A Single-Strand Annealing Protein Clamps DNA to Detect and Secure Homology |
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| Authors: | Marcel Ander Sivaraman Subramaniam Karim Fahmy A. Francis Stewart Erik Sch?ffer |
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| Affiliation: | 1. Nanomechanics Group, Biotechnology Center, TU Dresden, Dresden, Germany.; 2. Department of Genomics, Biotechnology Center, TU Dresden, Dresden, Germany.; 3. Division of Biophysics, Institute of Resource Ecology, Helmholtz-Zentrum Dresden-Rossendorf, Dresden, Germany.; 4. Cellular Nanoscience, Center for Plant Molecular Biology (ZMBP), Universität Tübingen, Tübingen, Germany.; Mount Sinai Hospital, CANADA, |
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| Abstract: | Repair of DNA breaks by single-strand annealing (SSA) is a major mechanism for the maintenance of genomic integrity. SSA is promoted by proteins (single-strand-annealing proteins [SSAPs]), such as eukaryotic RAD52 and λ phage Redβ. These proteins use a short single-stranded region to find sequence identity and initiate homologous recombination. However, it is unclear how SSAPs detect homology and catalyze annealing. Using single-molecule experiments, we provide evidence that homology is recognized by Redβ monomers that weakly hold single DNA strands together. Once annealing begins, dimerization of Redβ clamps the double-stranded region and nucleates nucleoprotein filament growth. In this manner, DNA clamping ensures and secures a successful detection for DNA sequence homology. The clamp is characterized by a structural change of Redβ and a remarkable stability against force up to 200 pN. Our findings not only present a detailed explanation for SSAP action but also identify the DNA clamp as a very stable, noncovalent, DNA–protein interaction. |
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