The Smc5/6 complex confers resistance to caffeine and genotoxic stress and plays a role in cell cycle regulation and cell survival in Drosophila melanogaster. Xiao Li¹, Ran Zuo², Stanley Tiong², Francesca Di Cara², Kirst King-Jones², Sarah C. Hughes¹, Shelagh D. Campbell², Rachel Wevrick¹. 1) Department of Medical Genetics, University of Alberta, Edmonton, Alberta, Canada; 2) Department of Biological Sciences, University of Alberta.

   The SMC5/6 complex consists of Smc5, Smc6 and Non-Smc-Element (Nse) proteins and is important for genome stability in many species. We identified inactivating mutations in CG5524 and MAGE, homologs of genes encoding Smc6 and Nse3 in yeast, from a genetic screen for mutants with reduced resistance to caffeine. Smc5 mutants are also caffeine-sensitive and Mage physically interacts with Drosophila homologs of Nse proteins, indicating that the structure of the Smc5/6 complex is conserved in Drosophila. Unlike their yeast counterparts, the Drosophila Smc5/6 complex is not essential under normal circumstances, although the mutants are hypersensitive to genotoxic agents such as ionizing radiation, camptothecin, hydroxyurea and MMS, consistent with a conserved role of the Smc5/6 complex in genome stability. We also show that they are not compromised for pre-mitotic cell cycle checkpoint responses. Rather, caffeine-induced apoptosis in these mutants is exacerbated by inhibition of ATM or ATR checkpoint kinases but suppressed by Rad51 depletion, suggesting a novel functional interaction involving homologous DNA repair pathways that deserves further scrutiny. We hypothesize that caffeine treatment and the loss of Smc5/6 synergistically misregulate Rad51 to cause apoptosis in Drosophila. In addition, overexpression of MAGE in Drosophila developmental eyes results in a small eye phenotype that can be suppressed by co-overexpression of cycE and overexpression of MAGE in S2 cells enhances their resistance to genotoxic agents, suggesting a role in cell cycle regulation and cell survival. Whether the other components of the Smc5/6 complex are involved is under investigation. Our insights into the SMC5/6 complex provide new challenges for understanding the role of this enigmatic chromatin factor in multi-cellular organisms.