Abstract
SLX4 disabled in Fanconi anemia group P is a multifunctional scaffold protein that coordinates the action of structure-specific endonucleases and other DNA repair proteins to ensure genome stability. We show here that SLX4 drives the regulated assembly of an extensive protein network cross-linked by SLX4 dimerization and SUMO-SIM interactions that yield liquid-like nuclear condensates. SLX4 condensates compartmentalize the SUMO system and the STUbL RNF4 to enhance selectively the modification of substrate proteins by SUMO and ubiquitin. Specifically, we find that the assembly of SLX4 condensates induces the processing of topoisomerase 1 - DNA protein crosslinks (TOP1cc), as well as the resection of newly synthesized DNA. This unanticipated function of SLX4 emerges from the collective behavior of proteins that compose SLX4 condensates in live cells. We conclude that SLX4 foci are functional compartments maintained by site-specific protein-protein interactions that control key biochemical reactions in DNA repair.