The Smc5/6 complex : the mysterious guardian of genome stability

Sammanfattning: An intricate network of proteins ensures the faithful transmission of genetic information through cell generations. The Structural Maintenance of Chromosomes (SMC) protein complex family plays a pivotal role in maintaining genome stability. Initially, the three eukaryotic SMC complexes, cohesin, condensin and Smc5/6 complex (Smc5/6) were identified for their functions in chromosome cohesion, condensation and recombination. Later, it was shown that SMC complexes also control replication and transcription. Another important group of proteins involved in the maintenance of genome stability are the topoisomerases. These enzymes control DNA topology to ensure faithful replication, transcription and chromosome segregation. Defects in processes that control genome maintenance lead to cell death and chromosomal aberrations, including aneuploidy and translocations, which are hallmarks of cancer cells. Therefore, it is essential to reveal the details of how genome stability is maintained in order to fully comprehend the underlying causes of tumor development. The aim of the projects described in this thesis was to increase the knowledge of Smc5/6, which is the least characterized of the three SMC complexes. Using the budding yeast Saccharomyces cerevisiae as model organism, our work shed light on the involvement of Smc5/6 in chromosome replication, segregation and, possibly, transcription. Moreover, in vitro analysis of purified Smc5/6 revealed new details of how the complex interacts with DNA. In Paper I, it was demonstrated that Smc5/6 accumulates onto chromosomes after sister chromatids are tethered by cohesin. Smc5/6 is also shown to facilitate segregation of short entangled chromosomes. Our data suggest that the chromosomal association of Smc5/6 occurs at sites where sister chromatids are entangled, and that entanglement is proportional to the level of superhelical stress. In Paper II, it was shown that Smc5/6 can both bind directly to, and topologically entrap DNA molecules in vitro. It was also demonstrated that Smc5/6 topologically entraps more than one DNA molecule at the time, stimulating their catenation by topoisomerase 2, which interacts with the complex. In Paper III, it was shown that Smc5/6 is recruited to the intergenic region between two highly transcribed genes. This suggests that Smc5/6 accumulates in regions of high transcription-induced superhelical stress.