Structural maintenance of chromosome (SMC) proteins play a key roles in the chromosome organization by condensing two meters of DNA into cell-sized structures considered as the SMC protein extrudes DNA loop. Recent sequencing-based high-throughput chromosome conformation capture technique (Hi-C) and single-molecule experiments have provided direct evidence of DNA-loop extrusion. However, the molecular mechanism by which SMCs extrude a DNA loop is still under debate. Here, we review DNA-loop extrusion studies with single-molecule assays and introduce recent structural studies of how the ATP-hydrolysis cycle is coupled to the conformational changes of SMCs for DNA-loop extrusion. In addition, we explain the conservation of the DNA-binding sites that are vital for dynamic DNA-loop extrusion by comparing Cryo-EM structures of SMC complexes. Based on this information, we compare and discuss four compelling working models that explain how the SMC complex extrudes a DNA loop.

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