Abstract
Homologous recombination (HR) repairs DNA double-strand breaks and stabilizes stressed replication forks, and HR deficiency promotes genome instability and cancer. HR requires assembly of RAD51 nucleoprotein filaments on single-stranded DNA (ssDNA), a process regulated by the human RAD51 paralogs RAD51C, XRCC3, RAD51D, and XRCC2. Here, using cryo-electron microscopy, we find that the RAD51–XRCC3–RAD51C complex (RAD51–X3C) assembles into an octamer in which XRCC3 engages the RAD51 DNA-binding surface and RAD51 subunits adopt a misaligned configuration incompatible with filament formation. These features define an autoinhibited RAD51–X3C state that limits nonproductive RAD51 binding to double-stranded DNA or RNA–DNA hybrids while preserving RAD51 availability for ssDNA-dependent strand exchange. We further show that the RAD51D–XRCC2 paralog complex remodels RAD51–X3C into a pentameric RAD51–X3CDX2 assembly by engaging the exposed RAD51C surface and disrupting contacts that stabilize the octamer. This remodeling exposes the RAD51 DNA-binding interface, enhances RAD51–ssDNA filament assembly, promotes strand exchange on RPA-coated ssDNA, and yields a filament-compatible paralog assembly that integrates into ssDNA-bound RAD51 filaments. Together, these findings establish paralog exchange as a mechanism that converts an autoinhibited RAD51–X3C octamer into an activated RAD51–X3CDX2 pentamer to regulate RAD51 filament formation during HR and replication fork preservation.