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).

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 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.

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