BCDX2–CX3 and DX2–CX3 complexes assemble and stabilize RAD51 filaments

The repair of DNA double-strand breaks by homologous recombination (HR) is essential for genomic integrity, and its dysregulation is a hallmark of cancer¹. Central to HR is the RAD51 recombinase, whose assembly into a nucleoprotein filament is governed by five RAD51 paralogs (RAD51B, RAD51C, RAD51D, XRCC2, XRCC3)². Mutations in any of these proteins predispose individuals to multiple cancers or genetic disorders^3-6. These paralogs are thought to form two functionally separate complexes, BCDX2 (RAD51B-C-D-XRCC2) and CX3 (RAD51C-XRCC3), that act independently at different stages of HR^7-11. Here, we demonstrate that all five paralogs can assemble into a single, ATP-dependent BCDX2-CX3-RAD51 supercomplex. The architecture of this assembly bound to single-stranded DNA (ssDNA) reveals a contiguous filament where the CX3 module stacks atop BCDX2, creating a protofilament template for RAD51 filament formation. We further identify a novel, RAD51B-independent DX2-CX3 complex (RAD51D-XRCC2-RAD51C-XRCC3) functioning as a stable RAD51 anchor on ssDNA, and we capture it in multiple states, including capping RAD51 filament segment. These distinct assemblies are differentially regulated by ATPase activity, defining a dynamic BCDX2-CX3 “loader” and a stable DX2-CX3 “anchor” that provide functional modularity to the HR machinery. This work provides a unifying mechanism for human RAD51 paralog function and delivers an atomic blueprint for interpreting disease-causing mutations.