Cleland immunoblotting unmasks unexpected cysteine redox proteoforms

Cysteine redox proteoforms are virtually unstudied because they are extremely difficult to detect. They are difficult to detect by immunoblotting when the polyethylene glycol (PEG)-payloads used to mobility-shift proteoforms into distinct bands block antibody binding. Here, we synthesised a novel compound to reversibly crosslink the PEG-payloads to oxidised cysteines using disulfide bonds. To reductively release the PEG-payloads from mobility-shifted proteoforms, we soaked the gel in Cleland’s reagent: 1,4-dithiothreitol (DTT). Hence, Cleland immunoblotting. Cleland immunoblotting unmasked hitherto undetectable cysteine redox proteoforms. In Xenopus oocytes, we detected 2 cdc20-specific coordinates in an otherwise abstract space housing 1,024 theoretical proteoforms. The coordinates mapped to the fully, all 10 cysteine residues, 100%-reduced and 100%-oxidised proteoforms. The unexpected absence of partially oxidised molecules along the 10-reaction path to the 100%-oxidised proteoform implies novel biology. By providing the technological means to detect cysteine redox proteoforms, Cleland immunoblotting opens new avenues of discovery.