Abstract
Chromosomes interact through their kinetochores with microtubule plus ends and they are segregated to the spindle poles as the kinetochore microtubules shorten during anaphase A of mitosis. The molecular natures and identities of coupling proteins that allow microtubule depolymerization to pull chromosomes to poles during anaphase have long remained elusive. In budding yeast, the ten-protein Dam1 complex is a critical microtubule-binding component of the kinetochore that oligomerizes into a 50-nm ring around a microtubule in vitro. Here we show, with the use of a real-time, two-colour fluorescence microscopy assay, that the ring complex moves processively for several micrometres at the ends of depolymerizing microtubules without detaching from the lattice. Electron microscopic analysis of 'end-on views' revealed a 16-fold symmetry of the kinetochore rings. This out-of-register arrangement with respect to the 13-fold microtubule symmetry is consistent with a sliding mechanism based on an electrostatically coupled ring-microtubule interface. The Dam1 ring complex is a molecular device that can translate the force generated by microtubule depolymerization into movement along the lattice to facilitate chromosome segregation.
