-DsbA2 from Wolbachia pipientis is a homotrimeric disulfide isomerase

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 alpha-DsbA1 and alpha-DsbB form a redox relay pair analogous to E. coli DsbA/B. The third enzyme alpha-DsbA2 incorporates a DsbA-like sequence but does not interact with alpha-DsbB. In comparison with other DsbA enzymes, alpha-DsbA2 has ~50 extra N-terminal residues. The crystal structure of alpha-DsbA2DeltaN, the N-terminally truncated form in which these residues are removed confirms the DsbA-like nature of this domain. However, alpha-DsbA2 does not have DsbA-like activity: it is structurally and functionally different as a consequence of its N-terminal residues. First, alpha-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 alpha-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.