Repair of oxidized methionine residues in the chaperone Spy maintains periplasmic proteostasis under chlorite stress in Escherichia coli.

The bacterial cell envelope is exposed to various stresses, including oxidative stress caused by different types of oxidants, such as reactive oxygen species (ROS) and reactive chlorine species (RCS). In Escherichia coli, the reduction of chlorate into chlorite, a toxic RCS compound, induces the expression of the MsrPQ system, which repairs periplasmic proteins oxidized at methionine residues (methionine sulfoxide, Met-O). In this study, using a proteomic-based approach, we show that chlorite stress also triggers the overproduction of the periplasmic molecular chaperone Spheroplast Protein Y (Spy). This response is mediated by the activation of the BaeSR two-component system. Furthermore, both in vivo and in vitro evidence reveal that Spy's susceptibility to oxidation is critical for its chaperone activity. We demonstrate that the MsrPQ repair system ensures Spy's functionality by reducing its Met-O, thereby safeguarding its role in periplasmic protein homeostasis. Overall, this work reveals Spy as a key target of chlorite-induced oxidative damage and underscores the essential role of MsrPQ in preserving periplasmic protein quality control.