N‐chlorination, a Reversible Post‐translational Modification That Activates Chaperone Function in RidA

RidA from Escherichia coli is a member of the highly conserved YjgF/YER057p/UK114 protein family. Members of this family have a multitude of assigned functions, which include translational inhibition (human), Hsp90‐like chaperone activity (fruit fly), and enamine/imine deaminase activity in the branched‐chain amino acid synthesis (Salmonella enterica). In the present study, we describe a novel function of RidA from E. coli . When treated with HOCl (household bleach), RidA is converted into a highly effective chaperone holdase preventing aggregation of the model substrate citrate synthase. Activation of RidA is reversed by treatment with DTT, ascorbic acid, the thioredoxin system, and glutathione. However, in contrast to Hsp33, another well‐described HOCl‐activated chaperone in E. coli , activation of RidA does not depend on cysteine modification. Instead, upon treatment with HOCl or chloramines, the amino group content of RidA decreases as a result of reversible N‐chlorination of positively charged residues. N‐chlorination increases surface hydrophobicity, which enables RidA to bind its unfolding client proteins. Deletion of ridA results in an HOCl‐sensitive phenotype of E. coli demonstrating its crucial role during HOCl stress. A proteomic analysis of RidA's client proteins revealed that N‐chlorinated RidA binds highly abundant cytosolic proteins to protect them from HOCl‐mediated precipitation. N‐chlorination of positively charged residues thus is a novel post‐translational modification that reversibly turns RidA into a chaperone holdase.