Structure of RADX and mechanism for regulation of RAD51 nucleofilaments.
Balakrishnan, S., Adolph, M., Tsai, M.S., Gallagher, K., Cortez, D., Chazin, W.J.(2023) Biorxiv 
- PubMed: 37786681 
- DOI: https://doi.org/10.1101/2023.09.19.558089
- Primary Citation of Related Structures:  
8U5Y - PubMed Abstract: 
Replication fork reversal is a fundamental process required for resolution of encounters with DNA damage. A key step in the stabilization and eventual resolution of reversed forks is formation of RAD51 nucleoprotein filaments on exposed ssDNA. To avoid genome instability, RAD51 filaments are tightly controlled by a variety of positive and negative regulators. RADX is a recently discovered negative regulator that binds tightly to ssDNA, directly interacts with RAD51, and regulates replication fork reversal and stabilization in a context-dependent manner. Here we present a structure-based investigation of RADX's mechanism of action. Mass photometry experiments showed that RADX forms multiple oligomeric states in a concentration dependent manner, with a predominance of trimers in the presence of ssDNA. The structure of RADX, which has no structurally characterized orthologs, was determined ab initio by cryo-electron microscopy (EM) from maps in the 2-3 Å range. The structure reveals the molecular basis for RADX oligomerization and binding of ssDNA binding. The binding of RADX to RAD51 filaments was imaged by negative stain EM, which showed a RADX oligomer at the end of filaments. Based on these results, we propose a model in which RADX functions by capping and restricting the growing end of RAD51 filaments.
Organizational Affiliation: 
Center for Structural Biology, Vanderbilt University, Nashville, TN 37240, USA.