Bacterial suppressor-of-copper-sensitivity (Scs) proteins exhibit diverse thiol-disulfide oxidoreductase cellular functions

Disulfide bond (Dsb) proteins catalyse oxidative protein folding governing bacterial survival and virulence. Dsb systems in Escherichia coli K-12 are well-studied, yet what determines dithiol oxidase or disulfide reductase activity remains unknown. Past studies suggest oligomerisation of periplasmic thiol oxidoreductases dictates the direction of thiol catalytic activity. Here, we studied three suppressor-of-copper-sensitivity C (ScsC) Dsb-like proteins known to exist in the reduced state and bind to copper. These proteins adopt different quaternary structures: Salmonella enterica ScsC (StScsC) is monomeric, while ScsC from Proteus mirabilis (PmScsC) and Caulobacter crescentus (CcScsC) are trimeric. When expressed in the model organism E. coli K-12, we showed that all three ScsC proteins exhibit both dithiol oxidation and disulfide reduction activity, despite structural differences. Interestingly, while ScsC reductase function was supported by the canonical E. coli DsbD reductase, oxidase activity depended on environmental oxidation. However, an engineered monomeric PmScsC synergises with E. coli DsbB to gain dithiol oxidase activity at the expense of reductase function. Thus, oligomerisation could be one mechanism by which ScsC proteins avoid interactions with the periplasmic thiol oxidase pathway. This tightly controls their re-oxidation and maintains ScsC proteins in the reduced state required for binding and sequestering toxic levels of cellular copper.