Biophysics of SMC complex function in chromosome organization and DNA repair

• Applicant: Professor Dr. Johannes Stigler
• Subject Area: Biophysics ; Biochemistry
• Term: since 2017
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 374605285

Chromosomes are important for the storage of genomic information but also provide the physical scaffold for transcription regulation and gene maintenance. The three-dimensional organization of chromosomes in an important aspect in all of these areas. Even in interphase, chromatin is hierarchically organized. SMC complexes like cohesin, condensin and Smc5/6 are key factors for chromosome organization in eukaryotes. Most SMC complexes interact with a series of additional regulatory subunits and it is possible that through these interactions, they can achieve a variety of diverse functions. For example, the tetrameric cohesin complex is responsible for establishing chromatin and DNA loops, as a fundamental block of chromosomal TADs (topologically associating domains). In addition, cohesin is responsible for maintaining cohesion between sister chromatids until the onset of anaphase. How precisely these functions are regulated is unclear, but it is possible that it is achieved through recruitment of additional subunits to the tetramer, through subunit exchange, or through posttranslational modifications. Similarly, the Smc5/6 complex is primarily implicated in processes related to DNA repair, but as for cohesin, its precise working principle and how its function is regulated by complex subunits is unclear. In the first years Emmy Noether funding, my group has embarked on studying the function of SMC complexes at the single molecule level. We focused our studies on cohesin and Smc5/6. We showed that single cohesin complexes, with the help of Scc2/4, form DNA loops in an ATP-dependent manner, and demonstrated the importance of the "elbow" feature for the folding of SMC coiled-coil arms. In addition, we showed that Smc5/6 specifically gets recruited to sites ssDNA-dsDNA junctions, in line with is proposed role of a DNA repair protein. Here, I propose investigating the effects of the Scc3 and Pds5 subunits to the cohesin complex, which are likely involved in complex regulation, but whose biophysical effects on cohesin are poorly understood. Furthermore, I propose investigating a potential mechanism for SMC motor function, using a single molecule force spectroscopic approach. Third, I propose studying the downstream activation of SUMO ligase function of Smc5/6 complexes, as well as the effect of the Nse5/6 subunit in the DNA lesion recruitment mechanism of Smc5/6. Together, these experiments will address many unresolved questions about the function of SMC complexes in chromosome organization.

• DFG Programme: Independent Junior Research Groups