Site-specific cysteine oxidation regulates 26S proteasomes

The main machinery responsible for cellular protein maintenance is the ubiquitin-proteasomal system. The main task of the system is a fast and efficient degradation of proteins not needed anymore in cellular metabolism. It is accepted that upon oxidative stress, the proteasome suffers a series of functional alterations, including a reversible, oxidation-triggered 26 S proteasome disassembly into its catalytic (CP) and regulatory (RP) particles. Formation of low fluxes of oxidants were stimulated with antimycin or palmitic acid in pancreatic MIN6 cells, inducing a partial increase of dimerization of mitochondrial and citosolic peroxiredoxins. Under this conditions, no significant increase of total protein thiol oxidation was detected. However, ATP-stimulation of 26 S proteasomes was inhibited while the particle remained fully assemblied. Discrete cysteine residues of both CP and RP were identified oxidized by a cysteine-targeted proteomics approach. Such results may rely on differential reactivity of cysteines in the subunits and suggest specific regulation of particles by discrete cysteine oxidation. Further studies will elucidate the type of modification on each cysteine residue and the impact. Our results suggest a novel mechanism of redox regulation of 26 S proteasome that precedes oxidation-driven 26 S particle disassembly.