Firrm functions as a multiprotein interaction platform that coordinates Mcm8-9-Hrob recruitment and Fignl1-mediated Rad51 unloading
Rad51 forms nucleoprotein filaments to catalyze homology search and strand exchange. The transition to recombination-associated DNA synthesis is accompanied by Rad51 unloading and recombination helicase recruitment. However, how these reactions are coordinated remains unclear. Using Xenopus egg extracts, we identify a mechanistic coupling between Rad51 unloading and recombination helicase recruitment. Proteomic analysis revealed that Rad51 filaments assembled on double-stranded DNA recruit a limited set of recombination regulators, including C1orf112/Firrm, Fignl1, Mcm8, Mcm9, and Hrob. Firrm forms a stoichiometric complex with Fignl1 and directly binds short acidic motifs in the intrinsically disordered regions of both Mcm9 and Hrob. The Firrm-Mcm9 interaction promotes recruitment of Mcm8-9 and Hrob to Rad51 filaments. Purified Mcm8-9 stimulates the ATPase activity of Fignl1 through Firrm in a DNA-dependent manner. Rad51 unloading from double-stranded DNA depends primarily on Fignl1 ATPase activity and is stimulated by Firrm-Mcm9 interaction. In human cells, FIRRM-FIGNL1 and MCM8-9-HROB act in the same genetic pathway to confer resistance to DNA interstrand crosslinks. These results support a model in which Rad51 filaments and Firrm-Fignl1 promote recruitment of Mcm8-9-Hrob, which, in turn, stimulates Fignl1 ATPase activity, thereby coupling recombination helicase recruitment with Rad51 unloading.