Proteasomal surveillance of exposed cysteines.

UNLABELLED: Cysteine residues are critical for both the catalytic activity and structural stability of many proteins, yet their high chemical reactivity threatens proteome integrity. Whether cells have specialized mechanisms to recognize and eliminate proteins with aberrant or exposed cysteines is unknown. By systematically analyzing the stability of thousands of cysteine-containing peptides, here we identify two complementary surveillance mechanisms targeting proteins with exposed cysteine residues. First, a set of E3 ubiquitin ligases - including AMBRA1, TRPC4AP, FBXO3, and FBXO21 - detects double cysteines and targets proteins for ubiquitin-dependent proteasomal degradation. Second, the PSMD14/Rpn11 subunit of the 26S proteasome recognizes exposed cysteines in a ubiquitin-independent manner. Both pathways function in quality control to clear misfolded proteins exposing cysteine residues, including zinc finger-containing proteins. Mutational escape from the ubiquitin-independent pathway through dysregulation of ornithine decarboxylase or FOS levels is responsible for two human diseases, Bachmann-Bupp Syndrome and osteoblastoma, respectively. Our findings reveal conserved mechanisms for degrading proteins with exposed cysteines and uncover an alternative mode of substrate recognition by the 26S proteasome.

SHORT SUMMARY: Cysteine exposure acts as a conserved degradation signal that triggers two modes of turnover by the 26S proteasome: ubiquitin-dependent and direct, ubiquitin-independent degradation. These findings demonstrate the importance of maintaining cysteine homeostasis and reveal an alternative mode of proteasomal degradation.