Cysteine 62 of Bax is critical for its conformational activation and its proapoptotic activity in response to H2O2-induced apoptosis.

Bax is activated and translocated onto mitochondria to mediate cytochrome c release and apoptosis. The molecular mechanisms of Bax activation during apoptosis remain a subject of debate. We addressed the question of whether reactive oxygen species could directly activate Bax for its subsequent translocation and apoptosis. Using the SW480 human colon adenocarcinoma cell line stably expressing Bax fused to GFP, we showed that H2O2 induces Bax conformational change, mitochondrial translocation, and subsequent oligomerization at mitochondria. We found that H2O2-induced Bax activation is dependent on the conserved cysteine residue 62 of Bax. Mutation of cysteine 62, but not cysteine 126, to serine or alanine abolished its activation by H2O2 but not other death stimuli, both in SW480 and Bax-deficient HCT116 cells, whereas wild type Bax sensitizes these cells to apoptosis. Cysteines of Bax could chemically react with H2O2. Mutation of Bax BH3 domain in the presence of cysteine 62 also abolished Bax proapoptotic activity. We conclude that reactive oxygen species could be a direct signal for Bax activation by reacting with cysteine residues. Our results identify a critical role of cysteine 62 in oxidative stress-induced Bax activation and subsequent apoptosis.