Structural and functional characterisation of ScsC, a periplasmic thioredoxin-like protein from Salmonella enterica serovar Typhimurium

AIMS: The prototypical protein disulfide bond (Dsb) formation and protein refolding pathways in the bacterial periplasm involving Dsb proteins has been most comprehensively defined in Escherichia coli. However, genomic analysis has revealed several distinct Dsb-like systems in bacteria including the pathogen Salmonella enterica serovar Typhimurium. This includes the scsABCD locus, which encodes a system that has been shown via genetic analysis to confer copper tolerance but whose biochemical properties at the protein level are not defined. The aim of this study was to provide functional insights into the soluble ScsC protein through structural, biochemical, and genetic analyses.

RESULTS: Herein, we describe the structural and biochemical characterisation of ScsC, the soluble DsbA-like component of this system. Our crystal structure of ScsC reveals a similar overall fold to DsbA, although the topology of beta-sheets and alpha-helices in the thioredoxin domains differ. The midpoint reduction potential of the CXXC active site in ScsC was determined to be -132 mV versus normal hydrogen electrode. The reactive site cysteine has a low pKa, typical of the nucleophilic cysteines found in DsbA-like proteins. Deletion of scsC from S. Typhimurium elicits sensitivity to copper (II) ions, suggesting a potential involvement for ScsC in disulfide folding under conditions of copper stress.

INNOVATION AND CONCLUSION: ScsC is a novel disulfide oxidoreductase involved in protection against copper ion toxicity.