Submolecular video-imaging of the Smc5/6 complex topologically bound to DNA

Structural maintenance of chromosomes (SMC) complexes, including cohesin and condensin, are ring-shaped ATPases that organize chromosome architecture by holding DNA segments together. However, the dynamic mechanisms underlying their DNA binding and processing remain unclear. Using high-speed atomic force microscopy, we directly visualized the eukaryotic Smc5/6 complex on DNA at submolecular resolution. Smc5/6 adopted an "I"-shaped conformation with closely aligned SMC arms and transitioned to an "O"-shaped open ring upon ATP binding. ATP-bound Smc5/6 stably associated with DNA through its ATPase head domains, whereas ATP hydrolysis promoted DNA entrapment within the SMC compartment and repositioning near the hinge. Smc5/6 also tethered two DNA segments together and stabilized twisted DNA structures, leading to DNA compaction. Our findings provide a visual demonstration of how an SMC complex employs its ring architecture to facilitate distinct DNA binding modes, deepening our understanding of the ATP-dependent dynamic transactions of Smc5/6 during DNA binding.