The trimeric Shu complex in C. elegans is an ATPase that remodels RAD51 filaments in the homologous recombination-associated DNA damage response.

Homologous recombination (HR) is critical for error-free lesion bypass. This error-free lesion tolerance pathway is initiated by the RAD51 recombinase, which forms nucleoprotein filaments on single-stranded DNA (ssDNA) to facilitate template-directed repair using homologous sister chromatids. RAD51 filaments are tightly regulated by RAD51 mediator proteins. As such mediators, Shu complexes facilitate HR-directed DNA damage tolerance and are evolutionarily conserved from yeast to humans. The Caenorhabditis elegans (C. elegans) Shu complex is a heterotrimer consisting of three protein subunits: RFS-1, RIP-1, and SWS-1. However, the biochemical properties of this trimeric complex remain unclear. Here, we report the biochemical characterization of the C. elegans Shu complex and interactions with DNA, ATP, and RAD-51 filaments. We first revealed that the Shu trimer preferentially binds DNA with an exposed 5' end, particularly favoring a fork-shaped double-stranded DNA (dsDNA). Then, we found that the RFS-1/RIP-1/SWS-1 trimer binds to ATP and exhibits DNA-dependent ATPase activity. Through site-specific mutagenesis, we identified the catalytic residues in the RFS-1 domain and validated the ATPase activity. Using fluorescence-based assays, we further demonstrated that the Shu trimer remodels RAD-51 filaments in an ATP-hydrolysis-dependent manner and stabilizes the filaments in an ATP-binding-dependent manner. Using CRISPR-Cas9-induced disruption of the Walker motif in rfs-1, we demonstrate that the ATPase activity of the C. elegans Shu complex is critical for organismal survival under camptothecin-induced genotoxic stress. These findings provide key mechanistic insights into how the C. elegans Shu complex regulates RAD-51 filament dynamics and primes the filaments for downstream HR-mediated DNA repair processes.