A conserved SUMO pathway repairs topoisomerase DNA-protein cross-links by engaging ubiquitin-mediated proteasomal degradation.

Topoisomerases form transient covalent DNA cleavage complexes to perform their reactions. Topoisomerase I cleavage complexes (TOP1ccs) are trapped by camptothecin and TOP2ccs by etoposide. Proteolysis of the trapped topoisomerase DNA-protein cross-links (TOP-DPCs) is a key step for some pathways to repair these lesions. We describe a pathway that features a prominent role of the small ubiquitin-like modifier (SUMO) modification for both TOP1- and TOP2-DPC repair. Both undergo rapid and sequential SUMO-2/3 and SUMO-1 modifications in human cells. The SUMO ligase PIAS4 is required for these modifications. RNF4, a SUMO-targeted ubiquitin ligase (STUbL), then ubiquitylates the TOP-DPCs for their subsequent degradation by the proteasome. This pathway is conserved in yeast with Siz1 and Slx5-Slx8, the orthologs of human PIAS4 and RNF4.