Allosteric activation of the SPRTN protease by ubiquitin maintains genome stability
The DNA-dependent protease SPRTN maintains genome stability by degrading toxic DNA-protein crosslinks (DPCs). To understand how SPRTNs promiscuous protease activity is confined to the cleavage of crosslinked proteins, we reconstitute the repair of DPCs including their modification with SUMO and ubiquitin chains, using recombinant human proteins. We discover that DPC ubiquitylation strongly activates SPRTN independently of SPRTNs known ubiquitin-binding domains. Using protein structure prediction, MD simulations and NMR spectroscopy we reveal that ubiquitin binds to an interface at the back of SPRTNs protease domain, promoting an active conformation. Replacing key interfacial residues prevents ubiquitin-dependent activation of SPRTN, which leads to genomic instability and cell cycle defects in cells expressing hypomorphic SPRTN variants that cause premature aging and liver cancer in Ruijs-Aalfs syndrome patients. Collectively, our results demonstrate that SPRTN activation is coupled to the modification of the crosslinked protein, explaining how specificity is achieved during DPC repair.