Salmonella enterica BcfH is a trimeric thioredoxin-like bifunctional enzyme with both thiol oxidase and disulfide isomerase activities.

Abstract

AIMS\nThioredoxin fold proteins are ubiquitous in nature. This redox scaffold has evolved to enable a variety of functions, including redox regulation, protein folding and oxidative stress defense. In bacteria, the thioredoxin-like disulfide bond (Dsb) family mediates the oxidative folding of multiple proteins required for fitness and pathogenic potential. Conventionally, Dsb proteins have specific redox functions with monomeric and dimeric Dsbs exclusively catalyzing thiol oxidation and disulfide isomerization, respectively. This contrasts with the eukaryotic disulfide forming machinery where the modular thioredoxin protein PDI mediates thiol oxidation and disulfide reshuffling. In this study we identified and structurally and biochemically characterized a novel Dsb-like protein from Salmonella enterica termed BcfH and defined its role in virulence.\n\n\nRESULTS\nIn the conserved bcf (bovine colonization factor) fimbrial operon, the Dsb-like enzyme BcfH forms a trimeric structure, exceptionally uncommon among the large and evolutionary conserved thioredoxin superfamily. This protein also displays very unusual catalytic redox centers, including an unwound α-helix holding the redox active site and a trans-proline instead of the conserved cis-proline active site loop. Remarkably, BcfH displays both thiol oxidase and disulfide isomerase activities contributing to Salmonella fimbrial biogenesis.\n\n\nINNOVATION AND CONCLUSION\nTypically, oligomerization of bacterial Dsb proteins modulates their redox function, with monomeric and dimeric Dsbs mediating thiol oxidation and disulfide isomerization, respectively. The present study demonstrates a further structural and functional malleability in the thioredoxin-fold protein family. BcfH trimeric architecture and unconventional catalytic sites permit multiple redox functions emulating in bacteria the eukaryotic protein disulfide isomerase dual oxido-reductase activity.