PapD chaperone function in pilus biogenesis depends on oxidant and chaperone-like activities of DsbA.
Adhesive P pili of uropathogenic Escherichia coli were not assembled by a strain that lacks the periplasmic disulfide isomerase DsbA. This defect was mostly attributed to the immunoglobulin-like pilus chaperone PapD, which possesses an unusual intrasheet disulfide bond between the last two beta-strands of its CD4-like carboxyl-terminal domain. The DsbA-dependent formation of this disulfide bond was critical for PapD's proper folding in vivo. Interestingly, the absence of the disulfide bond did not prevent PapD from folding in vitro or from forming a complex with the pilus adhesin in vitro. We suggest that DsbA maintains nascently translocated PapD in a folding-competent conformation prior to catalyzing disulfide bond formation, acting both as an oxidant and in a chaperone-like fashion. Disulfide bond formation in pilus subunits was also mediated by DsbA even in the absence of PapD. However, the ability of pilus subunits to achieve native-like conformations in vivo depended on PapD. These results suggest that a productive folding pathway for subunits requires sequential interactions with DsbA and the PapD chaperone.