Mechanism of Action
It is not clear how the cohesion ring links sister chromatids together. There are two possible scenarios:
- Cohesin subunits bind to each sister chromatid and form a bridge between the two.
- Since cohesin has a ring structure, it is able to encircle both sister chromatids.
Current evidence suggests that the second scenario is the most likely. Proteins that are essential for sister chromatid cohesion, such as Smc3 and Scc1, do not regulate the formation of covalent bonds between cohesin and DNA, indicating that DNA interaction is not sufficient for cohesion. In addition, disturbing the ring structure of cohesin through cleavage of Smc3 or Scc1 triggers premature sister chromatid segregation in vivo. This shows that the ring structure is important for cohesin’s function.
Even though the ring hypothesis appears to be valid, there are still questions about the number of rings required to hold sister chromatids together. One possibility is that one ring surrounds the two chromatids. Another possibility involves the creation of a dimer where each ring surrounds one sister chromatid. The two rings are connected to each other through formation of a bridge that holds the two sister chromatids together.
The cohesion complex is established during the initial stages of S-phase. The complexes associate with chromosomes before DNA replication occurs. Once cells start replicating their DNA, cohesin rings close and link the sister chromatids together. Cohesin complexes must be present during S-phase in order for cohesion to take place. It is unclear, however, how cohesin is loaded on the chromosomes during G1. There are two proposed hypotheses so far:
- The ATPase domain of the SMC proteins interacts with DNA and this interaction initially mediates the loading of cohesin complexes on chromosomes.
- Several proteins aid in the loading process. For example, Scc2 and Scc4 are both required for cohesin to load in budding yeast.
Read more about this topic: Cohesin
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