18.3 : Cohesins

During S-phase of the cell cycle in eukaryotes, chromosomal duplication produces two identical copies of each chromosome called sister chromatids.

The sister chromatids formed are held together by a set of protein complexes called cohesins. The cohesin rings exist as clamps around the sister chromatids at multiple locations along their length, preventing them from drifting apart.

Cohesin complexes contain four subunits: Smc1, Smc3, Scc1, and Scc3. The Smc1 and Smc3 are coiled-coil proteins with a hinge domain at one end and an ATPase head domain at the other end. The hinge domains of Smc1 and Smc3 bind directly to each other, while an Scc1 subunit connected to an Scc3 subunit bridges the head domains of Smc1 and Smc3, forming a ring-like structure.

The hinge domain of the cohesin ring-structure can be triggered to open and close, facilitating cohesin loading on the chromosomes.

As cells progress through mitotic prophase, sister chromatid resolution takes place, involving dissociation of cohesin rings along the chromosomal arms while preserving those bound to the centromere region. The differential removal of cohesins causes the sister chromatids to become partially separated along their arms while remaining bound at their centromere.

Chromatid cohesion at the centromere facilitates the bi-orientation of chromosomes on the mitotic spindle during metaphase, ensuring correct microtubule attachment to the kinetochores of the sister chromatids.

At the onset of anaphase, a protease enzyme, separase, cleaves the Scc1 subunit, leading to the dissociation of cohesin from the chromosome.

Cohesin dissociation permits the segregation of sister chromatids during anaphase, where they are pulled apart by mitotic spindles to opposite poles of the cell, eventually leading to cell division and formation of two daughter cells.