Bub1 and Sgo1 modify the cohesin barrel in response to altered microtubule dynamics during mitosis. Phosphorylation of histone H2A by Bub1 and the recruitment of Sgo1 within the kinetochore/pericentric region result in a conformational change in the inner kinetochore and pericentric cohesin. The paradox in these results is that the pericentric chromatin expands radially from the spindle axis, while the inner kinetochore contracts toward the spindle axis. We propose that this change is due to the physical expansion of pericentric chromatin and cohesin surrounding the kinetochore and spindle microtubules (, ). Increasing the area occupied by pericentric chromatin could exert an isotropic force that expands the outer dimension (pericentric chromatin and cohesin) and contracts the inner dimension (inner kinetochore) of the cylindrical array of DNA and cohesin (, bottom). In metaphase, pericentric cohesin together with histones and condensin restrain the pericentric chromatin to an entropically disfavored area proximal to the spindle [11
]. Chromatin modification provides a mechanism to tune the force exerted from the constrained DNA polymer and maintain force balance with spindle microtubules.
Physical properties of the chromatin spring
Model for how chromatin structure modification can be mechanically amplified around the metaphase spindle
Upon exposure to agents that damage the spindle or chromosome attachments to the spindle, it may be important for the cell to modulate the chromatin spring in an effort to maintain force balance. The chromatin spring behaves like a worm-like chain in vivo
]. The parameters that dictate the spring constant are the chain length (Lc) and persistence length (Lp)(). Persistence length is the length scale over which the ends of links in the chain are correlated. The force required to extend a polymer chain is inversely proportional to the persistence length (F ~ KB
T, Boltzmann constant). This makes intuitive sense if one considers a polymer chain. The shorter the persistence length the greater the number of available states for the random coil (# of entropic states, ); the longer the persistence length the fewer number of states for the random coil (). Less force is required to extend a chain that has fewer available states, and therefore the spring constant decreases. The persistence length also dictates the physical size of the random coil (radius of gyration Rg ~ √(Lc Lp)). Increase of persistence length would change the dimensions of the pericentric chromatin (, ) and soften the spring. We propose that the cell is able to tune the chromatin spring constant in response to mitotic spindle damage. In addition, the reduction in spring constant would also lead to a reduction in tension–based rescue mechanisms that regulate microtubule growth [38
]. It may be important for the cell to shift the equilibrium to shorter kinetochore microtubules to maintain critical concentrations of tubulin polymer for the integrity of interpolar microtubules. The recruitment of additional Sgo1 to the kinetochore and pericentromere when the spindle is perturbed is suggestive of the continuous modulation of pericentric chromatin in response to damage. A quantitative increase in chromatin modification could reflect a biological rheostat for regulating the spring as a function of the extent of spindle damage and/or chromosome loss.
The mechanism by which Bub1 kinase and Sgo1 modulate chromatin structure and promote chromosome bi-orientation is poorly understood. Bub1 resides on the chromatin side of the kinetochore, several nanometers from the Histone H3-variant, Cse4 (). Sgo1 resides another 60nm towards the sister chromatid axis (). It has been previously proposed that Sgo1 is responsible for the intrinsic property of the kinetochore to bi- orient although a mechanism for this geometric bias has not been well understood [15
]. Sgo1 resides at a critical junction defined by the packing ratio of pericentric chromatin. The packing ratio of pericentric chromatin from 1.7kb to 8.8kb is 106bp/nm [11
]. From 1.7kb to the position of the centromere (CEN, Cse4) the packing ratio is 25bp/nm, equivalent to nucleosomal chromatin. The position of Sgo1 coincides with this transition (, S4
), and indicates that Sgo1 may stabilize the C-loop cruciform structure for geometric bias of sister centromeres [15
Our understanding of kinetochore geometry and pericentric spring function derive largely from the discovery of Shugoshin (Sgo1) and the mechanism by which cohesin, condensin and centromere loops generate a molecular spring. The molecular spring, comprised of the DNA worm-like chain and cohesin and condensin protein springs is important for generating a counterforce to spindle microtubules such that tension can be measured between sister kinetochores [11
]. The pericentric region is also responsible for predisposing replicated centromeres to lie on the surface of the chromosome. This study provides mechanistic insight into the role of H2A phosphorylation and recruitment of Sgo1 in tuning the chromatin spring in vivo