Tension management in the kinetochore

Abstract

The kinetochore is the protein machine built at the centromere that integrates mechanical force and chemical energy from dynamic microtubules into directed chromosome motion. The kinetochore also provides a powerful signaling function that is able to alter the properties of the spindle checkpoint and initiate a signal transduction cascade that leads to inhibition of the anaphase promoting complex and cell cycle arrest. Together, the kinetochore accomplishes the feat of chromosome segregation with unparalleled accuracy. Errors in segregation lead to Down's syndrome, the most frequent inherited birth defect, pregnancy loss, and cancer. Over a century after the discovery of the kinetochore, an architectural map comprising greater than 100 proteins is emerging. Understanding the architecture and physical biology of the key components provides new insights into how this fascinating machine moves genomes.

Figure 1. Material properties of the major spindle components

The mitotic spindle is composed of proteins and DNA. The dominant protein in the spindle is tubulin, organized into an equilibrium polymer known as microtubules. DNA is wrapped around histone protein into nucleosomes, which are further compacted into the chromosome. DNA behaves as a worm-like chain and adopts a random coil due to thermal fluctuations of the chain.

Figure 2. The protein architecture of microtubule attachment sites at the budding yeast and HeLa kinetochores is highly conserved

(A) 3D visualization of the metaphase budding yeast kinetochore–microtubule attachment, as predicted by the protein localization data, assuming a symmetric arrangement of kinetochore protein complexes around the cylindrical microtubule lattice. Black stars indicate the positions of fluorescent labels used in distance measurements. Dashed lines indicate established biochemical interactions between two protein complexes (adapted from Figure 3 in [22]). (B) Yeast measurements are derived from kinetochore proteins labeled at the carboxyl or amino terminus with fluorescent proteins. In the case of many HeLa kinetochore measurements, antibodies recognized other regions within the proteins. The differences arising from this experimental factor have not been quantified. Letters in the bracket next to the KNL-1 protein indicate amino terminus (N) and middle protein (M).

Figure 3. A schematic diagram of force generation in a mitotic spindle

(A) The contribution of chromatin to the force balance in the spindle has been postulated as an inward force linearly related to extension (‘Hookean’ spring). (B) Based upon the distribution of cohesin [17] and pericentric chromatin [65], there may be force generation from the repulsion of polymers in a confined space (see text and [64]).