Long tethers provide high-force coupling of the Dam1 ring to shortening microtubules.

Microtubule kinetochore attachments are essential for accurate mitosis, but how these force-generating connections move chromosomes remains poorly understood. Processive motion at shortening microtubule ends can be reconstituted in vitro using microbeads conjugated to the budding yeast kinetochore protein Dam1, which forms microtubule-encircling rings. Here, we report that, when Dam1 is linked to a bead cargo by elongated protein tethers, the maximum force transmitted from a disassembling microtubule increases sixfold compared with a short tether. We interpret this significant improvement with a theory that considers the geometry and mechanics of the microtubule-ring-bead system. Our results show the importance of fibrillar links in tethering microtubule ends to cargo: fibrils enable the cargo to align coaxially with the microtubule, thereby increasing the stability of attachment and the mechanical work that it can do. The force-transducing characteristics of fibril-tethered Dam1 are similar to the analogous properties of purified yeast kinetochores, suggesting that a tethered Dam1 ring comprises the main force-bearing unit of the native attachment.