The mechanism of 3-D chromatin organization by cohesin revealed by single molecule spectroscopy in live cells

The Structural Maintenance of Chromosome complex cohesin mediates the 3-dimentional organization of chromatin. It ensures the fidelity of chromosome segregation in mitosis by tethering the sister chromatids. In addition to its mitotic functions, cohesin involved in DNA repair and transcription regulation. However, the mechanism by which cohesin works is elusive and under continuous debate. One of the long lasting arguments in the field engages cohesin stoichiometry. Models that involve either cohesin monomers or dimers have been proposed. We addressed this dispute by using single molecule spectroscopy and showed that in cells under all experimental conditions, cohesin is an exclusive monomer. In addition, our investigations revealed that the poorly studied Scc3 subunit of cohesin mediates the interaction with the Scc2 loader. This interaction is important for proper cohesin loading onto the chromatin. However, we show that in addition, it blocks premature non-canonical formation of cohesin-DNA complexes. These findings provide a rationale to the suggested Scc2-Pds5 switch that differentiates cohesin loading and cohesion maintenance, and offer a mechanistic insight into the tethering activity. Altogether, our data reveals new and important details on the molecular basis of cohesin activity and provides a new insight into the mechanisms that governs the spatial organization of chromatin in eukaryotic cells.