AbstractThe oxidation of a key cysteine residue (Cys106) in the parkinsonism‐associated protein DJ‐1 regulates its ability to protect against oxidative stress and mitochondrial damage. Cys106 interacts with a neighboring protonated Glu18 residue, stabilizing the Cys106‐SO2− (sulfinic acid) form of DJ‐1. To study this important post‐translational modification, we previously designed several Glu18 mutations (E18N, E18D, E18Q) that alter the oxidative propensity of Cys106. However, recent results suggest these Glu18 mutations cause loss of DJ‐1 dimerization, which would severely compromise the protein's function. The purpose of this study was to conclusively determine the oligomerization state of these mutants using X‐ray crystallography, NMR spectroscopy, thermal stability analysis, circular dichroism spectroscopy, sedimentation equilibrium ultracentrifugation, and cross‐linking. We found that all of the Glu18 DJ‐1 mutants were dimeric. Thiol cross‐linking indicates that these mutant dimers are more flexible than the wild‐type protein and can form multiple cross‐linked dimeric species due to the transient exposure of cysteine residues that are inaccessible in the wild‐type protein. The enhanced flexibility of Glu18 DJ‐1 mutants provides a parsimonious explanation for their lower observed cross‐linking efficiency in cells. In addition, thiol cross‐linkers may have an underappreciated value as qualitative probes of protein conformational flexibility.image DJ‐1 is a homodimeric protein that protects cells against oxidative stress. Designed mutations that influence the regulatory oxidation of a key cysteine residue have recently been proposed to disrupt DJ‐1 dimerization. We use cysteine cross‐linking and various biophysical techniques to show that these DJ‐1 mutants form dimers with increased conformational flexibility.