Correct chromosome segregation depends on mechanisms sensing the attachment of chromosomes to the spindle. Lack of tension between sister kinetochores produces a conformational change in the kinetochore, but a detailed mechanistic understanding of this process is lacking. By visualizing GFP-tagged proteins in budding yeast, Bloom and colleagues now describe how reduced microtubule dynamics alters kinetochore and pericentric chromatin geometry (Curr. Biol. 22, 1–11; 2012)

The authors find that reducing microtubule dynamics with low concentrations of benomyl leads to a contraction of inner kinetochore height. Visualizing cohesin, a chromatin-associated protein complex, reveals that the cohesin barrel instead expands in response to benomyl. Using mutant strains, the authors find that both inner kinetochore contraction and cohesin expansion require Shugoshin 1 (Sgo1, a protein that protects cohesin from degradation), Bub1 kinase (known to recruit Sgo1) and Bub1-mediated phosphorylation of histone H2A, but not the spindle assembly checkpoint. The pericentric chromatin itself also expands following benomyl treatment, as observed by imaging centromere-tagged lacO arrays, and this is associated with suppressed chromatin dynamics. The authors suggest that the physical expansion of pericentric chromatin and cohesin exerts a force on the inner kinetochore, causing its contraction. Statistical maps of Bub1 and Sgo1 distribution further reveal that benomyl increases Sgo1 recruitment to the pericentric region and alters Bub1 distribution. Thus, Bub1 and Sgo1 are involved in translating the response to altered microtubule dynamics into geometrical changes in pericentric chromatin and cohesin structure.