The Saccharomyces cerevisiae Shu complex, consisting of Shu1, Shu2, Csm2 and Psy3, promotes error-free homologous recombination (HR) by an unknown mechanism. Recent structural analysis of two Shu proteins, Csm2 and Psy3, has revealed that these proteins are Rad51 paralogues and mediate DNA binding of this complex. We show in vitro that the Csm2–Psy3 heterodimer preferentially binds synthetic forked DNA or 3′-DNA overhang substrates resembling structures used during HR in vivo. We find that Csm2 interacts with Rad51 and the Rad51 paralogues, the Rad55–Rad57 heterodimer and that the Shu complex functions in the same epistasis group as Rad55–Rad57. Importantly, Csm2’s interaction with Rad51 is dependent on Rad55, whereas Csm2’s interaction with Rad55 occurs independently of Rad51. Consistent with the Shu complex containing Rad51 paralogues, the methyl methanesulphonate sensitivity of Csm2 is exacerbated at colder temperatures. Furthermore, Csm2 and Psy3 are needed for efficient recruitment of Rad55 to DNA repair foci after DNA damage. Finally, we observe that the Shu complex preferentially promotes Rad51-dependent homologous recombination over Rad51-independent repair. Our data suggest a model in which Csm2–Psy3 recruit the Shu complex to HR substrates, where it interacts with Rad51 through Rad55–Rad57 to stimulate Rad51 filament assembly and stability, promoting error-free repair.

The conserved budding yeast Rad51 paralogues, including Rad55, Rad57, Csm2 and Psy3 are indispensable for homologous recombination (HR)-mediated chromosome damage repair. Rad55 and Rad57 are associated in a heterodimer, while Csm2 and Psy3 form the Shu complex with Shu1 and Shu2. Here we show that Rad55 bridges an interaction between Csm2 with Rad51 and Rad52 and, using a fully reconstituted system, demonstrate that the Shu complex synergizes with Rad55–Rad57 and Rad52 to promote nucleation of Rad51 on single-stranded DNA pre-occupied by replication protein A (RPA). The csm2–F46A allele is unable to interact with Rad55, ablating the ability of the Shu complex to enhance Rad51 presynaptic filament assembly in vitro and impairing HR in vivo. Our results reveal that Rad55–Rad57, the Shu complex and Rad52 act as a functional ensemble to promote Rad51-filament assembly, which has important implications for understanding the role of the human RAD51 paralogues in Fanconi anaemia and cancer predisposition.

The Saccharomyces cerevisiae Shu2 protein is an important regulator of Rad51, which promotes homologous recombination (HR). Shu2 functions in the Shu complex with Shu1 and the Rad51 paralogs Csm2 and Psy3. Shu2 belongs to the SWS1 protein family, which is characterized by its SWIM domain (CXC...X n ...CXH), a zinc-binding motif. In humans, SWS1 interacts with the Rad51 paralog SWSAP1. Using genetic and evolutionary analyses, we examined the role of the Shu complex in mitotic and meiotic processes across eukaryotic lineages. We provide evidence that the SWS1 protein family contains orthologous genes in early-branching eukaryote lineages (e.g., Giardia lamblia), as well as in multicellular eukaryotes including Caenorhabditis elegans and Drosophila melanogaster. Using sequence analysis, we expanded the SWIM domain to include an invariant alanine three residues after the terminal CXH motif (CXC. . .X n . . .CXHXXA). We found that the SWIM domain is conserved in all eukaryotic orthologs, and accordingly, in vivo disruption of the invariant residues within the canonical SWIM domain inhibits DNA damage tolerance in yeast and protein-protein interactions in yeast and humans. Furthermore, using evolutionary analyses, we found that yeast and Drosophila Shu2 exhibit strong coevolutionary signatures with meiotic proteins, and in yeast, its disruption leads to decreased meiotic progeny. Together our data indicate that the SWS1 family is an ancient and highly conserved eukaryotic regulator of meiotic and mitotic HR.KEYWORDS DNA repair; Shu complex; budding yeast; evolutionary rate covariation; homologous recombination H OMOLOGOUS recombination (HR) is an error-free DNA repair pathway that is essential both in mitotic cells to ensure DNA stability and in meiotic cells for faithful chromosome segregation. HR begins with double-strand break (DSB) formation and DNA end processing that give rise to 39 singlestranded DNA (ssDNA) overhangs (Heyer et al. 2010). A key step in HR is the formation of RecA-like filaments on these ssDNA ends. During mitosis in the budding yeast Saccharomyces cerevisiae, the RecA-like protein Rad51 coats the ssDNA, whereas during meiosis, both Rad51 and another RecA-like protein, Dmc1, form on ssDNA ends (Lin et al. 2006;Heyer et al. 2010). Formation of RecA-like filaments on the DNA is essential for the homology search and strand-invasion steps that define HR. Therefore, regulation of RecA filament formation is critical for accurate repair of DNA damage and chromosome segregation. Given their importance, both Rad51 and Dmc1 are extremely well-conserved descendants of the archaeal protein RADA (Diruggiero et al. 1999;Komori et al. 2000;Lin et al. 2006;Chintapalli et al. 2013).There are many proteins that both promote Rad51 filament formation and disassemble inappropriate filaments. Interestingly, in many organisms, the proteins that stabilize Rad51 filaments themselves share structural homology with Rad51 and evolved from the archaeal RADB homolog (Lin et al. 2006). In humans, these Rad51 paralogs...

DNA damage must be repaired in an accurate and timely fashion to preserve genome stability. Cellular mechanisms preventing genome instability are crucial to human health because genome instability is considered a hallmark of cancer. Collectively referred to as the DNA damage response, conserved pathways ensure proper DNA damage recognition and repair. The function of numerous DNA damage response components is fine-tuned by posttranslational modifications, including ubiquitination. This not only involves the enzyme cascade responsible for conjugating ubiquitin to substrates but also requires enzymes that mediate directed removal of ubiquitin. Deubiquitinases remove ubiquitin from substrates to prevent degradation or to mediate signaling functions. The Saccharomyces cerevisiae deubiquitinase Ubp7 has been characterized previously as an endocytic factor. However, here we identify Ubp7 as a novel factor affecting S phase progression after hydroxyurea treatment and demonstrate an evolutionary and genetic interaction of Ubp7 with DNA damage repair pathways of homologous recombination and nucleotide excision repair. We find that deletion of UBP7 sensitizes cells to hydroxyurea and cisplatin and demonstrate that factors that stabilize replication forks are critical under these conditions. Furthermore, ubp7⌬ cells exhibit an S phase progression defect upon checkpoint activation by hydroxyurea treatment. ubp7⌬ mutants are epistatic to factors involved in histone maintenance and modification, and we find that a subset of Ubp7 is chromatin-associated. In summary, our results suggest that Ubp7 contributes to S phase progression by affecting the chromatin state at replication forks, and we propose histone H2B ubiquitination as a potential substrate of Ubp7.