Cell cycle-dependent S-sulfenyl proteomics uncover a redox switch in p21-CDK feedback governing the proliferation-senescence decision

Cells are constantly exposed to reactive oxygen species (ROS) from both intrinsic and extrinsic sources. ROS influence proliferation and cell fate through cysteine oxidation (S-sulfenylation), but specific targets and mechanisms remain unclear. Here, we use redox proteomics to identify cell cycle-coordinated S-sulfenylation and investigate its role in cell cycle decision-making. We find that oxidation of a single cysteine (C41) on the CDK inhibitor p21 during G2 phase determines whether cells continue to proliferate. Preventing C41 oxidation redirects p21 from CDK4-Cyclin D to CDK2-Cyclin A, affecting a negative feedback loop that regulates p21 stability. When C41 cannot be oxidized in G2, daughter cells inherit more p21 from their mother, which decreases proliferation and induces senescence upon irradiation. We therefore identify a redox switch in a core cell cycle regulator that governs the decision to proliferate or exit the cell cycle and present a cell cycle-resolved S-sulfenyl proteome as a valuable resource.