Mechanism of outer kinetochore assembly on microtubules and its regulation by mitotic error correction

Kinetochores couple chromosomes to the mitotic spindle and transduce the energy of microtubule depolymerization to segregate the genome during cell division. Kinetochore – microtubule attachments are often initially erroneous and subject to a mitotic error correction (EC) mechanism that drives their turnover until biorientation is achieved. How this is accomplished and regulated, and how kinetochore-mediated chromosome segregation occurs at a molecular level remain major outstanding questions. Here we describe the cryo-electron microscopy (cryo-EM) structure of the budding yeast outer kinetochore Ndc80 and Dam1 ring complexes assembled onto microtubules. We observe coordinated interactions of the outer kinetochore complexes through multiple interfaces, in addition to a short staple within the Dam1 subunit that facilitates Dam1c ring assembly. Perturbation of these interfaces results in loss of yeast viability. Force-rupture assays indicated this is a consequence of substantial reductions in kinetochore – microtubule binding strength. EC-mediated phosphorylation of Ndc80c-Dam1c interfaces would drive complex disassembly, whereas Dam1 staple phosphorylation would promote Dam1c ring disassembly, explaining how kinetochore – microtubule attachments are destabilized and reset by the EC mechanism. One-Sentence Summary Phosphorylation disrupts the outer kinetochore to regulate kinetochore-microtubule attachments in mitotic error correction.