Bacteria use foldases such as Disulfide bond (Dsb) forming enzymes to catalyse the oxidative folding of many toxins and secreted proteins required for virulence and fitness (1). In addition to the classic Dsb system, which includes an oxidative DsbA/DsbB pathway and an isomerase DsbC/DsbD pathway, genomic analysis of the human pathogen Salmonella enterica Typhimurium revealed a second Dsb-like system encoded by the scsABCD locus. This locus contains genes encoding for Scs(A-D) proteins that have been associated with an increased tolerance to copper and some encompass a thioredoxin-like fold (2).
This project focuses on the detailed characterisation of the novel Dsb-like Scs system in S. Typhimurium. Our work shows that the catalytic N-terminal domain of ScsB (N-ScsB) is highly reducing in the oxidative environment of the periplasm. Furthermore, the ability of N-ScsB to transfer electrons to periplasmic partner proteins was investigated. Our results demonstrate that Salmonella ScsC, a reductase encoded in the scs operon that structurally resembles DsbA oxidases (3), is a substrate of ScsB. Interestingly, N-ScsB does not efficiently reduce the different DsbA oxidases present in Salmonella. These different protein interactions were also thermodynamically characterised using Isothermal Titration Calorimetry (ITC). Further structural and biochemical studies will define the copper binding site and how these proteins contribute to Salmonella copper tolerance.