Shaping chromosomes by DNA capture and release: gating the SMC rings

- SMC action can be explained by DNA co-entrapment and DNA loop extrusion.
- SMC loading and release controls chromosome superstructure.
- Cohesin unloads from chromosomes by the tightly regulated opening of a DNA exit gate.
- Similarities and differences in cohesin loading and unloading are discussed.

SMC proteins organize chromosomes to coordinate essential nuclear processes such as gene expression and DNA recombination as well as to segregate chromosomes during cell division. SMC mediated DNA bridging keeps sister chromatids aligned for much of the cell cycle, while the active extrusion of DNA loops by SMC presumably compacts chromosomes. Chromosome superstructure is thus given by the number of DNA linkages and the size of chromosomal DNA loops, which in turn depend on the dynamics of SMC loading and unloading. The latter is regulated by the intrinsic SMC ATPase activity, multiple external factors and post-translational modification. Here, I highlight recent advances in our understanding of DNA capture and release by SMC-with a focus on cohesin.