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 elusive¹. In budding yeast, the ten-protein Dam1 complex is a critical microtubule-binding component of the kinetochore² that oligomerizes into a 50-nm ring around a microtubule in vitro^{3, 4}. 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.

1. Department of Molecular and Cell Biology,
2. Lawrence Berkeley National Laboratory,
3. Howard Hughes Medical Institute, University of California Berkeley, Berkeley, California 94720-3202, USA

Correspondence to: Georjana Barnes¹ Correspondence and requests for materials should be addressed to G.B. (Email: gbarnes@socrates.berkeley.edu).

Received 22 September 2005 | Accepted 9 November 2005 |

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