Structure-function relationships of SMC protein complexes for DNA loop extrusion

Structural Maintenance of Chromosome (SMC) complexes are vital for chromosome organization. They extrude DNA loops to compact 2 meters of DNA into a micrometer-sized chromosome structure. The DNA loop extrusion process is believed to be a universal mechanism of SMC complexes for spatiotemporal chromosome organization conserved in almost all species from prokaryotes to eukaryotes. However, the molecular mechanism of DNA loop extrusion by SMC complexes is under debate; various tentative mechanistic models have been suggested, but there is no clear consensus. Here, we review the structural studies of various SMC complexes from prokaryotes to eukaryotes to understand the structurefunction relationships of SMC complexes involved in DNA loop extrusion. We introduce controversial observations of the conformations of SMC complexes based on previous reports and discuss various proposed mechanisms of DNA loop extrusion suggested by experimental observations of the conformations of diverse SMC complexes.