Cohesin

·     Cohesines are a member of a big protein family called SMC (Structural Maintenance of Chromosomes), who have a very important role in structural and functional organization of chromosomes.

·      SMC proteins have a crucial role in chromosome segregation and DNA repair. These proteins are interesting because of, among other aspects, their unique structure. They are dimers formed by two long coiled-coil motifs (a rod like structural shape that is formed by two long α-helices twisted around each other) connected by a non-helical sequence. An ATPase domain is created by folding a SMC monomer back to itself. This folding also creates a hinge domain at the other end. In this hinge domain, two monomers associate with each other, creating a V-shaped molecule. There are many possible structures for the SMC to possess, thanks to their high flexibility of dimers conformation.

·      Cohesines are composed by four subunits: Scc1, Scc3, Smc1 and Smc3. These last two subunits are members of SMC protein family, having therefore, ATPase domains in one end each. The two ATP domains are able to bind in the presence of ATP, thus resulting a ring-shaped structure. Scc1 and Scc3 (Spirochetal Coiled-Coil) are responsible for this binding and are also responsible of maintaining and stabilizing it. Scc1 also plays and important role in chromosome replication as it controls the separation of sister-chromatids. The cohesin ring structure is important as it keeps the sister-chromatids together during metaphase, ensuring that they travel to opposite polls of the cell (both in mitosis and meiosis). The structure facilitates as well, the spindle attachment onto chromossomes (process responsible for chromosome segregation to cell-daughters during cell division) and DNA damage repair.

Fig1: Cohesin structure

The following link is a short animation that explains the role of cohesin in cell division:

http://highered.mcgraw-hill.com/sites/9834092339/student_view0/chapter10/the_function_of_cohesin.html